Pitting corrosion behavior of additively manufactured spherical WC/W2C-reinforced stainless steels in chloride-containing solution

Stainless steels suffer from pitting corrosion in chloride-containing environments. Sulfide inclusions, such as MnS, are known to act as the initiation sites of pitting on stainless steels. In our previous works, we studied the pit initiation behavior at the MnS inclusions in stainless steels. The pit initiation mechanism was found to be as follows: 1) the dissolution of the MnS inclusions in chloride-containing solutions leads to the deposition of elemental sulfur on and around the inclusions; 2) the synergistic effect of the elemental sulfur and chloride ions causes the dissolution of the steel matrix at the MnS/steel boundaries, resulting in the formation of trenches; 3) the hydrolysis reaction of Cr3+ released from the steel matrix dissolution decreases the pH in the trenches, and at the same time, the electrode potential at the bottom of the trenches decreases due to the IR-drop. Finally, the pit initiation is defined as the local transition from the passive to active state at the bottom of the trenches. It would appear that inhibiting the dissolution of the steel side at the MnS/steel boundaries improves the pitting corrosion resistance of stainless steels in chloride-containing environments. In recent years, it has been reported that low-temperature carburizing treatments which result in interstitial carbon content of more than 10 at% with no precipitation of carbides improve the corrosion resistance of stainless steels; however, the mechanism is not well understood. In addressing the improvement of the pitting corrosion resistance of stainless steels, it is necessary to understand the mechanism of the effect of low-temperature carburizing treatments on the pit initiation at the MnS inclusions in stainless steels. The anodic polarization curves of a micro-scale electrode area with a MnS inclusion of the untreated and the carburized re-sulfurized Type 304 stainless steels (Mn: 1.32, S: 0.029 mass%) were measured in 0.1 M NaCl. Figure 1 shows the microscopic anodic polarization curves and the surface appearances of the inclusions after the polarization. The surface dissolution of the MnS inclusions resulted in broad peaks around 0.4 V in the anodic currents both of the untreated and the carburized specimens (Fig. 1a). In the polarization curve of the untreated specimen, sharp increases in current density due to the pit initiation were measured in the potential range of the dissolution of the MnS inclusion. These pits were initiated at the MnS inclusion marked by the arrows in Fig. 1b. The current peaks in the polarization curve of the carburized specimen were much smaller than those indicating the pit initiation on the untreated specimen. Neither metastable nor stable pits were initiated at the MnS inclusions in the carburized specimen (Fig. 1c). Figures 1d-e display enlarged views of the areas enclosed by the broken lines in Figs. 1b-c, respectively. A wide and deep trench was formed at the MnS/steel boundary on the untreated specimen (Fig. 1d). In contrast, a narrow and shallow trench was formed at the boundary on the carburized specimen (Fig. 1e). These results suggest that the improved resistance to pitting corrosion at MnS inclusions after the carburizing treatment can be attributed to the reduced scale of the trenches formed at the MnS/steel boundaries in chloride-containing solutions. It is assumed that the dissolution of the steel matrix forming the trenches at the MnS/steel boundaries is caused by the synergistic effect of elemental sulfur and chloride ions. To compare the active dissolution rate of the untreated and the carburized stainless steel matrix in a sulfur suspension with chloride ions, macroscopic anodic polarization curves were measured. The low-sulfur Type 304 stainless steel (Mn: 0.82, S: 0.0002 mass%) was used. The sulfur suspension was produced by the acidification of a thiosulfate-containing solution. It has been reported that the dissolution of the MnS inclusions produces thiosulfate ions (S2O3 2-) and hydrogen ions which decrease the solution pH around the inclusions to approximately 3. The polarization curves were therefore measured in 3 M NaCl – 1 mM Na2S2O3 solution at pH 3.0 (Fig. 2). The active dissolution rate at -0.3 V of the untreated stainless steel was about 500 A/m2, whereas that of the carburized stainless steel was reduced to about 5 A/m2, approximately one hundredth that of the untreated stainless steel. From these results, we can conclude that the carburizing treatment inhibits the active dissolution rate of stainless steel matrix in solutions with elemental sulfur and chloride ions, resulting in a reduction in the dissolution depth of the trenches at the MnS/steel boundaries. Therefore, no pit initiation occurs at the MnS inclusions on the carburized stainless steel in chloride-containing solutions. Figure 1

