Crevice Corrosion Tests Research Articles

Influence of protein migration on the fretting-corrosion behavior of 316 L stainless steel with artificial crevices

Fretting-crevice-corrosion at the modular interface of artificial joints can cause failure of the joint replacement. In this study, long-term fretting crevice corrosion tests of 316 L stainless steel were carried out at the center of an artificial crevice in a saline solution with and without proteins. The results showed that fretting initiated significant crevice corrosion without proteins and corrosion of large areas was observed in the non-contact area inside the crevice. In the presence of proteins, proteins can migrate into the crevice and block the crevice mouth. The enriched proteins had a lubricating effect and inhibited the occurrence of crevice corrosion.

Additive Manufacturing: Corrosion Proofing by Infusion of Interstitial Solute—Exemplified for Alloy 22

The corrosion resistance of Cr-containing alloy parts made by additive manufacturing can be significantly improved by a post-treatment of gas-phase-based infusion of concentrated interstitial solute (carbon and nitrogen). We demonstrate this universal approach for the example of low-temperature nitrocarburization by solid-reagent pyrolysis applied to Alloy 22 (UNS N06022) parts made by laser powderbed fusion. We show that the post-treatment improves the crevice-corrosion resistance of these parts, as well as the corrosion resistance of corresponding parts made from wrought Alloy 22 to surpass the maximum crevice corrosion test temperature specified in ASTM G48-D, whereas non-treated samples typically fail well below. Similarly, cyclic potentiodynamic polarization testing (ASTM G61-86) demonstrates that the post-treatment makes the additively manufactured alloy and the wrought alloy more corrosion-resistant than the non-treated wrought alloy.

Introduction of Cu2+ to the inside of the crevice by chelation and its effect on crevice corrosion of Type 316L stainless steel

The chelated complex of Cu2+: [Cu(EDTA)]2− was used to introduce Cu2+ from outside to the inside of crevice. The introduced Cu2+was expected to act as an inhibitor for the crevice corrosion on stainless steels. Crevice corrosion tests confirmed the introduction of Cu2+ to the inside of the crevice via electromigration of [Cu(EDTA)]2−. Migrated [Cu(EDTA)]2− reacted with H+ and inhibited decrease in pH inside the crevice, where Cu2+ was separated from [Cu(EDTA)]2− and suppressed active dissolution of the stainless steel.

The Effect of [Cu(EDTA)]2− on Crevice Corrosion of Type 316L Stainless Steel

Alloyed Cu is known to inhibit the growth of pitting corrosion on stainless steels after pitting initiation. The Cu2+ dissolved from the stainless steel matrix acts as an inhibitor in acidic chloride environments which is formed in pits. The Cu2+ suppresses active dissolution rate inside the pits1. This suppression effect is supposed to be also effective for active dissolution which promotes crevice corrosion. Therefore, if it is possible to introduce Cu2+ to the inside of a crevice from the outside, introduced Cu2+ is supposed to inhibit crevice corrosion on stainless steel. However, diffusion of ions between inside and outside of the crevice is restricted by its geometry, and Cu2+ does not migrate to inside of a crevice according to the electroneutrality principle. [Cu (EDTA (ethylenediaminetetraacetic acid))] 2− is a chelated Cu2+ which has negative charge, and is expected to migrate to inside of a crevice from the outside by electrochemical migration. In addition, it is reported that Cu2+ in [Cu(EDTA)]2− could be easily substituted by Fe2+ in low pH2. Therefore, Cu2+ is considered to be introduced to the inside of the crevice as [Cu(EDTA)]2−, and to affect crevice corrosion of stainless steel. In this study, in situ observation of an inside of the crevice3 was performed to analyze the effect of [Cu(EDTA)]2− on crevice corrosion. Type 316L stainless steel was used as specimen. The crevice corrosion tests were conducted in a flow cell3. 0.1 M NaCl or 0.1 M NaCl-10 mM [Cu(EDTA)]2− was flowed through the flow cell. The crevice was made by pressing the glass plate on the specimen surface. The specimen surface was perfused with 0.1 M NaCl before the glass plate was pressed. Therefore, inside of the crevice was filled with 0.1 M NaCl. The specimen was polarized at 0.35 V (vs. Ag/AgCl, Sat. KCl). After the crevice corrosion initiation, the different propagation morphology was observed between 2 conditions. In the case of 0.1 M NaCl, the gas bubbles were observed with corrosion propagation. On the other hand, little gas bubbles were observed in the case of 0.1 M NaCl-10 mM [Cu(EDTA)]2−. The gas bubbles observed in 0.1 M NaCl were considered to be H2 generated by the H+ reduction reaction inside the crevice. It was reported that the reduction reaction inside the crevice was changed by migrated anions which reacted with H+ inside the crevice3. Therefore, the results indicate [Cu(EDTA)]2− migrated to the inside of the crevice from the outside, and affected the crevice corrosion morphology.References T. Sourisseau, E. Chauveu, and B. Baroux, Corros. Sci., 47, 1097-1117(2005).W. Guan, B. Zhang, S. Tian, and X. Zhao, Appl. Catal. B, 227, 252-257 (2018). T. Aoyama, Y. Sugawara, I. Muto, and N. Hara, J. Electrochem. Soc., 166, C250-C260 (2019).

