2023 Volume 64 Issue 9 Pages 2051-2058
This paper reviews recent studies on the relationships between local electrochemical properties at the steel/inclusion boundary and pitting corrosion resistance of stainless steels. The effect of Mo addition to the steel matrix on the local dissolution behavior at the steel/inclusion boundary is explained. The inhibition of inclusion dissolution is beneficial for improving the pitting corrosion resistance of stainless steels. The importance of spark plasma sintering and microelectrochemical techniques in the research on localized corrosion processes are briefly discussed. We also discuss novel methods for improving the pitting corrosion resistance of stainless steels, such as the addition of corrosion inhibitors.
This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 71 (2022) 401–408. The title and abstract were slightly modified.
Fig. 4 Schematic of the fabrication method of specimens for corrosion evaluation by spark plasma sintering.
Stainless steels are widely used in structural components for development of infrastructure owing to their superior corrosion resistance. However, localized corrosion such as pitting corrosion, crevice corrosion, and stress corrosion cracking occurs in severely corrosive environments, leading to the failure of critical components.1) Alloying with corrosion-resistant elements and reducing impurities are the common methods used to improve the pitting corrosion resistance of stainless steels. For example, highly alloyed stainless steels containing corrosion-resistant elements such as Ni and Mo have been developed and utilized successfully in severely corrosive environments. However, the addition of large amounts of alloying elements impairs the mechanical properties of stainless steels besides causing wastage of resources.
Corrosion-susceptible sites such as nonmetallic inclusions and grain boundaries are inevitably present on stainless steel surfaces. Among these, sulfide inclusions such as manganese sulfide (MnS) are known to initiate pitting corrosion.2–5) Reducing sulfide inclusions can effectively improve the pitting corrosion resistance; however, complete removal of the inclusions is unfeasible. The prospect of reducing impurities to achieve resource conservation and improve pitting corrosion resistance is questionable. Hence, alternate techniques for improving the pitting corrosion resistance of stainless steels need to be developed.
The modification of chemical composition and morphology of inclusions has been explored as an alternate approach to reduce pitting corrosion of stainless steels.6,7) Significant advancements have been made in this regard through the application of microelectrochemical techniques.8,9) This paper reviews the local electrochemical properties of sulfide inclusions and the recent developments in improving the pitting corrosion resistance of stainless steels with a focused discussion on the findings of our research.
Alloying with a small amount of Mo is known to enhance the pitting corrosion resistance of stainless steels. Several mechanisms have been proposed to explain the inhibitory role played by Mo in pitting corrosion. The presence of Mo in passive films prevents film breakdown and promotes the repair of the passive films.10) Furthermore, dissolved Mo species reduce the active dissolution rate of bare stainless steel surfaces in highly acidic environments inside the pits.11,12) While the effects of Mo on passive films and the active dissolution of stainless steels have been intensively studied, few studies have focused on the effect of Mo on the inhibition of pitting at MnS inclusions.
