1 Introduction

The excellent abrasion and wear-resistant High Chromium White Irons (HCWI) hardfacing alloys used in sugarcane rolls, pump impeller components, oil and gas pipelines and also to rebuild screw barrels in plastic industry are exposed to strong acidic environment and there are limited studies on the corrosion behaviour of hardfacing alloys [1–4]. HCWI components for crushing and grinding mill liners, deep-sea drilling operations, seawater valves and subsea oil operations are exposed to neutral and seawater environment [5, 6]. Investigation done by Rogers [7] on corrosion of hardfacing alloys in marine environment showed that corrosion occurred in the chromium-depleted region of the austenite matrix while forming chromium carbides. This type of corrosion was also found in valves depending on the exposed time under seawater. Galvanic couple is formed between the carbide and matrix, where chromium-depleted austenite matrix acts as an anode and carbide acts as a cathode in seawater environment.

Similarly, the studies conducted for HCWI in acidic [1, 8–13] and neutral [8, 14–19] environments proposed that preferential corrosion of eutectic austenite or martensitic matrix occurred due to the formation of galvanic cell between carbides and eutectic austenite or martensite matrix. Zhang et al. [13] conducted experiment at pH 1.5 acidic solution by separating carbides and eutectic austenite matrix from HCWI hardfacing alloys. It was found that corrosion occurs mainly due to potential difference between eutectic austenite matrix and carbides. In pH 1.5 solution, carbides attained more noble potential than eutectic austenite matrix. Other investigators [8, 11, 17, 20–22] proposed that HCWI corrosion occurred at interface between carbides and eutectic austenite matrix for both acidic and neutral environments. They also suggested that the cause of corrosion was due to the depletion of chromium below 12 % at interfaces while forming carbides, which hindered the formation of passive film.

Aziz et al. [8] found that corrosion was strongly dependent upon the corrosive environment, and the evolution of hydrogen in acidic medium prevented the formation of passive film on HCWI surface. Tang et al. [9, 10] found that in acidic solution of 0.5 M H2SO4, the corrosion rate decreased with increasing percentage of carbon which resulted in HCWI to form more chromium carbides which decreases the volume fraction of austenite matrix. The higher carbon content increased the area ratio of cathode chromium carbides to anode eutectic austenite matrix in acidic environment.

Only limited studies [23, 24] suggested the effect of chloride ions in the microstructure of HCWI. It was proposed that chloride ions did not allow the eutectic austenite matrix to form a passive film on the surface. Tian et al. [11] found that due to diffusion control of chloride ions, the corrosion rate of HCWI was significantly lesser for higher chloride concentration than lower chloride concentration. Tang et al. [9] showed that the volume loss of HCWI was significantly higher at neutral pH 7 than at pH 3 and 5. It was due to the formation of weakened passive layer of HCWI in neutral solution.

Despite the significant findings on corrosion behaviour of HCWI, many authors did not discuss a method to predict corrosion behaviour. This is where the application of Pourbaix or E–pH diagrams fits in. There are only limited studies in the use of Pourbaix diagrams to predict a trend for the corrosion behaviour of hardfacing alloys in both acidic and neutral environments. Pourbaix diagrams can not only be used to predict the corrosion behaviour of materials, but also provide a simplified explanation of complicated chemistry and thermodynamics. Pourbaix diagrams can also be used to diagnose past and present problems in a thoughtful manner. They can also be used to design suitable experiments in an electrolyte of interest. Some investigators have used the Pourbaix diagrams of metals to explain the behaviour of corrosion of metals in natural sea water and aqueous sulfidic solution [25, 26]. With this in view, Varmaa et al. [27, 28] developed Pourbaix diagrams for carbides in order to explain the trend observed in the corrosion behaviour of carbides. In this experimental study, the corrosion of hardfacing alloys based on HCWI, in acidic and neutral environments, will be explained through the use of Pourbaix diagrams of carbides.

