Influence of heat treatment on the corrosion properties of CuAlMn shape memory alloys

3.1 Electrochemical investigation

The open circuit potential of the materials under investigation was recorded over 60 min immersion in chloride solution deaerated by purging with Ar, and the results of these investigations in 0.9% NaCl solution are presented in Figure 1.

Figure 1:

Variation of open circuit potential of the CuAlMn alloy in 0.9% NaCl solution, as-cast, annealed and annealed and aged state.

The E_OC values of heat-treated CuAlMn alloy samples have positive values compared with E_OC of as-cast samples. This trend is also visible in investigations in 0.1% and 1.5% NaCl solution. The steady state is reached within 60 min of electrode immersion. With increasing the NaCl concentration, stabilisation of E_OC occurs at more negative potential values.

The linear polarisation technique is the DC nondestructive technique that is based on the theory that within 10–20 mV of the corrosion potential, small changes in the potential of a freely corroding electrode have a linear relationship with the current change and so is proportional to the corrosion current. Linear polarisation measurements were performed in order to determine the polarisation resistance (R_p) that represents the resistance of metal to corrosion and is defined by the slope of the polarisation curve near the corrosion potential, by equation (1):

Results of linear polarisation investigations for CuAlMn alloy in 0.1% NaCl solution are presented in Figure 2, and the values of corrosion resistance for the CuAlNi samples are shown in Table 1.

Figure 2:

Linear polarisation curves for CuAlMn alloy, as-cast, annealed and annealed and aged state, in 0.9% NaCl solution.

From Figure 2 and Table 1, it can be seen that the heat treatment alloy samples have higher slopes of the polarisation curves, resulting in higher values of R_p. It is also apparent that with the increase of the chloride ions concentration, the value of polarisation resistance decreases, which indicates a more intense corrosion of the alloy. Similar influence of heat treatment and chloride concentration on corrosion resistance was observed in previous investigations on CuAlNi shape memory alloy (Vrsalović et al., 2017, 2018).

The last applied electrochemical method was a potentiodynamic polarisation method that was carried out in a wide range of potentials to gain insight into the anodic behaviour of CuAlMn subjected to different heat treatment. Figure 3 shows the potentiodynamic polarisation curves for CuAlMn alloy in 0.1% NaCl solution, while Figure 4 shows the influence of chloride concentration on the polarisation curves for the investigated alloy in as-cast state.

Figure 3:

Potentiodynamic polarisation curves for CuAlMn alloy, as-cast, annealed and annealed and aged state, in 0.9% NaCl solution.

Figure 4:

Effect of chloride ion concentration on potentiodynamic polarisation curves for CuAlMn alloy, as-cast state.

The potentiodynamic polarisation curve is usually presented in a semi-logarithm plot, composed of cathodic and anodic branches, which are the result of the electrochemical reactions in the system. The most common cathodic reaction is the reaction of hydrogen evolution (for acidic or deaerated solutions) or oxygen reduction, while the most common anodic reaction is oxidation reaction or alloy corrosion. From Figure 3, very small differences can be seen between the polarisation curves of as-cast CuAlMn alloy and heat-treated CuAlMn alloy, which indicate a very similar corrosion behaviour. The main differences can be seen in smaller values of anodic current densities for heat treatment alloy samples. In the area of high anodic potentials, anodic current densities have almost identical values, which indicate the same corrosion behaviour. Increase in chloride concentration leads to increase in the values of anodic current densities, which are clearly seen in Figure 4. From Figure 4, it can be seen that all three anodic branches of polarisation curves consist of the apparent Tafel region followed by a pseudo passive region in which the anodic current densities have been reduced to a certain extent because of the formation of a layer of low soluble corrosion products on the surface of the alloy. Similar anodic behaviour has been found in corrosion investigations of Cu, CuAl, CuAlAg, CuAlNi and CuZnNi alloys (Benedetti et al., 1995; Kear et al., 2004; Gojić et al., 2011; Satish et al., 2014; Vrsalović et al., 2018). In the mentioned investigations, an increase in anodic current density can be observed above a certain anodic potential due to the dissolution of corrosion products from the surface, which represent the third region. In the anodic polarisation curves presented in Figures 3 and 4, this third region is missing probably because of the more compact and protective surface film. This could be attributed to the presence of Mn as an alloying element in the investigated alloy. According to the literature, the addition of Mn in small amounts affects the microstructure of the alloy; i.e. it reduces the size of the crystal grain. Namely, Mn diffuses easily and rapidly disperses through the mass of alloy, accumulates on grain boundaries and prevents further grain growth (Sampath, 2005; Saud et al., 2015a). Studies have shown that such refining of microstructures, with the enhancement of mechanical properties, also increases the corrosion resistance of the alloy (Saud et al., 2015a). Namely, the fine microstructure positively affects the compactness and stability of the passive film formed on the alloy. Because of the high affinity of Mn for oxygen, the surface passive film on Cu-Al-Mn alloy contains Mn oxide, which is proven by EDS and XPS surface analyses. The shape of the anodic branch of the curve differs depending on the concentration of chloride ions (Figure 4). It seems that with the increase of chloride ions, a pseudo passive area starts to form at lower anodic potential (around 0.1 V for 1.5% NaCl solution, around 0.2 V for 0.9% NaCl solution and around 0.8 V for 0.1% NaCl solution). Corrosion parameters obtained from potentiodynamic polarisation measurements were presented in Table 2.