Microelectrochemical techniques enable detailed analysis of pitting at MnS inclusions and can contribute to the design of stainless steels with high pitting corrosion resistance. Nishimoto et al. investigated the effect of Mo alloying on the pit initiation behavior at MnS inclusions using Mo-free (Fe–18Cr–10Ni–0.03S) and Mo-added (Fe–18Cr–10Ni–2.3Mo–0.03S) stainless steels.13) Energy-dispersive X-ray spectroscopy (EDS) analysis revealed no difference in the chemical composition of MnS inclusions between the Mo-free and Mo-added stainless steels; Mo was detected only in the stainless steel matrix and not in the MnS inclusion. Figure 1 shows the potentiodynamic polarization curves of the Mo-free and Mo-added stainless steels measured in 0.1 M NaCl and scanning electron microscope (SEM) images of the inclusions after polarization.13) The anodic polarization curves in Fig. 1(a) were measured using electrode areas of approximately 100 µm × 100 µm containing a single MnS inclusion.14,15) Polarization began at −0.2 V (vs. Ag/AgCl (3.33 M KCl)), and the potential was increased in the anodic direction. For both the specimens, cathodic currents flow initially, and anodic currents are recorded above approximately 0 V. All potentials reported hereafter refer to the Ag/AgCl (3.33 M KCl) electrode. The current density gradually increases above approximately 0 V, reaching approximately 1.0 × 10−1 A m−2 at 0.35 V. According to previous reports, the passive current density of Fe–18Cr–8Ni stainless steels in NaCl solutions, measured at electrode areas of approximately 100 µm × 100 µm without inclusions, is approximately 1.0 × 10−2 A m−2;16–19) this is one order of magnitude smaller than the aforementioned value. Therefore, the gradual increase in the anodic current density in Fig. 1(a) is attributed to the electrochemical dissolution of MnS.17,20) However, all the MnS is not dissolved, and a significant portion remains (with slightly dissolved surfaces), as evident from the SEM images in Figs. 1(b) and 1(c). In addition, trenches are formed at the boundaries between the MnS inclusion and the steel matrix.18,21–23) In the Mo-free stainless steel, current spikes are observed at 0.37–0.4 V, indicating the initiation and repassivation of metastable pits. The continued increase in the current density at 0.41 V is attributed to the initiation of a stable pit. In contrast, the Mo-added stainless steel exhibits a gradual increase in the current due to the dissolution of MnS, with no current spikes or pits (as observed by SEM). At approximately 0.5 V, the anodic current density decreases to the level of the passive current density of the stainless steel matrix. These findings suggest that the superior pitting corrosion resistance of the Mo-added stainless steel is not due to the suppression of MnS dissolution.
(a) Potentiodynamic polarization curves of a small area with the MnS inclusion in Mo-free and Mo-added stainless steels measured in 0.1 M NaCl. Scanning electron microscopy images of the inclusions in (b) Mo-free and (c) Mo-added stainless steels after polarization. [Adapted under the terms of the CC BY 4.0 Creative Commons License, Copyright 2019, The Authors, published by The Electrochemical Society.]13)
Chiba et al. proposed the following mechanism for the initiation of pitting at MnS inclusions on Fe–18Cr–8Ni stainless steels in NaCl solutions:18) (1) the anodic dissolution of MnS occurs in the passive region of the stainless steel; (2) the disproportionation reaction of S2O32−, a dissolution product of MnS, leads to the deposition of S on and around the inclusions; (3) the synergistic effect of S and Cl− ions causes depassivation of the stainless steel matrix surrounding the MnS inclusions, thereby forming trenches at the MnS/steel matrix boundaries; (4) local active dissolution (pitting) occurs inside the trenches as a result of iR-drop and acidification caused by the hydrolysis reaction of metal ions, resulting in the formation of pits. According to Fig. 1, the addition of Mo in the stainless steel has no effect on the dissolution of the MnS inclusions; however, Mo may inhibit the formation of trenches at the MnS/steel matrix boundaries. This hypothesis was tested by recording the potentiodynamic anodic polarization curves in 0.1 M NaCl using microscale electrode areas containing a single MnS inclusion; the polarization was stopped at 0.35 V to characterize the trench morphology just before the initiation of pitting (Fig. 2).13) In the Mo-free stainless steel (Fig. 2(b)), dissolution occurs at the boundary between the MnS inclusion and the steel matrix, and deep trenches are formed. In contrast, little or no trenching is observed in the Mo-added stainless steel, and only slight dissolution of the inclusion surface is observed (Fig. 2(c)). The inhibition of trenching plays an important role in improving the pitting corrosion resistance by Mo alloying. The addition of Mo has also been reported to prevent the depassivation of stainless steel in solutions containing S and Cl− ions.13)
