A review on the corrosion resistance of electroless Ni-P based composite coatings and electrochemical corrosion testing methods

2.1 Effect of phosphorus content

The most common corrosive media using for corrosion testing is sodium chloride solution. Ni-P deposits in sodium chloride solution exhibits a better behaviour (a strong passivation tendency) than electroless nickel–boron coatings (Sudagar et al. 2013). Electroless Ni coatings with high phosphorus contents offer an excellent corrosion protection whereas moderate and lower amount of phosphorus contents are not preferred for unpropitious environments (Bigdeli and Allahkaram 2009; Elansezhian et al. 2008; Mallory and Hajdu 1990). Generally, 3–5 wt.% phosphorus content can be regarded as low phosphorus content and shows corrosion resistivity in concentrated caustic soda. 6–10 wt% phosphorus content can be taken as medium phosphorus content which provides moderate corrosive protection for some selective fields. Finally, high phosphorus content is measured as 11–18 wt% and it effectively works against chlorides, simultaneous mechanical stress and most of the applications (Alaneme et al. 2017; Sankara Narayanan et al. 2006). Moreover, electroless Ni-P coating can be graded as low (1–4 wt.% P), average (5–9 wt.% P) and high (10 wt% P or more) based on their phosphorus content, according to ASTM 733B-04 standard. The phosphorus content in electroless plating bath solution is considered to have a significant effect on both chemical (Schorr and Valdez 2016) and electrochemical (Court et al. 2000; Kerr et al. 1996, 1997) behaviours. The high weight percentage of phosphorus content can enhance the corrosion potential (E_corr) and significantly reduce corrosion current density (i_corr) by conducting the cathodic and anodic reactions during the corrosion process. Generally, the resistance of any alloy to corrosion depends on the dissolution rate of the passive film. The electroless Ni-P coating exhibits higher corrosion resistivity due to its amorphous structure (Fayyad et al. 2018). As a result, it can enhance the phosphorus content on the surface layer. Amorphous alloys provide greater resistance from corrosion damage than comparable polycrystalline materials because of the absence of grain and grain boundaries.

The SEM micrographs of electroless Ni-P coating have been shown in Figure 1. According to the findings of Lu and Zangari (Lu and Zangari 2002), the variation in surface morphologies of the electroless Ni-P coatings appear due to using different concentrations of phosphorus. Electroless Ni coating with high phosphorus content exhibits a smooth surface with uniform particle size (Figure 1A) whereas an electroless Ni coating with low phosphorus is observed with a fairly rough surface comprising of spherical particles of asymmetrical sizes (Figure 1B). The existence of cauliflower-like nodules (Figure 1A) is noticed in the electroless Ni-P coated surface where phosphorus contents remain high. Due to the increased phosphorous content of the plates, the nickel lattice disorder can cause a shift from crystalline to amorphous structure. By contrast, the average and low contents of phosphorus show microcrystalline and crystalline structure. The electroless Ni coated surface having low phosphorus content delineates Ni (111) and Ni (200) lattice structure with sharp peaks which is confirmed by Shibli and Dilimon from the X-ray diffraction pattern (Shibli and Dilimon 2007) and reveals its crystalline nature. On the other hand, in the case of medium phosphorus content, they found more blurred and diffused peaks and most remarkably no peaks are identified for the Ni-P plates with high phosphorus content due to their amorphous structure in the XRD sequence.

Figure 1:

SEM micrographs of electroless Ni-P coated surface: (A) high (13.30 wt%) P content and (B) low (3.34 wt%) P content (Sankara Narayanan et al. 2006). (Reprinted with permission, Copyright 2006 Elsevier).

According to Bai et al. (Bai et al. 2003), precise phosphorus content of the electroless Ni-P coating has a significant impact on decreasing the rate of corrosion and positively shift the corrosion potential in brine media. Expanded nickel corrosion provides basic requirements for phosphorus concentration and thus for the development on the surface of stable intermediate compounds Ni_xP_y, which serve as a passive film barrier. Phosphorus containing in the sodium hypophosphite react with water and generates phosphorus oxidation by forming a thin film of hypophosphite anions (H₂PO₂^─) on the coated substrate. The presence of hypophosphite and phosphate anions, as shown in Equations (1) and (2), were identified by X-ray photoelectron spectroscopy (XPS) measurement on the coated surface (Diegle et al. 1988).

During anodic polarization performed in the corrosive media (3.5 wt% NaCl solution), the hypophosphite layer is formed by maintaining at the pH 5.9 (Bonin et al. 2018). The formation of phosphate anion may also have been anticipated because of applying high overpotential (+250 mV) from the open circuit potential. The electrochemical formation of Ni²⁺ is achieved through anodic dissolution in equilibrium aqueous electrochemical system and the existence of H₂PO₄^─ is confirmed by comparing Pourbaix diagram. The Ni hydration is stymied due to the formation of hypophosphite layer which can block the water molecules to reach the electrode surface and thus it cannot interact with Ni. Therefore, the accumulation of phosphorus on the electrode surface results in increased corrosion resistance for electroless Ni-P coatings (Balaraju et al. 2006a). It can be observed that the corrosion resistance increases when the electroless Ni–high P coating forms the outer layer of the graded Ni-P coating due to the barrier properties of the underlying Ni–high P layer (Clausmeyer et al. 2016). The existence of enriched phosphorus is introduced to us by Kouwe (van der Kouwe 1993) in a different way. He used glow discharge optical emission spectroscopy to prove the presence of phosphorus in electroless Ni-P coating. The enriched phosphorus in the electroless plating bath solution can be hydrolyzed easily but the hydrolysis of Ni is not favorable at that condition due to the increment of P content. Several studies have pointed out the variation in corrosion resistance with different amount of phosphorus present in the electroless coating solution (Bai et al. 2003; Lu and Zangari 2002; Raicheff and Zaprianova 2000). In addition, a number of researchers also agree to these effects, including Lu (Lu and Zangari 2002), Lee (Lee and Liang 1991) and Aoki (Aoki and Takano 1986) by showing lower i_corr and better E_corr value with electronic effects, structure and homogeneity of the coated surface. The phosphorus content range 13–25 wt% in electroless coating acts as an electron acceptor and can gain 0.4 to 0.8 electrons (Okamoto et al. 1980) from Ni atom and shows amorphous structure.

The corrosion current density (i_corr) for 18 wt% phosphorus content exhibited 0.82 × 10⁻³ mA/cm² (Shibli and Dilimon 2007) while it showed 8.7 × 10⁻³ mA/cm² for using 10 wt% phosphorus content (Shibli and Dilimon 2007) in electroless nickel plating bath solution which is nearly 10 times higher. This can be validated by the theoretical fact that lower i_corr value gives higher corrosion resistance. Only 8% difference in phosphorus content can make a huge difference in corrosion resistance. It is evident from Sankara Narayanan et al. (2006) that electroless Ni-P coatings with higher phosphorus content (13.30 wt%), corrosion current density value is 0.60 × 10⁻³ mA/cm² whereas, with lower phosphorus (3.34 wt%) content, corrosion current density value has increased to 4.22 × 10⁻³ mA/cm². Figure 2 shows that impact of phosphorus content on the corrosion current density in both non-aerated and aerated 3.5 wt% NaCl solution. In non-aerated condition as phosphorus content increases, the corrosion current density decreases which results in the higher corrosion resistance. However, the lowest values of corrosion current densities have been observed for aerated condition. The corrosion current density increases slightly in an aerated condition as the phosphorus content increases.

Figure 2:

Effect of phosphorus content on current density in non-aerated (Balaraju et al. 2006a; Cui et al. 2006; Sankara Narayanan et al. 2006; Shibli and Dilimon 2007) and aerated conditions (Ashassi-Sorkhabi and Rafizadeh 2004).

Balaraju et al. (2006a) showed that for Ni-P coating at 11.36 wt% phosphorus, the value of corrosion current density is 0.456 × 10⁻³ mA/cm². This result exhibited a 33% deterioration in corrosion protection when he added extra element (tungsten) in the Ni-P coating solution and made it ternary alloy composite coating. This deterioration occurs due to the decrement of almost 6 wt% phosphorus content. However, for quaternary alloy (Ni-W-Cu-P) composite coating, though the presence of phosphorus content is only about 4.92 wt%, the presence of 4.08 wt% copper content in the coating solution exhibits a better corrosion resistance than ternary alloy (Ni-W-P) noting the corrosion current density is 0.481 × 10⁻³ mA/cm². As a result, if the content of tungsten keeps slightly less than 3 wt% and simultaneously phosphorus content raises to 7–9 wt.%, then the corrosion resistance must be higher.

Another influence on corrosion resistance of the weight percentage of phosphorus decrement in electroless Ni-P plating was found (Ghavidel et al. 2020). The corrosion current density showed higher corrosion safety when 9.71 wt% phosphorus content was used. However, after using 2 gm SiC nanoparticle in electroless Ni-P coating, it lowers the corrosion resistance. This phenomenon happens due to the incorporation of new nanoparticle which lowers the phosphorus content in an overall solution. This can be alleviated by adopting proper concentration of SiC nanoparticles and sodium hypophosphite in electroless plating bath solution. There must be a balanced addition of sodium hypophosphite and SiC nanoparticles in order to obtain the best protection against corrosion where phosphorus content remains at least 9 wt% (Ghavidel et al. 2020). Less than 9 wt% of P content can make grow the dislocations and crystals boundaries at the interface of particles and Ni-P matrix. In addition, the compact structure of the Ni-P coating can be damaged by using an inappropriate proportion of SiC nanoparticles and thus the accumulation of particles in the coating can create microholes and porosities in a high proportion. In fact, the concentration of SiC nanoparticles and its particle size show some inconsistent value of corrosion current density which will be discussed later part in this article.

The higher double layer capacitance (C_dl) value shows better corrosion protection. A total of 28 μF/cm² double layer capacitance was measured in 5 wt% NaCl corrosive media with 10–11 wt % phosphorus content in electroless Ni-P coating (Zeller 1991). However, in 1.88 wt% NaOH deaerated condition, Lo et al. (1995) found that the C_dl values are quite remarkable ranging 100–120 μF/cm² by using the phosphorus content 11.8–12.8 wt%. The corrosive safety could be more improved if NaCl solution was used instead of NaOH solution as an electrolyte. It can therefore be concluded from the C_dl values that the electroless Ni–high P coating is comparatively less porous and shows better corrosion resistance than electroless Ni–medium P and Ni–low P coatings.