Visualization of pH Distributions inside the Crevice of Stainless Steel in Corrosive Environments

Stainless steels are known for its excellent corrosion resistance. However, stainless steels sometime suffer from crevice corrosion in chloride environments. The distribution of pH inside the crevice is recognized to be the important factor in crevice corrosion of stainless steels [1, 2]. This study focused on visualization of pH inside the crevice in corrosive environments.An imaging plate was prepared by a sol-gel method. Ethyl violet was used to assess the pH values. The fabricated imaging plates were calibrated by 0.01 M NaCl solution at pH 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0. The imaging plate was immersed in the 0.01 M NaCl solutions, and the color change was observed by an optical microscope. Type 304 stainless steel (Fe-18mass%Cr-8mass%Ni) was used as the specimens. The size of the specimen for the crevice corrosion test is 3 mm × 3 mm×17 mm. The specimen surfaces were polished with a 1 µm diamond paste. The scan rate of potentiodynamic polarization measurements was set to 23mV min- 1. The specimen for the polarization measurements is 17 mm × 25 mm × 3 mm in size. The electrode area was 1 cm2. The electrolyte used for the potentiodynamic polarization was 0.01 M NaCl (pH 3.0). The electrolyte was deaerated before the polarization measurements. The reference electrode was Ag/AgCl (3.33 M KCl). The counter electrode was Pt. The crevice corrosion tests were performed under a potentiostatic polarization. The applied potential was 0.3 V. The crevice was made between the steel surface (3 mm × 3 mm) and the imaging plate. Optical microscope images of the surface were taken every 30 seconds. The pH values inside the crevice were calculated from the R (red), G (green), and B (blue) values of each pixel in the images.In calibration, the color of the imaging plate changed from violet to yellow, suggesting that G values of imaging plate mainly responded to pH. The pitting potential of the specimens was 0.4 V. Therefore, 0.3 V was suitable for the crevice corrosion test. Because 0.3 V (Ag/AgCl (3.33 M KCl)) was lower than the pitting potential. The visualization of pH in the crevice corrosion was conducted. The pH decreased during the incubation, initiation, and growth of crevice corrosion. The pH was the important factor in the initiation and growth of crevice corrosion.[1] T. Kaji, T. Sekiai, I. Muto, Y. Sugawara, N. Hara, Visualization of pH and pCl Distributions: Initiation and Propagation Criteria for Crevice Corrosion of Stainless Steel, J. Electrochem. Soc., 159 (2012), C289-C297.[2] M. Nishimoto, J. Ogawa, I. Muto, Y. Sugawara, N. Hara, Simultaneous visualization of pH and Cl- distributions inside the crevice of stainless steel, Corros. Sci., 106 (2016), 298-302.

Crevice and pitting corrosion of low temperature plasma nitrided UNS S32750 super duplex stainless steel

The UNS S32750 super duplex stainless steel (SDSS) was low temperature plasma nitrided at 350 °C and 400 °C for 4 h. Pitting and crevice corrosion resistance was evaluated using electrochemical techniques in a NaCl neutral solution. Nitriding treatments resulted in increased in hardness, general corrosion resistance and in the passive layer breakdown potential. Regarding crevice corrosion tests, the nitrided surfaces showed lower current densities during the potentiostatic polarization and lower corrosion depths when compared to the untreated surface. The nitrogen played a fundamental role in the passivation behavior and in the susceptibility to repassivation of localized corrosion.

