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Investigation on corrosion performance of multilayer Ni-P/TiO2 composite coating on steel

  • M. Edwin Sahayaraj EMAIL logo , J.T. Winowlin Jappes , I. Siva and N. Rajini
Published/Copyright: December 2, 2014
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Science and Engineering of Composite Materials
From the journal Science and Engineering of Composite Materials Volume 23 Issue 3

Abstract

Electroless nickel coating treatment improves the corrosion resistance of mild steel. This work aims at studying the corrosion behavior of electroless Ni-P/TiO2 composite and multilayer coatings applied to the mild steel substrate as their as-plated state and furnace annealed at various temperatures and compared both the coatings’ corrosion performance. The corrosion behavior of the deposits was evaluated by potentiodynamic polarization studies in 3.5 wt% sodium chloride solution. The results showed that the corrosion resistance of the multilayer coating was two times higher compared to the composite coatings. Further, the corrosion mechanism was discussed in terms of microstructure, phase transformation, grain size, and microstrain.

Keywords: corrosion resistance; electroless nickel; heat treatment; multilayer coating; TiO2 composite coating

1 Introduction

The electroless deposition process significantly meets the future needs of the surface-modifying industries like chemical, automotive, food, and textile industries due to its ease in operation, low equipment cost, and outstanding properties in wear, corrosion, soldering, electricity, magnetism, and metallization of nonmetal materials [1, 2]. One of the widely used plating baths for producing electroless Ni-P coatings is based on hypophosphite as the reducing agent [3, 4]. The characteristics of the Ni-P deposits are dependent on their phosphorous content, and the deposits with high phosphorous content were in amorphous structures [5, 6]. The investigators [7, 8] suggested that the EN deposits annealed at temperatures above ambient might lead to improve the mechanical and chemical properties of the coating by the precipitation of Ni and Ni3P phases. However, these chemical and mechanical properties were degraded with excessive heating at elevated temperatures due to the coarsening of nickel and nickel phosphides [9].

The properties of the coatings can be further improved by the incorporation of metallic and ceramic particles like Zn, Cu, W, Mo, TiO2, ZrO2, etc., into the Ni matrix [10–12]. Titanium oxide is widely used in various engineering applications due to its physical and chemical properties, and these TiO2 particles co-deposited into the Ni-P coating could improve the electrochemical activities. Therefore, this coating could be used in chlor-alkaline industries [13]. The thermal treatment reduced the corrosion resistance of these coating by producing microvoids [14, 15], but the addition of Zn [16] and Al2O3 [17] particles into the Ni-P-TiO2 matrix could enhance the corrosion resistance by improving the adhesion and reduced microvoids in the deposits.

In general, the amount of phosphorous content present in the coating determines the corrosion properties of the coating, and the incorporation of TiO2 particle decreases the phosphorous content of the Ni-P deposits [18]. One of the possibilities to improve the corrosion resistance of the electroless nickel coating could be done by using multilayer techniques. These coating layers produce different corrosion potentials, which have provided an effective solution to improve the corrosion resistance of the coating [19]. In this work, electroless Ni-P/TiO2 composite and multilayer coatings were prepared on the steel specimen and were heat treated at various temperatures to study the corrosion resistance of the coating on the 3.5 wt% of sodium chloride solution, and their corrosion resistance in terms of crystallinity, microstructure, grain size, and microstrain were compared.

2 Materials and methods

2.1 Production of electroless Ni-P/TiO2 composite and multilayer coatings

The mild steel samples of 20 mm diameter and 3 mm thickness were used in this process. The resulting surfaces of the samples were polished, and then they were subjected to pickling treatment using 10% hydrochloric acid. Thereafter, the treated samples were rinsed and were subsequently cleaned by acetone. The TiO2 particles (Merck product, anatase type) of 200 nm–300 nm, 0.01 g, were spread over the cleaned surface to produce Ni-P-TiO2 composite coatings. Deposits were, then, prepared using the optimized bath (chemicals of Merck product) compositions and operating conditions as mentioned in Table 1. Next, for producing the Ni-P-TiO2-Ni-P multilayer coating, the prepared Ni-P-TiO2 composite coatings were once again coated by Ni-P using the same electroless nickel bath mentioned in Table 1.