One of the criteria among the requirements of surgical instruments is that it should provide low reflection from the top surface of the tools. The higher brightness of the tools’ surface can irritate visual perception of the surgeon. A black electroless Ni-P coating can be used as a potential safeguard to reduce this kind of disruption. Acid etching process in which substrates are submerged into a 9 M nitric acid solution is needed to prepare the black surface. However, the incorporation of higher phosphorus content can hinder the etching procedure. The formation of nickel oxides (NiO, Ni₂O₃) and nickel phosphate are responsible for low reflective black color of Ni-P coating. In the case of low reflective black surface, phosphorus content should remain average to get both black surface and corrosion protected surface. Therefore, optimum phosphorus content ranging 3–7 wt.% is suitable (Cui et al. 2006) for the acid etching. Black Ni surface with lower reflectance can be obtained by incorporating mild phosphorus content in electroless plating bath solution. The corrosion resistance of high phosphorus electroless nickel deposits reduces after black treatment and can be improved by depositing low phosphorus content. However, in normal condition, higher phosphorus is suitable for highest corrosion resistance and the R_c value is 5772 Ωcm² but after black treatment the corrosion resistance is reduced in spite of having high phosphorus content and shows 3012 Ωcm² (Cui et al. 2006). It is prudent to keep the phosphorus content slightly lower than traditional weight percentage to get proper adhesion and stability of black coating. Higher optical properties (solar absorbance) are provided by the blackened electroless nickel due to its strong adhesion, stability and uniformity. Therefore, it is commonly used for solar applications (Farag 2020; Uma Rani et al. 2010) in adverse space conditions.

2.2 Effect of adding alloying elements

The incorporation of third element in electroless Ni-P based plating bath solution influences the corrosion behaviour and coating properties as well. The incorporation of tin or copper in electroless Ni-P coating raises the thermal stability of amorphous state and ensures the improvement of corrosion resistance (Georgieva and Armyanov 2007; Wang et al. 1999). The addition of 17.2 wt% (Liu and Zhao 2004) copper in the Ni-P coating solution can exhibit the best anticorrosion properties. If the copper element is included in quaternary alloy of electroless Ni-P coating, then above 4 wt% (Balaraju et al. 2006a; Bastidas et al. 2019; Shinato et al. 2020) Cu content is regarded as the best concentration for corrosion protection. The inclusion of Cu in electroless Ni-P-PTFE composite matrix can also enhance the corrosion resistance along with coating deposition rate based on the finding of Zhao (Zhao et al. 2004) where 0.5 gm/L copper sulfate pentahydrate is used in electroless plating solution. The addition of Cu element to the Ni-P coating matrix can speed up the selective dissolution of Ni, resulting in enrichment of phosphorus and copper elements in the coating surface layer, thus increasing the corrosion resistance of Ni-Cu-P composite coating. This coating is not only used for the practical condensation of flue gas but also for potential use in heat exchangers (Liu et al. 2010). Ternary tungsten alloys (10 wt% W) have an improvement in corrosion resistance and alloy with up to 40 wt% tin (Sn) is considered strong resistant to corrosive materials (Sudagar et al. 2013).

Balaraju et al. (2006a) found the lower corrosion current density (i_corr) and higher charge transfer resistance (R_ct) in case of deaerated condition in 3.5 wt% NaCl solution compared to aerated condition (Figure 3). Electrolyte containing dissolved oxygen can form OH^─ through cathodic reaction in aerated condition which is accessible to the atmosphere. The cathodic depolarization reaction reduces the charge transfer resistance in aerated condition. More capacitive corrosion behaviour is noticed in deaerated condition which shows the greater homogeneity of the coated surface. The charge transfer resistance (R_ct) is higher, and corrosion current density is lower for quaternary Ni-W-Cu-P composite matrix which provides better corrosion protection than ternary Ni-W-P composite coating. This improved corrosion protection can be gained due to the incorporation of copper in electroless coating composite matrix which can improve crystallinity and provide smooth coated surface compared to coarse nodular ternary Ni-W-P deposit. Copper element has a significant influence on corrosion resistance in both aerated and deaerated conditions. There is a difference in wt% of phosphorus in terms of using ternary and quaternary alloys to obtain the better protection from corrosion (Table 1). Though the phosphorus content in Ni-W-Cu-P coating matrix is much lower than binary (Ni-P) coating, the corrosion resistance of Ni-W-Cu-P coating and Ni-P coating is nearly similar to each other due to the inclusion of copper element in quaternary alloy composite matrix. For ternary and quaternary alloys 4.5 wt% of tungsten is considered as the optimum percentage for corrosion protection.

Figure 3:

Corrosion current density values of different alloys in deaerated and aerated conditions (Balaraju et al. 2006a).

Inclusion of copper in Ni-W-P bath leads to quaternary Ni-W-Cu-P deposit with improved crystal structure by decreasing phosphorus content (Balaraju and Rajam 2005). The applicability of electroless Ni-W-P ternary composite matrix in different sector is really impressive (Zhou et al. 2019). It can protect the biodiesel storage container from corrosive damage (Sukkasi et al. 2011). Moreover, by adopting laser surface treatment on Ni-W-P composite coating can show amorphous structure with nanocrystalline Ni and Ni₃P phase resulting in improved anticorrosion performance. The Ni-Mo-P coating also had an average corrosion-resistant performance and its corrosion current density in 3.5 wt% NaCl solution is 14.49 × 10⁻³ mA/cm² (Zhao et al. 2020).

2.3 Effect of addition of particles

2.3.1 SiC particles

Bigdeli (Bigdeli and Allahkaram 2009) confirmed that the inclusion of SiC nanoparticles in Ni-P coatings provided better protection against corrosion which could be due to a reduction in the effective metallic area available for corrosion in Ni-P-SiC composite coating. The different concentration of SiC nanoparticles using in electroless plating bath solution has an astounding effect on corrosion resistance. Ghavidel et al. (2020) showed that by addition of 1 gm/L SiC nanoparticle with 40–60 nm particle size in electroless Ni-P plating bath solution, corrosion resistance is higher but when he added 2 gm/L SiC nanoparticles in electroless bath solution, it deteriorates the corrosion properties. The surface morphology of Ni-P coating matrix with 1 gm/L SiC particles has been shown in Figure 4A. No pores, cavities and cracks can be seen in that SEM image. It can be clearly observed that the electroless Ni-P-SiC (1 gm/L SiC) composite coating is uniformly coated on the substrate. The most striking feature is that the Ni-P-SiC (1 gm/L SiC) composite coating provides not only barrier property but also align mechanical interlock between the substrate and the coating layer. As a consequence, the coating develops a robust adhesion and exhibits better corrosion protection. However, the addition of 2 gm/L SiC nanoparticles in Ni-P coating provides non-uniform surface structure and particles are concentrated in certain regions as shown in Figure 4B. In addition, the enhanced concentration of SiC nanoparticles can increase the number of particles which is responsible for the particles collision and agglomeration and creates porosity. It results in somewhat flatter granules and is therefore associated with coarser flecks. This is widely reported and extensively explored in the literature by some researchers (Bigdeli and Allahkaram 2009; Kubisztal et al. 2018) that the maximum concentration of SiC nanoparticles in the plating bath solution can aggregate the particles due to short interparticle gap. The use of relatively large concentrations of SiC nanoparticles in the coating results in dislocations and crystal boundaries occurring at the particle interface. The minimum corrosion resistance of Ni-P-SiC (2 gm/L SiC) composite coating is due to the highest degree of particle accumulation in its morphology compared to the others.

Figure 4:

SEM micrograph of (A) 1 gm/L and (B) 2 gm/L SiC nanoparticles in electroless Ni-P plating bath solution (Ghavidel et al. 2020). (Reprinted with permission, Copyright 2020 Elsevier).

Zhang et al. (2008) reported that the corrosion resistance of Ni-P-SiC composite coating decreases gradually with increasing SiC concentration in the coating solution. Ma et al. (2014) also showed the lower corrosion performance when 6 gm/L SiC powder was added into the plating bath solution and provided corrosion current density value of 306 × 10⁻³ mA/cm² which was really high. The coating process was carried out by sealing the specimens in an evacuated tempered glass tube and heating them to different temperatures. The Ni-P-SiC (6 gm/L SiC) composite coating gives worse corrosion resistance compared to Ni-P-SiC (1 gm/L SiC) coated substrate due to using lower size (30 nm) of SiC particles in Ni-P-SiC (6 gm/L SiC) composite coating.

Calderón et al. (2014) showed the variation in corrosion resistance at different concentration of SiC with 25 nm particle size in electroless Ni-P coating. He reported that 70 gm/L SiC particles gives better corrosion resistance than 20 gm/L and 50 gm/L SiC particles in electroless Ni-P-SiC composite coating. Ahmadkhaniha et al. (2018) used 20 gm/L SiC particles with average particle size of 50, 100 and 500 nm in electroless Ni-P plating bath solution. He showed the best corrosion resistance for bigger SiC particles (500 nm) noting, corrosion current density is only 0.72 × 10⁻³ mA/cm². He showed that by using 50 and 100 nm SiC particle size, they have a bit higher corrosion rate than using 500 nm SiC particles. Calderón used same concentration of SiC particles (20 gm/L) as Ahmadkhaniha, but there is a difference in their corrosion current density (i_corr) value only because of using different size of SiC nanoparticles.