In situ marine exposure study on corrosion behaviors of five alloys in coastal waters of western Pacific Ocean

The aim of this study is to investigate the effects of seawater retention inside subsea oil and gas pipelines on the corrosion of several corrosion resistant alloys commonly used in seawater. Pitting corrosion, crevice corrosion, and electrochemical test samples were immersed in coastal waters at the depth of about 170 m in South China Sea (western Pacific Ocean). Assessments of the pitting and crevice corrosion for different alloys were carried out through electrochemical measurements and surface characterizations. The results showed that, the order of the pitting corrosion resistance of five alloys in marine environment was: 825 Ni-based alloy ∼ 2507 super duplex stainless steel > 2205 duplex stainless steel ∼ 317L ∼ 316L, while the crevice corrosion resistance was: 825 Ni-based alloy ∼ 2507 super duplex stainless steel > 2205 duplex stainless steel > 317L > 316L. The pitting initiations of 2205 duplex stainless steel, 2507 super duplex stainless steel, and 825 Ni-based alloy were later than those of 316L and 317L stainless steels. After the exposure for 592 d, the passivation for 316L, 317L and 2205 stainless steels were completely broken down while 2507 super duplex stainless steel and 825 Ni-based alloy still had a wide and stable passive zone. In crevice corrosion samples, the most severe corrosion was observed in the crevice edge area. A boat shaped corrosion pit was found in the inner coverage area far from the edge of crevice, which was probably related to the distribution of microorganisms in this area.

Visualization of pH and Cl− Distributions inside Crevice of Type 430 Stainless Steel

The decrease in pH caused by the hydrolysis of metal ions inside crevice is widely accepted to be the predominant factor affecting the initiation of crevice corrosion for stainless steels. The accumulation of chloride ions inside crevice is also thought to be of great importance for the initiation of crevice corrosion. 1, 2 In this study, a simultaneous measurement technique for pH and Cl− concentration was applied to the visualization of pH and Cl− distributions inside crevice in the initial stage of crevice corrosion of Type 430 stainless steel. A sensing layer with fluorescent dyes was made on one side of a SiO2 plate using the sol-gel SiO2 coating technique. Quinine sulfate (fluorescent dye) was used to measure Cl− concentration. The excitation wavelength was 350 nm. The emitted light was filtered through a 420-nm long-pass filter, since quinine sulfate has a peak wavelength of 451 nm (blue fluorescence). 1 Tb3+-DPA (dipicolinic acid) complex was used as the fluorescent dye for pH measurements. 2 The excitation wavelength of this dye was 270 nm, and the emitted light was filtered through a 510–550-nm band-pass filter. In this case, green fluorescence was observed. The crevice specimens were prepared from Type 430 stainless steel (Fe-16.5Cr). A crevice (3 mm × 3 mm) was made between the stainless steel and the sensing plate. 1,2 Crevice corrosion tests were carried out in a naturally aerated 0.01 M NaCl at pH 3.0, and the specimen was polarized at 0.0 V (vs. Ag/AgCl, 3.33 M KCl). Before the initiation of crevice corrosion, pH inside the crevice gradually decreased with time. And also, Cl− concentration increased with time. At pH 1.8 and ca. 0.1 M Cl−, crevice corrosion initiated, and a large pH drop and a large Cl− increase were observed.

Nanometer Resolved Real Time Visualization of Acidification and Material Breakdown in Confinement

AbstractLocalized surface reactions in confinement are inherently difficult to visualize in real‐time. Herein multiple‐beam‐interferometry (MBI) is extended as a real‐time monitoring tool for corrosion of nanometer confined bulk metallic surfaces. The capabilities of MBI are demonstrated, and the initial crevice corrosion mechanism on confined nickel and a Ni75Cr16Fe9 model material is compared. The initiation of crevice corrosion is visualized in real time during linear sweep polarization in a 1 × 10−3 m NaCl solution. Pre‐ and post‐experiment analysis is performed to complementarily characterize the degraded area with atomic force microscopy (AFM), optical microscopy, nano‐Laue diffraction (nano‐LD), scanning electron microscopy (SEM)/electron backscatter diffraction (EBSD), and X‐ray photoelectron spectroscopy (XPS). Overall, Ni75Cr16Fe9 displays a better corrosion resistance; however, MBI imaging reveals 200 nm deep severe localized corrosion of the alloy in the crevice opening. Chromium rich passive films formed on the alloy contribute to accelerated corrosion of the confined alloy by a strongly acidifying dissolution of the passive film in the crevice opening. Nano‐LD further reveals preferential crystallographic defect and corrosion migration planes during corrosion. MBI provides nanometer accurate characterization of topologies and degradation in confined spaces. The technique enables the understanding of the initial crevice corrosion mechanism and testing modeling approaches and machine‐learning algorithms.