2 Experimental Procedure

Hardfacing alloys based on HCWI were deposited on low carbon steel using shielded metal arc welding. The typical compositions of the deposited hardfacing alloys are given in Table 1. Alloy 1 with boron element mainly forms eutectic martensitic matrix along with chromium carbides, while Alloy 2 had a number of elements, such as niobium, molybdenum, and tungsten forms complex carbides with eutectic austenite matrix [29, 30]. These elements would form carbides in preference to chromium, and hence this sample was likely to contain complex carbides. The recommended welding parameters provided by the manufacturer of the hardfacing alloys are given in Table 2. To avoid dilution, welding was performed in four layers. Samples cut from the weld deposits were prepared for microstructural observations. The samples were etched in 20 ml distilled water, 10 ml HCl, 30 ml HNO3, and 5 g of FeCl3 [31].

Table 1 Typical Chemical compositions (wt%) of hardfacing alloys
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Table 2 Welding Parameters
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Corrosion tests were conducted using electrochemical methods with a Gamry Reference 3000 potentiostat and a three-electrode cell. The HCWI (sample area = 2.85 cm2) was the working electrode exposed to electrolytes with selected pH, as shown in Table 3. pH 1.5, 2 and 7 were selected for explaining the trend of corrosion behaviour of HCWI in different regions of Pourbaix diagram of carbides [27, 28]. Solutions with pH 1.5 and 2 were prepared from diluting sulphuric acid with distilled water. The effect of chloride ions was analysed in pH 7 NaCl solution. The graphite and silver/silver chloride were the counter and reference electrodes respectively. Before measuring corrosion potential, the electrolyte in the corrosion cell was deaerated using nitrogen gas for 30 min as described in the ASTM Standard G5-94 [32]. The open-circuit potential (Ecorr) was measured after 10 min, and then the potentiodynamic test was conducted at a scan rate of 0.17 mV/s [14]. The potentiodynamic test was continued till the sample reached the final potential 2 V. The final potential 2 V was selected on the basis of maximum potential used in the Pourbaix diagram of carbides [27, 28]. Then the microstructure was examined using an optical microscope by removing the corrosion product using rubber cork under running tap water [2]. The sample weight and pH of electrolyte were measured before and after corrosion test for identifying the changes that happened during the potentiodynamic test.

Table 3 Composition of electrolytes for anodic polarisation tests
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The hardness was measured before and after corrosion test on HCWI samples using Vickers hardness testing machine at 10 g load. The 10 g load was chosen to measure the hardness of carbides and eutectic austenite/martensitic matrix separately. The Scanning Electron Microscopy (SEM, Philips XL 30 series) was used to study the microstructure and Energy Dispersive X-ray Spectrometry (EDS) was used to study the chemical composition of the microstructural phases. The Inductively Coupled Plasma Mass Spectrometry (ICPMS) was used to analyse the chemical composition of the electrolyte after the corrosion tests. Sample with higher dissolved solids was analysed using Inductively Coupled Plasma Optical Emission Spectrometry (ICPOES).

3 Results

3.1 Microstructure Characteristics of HCWI Alloys

Figure 1 shows the microstructure of alloy 1 hardfacing sample. The microstructure shows primary carbides surrounded by eutectic carbides and martensite. The boron content present in alloy 1 helps to alter the eutectic matrix from austenite to martensitic [29, 30]. The primary carbides were polygonal and some needle shaped depending on the direction of solidification. The primary and eutectic carbides are of the type M7C3 and M23C6, respectively. The M7C3 carbides were considered to be more stable than M23C6 carbides [15, 33–38]. Figure 2 shows the microstructure of alloy 2 sample. The microstructure shows again primary carbides surrounded by eutectic carbides and austenite. The microstructure also shows some light regions of niobium carbide (NbC) [35]. The niobium element in alloy 2 helps to enrich the chromium in matrix which increases the austenite formation temperature and forms austenite matrix. The formation of reinforced NbC in alloy 2 helps prevent the detachment of chromium-rich M7C3 primary carbides [35, 39].