If high concentration of SiC nanoparticles along with smaller particle size is immersed in the coating composite matrix, they do not directly contribute to barrier properties from corrosion damage due to the enriched number of SiC particle agglomeration. Therefore, it can be concluded that a correlation exists among the concentration, particle size and number of SiC particles. The number of SiC particles goes higher when smaller particle sized SiC are incorporated in a large concentration in electroless Ni-P plating bath solution. This phenomenon (Table 2) has occurred since smaller particle size and higher concentration of SiC nanoparticles can occupy more space in electroless plating bath solution. The large number of SiC particles reduces matrix stability and consistency while their interaction with the matrix can inhibit the formation of a passive layer resulting in SiC particles collision and agglomeration being initiated and eventually causes porosity. However, small number of SiC particles prevents the particles collision and agglomeration. Bigger size of SiC particles (like 500 nm) results in bigger passive range with small number of SiC particles which can be related to the more coverage of Ni-P matrix. Higher concentration of SiC particle is not suitable for better corrosion resistance. In fact, several researchers depicted different opinion about SiC particle size and its concentration on corrosion resistance (Table 3).

Table 2:

SiC particles phenomena on corrosion resistance.

Addition in coating solution		Outcome
SiC concentration (gm/L)	SiC particle size (nm)	Number of SiC particles	Agglomeration and collision	Corrosion resistance
High	Low	Increased	Increased	Decreased
Low	High	Decreased	Decreased	Increased

Table 3:

List of corrosion current density values by using different concentrations and particle sizes of SiC in electroless Ni-P plating bath solution.

SiC concentration (gm/L)	SiC particle size (nm)	Corrosion current density, icorr (mA/cm2)	References
0.5	50	1.9 × 10–3	Ghavidel et al. (2020)
1	50	1.2 × 10–3
2	50	19.81 × 10–3
4	50	2.3 × 10–3
20	25	38 × 10–3	Calderón et al. (2014)
50	25	23 × 10–3
70	25	13 × 10–3
20	50	6.4 × 10–3	Ahmadkhaniha et al. (2018)
20	100	3.4 × 10–3
1	500	1.68 × 10–3	Wang et al. (2013)
4	500	1.36 × 10–3
6	500	1.72 × 10–3
8	500	1.85 × 10–3
2.40	40	2.35 × 10–3	Bigdeli and Allahkaram (2009)

Therefore, it is highly difficult to prognosticate which concentration and particle size would be the best for the lowest corrosion current density. Hence, an optimization can fix this issue with proper modeling. The optimization was carried out using a design expert software adopting response surface methodology which can provide a better result and exhibit the optimum value of SiC particle size and its concentration for enhanced corrosion protection. The experiment was based on central composite design (CCD) to study the combined effects of two independent variables i.e. concentration of SiC nanoparticles and size of SiC nanoparticles. Statistical analysis was conducted to optimize the corrosion resistance of electroless Ni-P-SiC composite coating. 11 set of experimental variables has been listed in Table 4.

Table 4:

Eleven sets of experimental variables were selected from Table 3 for the central composite design.

Run	Concentration of SiC particle (gm/L)	Size of SiC particle (nm)	Corrosion current density, icorr (mA/cm2)
1	0.5	50	1.9 × 10–3
2	1	50	1.2 × 10–3
3	1	500	1.68 × 10–3
4	2.4	40	2.35 × 10–3
5	4	50	2.3 × 10–3
6	4	500	1.36 × 10–3
7	6	500	1.72 × 10–3
8	8	500	1.85 × 10–3
9	20	25	38 × 10–3
10	20	50	6.4 × 10–3
11	20	100	3.4 × 10–3

ANOVA analysis for quadratic model of electroless Ni-P-SiC composite coating has been shown in Table 5. Analysis of variance (ANOVA) was employed to determine the significant parameters that affect the corrosion current density. It uses the concept of lower p-value and higher F-value to find the significant factors. The p-value is a parameter by which the null hypothesis can be rejected. If the p-value is less than 0.05, then the parameter is considered as significant. The F-value is the ratio of the summation of square of the factors to the variance of the errors. Hence, a higher value of F will suggest a relatively better factor with respect to others. Here, the p-value of the model was 0.0115 which is regarded as significant model terms. Model F-value of 4573.65 implied that the model is significant. In this case B, B² and B³ are significant model terms. Values greater than 0.1000 indicate the model terms are not significant. The higher F-value (7558.02) of SiC particle size indicates that particle size has greater influence on corrosion resistance than the concentration of SiC particle. Using the Design Expert software, the results were completely analyzed via ANOVA. Equations (3) and (4) represent the final equations in terms of coded factors and actual factors, respectively.

Table 5:

ANOVA analysis for quadratic model of electroless Ni-P-SiC composite coating.

Source	Sum of squares	df	Mean square	F-value	p-Value
Model	1172.15	9	130.24	4573.65	0.0115
A-SiC concentration	1.19	1	1.19	41.83	0.0977
B-SiC particle size	215.22	1	215.22	7558.02	0.0073
AB	0.5849	1	0.5849	20.54	0.1383
A2	0.2897	1	0.2897	10.17	0.1934
B2	99.90	1	99.90	3508.15	0.0107
A2B	0.2595	1	0.2595	9.11	0.2036
AB2	1.88	1	1.88	65.90	0.0780
A3	0.3266	1	0.3266	11.47	0.1828
B3	159.73	1	159.73	5609.43	0.0085
Residual	0.0285	1	0.0285
Cor total	1172.17	10

Final equation in terms of coded factors:

(3)Corrosion current density =(365.14−36.57×A +442.64×B −24.73×AB −22.13×A2−327.68 ×B2−10.94×A2B +46.16×AB2−20.23×A3−460.27×B3)

Final equation in terms of actual factors:

(4)Corrosion current density =(39.9151+0.1209×A −1.6785×B −0.0448×AB +0.5655×A2+0.0203 ×B2−0.000485×A2B +0.000084×AB2−0.0218×A3−0.000034×B3)

Here, A and B represent the concentration of SiC particle and size of SiC particle, respectively. The equation can be used in terms of actual factors to make predictions about the response of each factor for given levels.

The predicted ramp response (Figure 5) implies that the best corrosion current density can be prognosticated by using 3.32 gm/L SiC concentration with 500 nm SiC particle size. The higher corrosion resistance can be achieved with larger SiC particles addition whereas the SiC nanoparticle concentration should keep low in range in electroless Ni-P plating bath solution. The blue dot (Figure 5C) indicates the lowest value of 1.14 × 10⁻³ mA/cm² corrosion current density. It can be clearly delineated from (Figure 6) that the blue zone is the safest zone for getting the best corrosion resistance by employing the corrosion current density value. The corrosion protection gets hampered when the concentration of SiC nanoparticles is higher and the least particle size of SiC nanoparticles is used in the plating bath solution. As seen in (Figure 6), this phenomenon can be clearly observed from the red zone of the 3D surface plot and contour plot.

Figure 5:

The predicted parameters (A, B) and outcome (C).

Figure 6:

(A) Second-order 3D response surface plot and (B) contour plot to show the variation of corrosion current density with concentration of SiC particle and size of SiC particle with indicating predicted optimum value.

In 3.5 wt% NaCl electrolyte (salty media), Ni-SiC-PTFE (Polytetrafluoroethylene) composite coating shows higher corrosion resistance than Ni-PTFE composite coating and this is confirmed by the corrosion resistance (R_c) value. The concentration of SiC particles were used as 8 to 10 gm/L with larger particle size (Huang et al. 2004). The enhancement of corrosion resistance of Ni-SiC-PTFE composite coating is mainly due to presence of small number of particles thus prevention of agglomeration of SiC and PTFE nanoparticles. The SiC and PTFE nanoparticles together can improve the surface autocatalytic properties triggered by thin Ni layer. The Ni-SiC-PTFE and Ni-PTFE composite coated surface are pore free, homogenous, and uniform in terms of their surface structure.

2.4 Effect of double layer and pretreatment

A double black layer of electroless Ni-P coating has some definite applications. In the case of sunlight absorptions, a double black Ni-P composite coating can be used with a coarse sandblast. Normally, 1–2 h is enough for single layer Ni-P coating deposition. Extended time (3 h) is normally required for double black layer coating deposition. It can be seen from the measurement of the corrosion rate in 1.35 wt% NaCl electrolyte that single layer Ni-P coating had a corrosion rate of 0.5 mm per year (mpy), while for the double layer Ni-P coating it was 0.4 mm per year (mpy) (Sosa Domínguez et al. 2017). The upper discussion demonstrates that the black Ni-P double layer coating provides higher corrosion resistance against unpropitious atmosphere. The free paths of the first layer become partially blocked between the electrolyte and the substrate due to the appearance of low frequency time-constant related to corrosion which is attributed by the second layer. However, another research opposes this hypothesis. There has been shown that as-plated single Ni-P coating exhibited higher corrosion resistance than duplex Ni-P coatings. The percentage of corroded surface after 336 h for single Ni-P coating is only 1.50%, while for duplex Ni-P coatings, the corroded surface was recorded as 8.68% after 336 h (Bonin et al. 2018). For deposition of duplex nickel–phosphorus/nickel–boron (Ni-P/Ni-B) coatings, samples are first immersed in Ni-P plating and after that samples are directly immersed into the nickel–boron (Ni-B) plating bath. The plating bath used for deposition is based on sodium hydroxide, nickel (II) chloride hexahydrate and sodium borohydride as reducing agent and mixed oxide tungsten as stabilizer. Thus, Ni-P/Ni-B duplex coating is formed and shows the value of 3.53 × 10⁻³ mA/cm² corrosion current density (Vitry et al. 2012b). The top layer has a greater influence on corrosion resistance in case of duplex coating (Hu et al. 2019). In other words, the top layer of electroless nickel coating is used for its barrier properties because the upper coating is free from moving porosities and other significant defects that might threaten its durability. Vitry et al. (2012b) and Narayanan et al. (2003) used same duplex Ni-P/Ni-B coating for their experiment. However, the corrosion current density value of their experiment was different due to the variation in coating thickness. The coating thickness of Vitry’s sample was 25 µm and showed higher corrosion resistance than Sankar’s sample where coating thickness was reported to be 20 µm. From the above findings it can be concluded that the higher coating thickness gives greater protection against corrosion.

The double nickel immersion pretreatment carries excellent corrosion properties with a value of 2.216 × 10⁻³ mA/cm² corrosion current density. In double Ni immersion pretreatment method, nickel immersion solution was conducted with 68 wt% concentrated nitric acid for 5 s (Yin and Chen 2013) to remove the nickel film obtained in the first nickel immersion process. The sample was then cleaned with distilled water, and activation was repeated and thus it is called double immersion pretreatment method.

