This study investigates the corrosion behavior of three stainless steel grades at two H2SO4 concentrations, namely 1 wt% and 10 wt%, with varying NaCl concentrations in the range from 500 mg/L to 10,000 mg/L. Dissociation of sulfuric acid yields the hydrogen (H+) and sulfate () ions, the former of which lowers the pH value of a solution while the latter increases the concentration of sulfate ions that act as a corrosion inhibitor. The equilibrium chemistry of the solutions was defined at the test temperatures of 22°C, 50°C, 90°C, and 130°C, and correlated with the observations on the electrochemical and microstructural examination of the materials. The results showed clear differences in the main corrosion form between the two H2SO4 concentrations. In 1 wt% H2SO4, pitting was the major form of corrosion attack in the presence of chlorides, whereas uniform corrosion dominated in 10 wt% H2SO4. The pitting corrosion tendency for the three stainless steel grades under various test conditions was consistent, but there were differences in their resistance to uniform corrosion. The chloride-to-sulfate activity ratio, , was found to be the key parameter in defining the occurrence of pitting corrosion for all three alloys. In H2SO4-NaCl systems, no pitting occurred at the activity ratio below 10, with higher values inducing pitting attack, particularly in 1 wt% H2SO4. The described novel results are presented and discussed in this paper.
Demand for metals is foreseen to increase globally in the coming years, putting pressure on to utilize more complex and heterogeneous raw materials.1-3 Hydrometallurgical processing is an essential part of sustainable metal production4-,5 and needs to accommodate varying raw material quality. Therefore, operating conditions in the hydrometallurgical processes are harsh. Sulfuric acid is a common leaching agent and introduces a low pH value, and the raw materials often contain chlorides and oxidizing ions, such as ferric cations, that end up in the leachant.6 Additionally, processes are run at elevated temperatures.6-8 When targeting efficient leaching of the raw materials, the construction materials used in the assets and hydrometallurgical equipment, such as tanks, containers, pipes, tubes, valves, and mixers, are equally subjected to the aggressive process environment with the risk of corrosion.
Outokumpu has put plenty of effort into defining the uniform CR of stainless steels at different sulfuric acid concentrations and temperatures.11,39 Other than this, corrosion studies performed for stainless steels (with the PREN of approximately 25 to 26) in sulfuric acid solutions with NaCl are not too many. The work by Hren, et al.,40 has concentrated on the development of a corrosion sensor rather than providing a thorough understanding of the corrosion behavior of stainless steels. Loto41 has studied the corrosion behavior of two grades of austenitic stainless steel, SS2562 (UNS N08904(1)) (EN1.4539 and PREN 36) and S31603 (EN1.4404 and PREN 28), in 2.5 M H2SO4 solutions with 3.5% NaCl as a function of temperature. Particularly for the grade S31603 (EN1.4404 and PREN 28), pitting corrosion was the primary corrosion mechanism at all temperatures, concentrating on grain boundaries. Pitting potential and the width of the passive range for S31603 decreased from 35°C to 45°C but further temperature raises did not have a profound influence on them. Khedr, et al.,23 have investigated the influence of chloride additions to 5 N, 7 N, and 10 N (2.5 M, 3.5 M, and 5 M, respectively) H2SO4 solutions on the corrosion behavior of the stainless steel of grade 1.4301 (PREN 18). Their results showed that the presence of chlorides introduced pitting corrosion in all cases except for 10 N H2SO4, i.e., at the highest sulfuric acid concentration. Laitinen42 has examined the localized corrosion of the stainless steel 1.4301 in 55°C H2SO4 solutions of pH 5 using the constant chloride concentration of 300 mg/L but varying sulfate additions. The results indicated that pitting corrosion occurred in the absence of additional sulfate and at the two lowest sulfate concentrations, 50 mg/L and 300 mg/L, but not at the highest sulfate concentration of 1,800 mg/L. Where the pits formed, they were deep and partly closed but did not contain any visible deposits. The inhibiting effect of sulfate ions on the pitting corrosion of stainless steel grade 1.4301 was proved valid with the molar ratio of [Cl−]/[SO42–]≤0.5. Except for the work by Laitinen,42 the inhibitive role of sulfate ions in the pitting corrosion of stainless steel has been demonstrated in neutral NaCl solutions supplemented with sulfates, typically added as Na2SO4, e.g., Pohjanne, et al.43 The challenge is that the critical [Cl−]/[SO42–] ratio for pitting corrosion inhibition from near-neutral (like pH 5) or neutral solutions cannot be directly transferred to acidic systems, like the ones in hydrometallurgy. Therefore, in order to use stainless steel assets for the hydrometallurgical processes, it is essential to define the material’s behavior under the respective process conditions. The work was motivated by the need to determine the safe operating conditions for the traditional S31603 type of stainless steel, an equivalent newer austenitic alloy with dissimilar alloying, and the corresponding duplex counterpart of S32101.