Fig. 1
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Microstructure of alloy 1 sample showing polygonal primary carbides surrounded by eutectic carbides and martensite

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Fig. 2
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Microstructure of alloy 2 sample showing polygonal primary carbides surrounded by eutectic carbides and austenite. Light regions are NbCs

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The SEM pictures of the alloy 1 and alloy 2 samples are given in Fig. 3i, ii. The point elemental analysis was done on primary carbides (a), carbide/matrix interfaces (b), matrix (c) and eutectic carbides (d) and the values are given in Table 4. In Fig. 3ii, the area marked d1 was found to be molybdenum carbide. In Fig. 3ii, the area marked a1 was found to be NbC. From Table 4, it can be seen that the primary and eutectic carbides (a and d, respectively) had high amounts of chromium in both the samples. Area b and c denote the carbide–austenite interface, and eutectic austenite or martensite matrix shows that the chromium is lower than primary carbides but higher amount of iron present in both the samples. The results of Table 4 show that larger amount of chromium was consumed while forming primary and eutectic carbides.

Fig. 3
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SEM analysis of i alloy 1 and ii alloy 2 hardfacing samples

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Table 4 EDS analysis (wt%) for hardfacing samples
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The hardness of carbide and matrix for both alloy 1 and 2 samples was measured with 10 g load, as shown in Table 5. The hardness of carbide and eutectic martensitic matrix for alloy 1 sample found to be higher than the alloy 2 sample. This may be due to addition of boron content which significantly increases the hardness of the microstructure of alloy 1. According to previous studies [22, 40], eutectic martensitic matrix of alloy 1 sample found to be harder than the eutectic austenite matrix of alloy 2 sample.

Table 5 Hardness of carbide and matrix of Alloy 1 and 2
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3.2 Corrosion Behaviour of HCWI Alloys

3.2.1 Potentiodynamic Tests

The potentiodynamic tests were conducted by exposing the HCWI samples to the selected electrolytes of pH 1.5, 2 and 7. The Rest Potential (RP) changes with the nature of electrolyte and pH for both the samples, alloy 1 and 2. In Fig. 4, the potentiodynamic curve of alloy 1 in H2SO4 of pH 1.5 exhibits an active behaviour rather than active–passive behaviour. The pitting can be noted on the curve especially at higher potentials. Figure 5 shows the microstructure of alloy 1 sample at Final Potential (FP) of the potentiodynamic curve in Fig. 4. It can be seen that the dissolution of eutectic martensitic matrix occurred in preference to carbides. The primary and eutectic carbides were seen to be protected from corrosion at FP in pH 1.5 solution. The potentiodynamic curve of alloy 2 in H2SO4 of pH 2 shows active–passive behaviour as shown in Fig. 6. The active behaviour of alloy 2 starts from RP until point 1, after that passive behaviour of alloy 2 was seen from points1 to 2 and then trans-passive region was observed from point 2 to FP. The microstructure of alloy 2 sample in Fig. 7 shows that the corrosion occurs in the eutectic carbides rather than the eutectic austenite matrix at FP of Fig. 6. The niobium and primary carbides were seen to be uncorroded at FP in pH 2 solution.

Fig. 4
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Anodic polarisation curve for alloy 1 in pH 1.5 solution

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Fig. 5
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Microstructure of corroded eutectic martensitic matrix of Alloy 1 after anodic polarisation test in pH 1.5 solution

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Fig. 6
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Anodic polarisation curve for alloy 2 in pH 2 solution

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Fig. 7
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Microstructure of corroded eutectic carbides of Alloy 2 after anodic polarisation test in pH 2 solution