(a) Potentiodynamic polarization curves of a small area with the MnS inclusion in Mo-free and Mo-added stainless steels measured in 0.1 M NaCl with polarization stopped at 0.35 V. Scanning electron microscopy images of the inclusions in (b) Mo-free and (c) Mo-added stainless steels after polarization. [Adapted under the terms of the CC BY 4.0 Creative Commons license, Copyright 2019, The Authors, published by The Electrochemical Society.]13)
As depicted in Fig. 1(c), trenches were formed at the boundary between the MnS and the steel matrix even in Mo-added stainless steel when the anodic polarization was conducted up to 0.6 V. The MnS inclusions continue to dissolve above 0.35 V, leading to an increase in the S concentration as the polarization curve continues to be measured. This results in the formation of trenches in the Mo-added stainless steel. Further increase in the Mo content is likely to inhibit the formation of trenches at high potentials. In addition, the active dissolution rate of stainless steel plays a crucial role in determining whether pitting (local active dissolution) occurs inside the trenches. Figure 3 shows the polarization curves of Mo-free and Mo-added stainless steels in 1 M HCl with suspended S particles.13) This solution simulates the environment inside the trenches formed around MnS inclusions — low pH with suspended S particles (a dissolution product of MnS). Owing to the difficulty in suspending dry S powder reagent in a solution, Na2S2O3 was dissolved in HCl solution, and S was suspended via disproportionation reaction. The addition of Mo causes the corrosion potential to shift towards higher values. On the cathodic side of the polarization curves, no significant difference in the rate of the hydrogen evolution reaction is observed between the Mo-free and Mo-added stainless steels; this indicates suppression of the active dissolution of stainless steel. Evidently, the active dissolution rate of stainless steel in an acidic solution containing S and Cl− ions, which simulates the environment inside the trenches at the MnS/steel matrix boundaries, is reduced by Mo alloying.
Potentiodynamic polarization curves of Mo-free and Mo-added stainless steels measured in 1 M HCl–1 mM Na2S2O3. [Adapted under the terms of the CC BY 4.0 Creative Commons license, Copyright 2019, The Authors, published by The Electrochemical Society.]13)
In summary, in the Mo-free stainless steel, the dissolution of MnS inclusions leads to the depassivation of the steel matrix surrounding MnS. Trenches are formed around MnS, and pits are generated inside the trenches as a result of local active dissolution. In the case of Mo-added stainless steel, the depassivation of the steel matrix is less likely to occur despite the dissolution of MnS, and the formation of trenches is suppressed. Even after the formation of trenches, the lower active dissolution rate of the Mo-added stainless steel inhibits the initiation of pitting inside the trenches. Thus, the addition of Mo does not inhibit the dissolution of MnS inclusions, but it enhances the corrosion resistance of the steel matrix in the presence of the dissolution products of MnS inclusions. However, from the perspective of the role of Mo in inhibiting pitting corrosion at MnS inclusions, the addition of large amounts of Mo into the stainless steel matrix is ineffective. Thus, to achieve resource conservation and improve the pitting corrosion resistance of stainless steels, it is essential to study the dissolution behavior of the inclusions.
Pitting corrosion at MnS inclusions is triggered by the dissolution products of MnS inclusions.18,24,25) Therefore, improving the dissolution resistance of the inclusions by controlling their chemical compositions is important. A systematic investigation of the relationship between the chemical composition and dissolution resistance of inclusions necessitates specimens containing various compositions of inclusions. Spark plasma sintering (SPS) enables facile fabrication of specimens containing inclusions with desired compositions.19,26,27) Figure 4 shows the schematic of the fabrication method of specimens using SPS. Specimens containing artificial sulfide inclusions are fabricated by mixing and sintering a small amount of sulfide powder with stainless steel powders corresponding to the metal matrix.
Schematic of the fabrication method of specimens for corrosion evaluation by spark plasma sintering.