NH4+ Generation: The Role of NO3− in the Crevice Corrosion Repassivation of Type 316L Stainless Steel

The role of NO3− in the repassivation of crevice corrosion of Type 316L stainless steel was investigated. In crevice corrosion tests, the solution was changed from 1 M NaCl to NaCl- NaNO3. NO3− led to complete repassivation, and after potentiostatic polarization in 2 M KNO3, NH4+ was detected in the crevice solution by ion chromatography analysis. Repassivation of the crevice corrosion was found to take place in two steps. In the first step, the estimated current density inside the crevice gradually decreased from ca. 5 mA cm−2 to ca. 5 μA cm−2. After that, the current density suddenly decreased to less than 0.1 μA cm−2. From the potentiodynamic polarization in acidic solutions simulated inside the crevice (pH 0.2) and in situ observations of the crevice corrosion morphology, the first step was thought to be generated by the suppression of active dissolution by NO3−. It would appear that the generation of NH4+ results in a pH increase and the further suppression of active dissolution, and then repassivation occurs.

Degradation behavior of metastable β Ti-15-3 alloy for fastener applications

Environmentally induced degradation is a serious issue governing the use of titanium alloy fasteners for marine applications. The workhorse alloy Ti6Al4V which has been the most used titanium alloy till date as fastener material has its limitations and search has been on for improved titanium-based materials. This paper deals with researches aimed at evaluating the metastable beta titanium alloy Ti15V3Cr3Al3Sn (Ti15-3) in solution annealed condition as a fastener material for medium strength applications. A variety of tests was carried out to rate the resistance of the alloy in this condition to seawater environments slow strain rate testing (SSRT), crevice corrosion testing, potentiodynamic polarization studies, and immersion testing. Limited SSRT was also done on Ti6Al4V for the sake of comparison. Analysis of test results shows that solution annealed Ti15-3 offers itself as a good candidate material for medium strength fastener applications in marine environments.

Change in pH and Chloride Concentration inside Crevice of Stainless Steels

Stainless steels sometimes suffer from crevice corrosion in chloride environments. The pH value and Cl- concentration are important factors in the initiation and growth of crevice corrosion. Nishimoto et.al.[1] fabricated a glass plate with a sol-gel SiO2 sensing layer, which contains a pH indicator and a Cl- sensitive florescent dye, and the pH and Cl- distributions in the crevice solution for stainless steel were visualized simultaneously. The specimens used in this study were cut from a Type 430 stainless steel sheet (0.035%C, 0.20%Si, 0.38%Mn, 0.019%P, 0.005%S, 0.19%Ni, 15.9%Cr in mass%). The specimens were heat-treated at 1123 K for 600 s and water-cooled. The size of the specimen was 4 mm×3 mm×20 mm. The specimen surfaces were polished by a 1 μm diamond past. To fabricate the sensing plate for the simulations measurements of pH and Cl- concentration, Terbium-dipicolinic acid complex (Tb-DPA) and quinine sulfate as the fluorescent dyes were chosen. Tb-DPA was used as the sensing dye for the pH measurements, and quinine sulfate was used to measure the Cl- concentration. Tb-DPA was formed by mixing Tb­­2(SO4)3 and dipicolinic acid. A sol-gel method was used to make the sensing layer, which contained two fluorescent dyes, onto one side of a quartz glass plate. Figure 1 shows the illustration of a crevice corrosion test cell. In crevice corrosion tests, the crevice was formed between the sensing plate and specimen. Potentiostatic polarization was performed at 0.15 V (vs. Ag/AgCl, 3.33 M KCl) in 10 mM NaCl (pH 3.0). During crevice corrosion tests, the surface images of the specimen were taken under UV irradiation, and pH and Cl- values were calculated from the fluorescence images. The details were described in the literature. [1] Figure 2 shows the surface appearance and the images of pH and Cl- distributions at the initiation point of the crevice corrosion at 1500 s. The pH and Cl- were calculated by the brightness of fluorescence images. A pit was initiated inside the crevice. Before the initiation of crevice corrosion, it was elucidated that the pH at the initiation point of gradually decreased with time, and that the Cl- concentration increased with time. On the basis of time variations of the pH and Cl- concentration, the initiation and growth mechanims of the crevice corrosion of stainales steels were determiend. References; Nishimoto, J. Ogawa, I. Muto, Y. Sugawara, N. Hara: Corros. Sci., 106, 298-302(2016) Figure 1