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In neutral solution at pH 7, Fig. 8 shows the potentiodynamic curve of alloy 1 at 2 M NaCl solution which exhibits active and slight passive behaviour from RP to FP. The presence of aggressive chloride ions in the solution does not allow the formation of passive film on the surface of HCWI. The microstructure of alloy 1 in Fig. 9 shows complete dissolution of eutectic martensitic matrix by protecting eutectic and primary carbides at FP of Fig. 8. In pH 7 buffer solution, Fig. 10 of alloy 2 shows passive–transpassive behaviour. The passive behaviour of alloy 2 in buffer solution starts from RP until point 1 and then the transpassive behaviour was observed from point 1 to FP. Figure 11 shows the microstructure of alloy 2 sample at FP of the potentiodynamic curve in Fig. 10. It can be seen that dissolution of eutectic and primary carbides with preference to eutectic austenite matrix. The NbCs were seen to be uncorroded at FP in pH 7 buffer solution.

Fig. 8
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Anodic polarisation curve for alloy 1 in pH 7 2MNaCl solution

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Fig. 9
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Microstructure of corroded eutectic martensitic matrix of Alloy 1 after anodic polarisation test in pH 7 NaCl solution

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Fig. 10
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Anodic polarisation curve for alloy 2 in pH 7 buffer solution

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Fig. 11
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Microstructure of corroded primary and eutectic carbides of Alloy 2 after anodic polarisation test in pH 7 buffer solution

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In summary, it can be said that in alloy 1 H2SO4 of pH 1.5 and in 2 M NaCl solution of pH 7, eutectic martensitic matrix has dissolved completely leaving eutectic and primary carbides with little corrosion, whereas, by exposing alloy 2 to H2SO4 solution of pH 2, eutectic carbide gets corroded in preference to eutectic austenite matrix. In addition to that alloy 2 at pH 7 of buffer solution, eutectic and primary carbides get corroded in preference to eutectic austenite and NbCs.

3.2.2 HCWI Samples and Electrolyte Analysis After Corrosion

The values for RP and corrosion current (Icorr) at FP of potentiodynamic curves for HCWI samples for selected environments are given in Table 6. Samples in pH 7 buffer solution showed the best corrosion resistance followed by samples in pH 2, 1.5 and 7 NaCl solutions, due to higher RP and lower Icorr at FP of potentiodynamic curves. Samples in pH 7 NaCl showed poor corrosion resistance due to the presence of aggressive chloride ions in the solution. The weight and pH analysis after corrosion test in Table 7 showed that poor corrosion resistance in pH 7 NaCl solution had net loss in sample weight with increase in pH value of the corroded solution. However, samples with good corrosion resistance of pH 7 buffer solution had net gain in sample weight and no change in pH value after corrosion. The net gain in sample weight shows that a passive layer probably formed.

Table 6 Rest potential (Ecorr) and corrosion current (Icorr) at final potential
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Table 7 Weight loss and pH analysis of Alloy 1 and 2 samples in 2 M NaCl and buffer solution before and after corrosion test
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The results of ICPMS (µg/L) and ICPOES (mg/L) test performed on electrolytes after corrosion test of HCWI in pH 7 NaCl and buffer solutions are presented in Table 8. There was significant dissolution of chromium and other elements such as manganese, silicon and vanadium at FP in pH 7 NaCl solution rather than in pH 7 buffer solution. Thus, these test results confirmed that corrosion was higher in pH 7 NaCl than in pH 7 buffer solution. The hardness test was done on carbide at 10 g load of alloy 2 sample after corrosion test in pH 7 buffer solution shows that there was a significant drop in hardness from 1711HV to 65HV as shown in Table 9. This may be as a result of the formation of corrosion products on the surface of the existing carbide.

Table 8 ICPMS and ICPOES analysis of 2 M NaCl and buffer solutions after corrosion test of Alloy1 and 2 samples
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Table 9 Hardness of primary carbide of Alloy 2 sample before and after corrosion test in pH 7 buffer solution
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The back scattered electron image of alloy 1 sample of pH 1.5 solution at FP in Fig. 12 shows that eutectic martensitic matrix dissolved completely leaving eutectic and primary carbides nearly intact. The secondary electron image of alloy 2 at pH 7 buffer solution in Fig. 13 shows that the carbides are hollow in shape and some cracks were found. These images of Figs. 12, 13 show that after corrosion of eutectic martensitic matrix, the carbides seem to be unsupported, and corrosion of carbides makes HCWI samples not useful any longer to resist wear.