Table 1

Chemical composition of the electroless nickel plating bath and its operating conditions.

Chemical compositionQuantity
Nickel chloride (NiCl2.6H2O)30 g/l
Sodium hypophosphite (NaH2PO2H2O)40 g/l
Sodium citrate (Na3C6H5O72H2O)30 g/l
Ammonium chloride (NH4Cl)50 g/l
Operating conditions
 Temperature80°C
 pH9–10

2.2 Heat treatment

The coated samples were heat treated using muffle furnace (‘Hasthas’ Scientific Instrument, Chennai, India). The heat treatment was carried out for 1 h at 250°C, 300°C, and 350°C, and then, the samples were cooled in air.

2.3 X-ray diffraction measurement

The X-ray diffraction (XRD) of the coatings was taken using a Siemens X-ray Diffractometer with monochromatic Cu-Kα radiation. Phases, grain size, and microstrain of the coatings were yielded from the diffractograms. The scanning angles (2θ) ranged from 10° to 90°, with a step size of 0.02° and a counting time of 2 s per step. The phases obtained from XRD were recognized by comparing them with the JCPDS files.

2.4 Corrosion test

The electrochemical analysis was done using Electrochemical Analyser (CH Instruments, Inc., Austin, TX, USA). The Tafel electrochemical measurements were obtained to study the corrosion resistance of the coatings. The criteria to evolve the linear polarization curves for the specimens were exposed to a 3.5% sodium chloride aqueous solution at room temperature with a constant scan rate of 2 mV/s. The cell configuration consisted of a reference saturated calomel electrode, a counter platinum electrode, and the testing specimen acted as the third electrode.

3 Results and discussion

The corrosion testing was carried out on the as-plated and furnace heat-treated composite and multilayer samples in the electrochemical analyzer. The 3.5 wt% sodium chloride solution was used as a corrosion medium for the electrochemical analysis. It could significantly accelerate the pitting corrosion by replacing oxygen molecules in water molecules in order to initially adsorb on the Ni-P coating surface and to form a soluble NiCl2 (Ni2++2Cl2←→NiCl2). The adsorbed chlorine ions could easily penetrate through the voids present in the coating surface causing pitting corrosion [20]. Figure 1A and B shows the electrochemical polarization curves for the as-plated and furnace-annealed electroless Ni-P-TiO2 composite and Ni-P-TiO2-Ni-P multilayer coatings in 3.5 wt% NaCl solution. The corrosion current density (Icorr) was calculated using the Tafel extrapolation method, and the corrosion resistance was calculated by the following equation:

Figure 1 (A) Polarization curves of the electroless Ni-P-TiO2 composite coatings in 3.5 wt% NaCl solution. (B) Polarization curves of the electroless Ni-P-TiO2-Ni-P multilayer coatings in 3.5 wt% NaCl solution.
Figure 1

(A) Polarization curves of the electroless Ni-P-TiO2 composite coatings in 3.5 wt% NaCl solution. (B) Polarization curves of the electroless Ni-P-TiO2-Ni-P multilayer coatings in 3.5 wt% NaCl solution.

(1)CR (mpy)=0.13Icorr(Eq. wt.)d. (1)

where Eq. wt is the equivalent weight, d is the density of the coating in g/cm3, and Icorr is the corrosion current in μA/cm2.