This study examines the corrosion behavior of three stainless steel grades of similar PREN but with dissimilar alloying and microstructure: S31603 (EN1.4432; austenitic), S31655 (EN1.4420; austenitic), and S32101 (EN1.4162; duplex) in two H2SO4 concentrations, 1 wt% and 10 wt%. Varying amounts of sodium chloride (500 mg/L to 10,000 mg/L) corresponding to 300 mg/L to 6,000 mg/L (ppm) chloride ions were added to the H2SO4 solutions. The test temperatures ranged from room temperature 22°C to 130°C. The tests involved electrochemical measurements (open-circuit potential [OCP] measurements and potentiodynamic polarization scans) and immersion experiments for 28 d to provide an understanding of the corrosion behavior of the three stainless steel grades. The concentrations of H2SO4 and NaCl and the test temperatures represent the industrial operation conditions in the hydrometallurgical process and provide the uniqueness for this work along with the approach, in which the solution equilibrium chemistry was investigated under each set of test conditions to obtain the chloride-to-sulfate activity ratio, , and correlated with the observations about the corrosion attack. Being motivated by the need to understand the link between the test conditions and the corrosion behavior of the materials, the research aimed at defining the prevailing corrosion form and key electrochemical characteristics for the three alloys as a function of test conditions. Ultimately, the target is to use the results for the computational prediction of corrosion of stainless steels under the hydrometallurgical process conditions. Earlier, e.g., Sridhar and Anderko44 have predicted the corrosion processes of selected stainless steel grades in sulfuric acid mixtures.
Materials and Test Conditions
Three grades of stainless steel: austenitic S31603 (EN1.4432), S31655 (EN1.4420), and austenitic-ferritic (duplex) S32101 (EN1.4162), were studied. The grade names in this paper were systematically presented according to the UNS designation. The actual composition of the grades together with the calculated PREN values, all in the range from 25 to 26, were given in Table 1. The test materials were supplied by Outokumpu Stainless AB, Sweden, as sheets of 2 mm (S31603) and 3 mm (S31655 and S32101) in thickness for the examinations at temperatures up to 90°C, and of 10 mm in thickness for the tests at 130°C. The surfaces of the specimens were wet ground to the 320 grit surface finish, ultrasonically cleaned in acetone, rinsed with ethanol, dried, and then let to oxidize in atmospheric air for at least 18 h.