Fig. 12
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Back scattered electron image of corrosion in eutectic martensite of Alloy 1 sample in pH 1.5 solution

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Fig. 13
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Secondary electron image of corrosion in carbide of Alloy 2 sample in pH 7 buffer solution

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3.2.3 Corrosion Analysis of Carbide and Eutectic Martensitic Matrix Samples

After the eutectic martensitic matrix had dissolved in the potentiodynamic tests, the potential of the eutectic and primary carbides alone in the pH 1.5 H2SO4 solution was measured for alloy 1 sample. For measuring these carbides potential, epoxy solution was placed on the surface of alloy 1 sample and this sample was repolished after the epoxy hardened [13]. Similarly, in order to measure the potential of eutectic martensitic matrix, the eutectic and primary carbides were dissolved in an alkaline solution. Again epoxy was applied to the sample surface and it was repolished. After preparing carbide and eutectic martensitic matrix sample, corrosion potential of the samples in pH 1.5 H2SO4 solution is shown in Table 10. The carbide sample showed nobler potential than a eutectic martensitic matrix sample in H2SO4 solution of pH 1.5. Similarly, for pH 7 NaCl solution, the eutectic martensitic matrix was not stable due to nobler potential of carbides. The carbides corroded in the pH 7 buffer solution same as in the alkaline solution of pH 14 [23, 41], whereas the eutectic austenite matrix sample showed nobler potential than a carbide sample as described in the literature [41] after extraction of carbide and eutectic austenite matrix sample in pH 14 solution.

Table 10 Corrosion potential for carbide and matrix of Alloy 1 sample in H2SO4 solution of pH 1.5
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4 Discussion

4.1 Galvanic Effect

The results showed that corrosion of HCWI in pH 1.5 and pH 7 NaCl solutions occurs mainly in eutectic martensite. In pH 2 solution, the corrosion occurs in eutectic carbides. In contrary, in pH 7 buffer solution, the corrosion occurred in primary and eutectic carbides. The results of extraction of carbide and matrix sample showed that corrosion occurs mainly due to galvanic effect. Due to the potential difference, the dissolution of either eutectic martensitic or carbides depends upon the nature of electrolyte and pH. So according to the nature of electrolyte, there would be a switch between the noble and active potentials. Other investigators found that in acid [1, 8–12] and neutral [8, 14–19] solutions, corrosion was due to galvanic effect for HCWI materials. Zhang et al. [13] found similar galvanic effect by conducting experiments on carbide and matrix samples. The findings of Salasi et al. [23, 24] in neutral solution also showed that corrosion of either eutectic austenite matrix or carbides depended upon the formation of passive layer on the matrix. The passive layer of matrix led to the corrosion of carbides.

The corrosion trend of HCWI hardfacing deposits for different pH can be explained through the use of Pourbaix diagrams. Since the alloys have significant amounts of elements other than chromium, the superimposition of Pourbaix diagrams of other elements on chromium carbide Pourbaix diagrams will be useful in explaining the stability of these alloys in high pH solutions.