It is evident from Table 2 that the corrosion current value increases with increasing chloride ion concentration, which indicates more intense corrosion of the alloy. Also with the increase of the concentration of chloride ions, the corrosion potential is shifted toward more negative values. The lowest values of corrosion current density and the most positive values of corrosion potential were obtained by heat-treated CuAlMn alloy samples for all NaCl concentrations, but differences in the corrosion current and corrosion potential values for as-cast CuAlMn alloy are small.

3.2 Surface investigation

Figure 5 shows SEM micrographs of the Cu-Al-Mn alloy after continuous casting and applied heat treatment procedure. The microstructure in as-cast state (Figure 5A) is significantly different in regard to the microstructure after annealing and ageing (Figure 5B, C). It can be noticed that a two-phase microstructure (probably residual β phase along with martensite phase) appears in Cu-Al-Mn alloy after casting.

Figure 5:

SEM micrographs of Cu-Al-Mn shape memory alloy in as-cast condition (A), after annealing at 900°C/30′ (B) and after aging at 300°C/60′ (C).

After heat treatment, the samples contain only the martensite microstructure (Figure 5B, C). By rapid cooling, the alloy undergoes ordering transitions β(A2)→β₂(B2)→β₁(L21), followed by martensite transformation β₁(L21)→β₁′ (Jiao et al., 2010). The martensite microstructure after aging (Figure 5C) forms as the completely needle-like shape martensite, whereas after annealing in some fields, the V-shape martensite was noticed. Self-accommodating zigzag martensite morphology is characteristic for the β₁′ martensite in shape memory alloys.

After the polarisation measurements, the CuAlMn electrode surface were ultrasonically treated in deionised water, dried in a desiccator and examined by optical microscopy at a magnification of 100 to determine the surface condition (Figure 6).

Figure 6:

Microscopic surface images of CuAlMn alloy after potentiodynamic polarisation measurement in 0.1% NaCl solution: (A) as-cast, (B) annealed and (C) annealed and aged state, with magnification of 200 times.

The rough surface of the CuAlMn alloy can be seen in microscopic images that are due to corrosion processes, and heat-treated samples of CuAlMn alloys do not show significant differences in the appearance of corroded surfaces. The appearance of a corroded alloy surface does not indicate the existence of pitting corrosion, which was a characteristic corrosion phenomena for CuAlNi alloy in similar corrosion investigations (Kear et al., 2004; Vrsalović et al., 2018). Instead of that, general corrosion seems to be a dominant corrosion type for the investigated CuAlMn alloy.

Detailed investigations of the alloy surface morphology after corrosion measurements were performed with SEM/EDS analysis, and the results were presented in Figures 7–9 and Tables 3 and 4.

Figure 7:

SEM surface images of CuAlMn alloy after potentiodynamic polarisation measurement in 0.1% NaCl solution: (A) as-cast, (B) annealed and (C) annealed and aged state.

Figure 8:

(A) SEM images of as-cast CuAlMn alloy after potentiodynamic polarisation measurements in 0.9% NaCl solution (pH=7.4, T=37°C) and (B) EDS analysis for position 2.

Figure 9:

(A) SEM images of annealed CuAlMn alloy after potentiodynamic polarisation measurements in 0.9% NaCl solution (pH=7.4, T=37°C) and (B) EDS analysis for position 1.