The pretreatment of a substrate by Zn before immersion in electroless Ni-P bath can improve the corrosion resistance. The thin layer of Zn is coated on the substrate by using electroplating method as pretreatment. The traditional and sonochemical warm water processing including as-synthesized Zn crystals for the transformation into the ZnO phases provides good corrosion resistance. It has been noted that corrosion resistance for Zn pretreated Ni-P coating was increased to 66% from normal Ni-P coating. The ZnO nanosheets are formed under the Ni-P coating matrix and acts as a barrier by raising the stability of ZnO nanoparticles which can keep the surface away from corrosive damages (Sharifalhoseini et al. 2018). From surface and grain boundaries, the additional scattering centers of ZnO is found which can create chemical effect on the improved corrosion resistance (Dhoke et al. 2009).

Organic and inorganic nanosized particles are being used comprehensively in a different engineering fields including photochemistry, electrochemistry, ultracapacitors and supercapacitors (Marzocchi et al. 2007; Sharifalhoseini and Entezari 2015; Xu et al. 2017). In addition, the substantial improvement in the strength and corrosion resistance of different materials changed by nanoparticles has made nanotechnology suitable for a wide range of engineering applications. ZnO is also used in biomedical applications due to its nontoxic and biocompatible feature (Wang et al. 2013; Xu et al. 2013). In addition, this ceramic oxide can be used as building blocks for electronic and optoelectronic devices (e.g. solar cells, lasers and ultraviolet light emitters) known for its high electron communication ability and near ultraviolet emission (Ameen et al. 2012; Li et al. 2015).

2.5 Effect of heat treatment

A number of studies have examined that heat treatment reduces the corrosion resistance of electroless Ni-P coating consistently. The increased corrosion phenomena are found due to the transition in microstructure of the heat-treated coatings. In as-deposited condition, electroless coatings usually show an amorphous structure that divulges greater corrosion resistance. However, due to the application of heat treatment, crystalline structures are found, and grain boundaries are formed as active sites for corrosion attack. There are different temperature ranges for heat treatment which reveal the variations in corrosion characteristics. It has been experimentally demonstrated that phosphorus content might be reduced from the coated substrate when more than 300 °C (Srinivasan et al. 2010) heat treatment is applied. This can be evident from previous discussion that low phosphorus content decreases the corrosion resistance. The decrement of phosphorus content of the coating reduces the corrosion resistance by forming nickel phosphide (Ni₃P) phase around 320 °C. Increased heat-treatment temperature can enhance the phosphorus segregation and form Ni₃P phase. Therefore, it is deduced that heat treated substrates are not generally amorphous. Heat treated coatings display lower corrosion resistance due to the grain boundary of its definite structure. The additional borders are normally seen between nodules in nanocomposite coatings that have the effect of subduing deformation and enable the formation of microcell corrosion. These microcell attacks on the surface of the object may increase with the lapse of time and can cause a severe damage to the object. The heat treatment affects the corrosion potentials and does not change the morphology (Figure 7) of the coating structures remarkably to form a passive region. Therefore, heat treatment vastly escalates the value of corrosion current density resulting in an enhancement in corrosion rate.

Figure 7:

Surface morphology of heat-treated Ni-P coating (Wu et al. 2015). (Reprinted with permission, Copyright 2015 Elsevier).

The electroless Ni–high phosphorus content commences to lose its amorphous structure at the temperature above 260 °C. With medium and high phosphorus content, intermediary metastable phases such as NiP₂ and Ni₁₂P₅ can evolve before stable Ni₃P phase is formed. Nickel–phosphorus (Ni₃P) stable phase is established when the temperature of the heat treatment exceeds 380 °C (Guo et al. 2003; Keong et al. 2003; Sahayaraj et al. 2014). The electroless Ni-P coating becomes crystalline upon heat treatment at 400 °C for 1 h and form face centered cubic (f.c.c) Ni and body-centered tetragonal (b.c.t) Ni₃P phases. After heat treatment, the electroless Ni-P coating possesses a strong orientation of Ni along (111) and (200) peak. During the heat treatment process, Ni-P coating matrix increases the proliferation of the Ni atom along (111) crystal among other significant phases which is responsible for improved corrosion current density. Unfortunately, in case of heat-treated electroless Ni-P coating, a mixture of crystalline Ni (111) and Ni₃P (231) phase is constructed with a greater number of grain boundaries that can drastically lower the corrosion resistance (Sankara Narayanan et al. 2006). The stage transition takes place within the surface, leading to the precipitation of the Ni₃P process at 400 °C. Moreover, it has been experimentally demonstrated from XRD analysis (Hari Krishnan et al. 2006; Sahoo and Das 2011) that as-plated Ni-P coating shows short range order broad peak. However, after heat treatment Ni phase peak shows sharper apex than Ni₃P phase peak (Vitry 2012a). The heat-treated Ni-P coated substrates do not make the transition of its surface to a passive form and the corrosion current density increases as the potential increases. As a result, the corrosion resistance reduces due to the drastic change of corrosion current density. The expose of corroded surface after 168 h for as plated Ni-P coating is only 0.27% and for heat treated Ni-P coating, it exhibited 10.87% (Bonin et al. 2018). Therefore, it can be inferred that heat-treated electroless Ni-P coatings are less defensive than as-plated coatings. Corrosion parameters of different electroless Ni-P based coatings in before and after heat-treated conditions have been shown in Table 6.

Heat treatment (HT) of the Ni-P coating upon twinning-induced plasticity (TWIP) steel was applied at 350 and 700 °C for 1 h in argon atmosphere (Hamada et al. 2015; Liu et al. 2016). The post heat treatments drastically increase the corrosion current density which is the sign of decreased corrosion resistance. As-plated Ni-P coating on TWIP steel shows a single large diffraction ridge which signifies the characteristics of fully amorphous structure. The large diffraction ridge is obtainable due to high phosphorus content with saturated amorphous domain (Hu et al. 2006). The most striking observation in this finding is 700 °C heat treatment provides higher corrosion resistance than the heat treatment carried at 350 °C. Thus, it can be deduced that there is no consistent relationship exists between the corrosion current density and the temperature of heat treatment for TWIP steel substrate. The corrosion rate of electroless Ni-P coating at 350 °C is much higher in 0.5 wt% H₂SO₄ solution due to the diffusion of iron and manganese from the TWIP steel surface. It is well known that manganese in the aqueous solutions appears to form manganese oxides (MnO) and give rise to Mn²⁺ ions with hydrogen evolution resulting in high corrosion rate.

The ternary Ni-P-SiC alloy composite coating shows some surprising corrosion properties in heat treated condition at different temperatures according to the finding of some researchers (Bigdeli and Allahkaram 2009; Ghavidel et al. 2020; Wang et al. 2013). The coating structure of as-deposited Ni-P-SiC composite matrix shows the amorphous nature and uniform distribution of SiC nanoparticles which indicates better corrosion protection. After applying higher temperature heat treatment, Ni reacts with SiC and nickel silicide is formed with free carbons. The electroless nanocomposite coating crystallizes into nickel crystal, nickel phosphide (Ni_xP_y), and nickel silicide when the heat-treated temperature rises to 400 °C. No phase transition takes place before 200 °C heat treatment and this phenomenon is confirmed by a single wide peak profile. The diffraction peak starts to disappear gradually at 600 °C (Chen et al. 2002) heat-treated condition. There have been studies of the analysis of nano-sized SiC particulates in the Ni-P coating matrix, the SiC nanoparticles have the significant influence on corrosion resistance (Allahkaram et al. 2011; Bigdeli and Allahkaram 2009; Jiang et al. 2020). Ghavidel et al. (2020) reported on his experiment that Ni-P coating and the incorporation of 40 nm sized SiC nanoparticles in Ni-P composite matrix showed poor corrosion resistance at 400 °C heat treated condition. However, both Ni-P coating and Ni-P-SiC composite matrix at 300 °C heat treated condition sharply reduces the corrosion current density, resulting in higher corrosion resistance. Heat treatment has adverse effect on corrosion resistance but at a certain limit of heat-treated temperature corrosion resistance can be increased. Heat treatment at ≤300 °C is favorable for corrosion protection and it has been already proved by previous research (Ghavidel et al. 2020; Vitry et al. 2012b; Wang et al. 2013). Wang et al. (2013) showed some different results by adopting two different heat-treated temperatures. He used 500 nm particle sized SiC nanoparticles and selected 275 and 400 °C as heat treated temperatures. He showed the highest corrosion resistance (0.07 × 10⁻³ mA/cm² current density) holds good for 400 °C heat treated temperature that is literally under debate while a number of studies suggesting that more than 300 °C temperature is not suitable for better corrosion protection (Bigdeli and Allahkaram 2009; Hamada et al. 2015; Narayanan et al. 2003; Novakovic and Vassiliou 2009; Wu et al. 2015).

Ma et al. (2014) showed in his research article that at 400 and 600 °C heat treated Ni-P-SiC coated samples reveal higher corrosion resistance than as-plated Ni-P-SiC coated sample. However, this result obtained by (Ma et al. 2014) highly contradicts with the findings of other researchers. Bigdeli and Allahkaram (2009) reported that the coating structure and improved density at 400 °C heat treatment, increased corrosion resistance of Ni-P and Ni-P-SiC composite coatings. In this experiment, nano-sized (average size 40 nm) β-silicon carbide particles were used for SiC codeposition. This result also contradicts with previous researchers’ outcomes. Most of the researchers showed that heat treatment (400 °C for 1 h) has adverse effect on corrosion resistance (Bigdeli and Allahkaram 2009; Hamada et al. 2015; Narayanan et al. 2003; Novakovic and Vassiliou 2009; Wu et al. 2015). Bigdeli et al. added hexadecyltrimethyl ammonium bromide (HTAB) for particles dispersion and surface charge adjustment in the Ni-P-SiC plating solution. That is quite uncommon addition in his experiment and has not been adopted by the other researchers. So, this may vary the results and exhibit quite opposite value. Moreover, Huang et al. (2004) showed also some different result on heat treatment effect. He showed that proper heat-treatment at 400 °C for 1 h significantly improves the corrosion resistance. But it contradicts from previous results and researchers’ inspections. Nevertheless, he selected an ethoxylate type additive (2,4,7,9-tetramethyl-5-decyne-4,7-diol ethoxylate) as surfactant for particle dispersion which is quite different addition from other researchers experiment. This additive may increase corrosion resistance despite of applying heat-treatment (400 °C and 1 h). In his experiment, he added 8–10 gm/L SiC powder in electroless Ni-P-PTFE coating bath solution which exhibited the highest corrosion resistance after applying heat treatment at 400 °C. The additional surfactant may activate the SiC and PTFE particles and retards autocatalytic chemical reactions between substrate and the electrolyte.