Two sulfuric acid concentrations, 1 wt% and 10 wt%, were studied. Varying amounts of NaCl, namely 500 mg/L, 1,000 mg/L, 2,000 mg/L, 5,000 mg/L, and 10,000 mg/L, were added to the sulfuric acid solutions. Chemicals of analytical grade purity and ion-exchanged water were used in the preparation of test solutions. Solution characteristics at the four test temperatures: 22°C , 50°C, 90°C, and 130°C, were shown in Table 2. The equilibrium chemistry was calculated using HSC Chemistry 10† software.45 Chloride concentrations varied from 300 mg/L to 6,000 mg/L, giving the equilibrium chloride activities, a(Cl−), in the range from 0.0001 to 0.0020. Equilibrium sulfate concentrations and, hence, sulfate activities, , showed a strong temperature dependency, yielding the ratio of chloride and sulfate activities, , in the range from 1 to 20 at room temperature and from 20 to 350 at the highest test temperature, 130°C. Calculated pH values ranged from 1.0 to 1.2 at the H2SO4 concentration of 1 wt% and from 0.01 to 0.2 in 10 wt% solution.
The test materials were subjected to microstructural characterization in order to obtain information on the overall material microstructure and composition of the passive film. For optical microscopy (OM) examinations, conducted using a Zeiss Axio Observer 7† optical microscope, austenitic alloys in the plane were chemically etched with V2A, while the duplex grade S32101 was electrolytically etched in oxalic acid and NaOH. The microstructure of the duplex grade was further imaged by scanning electron microscopy (SEM) using a Zeiss ULTRAplus† field emission (FE) SEM equipped with energy dispersive spectroscopy (EDS) capabilities. X-ray photoelectron spectroscopy (XPS) measurements were conducted for wet-ground specimens of all three stainless steel grades that were then ultrasonically cleaned in acetone, rinsed with ethanol, dried, and left to oxidize for at least 18 h (treatment like for the specimens for electrochemical measurements) using a ULVAC-Phi† Quantum 2000 instrument. The spectra were obtained using a monochromated Al Kα beam with a 100 µm spot size. In the case of duplex-grade S32101, such a spot size produced combined data for the austenitic and ferritic phases. The elemental analyses were performed for O (O1s), Fe (Fe2p3), Cr (Cr2p3), Ni (Ni2p3), Mo (Mo3d), and Mn (Mn2p3). The analysis depth was in the range of 3 nm to 5 nm, enabled by the sputtering with Ar+ with the acceleration voltage of 500 V. The measurements were followed by depth profiling for the same elements, at the sputtering rate of 1 nm/min. The depth profiles were collected every 30 s.
Electrochemical measurements up to 90°C were performed in an electrochemical glass cell with double walls, enabling the circulation of heated media in the casing for maintaining the desired test temperature. Heating was provided by an external heating unit. The counter electrode was either a platinum grit or a glassy carbon electrode, while a silver-silver chloride (Ag/AgCl electrode with saturated KCl solution) was used as a reference electrode. The studied test material as a plate specimen was located in the bottom of the cell with the help of flat rubber gaskets (that did not yield crevice corrosion, in contrast to conventional Avesta cell). The exposed area of the working electrode was 1 cm2 to 2 cm2, with the electrolyte volume being approximately 0.35 L. The measurements at 130°C were performed in a Cormet Hastelloy† C276 autoclave using cylindrical specimens with a diameter of 10 mm and height of 15 mm. The cylinder was of the material of interest, and a wire of grade S31603 (EN 1.4404) stainless steel was welded on the cylinder to provide an electrical connection. Special care was taken during the welding step to avoid the propagation of a heat-affect zone in the material. The weld area and the connector wire were isolated from the test medium using a polytetrafluoroethylene (PTFE, brand name Teflon†) sheath, leaving only the material of interest (3.2 cm2 to 4.5 cm2) exposed to the test solution of the volume of 1 L. Prior to the tests, the solution was warmed to the test temperature with an external resistance jacket. In the autoclave tests, platinum was used as a counter electrode and a silver-silver chloride (Ag/AgCl with saturated KCl) as a reference electrode. All potential values in this paper are reported against Ag/AgCl reference electrode (potential E = 0.210 V at 22°C).