UNS S20910 (XM-19) is an austenitic stainless steel with excellent corrosion resistance and high strength due to its high nitrogen content. In solution annealed condition, it exhibits better corrosion resistance than UNS S31603 and UNS S31703 while having approximately twice their yield strength at room temperature (> 345 MPa (50 ksi)). The tensile strength of UNS S20910 can be further increased by cold-working, which makes the material more versatile. This paper discusses the influence of strain hardening on the mechanical and corrosion properties of UNS S20910. The corrosion behavior was studied in terms of intergranular corrosion resistance, pitting corrosion and stress corrosion cracking susceptibility in chloride-containing solutions at different temperatures. UNS S20910 in strain hardened condition presents a fully austenitic microstructure with niobium-rich primary precipitates and Z-phase, a complex nitride. In addition, the material shows a yield strength and ultimate tensile strength higher than 890 MPa (129 ksi) and 1035 MPa (150 ksi), respectively, as well as an elongation above 20 %, and impact energy higher than 100 J (74 ft-lbs). Strain hardened material did not exhibit any sensitization; therefore, it retained its excellent pitting corrosion resistance in chloride-containing solutions, which decreases with increasing temperature, though. The material has also shown good resistance to stress corrosion cracking in brines at elevated temperature. It was confirmed that the strain-hardening process increases tensile strength without significantly affecting the corrosion properties of this stainless steel. Therefore, strain hardened UNS S20910 can be used in applications involving very aggressive environments where also high strength, good ductility and non-magnetic properties are required.

In this study the corrosion behavior of porous type(1) 316L stainless steel has been evaluated and compared to that of conventionally made wrought stainless steel. The corrosion behavior of this porous material was investigated by potentiodynamic anodic polarization, open-circuit potential measurements, and identification of corrosion products on the surface by x-ray photoelectron spectroscopic (XPS) analysis and scanning electron microscope (SEM) observations. The polarization studies indicated that the separation between the breakdown potential and the protection potential, ΔE = EB – EPP, increases, with decreasing pore size. The analysis of the surface film revealed the presence of nickel on the corroded surface. The reasons for the variation of the corrosion behavior with porosity have been examined. An attempt has been made to explain the mechanism of corrosion attack on porous stainless steels in chloride-containing solutions.

The crevice corrosion behaviors of 436 stainless steels in chloride-containing solutions with sulfate addition were studied using potentiodynamic, galvanostatic and repassivation potential measurements. The results of these electrochemical tests were compared and discussed. Galvanostatic test was proved to be the most powerful technique in detecting the crevice corrosion of 436 stainless steels, while the repassivation potential measurement was the most time-saving method in this study. Sulfate ions have inhibited the crevice corrosion of 436 stainless steels in chloride-containing solution, which may result from the effects of competitive adsorption and the IR drop mechanism.

Once crevice corrosion initiates, the hydrolysis of dissolved metal ions occurs to induce the acidification of solution inside the crevice and the migration of Cl- ions into the crevice from solution bulk. This Cl- ions migration promotes subsequent active dissolution and a further decrease of pH in the crevice. Therefore, the reduction of the concentration of Cl- ions is expected to be effective for suppressing crevice corrosion propagation. In this study, the influence of changes in solution chemistry on the propagation of crevice corrosion of stainless steel was investigated. After crevice corrosion was induced, in a chloride-containing solution the test solution was exchanged to chloride-free or chloride-containing solutions and corrosion propagation and repassivation behavior were monitored under the potentiostatic control. An austenitic stainless steel (0.017%C, 0.58%Si, 0.84%Mn, 0.028%P, 0.001%S, 12.15%Ni, 17.43%Cr, 2.08%Mo) was used to fabricate a crevice specimen schematically shown in Fig. 1. The steel was cut into 20 mm × 20 mm × ca. 5 mm, and the surface was polished down to 1 μm diamond paste. A through-hole with a diameter of 6 mm was formed at the center of the surface. Ultrasonic cleaning was carried out in ethanol, and then the specimen was passivated in 30 mass% HNO3 at 323 K for 1800 seconds. After that, the electrode surface area of the passivated specimen was polished down to 1μm diamond paste again. A lead wire was soldered to the electrode. An artificial crevice was formed between the electrode surface and a 20 mm × 20 mm × 2 mm polycarbonate sheet fixed by a bolt and a nut. The specimen was covered by insulating rubber except for the electrode surface. During the fabrication of the crevice, the electrode surface was kept being in contact with a NaCl solution (pH 5.0) dearated by N2 gas. The concentration of Cl- was adjusted to 20000 ppm. Potentiostatic polarization measurement at 0.3V (vs. Ag/AgCl) was conducted for 1000 seconds in a 20000 ppm Cl- containing NaCl (pH 5.0) at 298K to initiate crevice corrosion of the specimen. After that, the polarization was stopped and the test solution was exchanged to a chloride-free solution (test A) or a chloride containing-solution (test B), respectively. A Na2SO4 solution was selected as a chloride-free solution. The concentration of SO4 2- was adjusted to 20000 ppm. A chloride containing-solution was the same as the test solution used for the initiation of crevice corrosion. Subsequently, the potentiosstatic polarization at 0.3V was restarted and the change in current was monitored. After the test, the specimen surface was rinsed with deionized water and observed using an optical microscope. Figure 2a shows the time variation of current measured in the crevice corrosion tests. Figure 2b shows an enlarged view of the first 1000 seconds, that is, the current transient of the specimens in the NaCl solution. During this initial period, the currents gradually increased up to c.a. 1000 μA. It is therefore presumed that crevice corrosion occurred and propagated under the potentiostatic polarization at 0.3 V in the NaCl solution. After the solution was changed to the Na2SO4 solution, the current value increased to c.a. 700 μA until 4000 seconds passed and then decreased gradually to c.a. 30 μA. On the other hand, after the solution was exchanged to the NaCl solution, the current increased rapidly and remained at a high level until the end of the crevice corrosion test. These results suggest that the removal of Cl- ions from the bulk solution suppressed the propagation of crevice corrosion of stainless steel. Caption list: Figure 1 Schematic illustration of the specimen for crevice corrosion tests. Figure 2 (a) The time variation of current measured during the crevice corrosion tests at 0.3 V (b) An enlarged view of the first 1000 seconds of the crevice corrosion test. Figure 1