Conceptually similar outcome of heat-treated coating has also been carried out by Wu et al. (2015) at 400 °C heat treatment in which heat-treated Ni-P and Ni-P-TiO₂ sol composite coating possessed higher corrosion rate than as-plated composite coatings. Samples with as-plated Ni-P coatings, the corrosion current density is 1.33 × 10⁻³ mA/cm² but for heat-treated Ni-P coatings, corrosion current density is 1.48 × 10⁻³ mA/cm² which indicates that heat treatment adversely affects the corrosion resistance of as-plated coatings. Though heat treated Ni-P-TiO₂ sol composite coating showed lower corrosion resistance, its corrosion current density value is quite small noting only 2.03 × 10⁻³ mA/cm² (Wu et al. 2015). The lower current density value has been possible to obtain because of using TiO₂ sol in electroless plating bath solution and 6 °C/min (Wu et al. 2015) heat is applied isothermally in argon flow which is 99.99% pure. These conditional parameters can prevent oxidation and avoid consequent microcracks on the coated surface. Due to showing the lower current density for the Ni-P-TiO₂ sol composite coating, some researchers have selected 400 °C as ideal heat treatment temperature (Liu et al. 2007; Vojtěch et al. 2009). However, the optimum temperature for heat treatment should not exceed 300 °C to get better corrosion protection as it has been discussed earlier.

Vacuum heat treatment at 800 °C for 10 min duration of electroless Ni-P coating exhibits lower corrosion resistance than as-plated Ni-P coating. The addition of 0.5 gm/L TiO₂ in electroless Ni-P coating and applying vacuum heat treatment at similar conditions can slightly increase the corrosion resistance but still the corrosion resistance remains lower compared to as plated Ni-P coating. During heat treatment, the deposition of TiO₂ nanoparticles in the electroless plating solution may inhibit the growth of nodular shaped spherical grains. The vacuum heat treatment is carried out generally by a 300 ls⁻¹ diffusion pumping which can create a close high vacuum environment and then samples are thermally treated for 10 min using molybdenum (Mo) foil resistor (Novakovic and Vassiliou 2009). The thickness of Mo foil resistor is 0.15 mm and through this thin resistor alternate current (AC) can flow. A mixture of crystalline nickel and nickel–phosphides phase is generated after vacuum heat treatment so that areas with different active/passive corrosion cells are formed. On the surface of electroless Ni vacuum heat treated composite coating micro-voids are observed and these small pores can significantly reduce the corrosion resistance.

Heat treatment of the system at 180 °C with a duration of 4 h (Vitry et al. 2012b) causes a notable enhancement in the corrosion potential of the Ni-P/Ni-B duplex system and increases corrosion resistance (i_corr 1.10 × 10⁻³ mA/cm²) but for as-plated duplex Ni-P/Ni-B system, the i_corr value is 3.53 × 10⁻³ mA/cm² (Vitry et al. 2012b). The lower temperature (180 °C) heat treatment causes a slight densification of the duplex coating resulting in high corrosion resistance. The top layer (Ni–B) of the duplex Ni-P/Ni-B composite coating is not greatly modified by the low temperature heat treatment. Similar heat treatment temperature has also been selected by Delaunois and Lienard (2002) and Vitry et al. (2008) in which heat treatment is carried out under Ar (with 5% H₂) to prevent oxidation at 180 °C.

Among TiB₂, ZrB₂ and TiC particles on the stability of high phosphorus electroless nickel plating bath, TiC offers better corrosion resistance. Heat treatment at 400 °C for 1 h affects the corrosion protection (Huang et al. 2019).

There is an interesting scenario of heat treatment which has been observed from the upper discussions. Heat treated Ni-P coatings at 300 °C exhibited better corrosion resistance compared to other heat treatment temperatures and thus it can be selected as the standard heat-treated temperature. It is therefore clear from the upper discussion that heat treatment up to 300 °C is considerable for higher corrosion resistance, but more than 300 °C is not desirable at all.

Different corrosion parameters of electroless Ni-P based coatings immersed in 3.5 wt% NaCl solution have been shown in Table 7

2.6 Effect of corrosive media

Sodium chloride solution is the most common electrolyte used during the corrosion test for laboratory purposes. In 3.5 wt% NaCl solution, electroless Ni-P and Ni-P-SiC-PTFE composite coatings revealed higher corrosion resistance than in 2.67 wt% H₂SO₄ solution (Huang et al. 2004). The incorporation of either SiC or TiO₂ particles marginally increase the corrosion resistance of the coating in 3.5 wt% NaCl solution but adversely affects their corrosion resistance in 2.67 wt% H₂SO₄ solution. The previous study suggested that the lower corrosion resistance was resulted from the non-uniformity and lower density caused by co-deposition of PTFE particles in electroless Ni-P composite matrix (Wu et al. 2005). However, electroless Ni-P-PTFE coating surprisingly showed higher corrosion resistance in 2.67 wt% H₂SO₄ solution rather than 3.5 wt% NaCl solution, which exhibited completely opposite characteristics of other researchers’ findings. Hamada et al. (2015) used 0.5 wt% H₂SO₄ solution as corrosive media for his experiment. The Ni-P coating on a twinning-induced plasticity (TWIP) steel containing 25 wt% Mn and 3 wt% Al shows average corrosion resistivity in 0.5 wt% H₂SO₄ solution indicating 20 × 10⁻³ mA/cm² (Hamada et al. 2015) corrosion current density due to the absence of grain boundaries and surface defects. If 3.5 wt% NaCl electrolyte was used instead of 0.5 wt% H₂SO₄, the corrosion current density value might be less than 20 × 10⁻³ mA/cm² and the improved corrosion resistance could be reported. Therefore, it can be concluded that corrosion resistance in salty atmosphere provides better results than in acidic media.

Shibli and Dilimon (2007) used 32 wt% NaOH solution as an electrolyte and they showed corrosion current density value is 0.82 × 10⁻³ mA/cm². This superior corrosion resistance behaviour is possible due to the enrichment of phosphorus (18 wt%) content. If they used NaCl electrolyte instead of NaOH, then the corrosion current density could be lower than 0.82 × 10⁻³ mA/cm² and highest corrosion resistance could be achieved. The differences in the corrosion rates in various solutions are very large. Elsener et al. (2008) showed that the worst one is in the 5 wt% HCl, and the best is in the 10 wt% NaCl. The corrosion resistance in the four solutions for the Ni-Cu-P and Ni-Sn-Cu-P deposits increases according to the following sequence: 5 wt% HCl, 2.67 wt% H₂SO₄, 50 wt% NaOH and 10 wt% NaCl. The data showed that Sn does not usually improve the corrosion resistance of the Ni-P based alloy deposits in HCl, H₂SO₄, NaCl and NaOH solutions. However, phosphorus and copper content usually improve the corrosion resistance in the four solutions.

Charge transfer resistance (R_ct) for electroless Ni-Cu-P coated substrate in 5.35 wt% H₂SO₄ solution was 108 Ωcm² while it was 9210 Ωcm² in 3.5 wt% NaCl solution (Cissé et al. 2010). The result showed a marginal improvement of corrosion resistance in 3.5 wt% NaCl solution which is almost 90 times higher compared to acidic medium. The findings also showed a slight trend towards passivation in sulfuric acid and hydrochloric acid by the presence of a current plateau in anodic polarization in acidic media. There was no passivity pattern in the measurement range which is consistent with the work of Mimani and Mayanna (1996). However, in the anodic range, a substantial decrease in corrosion current densities is observed in the presence of alloy for (3.5 wt% NaCl) salty environment. Corrosion rate of electroless Ni-P coating in different environments has been shown in Table 8.

2.7 Effect of coating process parameters

2.7.1 Time

The corrosion current density of as-deposit samples inconsistently increases with increase of coating deposition time (i.e. decrease of phosphorus content). Thus, it increases the corrosion rate. From Figure 8, it must be wise to keep the coating deposition time in electroless Ni-P solution between 1 and 1.5 h.

Figure 8:

Effect of electroless Ni-P coating deposition time on corrosion rate (Ashassi-Sorkhabi and Rafizadeh 2004).

2.7.2 Temperature

Temperature is an important parameter which affects the rate of deposition of electroless Ni-P coating. The reactions occur during the deposition process are endothermic. As a consequence, the deposition rate increases by increasing the temperature. The impact of bath temperatures (Taheri 2002) on the deposition rate of the coating has been shown in Figure 9. Corrosion resistance increases with enhanced bath temperature at certain limit. The bulk of the acidic and salty baths are maintained at 80–90 °C while the alkaline baths can be run at lower temperature (40 °C).

Figure 9:

Effect of bath temperature of the coating solution on the coating deposition rate (Taheri 2002).

2.7.3 pH

Many of the reactions involved in the deposition process of electroless Ni-P coating are susceptible to changes in solution pH. By increasing the pH of the coating bath solution, the phosphorus release from the hypophosphite is retarded whereas nickel-reduction reaction is accelerated. Equation (7) is the simplified form of Equations (5) and (6), corresponds to nickel reduction while Equation (8) corresponds to phosphorus reduction. From Equation (7), it can be concluded that the higher the concentration of H⁺ in the electroless bath is, the less the deposition of the Ni will be. At lower pH, the percentage of Ni content in the coating is low and the phosphorus content remains high. As the pH increases, Equation (7) is shifted to the right direction, causing a decrease in the concentration of H⁺, i.e., an increase in the Ni content in the coating. On the contrary, as the pH of the electroless bath decreases, the concentration of the OH⁻ ions decrease and, consequently, the percentage of the P in the coating increases, which has been shown in Figure 10. Therefore, decreasing the pH of the solution increases the phosphorus content of the coating resulting in higher corrosion resistance.