4.2 Stability of Passive Formation of Matrix and Pourbaix Diagrams

From the Pourbaix diagram of chromium carbide (Cr7C3) as shown in Fig. 14, it can be seen for both acidic pH 1.5, 2 and neutral 7 solutions that the immune region is higher than for basic solution. At higher potentials for pH 1.5 and 2, Cr7C3 forms Cr2+ followed by Cr3+ and HCrO4 −. At higher potentials for pH 7, Cr7C3 forms Cr2O3 followed by CrO4 2−. The rest potentials measured for the hardfacing alloys at pH 1.5 and 2 lie in the Cr3+ corrosion region, whereas RP measured at pH 7 NaCl and buffer solutions lies in the Cr2O3 passive region. That is why due to Cr2O3 passive region, initial stage of alloy 2 potentiodynamic curve of pH 7 buffer solution in Fig. 10 shows passive behaviour. For samples in solutions of pH 1.5 and 2, the RP lies in Cr3+ corrosion region, initial stage of HCWI samples showed active behaviour as shown in Figs. 4 and 6. There is a sort of galvanic effect described in the previous section due to the presence of Fe in carbides and eutectic austenite. It would be useful to superimpose the Fe Pourbaix diagram [42] with Pourbaix diagram of Cr7C3 as shown in Fig. 14.

Fig. 14
figure 14

Pourbaix diagram of Cr7C3–Fe in pH 1.5, 2 and 7 with anodic potentials of HCWI [27, 42]

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In the superimposed Pourbaix diagram of Cr7C3 with Fe as shown in Fig. 14 shows at pH 1.5 starts from RP till FP, the potentials lie in the corrosion regions of iron (Fe2+, Fe3+ and FeO4 2−) and chromium. As a result of corrosion region of iron and chromium as shown in Fig. 14, potentiodynamic curve of HCWI at pH 1.5 in Fig. 4 shows only active behaviour. Thus, the higher amount of Fe in eutectic martensitic matrix as given in Table 4 was not stable at pH 1.5 forms active potential which leads to dissolution of eutectic martensitic matrix in preference to carbides as shown in Fig. 5. At pH 2, HCWI lies in the corrosion regions of chromium carbide, but due to the presence of passive region of Fe2O3 from points 1 to 2, the potentiodynamic curve shows slight passive behaviour. The formation of passive film Fe2O3 in eutectic austenite matrix attains nobler potential than carbides, which protects matrix from corrosion at pH 2 as shown in Fig. 7. However, in pH 2 solution corrosion in eutectic carbides rather than primary carbides occurred probably due to eutectic carbides generally being Cr23C6 carbides where the primary carbides are Cr7C3 [15, 33–38]. The stability of Cr23C6 eutectic carbides was less stable than Cr7C3 primary carbides as described in the literature [27] of Pourbaix diagrams of chromium carbides. The HCWI potentiodynamic curve of pH 7 buffer solution in Fig. 10 shows passive behaviour from RP till point 1, as their potentials are placed in the region of Fe2O3, as shown in Fig. 14. However, the potentials after point 1 till FP in pH 7 buffer solution show active behaviour. It is found that these potentials are located in the corrosion region of iron, as shown in Fig. 14. Thus, the high Fe eutectic austenite matrix attains noble potential in pH 7 buffer solution, due to the formation of passive film on matrix which leads to dissolution of eutectic and primary carbides as shown in Fig. 11.

However, the results obtained for HCWI in pH 7 NaCl solution show opposite behaviour compared to pH 7 buffer solution. Although the potentials of HCWI at pH 7 NaCl solution lie in the passive regions of Cr2O3 and Fe2O3, as shown in Fig. 14, the presence of aggressive chloride ions in the electrolyte did not allow the eutectic martensitic matrix to form passive layer. In that case, the eutectic martensitic matrix attains active potential and starts to corrode in preference to carbides, as shown in Fig. 9. Hence, the results of pH 7 buffer and NaCl solutions show that corrosion of HCWI depends not only on pH but also on the nature of electrolyte. The study conducted by Salasi et al. [23] of cast HCWI in alkaline solution (pH 12) with inclusion of chloride ions found that corrosion of eutectic austenite matrix happened mainly due to localised breakdown of passive layer. The same eutectic austenite matrix in alkaline solution was found to be relatively uncorroded resulting in the corrosion of carbides [23, 41].