The electrochemical measurements involved OCP measurements and cyclic potentiodynamic polarization measurements. In all cases, the electrolyte was purged with nitrogen gas (N2) before and during the measurements, in order to remove air. In 130°C experiments, in addition to purging the test solution with N2, the autoclave was closed and filled with N2 up to five bars and then released, the cycle which was repeated at least five times to deaerate the autoclave and ensure that there was no leakage. The electrochemical measurements under each set of test conditions were performed sequentially for one specimen. First, OCP measurements were performed for at least 1 h or until OCP stabilized (ΔE/Δt was less than 20 mV/20 min). Second, cyclic potentiodynamic polarization measurements were conducted from the cathodic potential of −0.25 VOCP up to the anodic value of +1.4 V at a scan rate of 0.167 mV/s. The cut-off current density value of 0.1 mA/cm2 was defined for the measurements. The measurements were performed using either a Gamry Reference 600† or a Princeton Applied Research VersaSTAT† 3 potentiostat. The following electrochemical parameters were then extracted from the measurement data: OCP at the end of the measurement period, corrosion potential (Ecorr), and corrosion current density (icorr) that were extrapolated from Tafel fits, critical passivation current density (icrit), passivation potential (Epass), passive current density (ipass), and breakdown potential (Eb). Where a positive hysteresis was detected in the cyclic potentidynamic curve, repassivation potential (Erep) also was defined. Finally, CR was calculated from the value of icorr according to Faraday’s law. The reliability of the results was verified by checking the consistency of observed trends. Where the positive hysteresis was detected in the cyclic potentiodynamic curve, at least two parallel measurements (often more) were run (with the average values being presented).
For validation of the results from electrochemical measurements, 28-d immersion experiments were done. The aim of the test was to examine, whether the same phenomena could be introduced by immersion experiments that were revealed by electrochemical measurements. The identical immersion test was conducted at four test sites, thus only one combination of material and test conditions was chosen for investigations. The stainless steel grade included in the immersion tests was S31603, with a surface area of at least 20 cm2. The specimen surfaces were treated similarly to the specimens for electrochemical tests. The electrolyte in the immersion experiments was 1 wt% H2SO4 with 2,000 mg/L NaCl. The solution was further supplemented by Fe2(SO4)3, in order to rise the redox of the solution high enough to induce pitting corrosion. In total, 5,000 mg/L Fe3+ was added as in Lindgren, et al.,27 The immersion experiments were performed at 90°C, thus the prepared solutions were heated, either in a heating cabinet or using a hotplate. In each case, at least two parallel weighed specimens were exposed to the test solution with the help of a PTFE specimen holder located at the bottom of a container.
After the experiments, the specimens were examined visually to detect the occurrence, form, and extent of corrosion. Surfaces of selected specimens were further investigated with SEM/EDS, using a Zeiss ULTRAplus SEM to define the corrosion form and investigate the details of corrosion attack. After the immersion tests, weight changes for the specimens were also defined.
XPS spectra with elemental depth profiles were measured to define the structure and thickness of passive films on the three materials, Figures 2 through 4. The passive films on all investigated stainless steels were of equal thickness of 2 nm, as estimated based on the depth where the concentration of oxygen was halved from the initial value. The passive film thickness is consistent with the literature, e.g., Gardin, et al.46 In all cases, the relative concentration of chromium in the vicinity of the surface was much greater than that at greater depths, suggesting chromium was enriched in the outer layer of the passive film. For the grade S31603, the maximum Cr/Fe ratio of 0.45 was detected at the depth of approximately 1 nm, in comparison to the corresponding value of 0.25 in the bulk of the alloy. For the two other grades, S31655 and S32101, the maximum Cr/Fe ratio was slightly higher, 0.55 (in comparison to 0.30 in the bulk of the alloys), and it occurred at the depths of 1 nm (S31655) and 0.5 nm (S32101). Chromium depth profiles (Figures 2[b], 3[b], and 4[b]) revealed that down to the depth of 1.5 nm, the highest-intensity peak was located at 577 eV to 576 eV, referring to Cr(OH)3 and Cr2O3, with the respective binding energies of 577.3 eV and 576.3 eV.47-51 Indeed, the experience has proved that Cr(OH)3 often coexists with Cr2O3 on wet-ground surfaces. At greater depths, the metallic state of chromium prevailed, as demonstrated by a major peak at the binding energy of 574 eV (574.1 eV).46,50 Iron depth profiles (Figures 2[c], 3[c], and 4[c]) disclosed the maximum peak at 711 eV to 710 eV until the depth of 1 nm, suggesting Fe2O3 (Fe3+) with the binding energy of 711.0 eV.52 In the austenitic grades S31603 and S31655, nickel and molybdenum were also detected in the passive film, but these existed at lower amounts than in the bulk alloy. It is emphasized that, although nitrogen is included in the alloys, its detection by XPS is challenging, due to the overlapping of the N1s and the Mo 3p3/2 signals. Altogether, the main finding was that in the Alloys S31655 and S32101 with higher chromium contents in the bulk alloy than in grade S31603, chromium was also enriched to a greater degree in the passive film.