(5)3NaH2PO2+3H2O +NiSO4→3NaH2PO3+H2SO4+2H2+ Ni

(6)2H2PO2−+Ni+++2H2O →2H2PO3− +H2+2H++ Ni

(7)Ni2++H2PO2−+H2O →Ni0+H2PO3−+2H+

(8)3H2PO2−→H2PO3−+H2O +2OH−+2P

Figure 10:

Effect of coating solution pH on phosphorus content (Fayyad et al. 2019).

2.7.4 Coating thickness

The coating thickness is one of the parameters which is responsible for the porosity of the coating surface. As the thickness of the coating increases, the pores and tiny holes on coating surface diminishes, leading to enhanced corrosion resistance. Thicker coatings passivate at the lower corrosion current density and thus possess greater resistance to corrosion. The coating thickness of electroless Ni-P coated substrate was reported to be influenced by phosphorus content (Shibli and Dilimon 2007). This refers to the reduction of the coating’s porosity due to increased coating thickness. There is a close relation between the thickness of the coating and its corrosion resistance, as shown in Table 9.

Table 9:

Electroless Ni-P deposits at various thicknesses.

Coating thickness (µm)	Corrosion current density, icorr (mA/cm2)	Corrosion potential, Ecorr (mV vs. SHE)	Phosphorus content (wt. %)	References
0	61 × 10–3	−298	–	Taheri (2002)
3	30 × 10–3	−184	–	Taheri (2002)
9.57	3.6 × 10–3	−183	10	Shibli and Dilimon (2007)
10.11	3.2 × 10–3	−330	13	Shibli and Dilimon (2007)
11.02	0.82 × 10–3	−270	18	Shibli and Dilimon (2007)

Summary of the coating parameters of electroless Ni-P coated substrates in NaCl solution have been shown in Table 10. As mentioned in the previous sections, there are various coating process parameters of electroless Ni-P based coatings which can highly influence the properties of corrosion resistance. Figure 11 demonstrates the range of these parameters that can effectively improve corrosion resistance of electroless Ni-P coating.

Table 10:

Summary of the coating parameters of electroless Ni-P coated substrates in NaCl solution.

Substrate	Coating solution		Elemental composition		Coating operational parameters			Results		References
	NiSO4 (gm/L)	NaH2PO2 (gm/L)	Ni (wt.%)	P (wt.%)	Time (min)	Temperature (°C)	pH	icorr (mA/cm2)	Ecorr (mV vs. SHE)
Mild steel	30	25	73.71	13.1	30	88	4.5	0.06 × 10–3	−137	Ashassi-Sorkhabi and Rafizadeh (2004)
	30	25	78.50	12.7	60	88	4.5	0.148 × 10–3	−57	Ashassi-Sorkhabi and Rafizadeh (2004)
	30	25	79.40	12.2	90	88	4.5	0.129 × 10–3	−101	Ashassi-Sorkhabi and Rafizadeh (2004)
	30	25	80.43	11.7	120	88	4.5	0.140 × 10–3	−53	Ashassi-Sorkhabi and Rafizadeh (2004)
	30	25	83.13	10.8	150	88	4.5	0.278 × 10–3	−216	Ashassi-Sorkhabi and Rafizadeh (2004)
	30	25	86.34	10.1	180	88	4.5	0.425 × 10–3	−312	Ashassi-Sorkhabi and Rafizadeh (2004)
	21	24	91	9	60	90	4.5	3.62 × 10–3	−113	Narayanan et al. (2003)
	30	25	88.10	8.30	60	85	4.5	0.98 × 10–3	−189	Shibli and Chinchu (2016)
	21	24	90.1	9.9	60	90	4.8	0.640 × 10–3	−126	Balaraju et al. (2006b)
	21.2	24	86.70	13.30	120	90	4.5	0.60 × 10–3	−170	Sankara Narayanan et al. (2006)
	21.2	12	93.30	6.70	120	90	4.5	1.17 × 10–3	−193	Sankara Narayanan et al. (2006)
	50	10	96.66	3.34	120	65	10	4.22 × 10–3	−295	Sankara Narayanan et al. (2006)
	20	14.84	88.64	11.36	120	88	8	0.456 × 10–3	−143	Balaraju et al. (2006a)
Copper	52.6	42.4	80.51	7.12	30	85	5.0	8.02 × 10–3	−	Shashikala and Sridhar (2020)
	25	20	90	8	60	90	9	0.285 × 10–3	16	Gawad et al. (2013)
Steel	39.50	20	92.20	6.70	45	80	6.5	5.4 × 10–3	−151	Promphet et al. (2017)
	25	30	92.88	7.12	60	70	8	3.41 × 10–3	−215	Sharifalhoseini et al. (2018)
Aluminium	27	30	90.42	7.85	50	91	5	5.11 × 10–3	−299	Yin and Chen (2013)
5083 Al alloy	30	20	85.67	14.3	60	86	4	6.36 × 10–3	−597	Lee (2012)
1050 Al alloy	28	20	93.5	6.5	120	85	4.7	1.0 × 10–3	−250	Khollari et al. (2019)
211Z Al alloy	25	30	93.94	6.56	120	90	6	1.33 × 10–3	−509	Wu et al. (2015)
AZ31 Mg alloy	20	22	90.29	9.71	60	75	7	1.7 × 10–3	−188	Ghavidel et al. (2020)
Carbon steel	30	40	92.40	7.60	60	88	5.5	5.26 × 10–3	−185	Tamilarasan et al. (2017)
	15	25	89.09	8.85	60	85	5	7.14 × 10–3	−160	Luo et al. (2018)
	25	30	93.20	6.30	120	85	5.5	4.0 × 10–3	−178	Luo et al. (2015)
	23	21	89	11	120	88	4.9	1.66 × 10–3	−79	Balaraju et al. (2001)
	15	30	89.09	10.91	180	85	8	4.50 × 10–3	−323	Fayyad et al. (2019)
	21	24	62.2	37.8	180	90	4.5	3.83 × 10–3	−228	Momenzadeh and Sanjabi (2012)
	21	24	84.3	15.7	180	94	4.5	6.81 × 10–3	−296	Novakovic and Vassiliou (2009)
Brass	30	25	92.50	7.5	240	90	5	2.21 × 10–3	−220	Gholizadeh-Gheshlaghi et al. (2018)
Polyester fabric	17.5	25	90.1	9.02	60	73	8	0.11 × 10–3	−58	Jiang and Guo (2011)
Polymer carbon	20	30	86	11.5	120	80	6.1	2.8 × 10–3	−299	Su et al. (2015)

Figure 11:

Factors influencing the corrosion resistance for electroless Ni-P based coatings.

3.1 Localized electrochemical impedance spectroscopy

The most recent development of electrochemical impedance spectroscopy (LEIS) is instigated by Lillard et al. (1992) which is further termed as localized electrochemical impedance spectroscopy (LEIS). The coated samples which have poor electrochemical activity can be measured by the LEIS system. An external voltage like previously mentioned in EIS method is applied to the LEIS system. A movable LEIS probe can measure the local potential difference. The schematic diagram for LEIS measurement has been shown in Figure 12. The electronic devices used in the LEIS setup are a gamry potentiostat which is integrated with a frequency response analyzer (FRA), two differential amplifiers, a LEIS probe and a computer monitor to visualize the data. Global and local impedances are recorded concurrently with a four-channel frequency response analyzer. Furthermore, transfer functions are generally analyzed by frequency response analyzer. A dual microprobe system has been introduced in this setup which is encompassed in the movable LEIS probe. The space between the two microprobes (micro reference electrodes) is denoted by ‘d’. The most modern LEIS instruments have a tolerance of around 1 nanovolt (nV). Differential amplifiers are used to monitor the data of local potential and current density by applying high input impedance. The two microprobes can sense the local potential difference from where current density can be calculated. The LEIS probe can travel in a three-dimensional motion and it is controlled by imposing digital signal. With providing information of position, velocity and direction, an encoder can regulate the positioning system of the LEIS probe (Cui et al. 2015; Huang et al. 2011). To enhance the ratio of signal and noise, the amplitude of sinusoidal voltage should be set as large as possible by performing the regulation of potentiostat. Some LEIS experiments have been conducted by using 50 acquisition cycles, seven points frequency per decade and 100 mV peak signal (Huang et al. 2007).

Figure 12:

A schematic diagram of LEIS setup.

The dual microprobe (microelectrodes) consists of metallic wires. Each microprobe has its own metallic wires and they are connected with the individual differential amplifier. The diameter of those metallic wires can be measured in micrometers which are very tiny in nature. Molten glasses are used to seal the wires having very thin internal diameter, and the sealing process is carried out by a nichrome wire with a current controllable resistance heater (Jaffe and Nuccitelli 1974). However, in the case of Ag/AgCl microelectrodes, epoxy resin is used instead of molten glass for sealing the silver wires. A thin layer of silver chloride is formed according to the following electrochemical reaction by anodizing the silver microelectrode in KCl solution:

It is necessary to first clean the silver microelectrode in 2 M KCl solution at the condition of 241.20 V_SHE at the rate of 100 mV/s. After passing 5–10 min, potentiodynamic oxidation occurs in that electrolyte with possessing 241.40 V_SHE (Huang et al. 2011). The electrochemical tests are usually conducted at room temperature using a standard three-electrode cell. The dimension variation of Ag wires can make difference the potential value. The potential difference can be obtained by a platinum grid (counter electrode) with respect to a reference electrode as shown in the schematic diagram (Figure 12). There is a major impact of the microprobe size and its position on of LEIS measurement. A small microprobe is always desirable for the experiment. The resistance of electrolyte must be reduced in order to decrease the contribution of high frequencies, in accordance with experimental results for different probe positions above the working electrode (substrate).

The local current density can be measured by the closest microprobe through controlling ohmic drop. The LEIS measurement can therefore be improved using local interfacial impedance to avoid high frequency. A recent study has indicated that the consequences of the probe size (Abreu et al. 2017). Some authors have also suggested using the micro capillary electrochemical setup to solve the probe dimension difficulty (Pilaski et al. 2002).