Visualization of Potential Distribution inside Crevice of Type 430 and 304 Stainless Steels

Stainless steels are widely used in industries because of their high anti-corrosive performance due to their passivation ability. However, crevice corrosion sometimes occurs in chloride environments. It is known that crevice corrosion is initiated when electrode potential inside crevice changes from the passive to active state. Therefore, the potential is the important factor to understand the mechanism of crevice corrosion. In this research, visualization of the potential inside the crevice for type 430 and 304 stainless steels was performed in 0.01 M NaCl at pH 3.0. An indium-tin oxide (ITO) glass with a sensing layer was applied for visualizing the potential. In the sensing layer, a redox indicator was entrapped. Figure 1 shows the schematic illustration of apparatus for the calibration procedure for color change of the plate. The ITO glass with the sensing layer (sensing plate) was used as a working electrode. In an electrolyte, a mirror polished Pt plate was put below the sensing plate to reflect the image of the sensing plate, and the optical microscope images were taken through the sensing plate by a CCD camera system. Electrode potential of the sensing plate was changed from -0.52 to 1.08 V in 0.01 M NaCl (pH 3.0 adjusted with HCl). All potentials reported in this work refer to Ag/AgCl (3.33 M KCl). Figure 2 shows the optical micrographs of the sensing plate placed above the Pt. It is clear that the color was changed from light green to dark blue with increasing electrode potential. The sensing plate was applied to crevice corrosion tests in 0.01 M NaCl (pH 3.0 adjusted with HCl). In the crevice corrosion tests, the sensing plate was used as a crevice former. The distributions of electrode potentials inside the crevice of the stainless steels were monitored from the color of the sensing plate. Figure 1

Crevice Corrosion of Stainless Steels 904L, 2205, and 2507 in High-Temperature Sulfuric Acid Solution Containing Chlorides: Influence of Metal Cations

Crevice corrosion influences the integrity of materials in many industrial sectors, such as hydrometallurgy. The conditions in hydrometallurgical processes are severe, with high concentrations of sulfuric acid leachant as well as high-chloride impurity, oxygen, and metal cation contents. In this study, crevice corrosion behavior of highly-alloyed austenitic and duplex stainless steels is investigated in aerated high-temperature sulfuric acid solution with 1 g/L chlorides in the presence of three types of metal cations: Fe3+, Cu2+, and Fe2+. The experimental work contains the determination of the oxidizing capacity of the solutions, immersion crevice corrosion tests followed by characterization of the corrosion attack, and potentiodynamic polarization measurements. The results show that the alloy with the highest PREN value, grade 2507, was not susceptible to crevice corrosion under the test conditions. In the case of grades 904L and 2205 of relatively equal PREN values, the extent and nature of crevice corrosion attack was different and dependent on the ratio of activities of chlorides to sulfates and the type of metal cations. The results are presented and discussed in the light of IR drop mechanism.