As stainless steels are passive alloys, in which localized corrosion is typically the major issue, cyclic potentiodynamic polarization measurements were chosen as the main method to define the corrosion behavior and yield electrochemical parameters that can be compared. Besides pitting corrosion, the measurements enable theevaluation of uniform CR via the parameters, particularly icorr derived by extrapolating the electrochemical data. In most cases, the uniform corrosion occurred as the uniform corrosion in active state (uniform active state corrosion), but there were also cases in which the uniform corrosion took place despite the passive state (uniform passive state corrosion); these are described in the following. We acknowledge that the extrapolation requires first-order kinetics in the system; this was met in many cases, but not in all. It is also emphasized that as polarization always deviates the system from the equilibrium, the CR defined by this approach is likely higher than under equilibrium conditions. However, the obeyed methodology enables thedefining of the dominating corrosion form in each case and to make an overall comparison between the stainless steel grades and test conditions, keeping in mind the large test matrix of the study.
1 wt% Sulfuric Acid
The values for Ecorr were lower than those for OCP for the austenitic grades at all temperatures and duplex specimens tested at the highest test temperature, Figure 6(b). In general, the main observation was that the data points could be categorized into three groups. For the austenitic grades S31603 and S31655 at temperatures up to 90°C, both Ecorr and OCP values fell around the range from −300 mVAg/AgCl to −200 mVAg/AgCl. For the duplex-grade S32101, both potential values were much higher than the corresponding values for the austenitic grades (Figure 6[b]) and Ecorr values were typically higher than OCP values; this is likely due to the passive film that may have evolved during the OCP measurement period. The third group of data points included the measurements performed at 130°C, with the values of Ecorr for all three grades being in the range from −450 mV to −300 mV, thus clearly lower than the OCP values. The obtained results reflect that at temperatures up to 90°C, the duplex-grade S32101 was more easily passivated than the austenitic grades S31603 and S31655, but at the highest test temperature, the potential values indicated an active rather than passive behavior for all three alloys. The behavior up to 90°C is consistent with the findings by Helmersson.53
10 wt% Sulfuric Acid
In such occasional cases where the polarization curves featured a positive hysteresis with passivity breakdown and repassivation, areas with a pit-like appearance, likely formed by the initiation of several pits and their growth together (Figure 17[c]), were detected in the surfaces together with the uniform corrosion mode (Figure 17[d]). At the highest test temperature, corrosion of the alloys was severe and took place uniformly on the surfaces (Figures 17[e] and [f]), as uniform passive state corrosion.