We investigated the unique passive behaviour of Hybrid steel in de-aerated sulfuric acid and aqueous sodium chloride solutions through corrosion tests, surface analysis, and thermochemical modelling. Electrochemical measurements confirmed that Hybrid steel possesses stainless steel characteristics, including passivity, breakdown, and pitting, akin to low-alloyed stainless steel. Synchrotron hard X-ray photoelectron spectroscopy revealed a dynamically protective nanoscale passive film composed of Fe, Cr, Ni, and Al oxides, contributing to its stainless nature. The presence of Al and Ni enhances Cr’s role in forming a spontaneously passive and protective surface, resulting in exceptional corrosion resistance in acidic and chloride-containing solutions. Hybrid steel’s surface oxides remain robust even beyond the Cr(III)-to-Cr(VI) redox potential, distinguishing it from other stainless steels. This work demonstrates the potential for designing sustainable stainless steel with high-strength properties without requiring the conventional Cr threshold concentration of 10.5 per cent.

The electrochemical behaviour of stainless steels following surface modification in ceriumcontaining solutions

A combination of mathematical modeling and experiments on single MnS inclusions was used to investigate the role of MnS inclusions on the initiation of pitting corrosion of 304 stainless steel. Isolation of single MnS inclusions in chloride-containing solutions with use of microcapillaries as electrochemical cells showed that the orientation of the MnS inclusion played a significant role in pit initiation. Large, shallow MnS inclusions failed to initiate pitting corrosion, while the same inclusions, oriented narrow and deep, consistently exhibited the onset of localized corrosion. The formation of a microcrevice was observed between the dissolving MnS inclusion and stainless steel. The microcrevice resulted in a locally occluded region where aggressive ions can accumulate. A mathematical model was developed to explore the local chemistry within a one-dimensional microcrevice of which one wall was MnS and the other was stainless steel. The results of the simulations supported the view of a critical solution chemistry of sulfur species, in a chloride environment, as a possible trigger mechanism for localized corrosion. A critical microcrevice geometry of approximately 1 μm was predicted to be sufficiently large to generate stable pitting in the system under study. © 2001 The Electrochemical Society. All rights reserved.

A new high-nitrogen austenitic stainless steel with excellent mechanical properties was tested for its resistance to stress corrosion cracking. The new conventional produced hybrid CrNiMnMoN stainless steel combines the excellent mechanical properties of CrMnN stainless steels with the good corrosion properties of CrNiMo stainless steels. Possible applications of such a high-strength material are wires in maritime environments. In principle, the material can come into direct contact with high chloride solutions as well as low pH containing media. The resistance against chloride-induced stress corrosion cracking was determined by slow strain rate tests and constant load tests in different chloride-containing solutions at elevated temperatures. Resistance to hydrogen-induced stress corrosion cracking was investigated by precharging and ongoing in-situ hydrogen charging in both slow strain rate test and constant load test. The hydrogen charging was carried out by cathodic charging in 3.5 wt.% NaCl solution with addition of 1 g/L thiourea as corrosion inhibitor and recombination inhibitor to ensure hydrogen absorption with negligible corrosive attack. Slow strain rate tests only lead to hydrogen induced stress corrosion cracking by in-situ charging, which leads to total hydrogen contents of more than 10 wt.-ppm and not by precharging alone. Excellent resistance to chloride-induced stress corrosion cracking in 43 wt.% CaCl2 at 120 °C and in 5 wt.% NaCl buffered pH 3.5 solution at 80 °C is obtained for the investigated austenitic stainless steel.