Effect of Nitrate Ions on Repassivation Behavior of Crevice Corrosion on Type 316L Stainless Steel

Once crevice corrosion is initiated on stainless steels in chloride environments, the hydrolysis of dissolved metal ions causes severe acidification inside the crevice, and Cl- ions migrate into the crevice from the bulk solution. This electromigration of Cl- ions promotes subsequent active dissolution inside the crevice. Therefore, neutralization of the pH inside the crevice is expected to be the predominant factor affecting the repassivation of crevice corrosion.To clarify the repassivation behavior of crevice corrosion, in situ observations were performed, and the quantitative relationship between current values and corrosion morphology was analyzed. In addition, to elucidate the effect of the solution composition on the repassivation behavior, an electrochemical flow cell was used. In this study, the effect of NO3 - ions on the repassivation behavior of Type 316L stainless steel was investigated. Type 316L stainless steel was used as specimens. The specimens were heat-treated at 1373 K for 3.6 ks and then water-quenched. A schematic illustration of the electrochemical flow cell is shown in Figure 1. An artificial crevice was formed between a glass plate and the specimen surface. The specimen was polarized at 0.25 V (vs. Ag/AgCl, 3.33 M KCl). Crevice corrosion tests were performed in a flowing 1 M NaCl solution at 298 K. In the case of the solution change experiment, the inlet solution was changed from 1 M NaCl to 1 M NaCl-1 M NaNO3. The solution was changed when the current exceeded 100 μA. The flow rate of the solution was 24 mL h-1 during the crevice corrosion tests. Since the volume of the cell was ca. 24 mL, it took about 1 h to change the solutions completely. During the crevice corrosion tests, the crevice corrosion behavior of the specimen surface inside the crevice was observed using an optical microscope, and the images were taken every 100 s. Figure 2a shows the time variations of the currents in the crevice corrosion test in 1 M NaCl, and Figure 2b shows the results of in situ observations of the specimen surface. From the beginning of the crevice corrosion test to ca. 250 s, the current decreased to ca. 4 μA with time. Around ca. 300 s, the current increased rapidly. This current increase was due to the initiation of crevice corrosion. At this time, two small dark spots were observed inside the crevice (Figure 2b1). These spots were the initiation sites of the crevice corrosion. As shown in Figures 2b1-b3, the crevice corrosion grew toward the crevice mouth. After that, the corroded areas proceeded along the crevice mouth (Figures 2b4-b7). Once the entire crevice mouth was corroded, the crevice corrosion propagated toward the inside of the crevice, and the current increased up to ca. 5 mA at ca. 10 ks. From the results shown in Figure 2, it was confirmed that the crevice corrosion was readily initiated in 1 M NaCl, and no spontaneous repassivation was generated in this solution. Figure 3a shows the time variations of the current in the solution change experiment from 1 M NaCl to 1 M NaCl-1 M NaNO3, and Figure 3b shows the results of in situ observations of the specimen surface. Around 300 s, the current increased rapidly due to the initiation of crevice corrosion. At 406 s, the current exceeded 100 μA, and the inlet solution was then changed to 1 M NaCl-1 M NaNO3. From 406 s to 3.75 ks the current increased to 2.9 mA. In this period, the corroded areas mainly grew along the crevice mouth as shown in Figures 3b2-b5. At 3.75 ks (Figure 3b6), the current started decreasing. Almost at the same time, the growth of the crevice corrosion along the crevice mouth was stopped. From this time, the crevice corrosion propagated toward the inside of the crevice. Afterwards, the current decreased to ca. 150 μA. After 26.1 ks, the current began decreasing again. As shown in Figure 3b8, no propagation of the corroded area was observed after 43.0 ks. This suggested that the surface of the corroded area was completely repassivated. This result indicated that NO3 - bring about the repassivation. Figure 1

Effects of Heat Treatment and Surface Finish on the Crevice Corrosion Resistance of Martensitic Stainless Steel

The present study aims to investigate the influence ofheat treatment and surface finish on the behavior of crevicecorrosion resistance of AISI 410 and 416 martensitic stainlesssteels thus, to quantify the conditions at which crevicecorrosion minimize as possible. The experimental work carriedout during this study involves material selection, chemicalcomposition tests, specimens preparation before heattreatments, austenitizing at temperature range (925-1010˚C)and for holding time periods of (30, 45 and 90 min), air and oilquenching followed by tempering at heating range of (205-605 ̊C) and for 45 min, micro hardness tests, specimensgrinding, surface roughness measurements, crevice corrosiontests, crevice evaluation and microstructure tests.Theoretically, empirical equations for crevice maximum depthunder the effect of surface roughness and hardness for bothAISI 410 and 416 steels were determined. While formicrostructure analysis, carbides average area was determinedby using the ImageJ analysis program and a mathematicalmodel was also predicted. Results showed that like hardness andsurface roughness increase crevice corrosion resistancedecreases. Therefore, material treated by annealing canminimize crevice corrosion rates more than that treated withhardening.