Table 3 presents a summary of the findings from the electrochemical measurements concerning the occurrence of major corrosion forms: pitting corrosion and uniform corrosion, and the rate of the latter. In Table 3, P in the cell indicates the susceptibility of the material to pitting corrosion under the specific combination of test conditions. In the categorization of the uniform CR, we have followed Outokumpu Corrosion Handbook11 in that the green color means that the material is compatible with the conditions and has a CR below 0.1 mm/y, yellow indicates that the material is expected to corrode at a rate between 0.1 mm/y and 1 mm/y, so it may be useable in some applications, but it is not completely resistant and red denotes that the material will corrode heavily at a rate above 1 mm/y. The results revealed that there were no significant differences between the test materials with respect to the tendency of pitting corrosion, which was expected as the materials have a very similar PREN number. However, there were differences between the test conditions, in particular between the two H2SO4 concentrations, in this respect: pitting in the materials was clearly more frequent in 1 wt% than in 10 wt% sulfuric acid solution. Only in 10 wt% H2SO4 solution minor dissimilarities in the pitting corrosion tendency between the materials were found: the duplex grade S32101 exhibited pitting corrosion only at the highest test temperature, while the two austenitic grades underwent pitting corrosion already at 90°C at least at the highest NaCl contents.
Although the presented corrosion rates were likely conservative due to the nature of the methodology, e.g., use of polarization, they enable the comparison between the three alloys. Indeed, the analysis revealed evident differences between the uniform corrosion rates of the materials. The highest overall corrosion rates were related to the austenitic grade S31603 consistent with the lowest Cr alloying in the bulk alloy and in the passive film, revealed by the XPS results. In 1 wt%, the uniform CR of S31603 was low (<0.1 mm/y) only at room temperature, but already at 50°C at the highest NaCl content was the uniform CR of the alloy at a moderate level (between 0.1 mm/y and 1 mm/y). At 90°C at the highest NaCl content and at 130°C, the uniform CR of the alloy was high (>1.0 mm/y). Based on the determined ipass values, uniform corrosion at the temperatures of 90°C and lower represented uniform active state corrosion, whereas uniform corrosion at the temperature of 130°C progressed as the uniform passive state corrosion of the material. The results from immersion tests confirmed not only the presence of pitting corrosion but also the simultaneous uniform corrosion at 90°C at the NaCl concentration of 2,000 mg/L detected in the electrochemical tests. In 10 wt%, the uniform CR of the alloy was low only occasionally at room temperature and high CR was detected already at the temperature of 50°C at the highest NaCl concentration. Again, at 130°C, uniform corrosion progressed as uniform passive state corrosion. Furthermore, at the H2SO4 concentration of 10 wt%, the most corrosion-resistant alloy was the duplex-grade S32101, which contained the highest chromium alloying. Although in 1 wt% sulfuric acid the corrosion rates for the Alloys S31655 and S32101 were equal and low up to 90°C, the differences between these alloys became evident in 10 wt% sulfuric acid. For the austenitic S31655, the corrosion rates were low only at room temperature and 50°C at low NaCl concentrations, and moderate or high under all other conditions. For the duplex grade, uniform corrosion rates were retained at a low level up to 90°C, the highest NaCl concentration. It is worth mentioning that the experience has shown that although the duplex grades of stainless steel passivate easier than the austenitic grades if the duplex grades for some reason start to corrode actively, the corrosion rates may be much higher than for the austenitic grades.
The analysis of the contribution of pH value to the corrosion phenomena is complex. Values for icorr and ipass were proved to be at least one order of magnitude higher in 10 wt% H2SO4 than in 1 wt% H2SO4 solution, which is likely related to the lower pH of the more concentrated solution. However, the increase in sulfate ion content also adds to the solution conductivity, thereby facilitating the dissolution. In this respect, we likely observed the combined effect of pH and conductivity in the values for icorr and ipass. Another approach to analyze the contribution of pH to the corrosion of the alloys is via investigation of corrosion morphologies. It is known that localized corrosion results in the acidification of the solution within the pit (or crevice) as a result of the hydrolysis of metal cations, which then leads to the further acceleration of the localized attack.63 In 1 wt% H2SO4 solution, pitting corrosion was essentially the dominant corrosion mechanism. The detected large pit size and their low number may be linked with the contribution of sulfate ions through the inhibition of pit initiation and growth, but it may equally be associated with the overall low pH of the solution, which supports the dissolution of the metal in the formed attack site. In contrast to the lower H2SO4 content, uniform corrosion was the prevailing corrosion form in 10 wt% H2SO4 solution. Earlier, Szklarska-Smialowska13 has concluded that at very low pH values, pitting transfers into uniform corrosion. It is therefore expected that the uniform dissolution of the passive film undermines the importance of individual passive film breakdown sites. As indicated above, both low pH and conductivity likely play a role here.