SPS enables faster consolidation of raw powders to a high density than conventional powder metallurgy methods.28,29) Many studies have been reported on the fabrication of sintered specimens with finely dispersed metal oxides to improve properties such as wear resistance.30,31) However, few researchers have utilized this technique to prepare model specimens for analyzing electrochemical properties of microstructural features, such as sulfide inclusions, in commercial stainless steels. This is probably because powder metallurgy materials often contain defects such as voids and cracks, and corrosion tends to occur at these defects.32,33) The presence of these defects complicates the analysis of the electrochemical properties of targeted inclusions using conventional macroscale electrochemical measurements with electrode areas of approximately 10 mm × 10 mm. Williams and Zhu fabricated stainless steel specimens with artificial inclusions by melting and solidifying sulfide powders in a cavity (1 mm diameter) drilled into a stainless steel rod.34) However, the artificial inclusions were much larger than the actual inclusions in commercial stainless steels. Nishimoto et al. developed a method to analyze local electrochemical properties at the boundaries between artificial sulfide inclusions and steel matrix using small electrode areas of approximately 100 µm × 100 µm to avoid defects associated with powder metallurgy processes.19,26,27)
3.2 Effect of Cr on dissolution of (Mn,Cr)S inclusionsMnS inclusions in stainless steels contain a small amount of Cr; hence, they are represented as (Mn,Cr)S inclusions.35) (Mn,Cr)S inclusions with higher Cr concentrations have been reported to exhibit better pitting corrosion resistance7,36) as Cr inhibits the dissolution of inclusions.37–41) However, analyses of the effect of Cr concentration on the dissolution of (Mn,Cr)S inclusions have remained qualitative. Therefore, the extent to which the Cr concentration prevents the dissolution of inclusions is unclear. Nishimoto et al. investigated the relationship between the Cr concentration and dissolution resistance of (Mn,Cr)S inclusions using a combination of SPS and microelectrochemical measurements.19) Figure 5 shows the optical micrographs of as-polished surfaces of different specimens.19) A commercial re-sulfurized type 304 stainless steel rod prepared by vacuum induction melting (Steel A) is shown in Fig. 5(a). The inclusions in Steel A are elongated along the rolling direction. Figure 5(b) shows a sintered specimen fabricated using gas-atomized type 304 and MnS powders (Steel B), and Fig. 5(c) shows a sintered specimen fabricated using gas-atomized type 304 and Cr2S3 powders (Steel C). The gray particles at the center of the images in Figs. 5(b) and 5(c) are artificial sulfide inclusions originating from the sulfide powders. By mixing and sintering the stainless steel and sulfide powders, artificial inclusions with sizes similar to those of actual inclusions in the commercial stainless steel rod were produced.
Optical micrographs of as-polished specimen surfaces of (a) Steel A: type 304 stainless steel rod, (b) Steel B: sintered specimen fabricated from type 304 stainless steel powders and MnS powders, and (c) Steel C: sintered specimen fabricated from type 304 stainless steel powders and Cr2S3 powders. [Reproduced under the terms of the CC BY 4.0 Creative Commons license, Copyright 2020, The Authors, published by Elsevier Ltd.]19)
Figure 6 shows the SEM images and the corresponding EDS maps of the inclusions.19) In Steel A (Fig. 6(a)), the Mn:Cr:S:O (relative atomic) ratio at point 1 was 47:5:48:<1, indicating the formation of (Mn,Cr)S inclusions with a small amount of Cr (approximately 5 at%). In Steel B (Fig. 6(b)), the Mn:Cr:S:O ratio at point 2 was 32:18:49:<1. The Cr concentration of the artificial inclusion in Steel B is higher than that of the actual inclusion in Steel A. Furthermore, the Cr concentration of each artificial (Mn,Cr)S inclusion in Steel B varied from approximately 10 to 28 at%. During the sintering process, Cr is likely to have diffused from the stainless steel powders to the sulfide powders. In Steel C (Fig. 6(c)), the Mn:Cr:S:O ratio at point 3 was 10:40:49:<1, indicating the formation of Cr-enriched (Mn,Cr)S inclusion. Here, Mn likely diffused from the stainless steel powders during the sintering process. The phase diagram of the CrS–MnS pseudobinary system indicates that MnS can contain Cr up to approximately 30 at% by replacing Mn atoms with Cr atoms, while CrS can hardly contain Mn.35) The Cr concentration of the artificial (Mn,Cr)S inclusions in Steel C is above the solubility limit of Cr in MnS; the artificial inclusions in Steel C have non-equilibrium chemical compositions. Thus, specimens containing (Mn,Cr)S inclusions with different Cr concentrations can be successfully fabricated by mixing and sintering the stainless steel and sulfide powders.