The substrate (base metal) corrosion rate is high with a value of 21.85 mpy for which the Icorr value is 47.26 μA/cm2. The as-coated Ni-P-TiO2 composite and Ni-P-TiO2-Ni-P multilayer coatings’ corrosion current densities Icorr were much lower than those of the substrate, and their corrosion resistances were found to be 5.266 mpy and 4.1412 mpy with Icorr of 4.0224 μA/cm2 and 2.354 μA/cm2, respectively. The X-ray diffraction patterns of electroless Ni-P/TiO2 composite and multilayer coatings in their as-plated and heat-treated conditions are depicted in Figure 2A and B. The obtained pattern showed that there was no significant difference exhibited between the plain and the composite/multilayer coatings other than TiO2 particles incorporating in the microstructure [21]. Also, the obtained broad peak angle (2θ) around 50° and 75° showed that the TiO2 particles were co-deposited into the Ni-P matrix, and its crystalline nature was suppressed by the nickel-phosphorous amorphous film as in the as-plated state. The amorphous alloys had higher corrosion resistance compared with the furnace-annealed deposits. That was due to the nonexistence of grain boundaries, which gave a glassy film structure that was conducive to corrosion resistance in the surfaces. The corrosion resistance of Ni-P-TiO2-Ni-P multilayer coating was higher than that of the Ni-P-TiO2 composite coating in its as-plated state. This could be due to the fact that the microvoids present in the composite coating were filled by the Ni-P second layer formed on the coating surface during multilayer deposition as shown in Figure 3A and B, which improved the passivity of the coating surface.

Figure 2 (A) XRD for the Ni-P-TiO2 composite coatings. (B) XRD for the Ni-P-TiO2-Ni-P multi-layer coatings.
Figure 2

(A) XRD for the Ni-P-TiO2 composite coatings. (B) XRD for the Ni-P-TiO2-Ni-P multi-layer coatings.

Figure 3 (A) SEM image of the as-plated Ni-P-TiO2 composite coating. (B) SEM image of the as-plated Ni-P-TiO2-Ni-P multilayer coating.
Figure 3

(A) SEM image of the as-plated Ni-P-TiO2 composite coating. (B) SEM image of the as-plated Ni-P-TiO2-Ni-P multilayer coating.

The corrosion resistances of both the composite and multilayer heat-treated coatings were lower than those of their as-plated state. This could be attributed to the changes in crystallinity from amorphous to crystalline nature. The effect of heat treatment temperature on the corrosion rate is presented in Figure 4. When increasing the heat treatment temperature, the corrosion rates of the furnace-annealed composite and multilayer coatings were decreased. This could be due to the fact that the increasing heat treatment temperature reduced the deposit defects by stress relief and reduced microvoids present in the coating by filling it up with the formation of Ni3P phases. The heat treatment temperature increased resulting in the increase in the number of Ni3P phases, which is evident in Figure 2A and B.

Figure 4 Effect of heat treatment temperature on the corrosion resistance of the Ni-P/TiO2 composite and multilayer coatings.
Figure 4

Effect of heat treatment temperature on the corrosion resistance of the Ni-P/TiO2 composite and multilayer coatings.

The corrosion resistances of the heat-treated multilayer coating were higher than those of the composite coatings for all the heat-treated temperatures, and the values were almost two times higher as shown in Figure 4. In general, the adherence of TiO2 particles co-deposited into the Ni matrix composite coating was very small because during deposition, no chemical reaction took place between the particle and the matrix, which affected the heterogeneity of the coating, and during heat treatment, it produced microcorrosion cells causing severe chemical attack, which, in turn, reduced the corrosion resistance of the composite coating [14, 22]. But in the multilayer coating, the formed Ni-P second layer enriched the phosphorous (passive element) contents on the outer layer surface. Also, the increasing heat treatment temperature accelerated the grain boundary diffusion causing the second layer nickel and phosphorous content to penetrate more easily into the first layer composite coating to develop a dense composite first layer. This improved the bonding strength of the TiO2 particle and homogeneity of the deposits and diminished the microcorrosion cells formed by the TiO2 particles. The obtained scanning electron micrographs as shown in Figure 5A and B is evidence of the above-mentioned fact. Therefore, the corrosion resistance of the multilayer coating increased.

Figure 5 (A) SEM image of the Ni-P-TiO2 composite coating heat treated at 350°C. (B) SEM image of the Ni-P-TiO2-Ni-P multilayer coating heat treated at 350°C.
Figure 5

(A) SEM image of the Ni-P-TiO2 composite coating heat treated at 350°C. (B) SEM image of the Ni-P-TiO2-Ni-P multilayer coating heat treated at 350°C.