The corrosion behavior of sodium-exposed stainless steels in chloride-containing aqueous solutions was investigated. Results showed that sodium-corroded Type 316 stainless steel (prototypic Liquid Metal Fast Breeder Reactor (LMFBR) fuel cladding) maintains its integrity after five months exposure in these solutions at 82°C and with chloride content up to 500 ppm. In contrast, sensitized and sodium mass transfer deposit-containing Type 304 stainless steel failed in the high chloride solution (500 ppm) within ten days at the same temperature. The failure was initiated by pitting and subsequently accelerated by intergranular attack. The results also show that high pH tends to reduce the susceptibility to failure while procedures commonly used for sodium removal have no significant effect on the water corrosion behavior of the test material. Based on the current results, it is concluded that water storage is feasible for spent fuels in a LMFBR reprocessing plant.

The corrosion behaviour of lean duplex S32101 (1.4162-X2CrMnNiN21-5-1) stainless steel was assessed in various corrosive environments relevant to the pulp and paper industry. Electrochemical techniques, including open-circuit potential measurements and cyclic polarisation, were used to evaluate the corrosion resistance of S32101 stainless steel in various acidic, saline, and industrial liquors such as black, green, and white liquors, as well as dissolved chlorine dioxide bleaching solutions. To evaluate the extent of damage and corrosion mechanisms, post-exposure surface analysis was conducted using scanning electron microscopy (SEM). The results showed that S32101 experienced pitting corrosion in chloride-containing solutions, particularly in salt and acidified-salt environments. Corrosion rates increased with rising temperatures across all solutions. The highest corrosion rate of 3.17 mm/yr was observed in the highly alkaline white liquor at 50 °C, whilst chlorine dioxide induced the least aggressive effects at all temperatures. The suitability of S32101 stainless steel in handling pulp and paper liquors is shown in its corrosion resistance against the bleaching medium and low-temperature saline solutions, but it is not recommended for prolonged exposure to high alkaline liquors or chloride-rich solutions.

During heat treatment surface oxide layers, usually called heat tints, are formed on metallic materials. These oxide layers are composed of elements that have been selectively oxidized from the base metal; in the case of high-alloy materials principally chromium, nickel and iron. On austenitic stainless steels, it is well known that the region beneath the oxide layer is depleted in one or more of the elements that are involved in the scale formation. Consequently, reduced corrosion resistance is expected. It is also known that defects and stresses within the heat tint layer limit their protectiveness. Therefore, heat tint layers are usually removed by mechanical and/or chemical treatments to avoid corrosion issues during service. Nevertheless, the same understanding on heat tints formed during aging of precipitation hardenable Ni-based alloys is still lacking. Ni-based alloys generally have better corrosion resistance than stainless steels and the chemical composition of their surface oxide layers differ from those typically formed on stainless steels. In the present work the effect of heat tints on the pitting corrosion resistance of the Ni-based alloy UNS N07718 has been evaluated by means of electrochemical methods including cyclic potentiodynamic polarization tests and electrochemical noise measurements, and exposure tests in chloride-containing solutions.

A vast literature can be found on pitting corrosion addressing the initiation and propagation of pits due to localized corrosion. However, most of is the work to date is devoted to the electrochemical and metallurgical aspects of the phenomenon. In this paper, we provide a brief review of the recent progress in characterizing and analyzing the effects of various microstructural features on pitting corrosion. The scope of the paper is limited to stainless steels in chloride-containing solutions. The review shows that initiation of pitting corrosion in stainless steels is affected by such microstructural features on the exposed surface as the crystallographic orientation, phase and grain boundaries, beside the well-known and much characterized sulphide inclusions. Similarly, pit growth kinetics and evolution has been shown to depend upon the presence of precipitates and secondary phase particles, grain boundaries, and other material interfaces. One outcome of the review is the identification of the need to complement the recent computational studies incorporating fully-coupled electrochemical, mass transfer and momentum field equations at the macroscale with similar modeling and analysis at microscale to fully understand the effects of microstructural features on stable pit growth.

Electrochemical investigations of pitting corrosion

AISI(1) type 304L (UNS(2) 30403) and 316L (UNS 31603) stainless steels were heat treated under controlled conditions to produce surface oxides similar to those formed during welding. The oxides were surface-analyzed by Auger electron spectroscopy (AES) and studied electrochemically in both a weak sulfuric acid and a chloride-containing solution. The reduction in corrosion resistance brought about by the presence of the high-temperature oxides on the alloy surfaces was attributed to chromium depletion in both the oxide and underlying alloy substrate. This depletion was due to the relatively high oxygen partial pressures, present even during inert gas welding, and also to direct evaporation of this element from the surface. AES and corrosion test results for the heat-treated specimens were then compared with those obtained on welded type 304 stainless steel.