From Figures 7–9, it is apparent that high anodic polarisation (up to 1.5 V) results in significant damage to the electrode surface. The cracked layers of corrosion products with heterogeneous structures are clearly visible on the surface of the electrode, as well as cracks and ruptures formed by their dissolution. The appearance of corrosion products varies depending on the heat treatment, which indicates its different chemical composition. Elemental analysis at specific points on the surface by energy dispersive X-ray spectroscopy has showed that the corrosion products consist mainly of aluminium and manganese oxides and chlorides, the percentage of which differs depending on the site where the analysis was performed. Significantly lower copper content on the surface was found on the surface of the heat-treated CuAlMn alloy sample compared with the surface of the as-cast CuAlMn alloy (Tables 3 and 4), which suggests the formation of a thicker layer of Al and Mn oxides on the heat-treated alloy.

In order to get more precise insight into the surface chemistry, we performed XPS analyses on surfaces of the as-cast and annealed samples. The XPS method is very surface sensitive as the information about chemical composition and oxidation states originates from the 3–5 nm thick surface layer. This is much less than by the EDS method where the analyses depth is about 1 μm. Figure 10 shows the wide energy XPS spectra obtained from the as-cast and annealed samples after corrosion test.

Figure 10:

XPS survey spectra from the as-cast and annealed samples.

Both samples are similar and contain O 1s, C 1s, Cl 2p, Cu 2p, Mn 2p and Al 2p peaks. From the intensities of these peaks, the chemical composition was calculated, and it is given in Table 5. We did not take into account carbon as it usually originates from the surface contamination. From the data given in Table 5, it can be seen that the surface of corrosion products is enriched in Al and Mn with respect to Cu, which confirms similar EDS results. XPS results show that the most abundant metallic element on the surface is Al.

To reveal the oxidation states of elements on the surface, high-energy resolution XPS spectra were measured on the as-cast and annealed samples. Spectra from both samples were very similar. Therefore, the XPS spectra Al 2p, Mn 2p and Cu 2p from the as-cast sample are only given in Figure 11.

Figure 11:

XPS high-energy resolution spectra Al 2p (A), Mn 2p (B) and Cu 2p (C) from the as-cast sample.

They were interpreted by help of reference Moulder et al. (1995). From Figure 10A, we can see that the Al 2p peak is at 74.2 eV, which can be assigned to the Al(3+) oxidation state in the surface of Al oxide. The Mn 2p peak (Figure 10B) is composed of a double peak. The main one Mn 2p_3/2 is at 642.0 eV, and this may be assigned to MnO₂ or MnCl₂, which cannot be clearly distinguished by XPS method. The Cu 2p spectrum (Figure 10C) consists of Cu 2p_3/2 peak and Cu 2p_1/2 peak separated by 19.9 eV. A closer look at the Cu 2p_3/2 peak of the as-cast sample reveals that it may be decomposed into two peaks at binding energy of 932.0 assigned to Cu₂O and at 935 eV assigned to CuCl. In both cases, Cu atoms are in the Cu(1+) oxidation state. The absence of satellite peak at 945 eV in the Cu 2p spectrum means that no or very little amount of Cu(2+) oxidation state is present.

AFM surface analysis was performed on CuAlMn alloy samples (as-cast and annealed state) after potentiodynamic polarisation measurements in 1.5 mol dm⁻³ NaCl solution. Before analysis, alloy samples were ultrasonically treated in deionised water and dried in a desiccator identical to the samples for the SEM/EDS analysis. The results of the investigations were presented in Figures 12 and 13.

Figure 12:

AFM images of as-cast CuAlMn alloy surface after polarisation measurements in 1.5 mol dm⁻³ NaCl solution and its roughness determined by line profile across the surface.

Figure 13:

AFM images of annealed CuAlMn alloy surface after polarisation measurements in 1.5 mol dm⁻³ NaCl solution and its roughness determined by line profile across the surface.

AFM analysis revealed significant roughness and cracks on the alloy surfaces due to intensive corrosion processes. The cracks between the flat regions are about 1 μm deep. Deeper cracks were observed on the annealed samples, which can be attributed to a somewhat thicker layer of aluminium and manganese oxide on the surface, which is cracked and partially dissolved because of intensive corrosion process in higher chloride concentration.