3.2 Scanning vibrating electrochemical technique

Scanning vibrating electrochemical technique (SVET) setup employs a conductive vibrating probe that determines the potential difference between the probe and the substrate in an electrolyte. The vibrating probe is mounted on a piezoelectric actuator (Figure 13). A measurement probe can be positioned closely above the surface of the sample. The scanning probe instrument consists of a very thin platinum tip. This tip of SVET probe measures potential difference (ΔV). The SVET probe is typically positioned 150 µm apart from the current source point which normally supplies 60 nA current to the system. The probe is wrapped with an insulated thin platinum–iridium (Pt-Ir) wire. The tip is spherical in shape with 10 µm diameter and coated with black platinum. The probe is positioned 100 µm above from the surface and the frequency of vibration probe normally remains at 398 Hz (Gnedenkov et al. 2016). The piezoelectric motor can move in all three dimensions. The piezo ceramic device controls the movement of the probe allowing vibration amplitudes (d) from 1 to 60 µm in the direction perpendicular (Z axis) to the sample surface. An amount of 100 µA current (Bastos et al. 2017) flows between the graphite electrodes (B to C). This current is naturally generated from electrochemical corrosion or biological process and is externally controlled by a galvanostat. Potential difference is calculated from the circuit at selected points (A to D). The reference microelectrodes which contain low impedance are used to measure the ohmic drop in solution. Two reference microelectrodes are kept in a fixed position and they are moved together. Due to the developed compact electric field in the electrolyte solution, the potential difference exists. The current density (I) can be easily determined by measuring the potential difference (ΔV), the amplitude of vibration (d) and the solution resistivity (ρ) with the help of Equation (13). To get the value of current density, the method is repeated in a settled framework and acquires the data from the plot of current density over a selected sample surface (Dolgikh et al. 2016). From this current (I), corrosion rate can be measured.

Figure 13:

Schematic presentation of SVET.

The SVET probe position is regulated by an auto-generated computer command. The close loop linear encoders can identify the exact position of the probe. The vibrating nature of the probe in the perpendicular direction to the substrate surface can generate a zone where AC signal is developed. A lock-in amplifier (LIA) device is used to extract the AC signal with its carrier wave from an extremely noisy environment and converts it to a DC signal by using input phase angle. The LIA is able to capture and demodulate not just small AC signals but also large signals. The LIA acts as a rectifier in this SVET setup. The input phase angle is usually determined by manually changing the phase input of lock-in amplifier until no response is detected. The optimum phase angle can be gained by adding 90°. Some commercial equipment can be able to acquire the reference phase angle directly. The converted DC signal which is obtained from LIA device can then be plotted to represent the distribution of local activity. The SVET signal chain configuration has been shown in Figure 14.

Figure 14:

SVET signal chain configuration.

The use of SVET in corrosion studies is subject to certain experimental drawbacks. The processing of data acquisition takes longer time in the matrix. Another difficulty can develop if the sample is not completely flat or is not placed properly accordance with the scanning probe (Jadhav and Gelling 2019). Larger amplitude vibrations may damage the substrate (McMurray et al. 2003) and inhibits the electrochemical corrosion reaction by rippling of the electrolytes resulting in an inaccurate value of current density. SVET methods can be applied to determine galvanic corrosion rate (Battocchi et al. 2005; Ogle et al. 2000; Simões et al. 2007), pitting corrosion (Dorman et al. 2017; Vuillemin et al. 2003; Williams et al. 2010), crevice corrosion (Isaacs 1996), stress corrosion cracking (Isaacs 1988), corrosion due to microorganisms (Abdul-Rani et al. 2019; Chen et al. 2015; Franklin et al. 1991; Iken et al. 2008), inorganic coatings (Manhabosco et al. 2015), coated materials (Ahmad et al. 2016; Bastos et al. 2010b), corrosion inhibitors (Bastos et al. 2010a; Kallip et al. 2012; Kumar et al. 2017; Toloei et al. 2013), corrosion of weldments (Bertoncello et al. 2015; Lollini et al. 2019) and corrosion of polymers (He et al. 2004).

3.3 Scanning ion-selective electrode technique

The scanning ion-selective electrode technique (SIET) works noninvasively and gives accurate measurements of the pH and the presence of specific ionic species just above surface of electrolyte (Fix et al. 2011). The schematic diagram of SIET setup has been shown in Figure 15A. The SIET probe contains a microelectrode (Figure 15B) consisting of a glass or plastic micropipette which is used for ion detection at the tip of the electrode along with pH variance detection (Cabrini et al. 2017). Ammann et al. (1985) had highlighted some technical studies on ion-selective microelectrodes. A selective ionophore-based membrane like oil is packed within a glass-capillary microelectrode. Between the ion-selective membrane and the metallic tip, an ion-to-electron transducer is mounted which is constructed from a conductive polymer poly (3-octylthiophene-2,5-diyl). The electrode’s ion-sensitive tip is approximately 10 μm long (Lamaka et al. 2010) and for various applications, this can range from 10 to 100 μm. A needle like Pt-Ir wire with an open tip is the base of the microelectrode. A competitive lateral resolution results from different diameter of the orifice of ion-selective glass-capillary microelectrode which ranges from 0.1 to 5 µm (Ammann et al. 1985).

Figure 15:

(A) Typical set up for SIET method (B) glass-capillary ion-selective microelectrode with liquid membrane.

Scanning ion-selective electrode technique functions as a micro-potentiometric instrument which works on an active solution surface to measure specific ions at a quasi-constant micro-distance. Potentiometric experiments are done under zero current circumstances in a two-electrode galvanic cell. A potentiometric cell is made up of a reference electrode and a microelectrode specific for ions. A SIET system includes the mentioned key elements: an ion-selective microelectrode positioned on a 3D computer-controlled stepper-motor system, which is used to place and transfer the microelectrode above the specimen. A video camera is located above the surface of the sample, fitted with a long-distance lens which provides magnification up to 400 times. It enhances and digitizes the potential difference evaluated inside this potentiometric cell. Ag/AgCl wires are used as a reference electrode. The micropipette including the reference electrode is placed on the computer-controlled 3D stepper motor. The sample is dissolved in the electrolyte and both electrodes are carried into the sample’s vicinity. Video-assisted camera optics captures the image data, help place the probe and hold the distance between sample and probe. Pre-assessment and post-assessment of SIET is extensively required for the microelectrode calibration. The travelling with motion and speed of the micropipette across the surface of the sample can be controlled by a computer-generated command. A 3D data represents the spread of pH or ionic concentration of sample. SIET tests are carried out in the context of potentiometry. Throughout this control mode the potential differences leading to variation in ionic concentration are evaluated. On the basis of its ability to allow the specific ions through the membrane, ionophore membranes are used in ion selective electrode. The chemical potential changes due to the diffusion of the ions through the membrane and thus potential variations are observed by means of the internal and external reference electrodes.

Usually, a silver chlorinated wire which is immersed into the electrolytes is used as a reference electrode. For SIET experiments, selective microelectrodes are generally placed 50 μm above from the tracked surface. According to the Nernst equation, calibration of H⁺-selective microelectrode is performed using the buffer solution. An ASET-2 (Science Wares) programming software can scan the respective area. The potential can be calculated by using 1 PΩ rated input impedance preamplifier (Gnedenkov et al. 2016). The ion selective electrodes have some drawbacks due to its physical structure. The micropipettes of glass are delicate and therefore fragile. During the analysis and in the calibration procedure, the transparent nature of glass makes it difficult to obtain the correct distance between the probe and the sample.

3.4 Scanning droplet cell technique

A scanning droplet cell (SDC) is a traditional three electrode experiment with a localized electrochemical cell scanned across the surface of the sample. A positive displacement pump is used in the SDC technique which can force the electrolyte from a gas-purged reservoir through a small diameter tube. The narrow tube generates a convection profile which can be controlled through pump rate. The electrolyte is then sprinkled into the single droplet on the surface of the substrate which acts as a working electrode. The cohesive forces between liquid molecules grasp this droplet on the surface and later can be dragged across the sample by dint of capillary action. As electrochemical reaction takes place between electrolyte and electrode, contact portion of the sample with droplet is measured. SDC experiments can be configured in one of the two ways. Firstly, by applying a constant bias such as potential or current and increment the position of the droplet creating a data map. Secondly, by applying a static or dynamic electrochemical signal to perform experiments such as Tafel or EIS as the droplet is held in a fixed position. Because the droplet is a self-contained, three electrode chemical cell. The force sensor which is mounted on a fixed position of the scanning flow cell (SFC) can measure the force of the droplet when pressed against the working electrode. The SFC consists of inlet and outlet channels which are intersected in a polycarbonate block and it also has an elliptical opening at the bottom of the cell body (Figure 16). The reference and the counter electrode are positioned directly into the channels of the SFC cell and the working electrode is located directly under the elliptical opening into an automatic three-dimensional stage. Ag/AgCl is used as reference electrode while platinum wire is used as counter electrode. To avoid the leakage of the electrolyte, a 150 µm (Schuppert et al. 2012) thick silicon gasket is attached at the opening of the electrode. The working electrode is 500 µm (Kulyk et al. 2015) far from the bottom of the cell. The droplet contact area and the capillary size should be as small as possible (Kollender et al. 2015; Snowden et al. 2010). Since the sample measurement area is very small, the induced current range is as little as in picoamperes. The high resolution potentiostat can measure this small amount of current. A constant polarization is applied during the time of scanning the SDC head and the droplet across the surface in one direction and then the responses (current, voltage and open circuit potential) are measured. Although the droplet motion on the surface is a dynamic process, the ohmic drop should be considered as well. Moreover, as it is a flow-type cell, it makes fast combination with downstream analysis or even with spectrometry. The SDC technique is suitable for both DC and AC measurements with the same configuration one after the other (Cherevko et al. 2014; Efaw et al. 2019).

Figure 16:

3D diagram of scanning droplet cell.