Effect of Sulfate Ions on Repassivation Behavior of Crevice Corrosion on Type 316L Stainless Steel

Crevice corrosion is known to be a serious problem of stainless steel structures in Cl- containing environments. The growth and repassivation behavior of crevice corrosion are significantly influenced by the solution composition of corrosion environments. To clarify the repassivation behavior of crevice corrosion, an in situ observation is a promising technique. This is because the quantitative relationship between current values and corrosion morphology could be analyzed. The current distribution inside the crevice can be estimated from the spreading of crevice corrosion depth. In this study, an in situ observation technique was applied to the study of the crevice corrosion of stainless steel. In addition, to clarify the effect of the solution composition outside the crevice on repassivation behavior, an electrochemical flow cell was newly designed. Type 316L stainless steel was used as specimens. The specimens were heat-treated at 1373 K for 3.6 ks and then water-quenched. An electrochemical flow cell was fabricated (Fig. 1). The crevice was made by pressing a glass sheet to the specimen surface, and the specimen was polarized at 0.25 V (vs. Ag/AgCl, 3.33 M KCl). Crevice corrosion tests were initially performed in a flowing 1 M NaCl solution (298 K). During the corrosion tests, the corrosion behavior of the specimen surface inside the crevice was observed using an optical microscope through the glass sheet, and then the time-lapse video was produced by capturing one frame per 10 seconds. In some cases, the flowing solution was changed from 1.0 M NaCl to 0.88 M Na2SO4, after the initiation of the crevice corrosion. The solution was changed when the current reached a value of 100 µA. Figure 2a shows the time variations of the current during the crevice corrosion tests. In the case of test A, the solution was not changed. In test B, the solution was changed from NaCl to Na2SO4 when the current reached the value of 100 µA. In both cases, the current first decreased to ca. 5 µA with the lapse of time and at about 400 s, it became to increase rapidly due to the initiation of crevice corrosion. In test A, the current increased up to ca. 5 mA. On the other hand, in test B where the solution was changed, the current decreased after reaching a maximum value of 3 mA at ca. 2.5 ks. The results of in situ observation of the specimen surface are shown in Fig. 2b. In test A, the corroded area propagated along the crevice mouth (Fig. 2b). At the beginning of crevice corrosion in test B, its propergation was similar to that of test A. When the current began to decrease at ca. 2.5 ks, the direction of corrosion propagation changed toward the inside of crevice (Fig. 2c). Finally, when the current decreased to ca. 1.2 µA, no lateral propagation of the corroded area was observed, suggesting that the repassivation inside the crevice was completed. Figure 1

Use of the Critical Acidification Model to Estimate Critical Localized Corrosion Potentials of Duplex Stainless Steels

Crevice corrosion affects the integrity of stainless steels used in components exposed to seawater. Traditionally, crevice corrosion testing involves the use of artificial crevice formers to obtain a critical crevice potential, which is a measure of the crevice corrosion resistance of the alloy. The critical acidification model proposed by Galvele predicts that the critical crevice potential is the minimum potential required to maintain an acidic solution with a critical pH inside either a pit or a crevice. Application of Galvele’s model requires an estimation of both the diffusion length and the i vs. E behavior of the metal in the solution inside the crevice. In this work, the crevice corrosion resistance of a 22%Cr duplex stainless steel (UNS S31803) and a 25%Cr super duplex stainless steels (UNS S32750) was investigated. The i vs. E response of the two stainless steels was determined in acidified solutions of various chloride concentrations, which simulate those found in an active crevice. Critical po...

Effects of alloyed Cr and Cu on the corrosion behavior of low-alloy steel in a simulated groundwater solution

The effects of Cr and Cu on the corrosion behavior of low-alloy steel have been studied by immersion tests and electrochemical measurements combined with scanning electron microscopy and analysis of polarization curves. The low-alloy steels exhibit higher uniform corrosion resistance than do carbon steel. Crevice corrosion morphology study indicated that the pit-like corrosion attack occurred only on the Cr-containing steels. The crevice corrosion tests indicated that Cr and Cu have synergistic effects on increasing the crevice corrosion of the steel.

The Effect of Change in Solution Chemistry of Bulk Solution on Crevice Corroson Propagation of Stainless Steel

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