Scanning electron microscopy images and energy-dispersive X-ray spectroscopy maps of the inclusions in (a) Steel A (b) Steel B, and (c) Steel C. [Reproduced under the terms of the CC BY 4.0 Creative Commons license, Copyright 2020, The Authors, published by Elsevier Ltd.]19)
Figure 7 shows the potentiodynamic anodic polarization curves of microscale electrode areas that contain a single (Mn,Cr)S inclusion.19) To elucidate the quantitative effect of the Cr concentration on the dissolution resistance of (Mn,Cr)S inclusions, the chemical composition of the inclusions chosen for polarization was first analyzed by EDS. Subsequently, the specimens were repolished using diamond pastes. A microscale electrode area with the inspected inclusion was then fabricated by masking. Figure 7 also shows the polarization curve measured at the electrode area without inclusions in Steel A; in this case, the passive current density of the stainless steel is measured from approximately 0 to 0.65 V. The slight increase in current density at approximately 0.65 V is attributed to transpassive dissolution. Further increase in the current density by an oxygen evolution reaction is observed above 1.1 V. No pitting is initiated at the area without inclusions, suggesting that the stainless steel matrix has inherently superior pitting corrosion resistance.16,18,42) In Steel A (approximately 5 at% Cr), a gradual current increase due to the dissolution of the (Mn,Cr)S inclusion is observed above approximately 0.2 V. Stable pitting is initiated at 0.6 V. The dissolution of the Mn-rich (Mn,Cr)S inclusion begins in the passive region of stainless steel, and pitting corrosion occurs after the onset of inclusion dissolution, as demonstrated by many researchers.14,17,43) In Steel B (approximately 18 at% Cr), a gradual increase in the current due to the dissolution of the (Mn,Cr)S inclusion is observed above approximately 0.4 V, and stable pitting is initiated at 0.69 V. The onset potential of the inclusion dissolution and pitting potential are higher than those of the (Mn,Cr)S inclusion in Steel A. In Steel C (approximately 40 at% Cr), the onset potential of the inclusion dissolution is approximately 0.7 V. This value is close to the potential of the passive-to-transpassive transition of type 304 stainless steel. Stable pitting occurs within the transpassive region.
Potentiodynamic polarization curves of microscale electrode areas measured in 0.1 M NaCl. The electrode areas included single (Mn,Cr)S inclusions: the Cr/(Mn + Cr) atomic ratios of the (Mn,Cr)S inclusions in Steel A, Steel B, and Steel C were 0.11, 0.36, and 0.81, respectively. The polarization curve of Steel A was also measured at the electrode area without large inclusions. [Adapted under the terms of the CC BY 4.0 Creative Commons license, Copyright 2020, The Authors, published by Elsevier Ltd.]19)
Figure 8 shows the relationship between the Cr concentration and the onset potential of the inclusion dissolution obtained from the above measurements.19) The horizontal axis shows the chemical composition of (Mn,Cr)S inclusions represented by the Cr/(Mn + Cr) atomic ratio. The onset potential of inclusion dissolution increases with increase in the Cr concentration in (Mn,Cr)S, reaching 0.6 V when the Cr/(Mn + Cr) ratio is 0.5. This potential is almost the same as the potential of the passive-to-transpassive transition of type 304 stainless steel. No further increase is observed even when the Cr/(Mn + Cr) ratio is greater than 0.5. The dissolution of nearly pure CrS inclusions begins at approximately 0.7 V.40) In 0.1 M NaCl, the Cr/(Mn + Cr) ratio should be 0.5 or greater to prevent inclusion dissolution in the passive region and to increase the pitting potential of type 304 stainless steel to the transpassive region. The inhibition of inclusion dissolution by Cr is attributed to the Cr-oxide enrichment in the oxide layers on inclusion surfaces.19,40,44)
Changes in the onset potential of dissolution of (Mn,Cr)S inclusions and the pitting potential as a function of the Cr/(Mn + Cr) atomic ratio of (Mn,Cr)S inclusions. [Reproduced under the terms of the CC BY 4.0 Creative Commons license, Copyright 2020, The Authors, published by Elsevier Ltd.]19)
The Cr concentration of (Mn,Cr)S inclusions in stainless steel fabricated by vacuum melting is known to decrease with increase in the amount of Mn added to stainless steel.35,36,45) This interesting phenomenon implies that sulfide inclusions can be modified by the addition of small amounts of alloying elements and manufacturing processes. In addition, controlling the chemical compositions of inclusions by heat treatment is believed to simultaneously improve corrosion resistance and other properties.46–49)