The obtained X-ray diffractograms shown in Figure 2A and B were used to calculate the crystallite size and microstrain for the heat-treated composite and multilayer coatings at various heat treatment temperatures using the Debye-Scherer equation and Stokes-Wilson expression. The results are shown in Figure 6A and B. It is observed that the heat treatment temperature increased to the enhanced nickel-phosphide grain size, and the microstrain relaxed due to the recovery of defects and grain growth. The multilayer coatings produced lesser microstrain than the composite coatings, and the grain size was found to be larger than the composite coatings for all the heat treatment temperatures. This lesser microstrain and larger grain sizes could be the prime reason for improving multilayer coating corrosion resistance [8].

Figure 6 (A) Grain sizes of the Ni-P-TiO2 composite and Ni-P-TiO2-Ni-P multilayer coatings for the heat treatment temperatures. (B) Micro strain of the Ni-P-TiO2 composite and Ni-P-TiO2-Ni-P multilayer coatings for the heat treatment temperatures.
Figure 6

(A) Grain sizes of the Ni-P-TiO2 composite and Ni-P-TiO2-Ni-P multilayer coatings for the heat treatment temperatures. (B) Micro strain of the Ni-P-TiO2 composite and Ni-P-TiO2-Ni-P multilayer coatings for the heat treatment temperatures.

4 Conclusion

  • The electroless composite and multilayer Ni-P/TiO2 coatings were successfully prepared on the mild steel substrate.

  • Both the coatings exhibited amorphous nature in their as-plated state, and there was no significant change in the structure and phase transformation caused by the co-deposited particles.

  • The surface morphology of the electroless composite and multilayer coatings was varied after heat treatment. This could be due to the formation of Ni3P phases and the increase in heat treatment temperature increasing the number of Ni3P phases.

  • Multilayer Ni-P-TiO2-Ni-P coatings offered higher corrosion resistance in their as-plated and heat-treated states compared with the composite Ni-P-TiO2 coatings.

  • The corrosion resistance of the heat-treated multilayer coating was found to be two times higher than that of the composite coating. This could be due to the grain boundary diffusion forming a dense composite film.

  • The increasing heat treatment temperature reduced the microstrain and improved the grain size in multilayer coatings compared to the composite coating. This could be beneficial for improving corrosion resistance in multilayer coatings.

Corresponding author: M. Edwin Sahayaraj, Noorul Islam Centre for Higher Education, Department of Automobile Engineering, Kumaracoil, Tamilnadu 629180, India, e-mail:

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Received: 2014-5-4
Accepted: 2014-9-1
Published Online: 2014-12-2
Published in Print: 2016-5-1

©2016 by De Gruyter

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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https://doi.org/10.1515/secm-2014-0134
Keywords for this article
corrosion resistance; electroless nickel; heat treatment; multilayer coating; TiO2 composite coating
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Articles in the same Issue

  1. Frontmatter
  2. Original articles
  3. In vitro degradation and bioactivity of poly(propylene fumarate)/bioactive glass sintered microsphere scaffolds for bone tissue engineering
  4. Properties enhancement in multiwalled carbon nanotube-magnetite hybrid-filled polypropylene natural rubber nanocomposites through functionalization and processing methods
  5. Synthesis of magnetic photocatalyst and sensitization properties of polypyrrole
  6. Effect of silane coupling agent on the mechanical properties of nitrile butadiene rubber (NBR)/organophilic montmorillonite (OMMT) nanocomposites
  7. Monitoring of jute/hemp fiber hybrid laminates by nondestructive testing techniques
  8. Size effects on the in-plane mechanical behavior of hexagonal honeycombs
  9. Investigation on corrosion performance of multilayer Ni-P/TiO2 composite coating on steel
  10. Investigation of pozzolanic activity of volcanic rocks from the northeast of the Black Sea
  11. The effect of delayed ettringite formation on fine grained aerated concrete mechanical properties
  12. Effect of early-age freeze-thaw exposure on the mechanical performance of self-compacting repair mortars
  13. Modeling cement hydration by connecting a nucleation and growth mechanism with a diffusion mechanism. Part I: C3S hydration in dilute suspensions
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