Concerning the reliability of the results, at least two parallel measurements were systematically performed at least in such cases where the polarization curve contained a hysteresis loop. The average value is given above. The scatter in the results was very low, as can be seen in the sequential data points that were often overlapping. For example, in the case of OCP of austenitic grades, the values under all test conditions for each type of test environment (1 wt% or 10 wt% H2SO4) typically fell within the 50 mV range and the differences detected between the test materials were greater essentially greater than 50 mV. Independent of which electrochemical parameter was examined, the observations could be explained and were supported by both literature and microstructural investigations.
The findings presented in this paper enable the following conclusions to be drawn:
Sulfuric acid concentration influences the corrosion performance of stainless steel via two mechanisms: inhibitive effect which makes the ratio of chloride and sulfate activities rather than solely the chloride activity an important parameter for pitting initiation, and pH effect, which contributes to the dissolution rate of the steel and, in particular, the passive film on the surface. In H2SO4-NaCl systems, no pitting corrosion was detected below the chloride to sulfate activity ratio of 10.
Pitting corrosion was the dominant corrosion form in 1 wt% sulfuric acid solution for all three studied alloys. The susceptibility of alloys to pitting corrosion increased with increase in temperature and NaCl concentration of the solution, consistent with raising chloride-to-sulfate activity ratio, and no significant differences in pitting tendency between the alloys could be detected. In each case, pits were few in number on the surfaces, and often contained several pit nuclei, resulting in a large pit size. At the lowest test temperatures, the pits were open and did not contain a cover. At higher test temperatures, the pits contained a lacy cover and were formed by several pits growing together into worm-like attack sites and craters. Pitting with the formation of the lacy cover was also evidenced in 28-d immersion experiments.
In 10 wt% sulfuric acid solutions, uniform corrosion was the prevailing corrosion form. There were significant differences in the uniform corrosion rates between the materials estimated from the potentiodynamic polarization curves, with the lowest rates being systematically related to the grade S32101 with the highest Cr content and duplex microstructure, and the highest rates being conversely associated with the grade S31603 with the lowest Cr concentration and austenitic microstructure. The observations strongly correlate with the levels of ipass measured under the test conditions. In such cases where the uniform CR was moderate, the grain boundaries were preferentially etched.
At the highest test temperature, 130°C, where the uniform CR was high for all test materials, the degradation progressed as the uniform passive state corrosion. This was justified by much higher ipass values than at the lower temperatures.
The results revealed the importance of understanding the equilibrium solution chemistry in the analysis of the material’s behavior in multispecies electrolytes. They also demonstrated that despite the nominal identical corrosion resistance, as evaluated based on PREN values, differences in the corrosion performance may exist and they become more evident when the operation environment becomes more demanding.
UNS numbers are listed in Metals & Alloys in the Unified Numbering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International.
The financial support by EIT Raw Materials and the partner organizations: VTT Technical Research Centre of Finland Ltd., Metso Outotec Finland Oy, ZAG Slovenian National Building, and Civil Engineering Institute, Tecnalia, Outokumpu Stainless AB, Boliden Harjavalta Oy, Ferritico AB, and Data Measuring Systems DMS, for the CORTOOLS project under the acceleration program (Project No. 18158) is gratefully acknowledged.