3.3 Addition of corrosion inhibitors to sulfide inclusionsIn section 3.2, the inhibition of inclusion dissolution by Cr and its effect on the pitting corrosion resistance was described based on the research on (Mn,Cr)S inclusions. In addition to Cr-rich inclusions, TiS and Ti4C2S2 inclusions have been reported to exhibit excellent dissolution resistance owing to the thermodynamic stability of their surface oxide film; they are less likely to function as pitting initiation sites.44,50) Moreover, atmospheric exposure of stainless steel has been found to increase the thickness of the oxide film on MnS surfaces and improve the dissolution resistance of MnS inclusions.51) However, corrosion protection by oxide films is limited as they cannot be maintained below the depassivation pH. Because stainless steels are utilized in various corrosive environments, it is difficult to prevent the inclusion dissolution by oxide films in all those environments.52–57) Electrochemical treatments have been proposed to remove inclusions from the surface of stainless steels to improve their pitting corrosion resistance.58,59) However, complete removal of all inclusions on the steel surfaces is difficult.
We have developed a novel method to improve the pitting corrosion resistance of stainless steel by adding elements that act as corrosion inhibitors to sulfide inclusions. Figure 9 shows the potentiodynamic polarization curves of microscale electrode areas with either MnS or CeS inclusions.60) Pitting potential at the CeS inclusion was 0.4 V higher than that at the MnS inclusion. The dissolution of the CeS inclusion proceeds below 0.5 V, which is almost the same potential as that of the MnS inclusion. In the case of MnS inclusions, pitting occurs in the potential range of the inclusion dissolution. However, in the case of CeS inclusions, pitting is inhibited up to high potentials even after the inclusion dissolution. This is because the dissolution products of CeS inclusions inhibit pit initiation.60) Figure 10 shows the polarization curves of microscale electrode areas with a single MnS inclusion measured in NaCl–CeCl3 solutions.60) It was found that Ce3+ ions, a dissolution product of CeS, improve the pitting corrosion resistance at sulfide inclusions. Thus, pitting corrosion can be inhibited even after the dissolution of the inclusions by adding elements that act as corrosion inhibitors to sulfide inclusions.
Potentiodynamic polarization curves of microscale electrode areas on type 304 stainless steels with either MnS or CeS inclusions measured in 0.1 M NaCl. [Adapted under the terms of the CC BY 4.0 Creative Commons license, Copyright 2017, The Authors, published by The Electrochemical Society.]60)
Potentiodynamic polarization curves of microscale electrode areas with single MnS inclusions in type 304 stainless steel measured in NaCl–CeCl3 solutions: (a) 0.1 M Cl−, and (b) 3 M Cl−. [Adapted under the terms of the CC BY 4.0 Creative Commons license, Copyright 2017, The Authors, published by The Electrochemical Society.]60)
Recently, the pitting corrosion resistance of stainless steels was improved by modifying their microstructures with secondary phases containing corrosion inhibitors.61,62) Further progress in this important study is required to develop a method to stop corrosion growth spontaneously.
Stainless steels contain nonmetallic inclusions and other microstructural features, such as precipitates, grain boundaries, and segregation. Elucidating the effects of alloying elements on the electrochemical properties of these microstructural features is expected to enable more efficient use of alloying elements and reveal new elements that contribute to corrosion resistance. For example, carbon is known to decrease the corrosion resistance of stainless steels; however, recent studies reveal that interstitial carbon contributes to the improvement of dissolution resistance of stainless steels and inhibits pitting corrosion.63–66) The pitting corrosion resistance of stainless steels varies with inclusion composition, and alloy design for modifying the inclusions has been found to be effective in the prevention of pitting. To conclude, a systematic investigation of the relationship between the electrochemical properties of microstructural features and the pitting corrosion resistance of stainless steels is essential to transform the experience-based knowledge into a well-established technology.