The superimposed Pourbaix diagram of Cr7C3 with NbC in Fig. 15 shows that the immune region of NbC is larger than the Cr7C3 immune region. The RP of pH 2 is located in the immune region, whereas RP for pH 7 buffer solution is located in the passive region of NbO. The higher potentials till FP of pH 2 and pH 7 buffer solution are placed in the passive region NbO. This is the reason why in the alloy 2 microstructure of Fig. 7 and Fig. 11 after potentiodynamic tests in pH 2 and pH 7 buffer solutions showed NbC to be protected from severe corrosion even at FP. The study conducted by Arsova et al. [43] also confirmed that stable NbO passive film was detected through Raman spectroscopy in H2SO4 solution.

Fig. 15
figure 15

Pourbaix diagram of Cr7C3 - NbC in pH 1.5, 2 and 7 with anodic potentials of HCWI [27, 28]

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The development of superimposed Pourbaix diagram shows that formation of passive layer of eutectic austenitic or martensitic matrix plays a significant role in corrosion of HCWI hardfacing alloys. The findings of Kim et al. [44] on passive films of AISI 316 stainless steel in acidic HCl and NaCl solutions show that densities (thickness) of passive film are lower for NaCl solution than in acidic solution. The effect of chloride ions may affect the density of the passive film. The study conducted by Landolt et al. [45] for analysing the effects of chloride ion in passive film of Fe–Cr alloys showed that chloride ions were incorporated only at the time of growth of passive films. However, the chloride ions had no effect on the passive films which had already formed. The findings from Dobbelaar et al. [46] of Fe–Cr alloys in 0.5 M H2SO4 showed that the contribution of iron in forming a passive layer at higher potential decreases the porosity of the passive film. The study [47] conducted for Fe–25Cr alloy in 0.1 M H2SO4 + 0.4 M Na2SO4 and 0.1 M NaOH solutions shows that there is mass loss in acidic solution. This shows that passive layer was not stable which leads to dissolution of alloys in the given solution.

The dissolution of either matrix or carbides in acid and neutral environments will have an effect of using HCWI hardfacing alloys for wear resistance. The matrix dissolution may lead to removal of carbides due to abrasive particles. Also the formation of passive layer Cr2O3 in chromium carbides can happen only when Cr7C3 dissociates to form Cr2O3 and carbon dioxide. However, the passive layer on either matrix or carbides, as seen through superimposed Pourbaix diagram of carbides [27, 28, 42], can be removed continuously through erosion-corrosion mechanism, which leads to severe material loss of HCWI hardfacing alloys. The only way to control the material loss due to erosion and corrosion by placing the HCWI hardfacing alloys in the immune region of the Pourbaix diagram. This can be done by cathodic protection of hardfacing alloys. Work is underway in this laboratory on conducting experiments for HCWI in different pH by placing at cathodic potentials and also introducing nitrogen during welding process. Nitrogen into the hardfacing alloys can form nitrides, carbo-nitrides or simply nitrogen as an alloying element. Nitrogen has been added in stainless steel to improve corrosion resistance [48, 49].

5 Conclusions

In this study, HCWI alloys were deposited on low carbon steel to investigate the behaviour of hardfacing overlays in acid and neutral solutions. The following conclusions were drawn from this study.

  1. 1.

    In acidic solution, at pH 1.5, the preferential corrosion occurs on eutectic austenite or martensite matrix, whereas at pH 2 the preferential corrosion happens on eutectic carbides for all the HCWI samples at higher potentials.

  2. 2.

    In neutral solutions, at pH 7, the preferential corrosion occurs in matrix in 2 M NaCl solution, whereas the carbides get corroded with preference to matrix in the buffer solution.

  3. 3.

    The corrosion mechanism for the HCWI in both acid and neutral solution was galvanic. The uncorroded matrix or carbides attain noble potential, which act as a cathode, and the corroded matrix or carbides attain active potential, which act as an anode.

  4. 4.

    The addition of chloride ions in neutral solution alters the potential difference between carbides and matrix.

  5. 5.

    Pourbaix diagrams were very useful in explaining the corrosion trend of the hardfacing alloys.