3.5 Scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) methods can analyse the electric potential of working electrode (substrate) with higher accuracy. Using this technique, it can easily access to scan the surface reactions in macroscopic scale. The experimental setup (Figure 17) of SECM consolidates a piezo-electric actuator, a biopotentiostat and an ultra-microelectrode (UME) which is used as a SECM probe. The SECM probes are made of a noble metal and they are normally rigid. Ultra-microelectrod tip is vertically positioned above the surface of the substrate and can capture the topographic view of the surface of the substrate. Substrate and UME tip is regarded as working electrodes whereas platinum electrode can be used as a reference electrode. DC voltage is generally applied to the SECM probe to measure the current response. The bipotentiostat polarizes the UME tip as well as the substrate individually and it can measure the currents. Potentiostat is used to amplify the probe signal which is later converted to a digital signal. One electrode is responsible for conducting electrochemical reactions between the electrolytes and the substrate whereas the other one can measure the response. By changing the polarization level, tip collection and tip generation data can be monitored. A stepper motor gets response through the signal of position controller by a computer-generated program. The positioning system can scan the location of the measuring tip and through generating a data map; it can calculate the electrochemical parameters. The electric signal is then transmitted to the mechanical energy and piezo-electric actuator can drive the UME probe. The two-dimensional (X and Y) movements of the probe are controlled by a position controller but the vertical movement of the SECM probe is conducted manually with respect to the substrate surface. Current is recorded according to SECM probe position and is scanned in either X or Y direction. The sensitivity of the SECM is 100 µV (Beheshti et al. 2016, 2020).

Figure 17:

Schematic diagram of SECM experimental setup.

3.6 Scanning Kelvin probe

Scanning Kelvin probe (SKP) is a nondestructive method which does not require any conducting path between the kelvin probe and the substrate. Kelvin probe can measure work function difference in different media like vacuum, humid air, open air or using a drop of electrolyte on the surface by a non-destructive capacitance method (Wicinski et al. 2016). The metallic probe is generally made from tungsten which is enclosed by brass container. The vibration frequency and the maximum extent of vibration of the probe can be regulated in terms of their required speed and predefined height. The probe vibration produces current that is quashed by external potential. The specific configuration of the SKP setup can measure the capacitive height at the time of collecting data which in turn allows the Kelvin probe to falter the specific profile. Two parallel capacitors plate are used to measure the capacitive height. Between two parallel plates, one plate is regarded as the Kelvin probe surface and another plate is considered as the surface of the substrate. When the needle of the Kelvin probe is near the substrate then the energy variances of the electrons are observed. The energy that is required to remove an electron from the surface of the conductor forms the concept of work function and this work function difference varies the potential of conductive materials. The flow of electron can shift from the higher work function to the lower work function depending on the conductors’ properties. The relative work function can be associated with the value of corrosion potential (E_corr). When two conductive surfaces come into electrical contact, the potential difference is induced. Thus, positive and negative charges are developed by the two different metals.

The respective potentials are denoted by E_F1 and E_F2 while Φ1 and Φ2 represent the work function of the respective plates. When two parallel conductors are not electrically connected then there will not generate any potential difference (V_C). There also shows a variation in charge and energy level between two plates (Figure 18A). On the other hand, charge and energy level appear in an equilibrium position when the plates are connected electrically. Figure 18B delineates the scenario of the connections between two plates. In this case, potential difference (V_C) is developed by the resultant flow of charge due to the electrical connection between the two conductors. The differences of work function can be equated by the multiplication of potential difference (V_C) and electron charge (e) and is represent by Equation (11).

Figure 18:

Energy and charge levels of the plates when they are (A) not electrically connected, (B) electrically connected.

An equal and opposite backing potential (V_B) is applied in the plates to neutralize the potential difference (V_C) during the vibration of the probe. To obtain the value of work function difference, the neutralize condition is essential during the measurements. This Volta potential can be termed as corrosion potential and determined by using following Equation (12).

where (φ1−φ2) is the work function difference between the substrate and Kelvin probe. K is constant whose value can vary for different probe material.

The height regulating scanning Kelvin probe (HR-SKP) system was introduced to obtain the potential difference for the rough, curved, and irregular surfaces. The experimental setup of HR-SKP has been shown in Figure 19. This type of Kelvin probe consists of different electromechanical parts including a stepper motor, piezo electric actuator, speaker, isolator and a needle. The Kelvin probe is normally instructed by an acoustic signal ranging 1–2 kHz frequency (Wapner et al. 2005). The sample chamber can be positioned by a computer-generated command to fix the position between the needle and the substrate surface. A thin wire made by nickel/chromium is used as needle. The appropriate positioning of the needle is controlled by three stepper motors which can provide three dimensional directions. For height regulation, fast and accurate movements piezo-electric actuator is used which can move in Z direction and set the position vertically from the substrate surface. A function generator generates sinusoidal current at a frequency of 10 Hz keeping 300 mV voltage for the distance control circuit. Two computer screens are used to visualize the performance of the two control circuits which are internally connected to each other. At the very beginning of the experiment, X and Y directed stepper motors are driven by computer command to locate the corresponding point of the substrate surface. The piezoelectric actuator is moved by a height regulating circuit. If precise height is needed and the height is not adjusted with the actuator, then additional Z-stepper motor can be used combinedly with piezoelectric actuator to adjust the height. Only the needle distance is set manually while other all commands are performed automatically with computer-generated program. A LIA is applied for the potential measurement. After positioning the probe in a correct point upon the substrate surface, the Volta potential is measured by the potential control circuit. The LIA then commands the probe and the needle is moved to a next position.

Figure 19:

Experimental setup of the HR-SKP.

This work function can be correlated to the corrosion or open circuit potential of bare or coated metals. One example of the application of the SKP is the study of the surface of a material beneath a coating to detect active corrosion sites or identify the potential for future corrosion site development. This technique has been additionally applied to the field of forensic, solar cell research and the development of organic light emitting diodes. Based on the principle of a capacitor, SKP allows the user to set and maintain the probe-to-sample distance enabling the researcher to negate topographical contribution across real world surface. In addition to being a non-destructive technique (NDT), SKP also allows for electrochemical analysis in the absence of electrolytes preserving the sample for future analysis. The SKP non-contacting optical surface profile (OSP) is an option which generates relative height information that can be used as the basis for constant distance mode operation. The OSP technique can also be used to map topographic changes on a sample surface to characterize corrosion pit size, number and depth.

3.7 Novel contactless technique

A modern measurement technique has recently been established that does not require direct physical contact between potentiostat and the substrate to calculate the polarization resistance (R_p). The system here developed is called the contactless technique for corrosion system. Normally, it is essential to establish a direct contact among all the electrodes to measure the polarization resistance and potential difference using suitable wires in order to polarize the substrate which acts as a working electrode. When a current flow between the electrolyte and the substrate, then it gets polarized and generates electrostatic separation. The effect can be accomplished either by putting the substrate in a magnetic field, or by using direct current (DC) or alternating current (AC). The approach is based on the observation that the electrode can be polarized by positioning it under the action of an external electrical field produced by the application of a current between two externals (A and C) graphite electrodes. By applying an external voltage or current to the substrate (M), the applied current can flow in parallel field lines across the electrolyte and electrostatically polarizes the substrate and thus measuring the response. The reference electrodes (REF_A and REF_C) can record the differences in potential. Figure 20 demonstrates the schematic representation of four-electrode system experimental setup, where all four electrodes are connected with a galvanostat. The system comprises substrate ‘M’ as the working electrode and tank is filled with the electrolyte. The most notable feature in this novel technique is that the substrate (M) is not connected electrically with the galvanostat and it is positioned vertically which is parallel to the reference and counter electrodes. Current is applied with a galvanostat/potentiostat by two graphite electrodes (A and C) to measure the response. Two saturated calomel reference electrodes (REF_A and REF_C) located adjacent to the bar are used to measure the voltage variation at both sides of the substrate (M) before and after the applied current. During corrosion process, the concentration of electrolyte may decrease due to the formation of soluble corrosive products.

Figure 20:

Schematic representation of electrode arrangements for novel contactless technique.

The measurement procedure of corrosion rate for this novel contactless method is performed by employing several steps. The equation R_e = V_Re/I_ap is used to calculate the electrolyte resistance. An external current (I_ap) (Andrade et al. 2008) is applied to the immersed substrate (M) in the electrolyte without touching it. This applied current polarizes the substrate by changing the potential through generating external electric field in the surrounding electrolyte. V_Re is measured at the absence of the substrate (M) in the electrolyte with the four-electrode arrangement. After that the substrate (M) is immersed into the electrolyte and V_e+M is measured utilizing the same arrangement. The voltage differences between the reference electrodes (REF_A and REF_C) is regarded as the value of V_e+M where V_e+M = V₂ − V₀. Before testing, the voltage drop (V₀) is measured between the reference electrodes and another voltage drop (V₂) between the reference electrodes is counted after applying I_ap current for a certain time usually taking less than 1 min to obtain the stable value of V₂. The substrate resistance (R_M) can be considered analogous to traditional polarization resistance (R_p). After several hours and days, V_e+M is then measured periodically to acquire R_M values and to analyze whether it is equivalent to R_p or not. Figure 21 shows the electrical circuit for the contactless method where current flows parallelly through the substrate bar and the electrolyte. Two different paths are assumed for the current where one polarizes the substrate bar (M) and the other travels through the electrolyte. The following expression can determine the corrosion resistance of the substrate bar:

where, R_e+M specifies the resistance when the substrate is present in the electrolyte, R_e is the electrolyte resistance in the absence of the substrate and R_M is the resistance of the substrate. Using the Stern–Geary relationship, the corrosion current (i_corr) can be determined from R_M (Stern and Geary 1957). The theoretical framework can be connected to the fact that the currents produced in the substrate are significantly proportional to the induced voltage due to the separation of charges, fulfilling the principle of the linear polarization method (Feliu et al. 1989). The ratio of the current and electric potential (induced voltage) is considered as the polarization resistance if the currents are directly proportional to the electric potential, which depends on the substrate condition. The feasibility of calculating the rate of corrosion without physical contact of the working electrode may unlock new horizon for smart applications and can be termed as non-destructive testing (NDT).

Figure 21:

Electrical model used to represent the configuration of non-contact method.

Summary of the latest electrochemical corrosion measuring techniques has been shown in Table 11.