Initiation Of Crevice Corrosion Research Articles

Probing and monitoring multiple forms of localised corrosion occurring concurrently on pipeline steels in marine environments

An electrode array probe, with a novel approach, has been specifically designed to enable the in-situ field monitoring of multiple forms of localised corrosion of steel exposed to marine environments. The initiation and propagation of localised corrosion within the crevice area as well as at the steel weldment were visualised and evaluated over one-year field exposure at two different marine locations in Australia. The results show the varied initiation and propagation behaviour of crevice corrosion and weldment corrosion on steel, which provide experimental evidence for ‘bi-modal’ behaviour and the change-over of corrosion modes in extended periods of marine exposure.

Numerical Analysis of Stainless Steel of Crevice Corrosion Initiation Behavior with Varying External Potentials and Initial Crevice Gaps

The potential, current density, pH, and crevice profile distribution within the crevice structure of stainless steel in seawater were numerically analyzed. The crevice corrosion phenomena were mathematically modeled with the initial boundary value problem of diffusion and static electrical field, respectively. This initial boundary value problem was discretized using a finite difference method. The predicts of the crevice corrosion behaviors, set for various external potentials and initial crevice gaps, were classified into three categories: the pitting type, the active type, and no corrosion. The pitting type crevice corrosion was predicted to occur in conditions where the external potentials were noble and the initial crevice gaps were narrow. The active type crevice corrosion was predicted to occur in conditions where the external potentials were less noble and the initial crevice gaps were narrow. The range of external potential conditions where crevice corrosion was predicted not to occur increased in conditions with wide initial crevice gaps. Crevice corrosion was predicted to occur at all external potentials in conditions where the initial crevice gaps were 2 μm or less.

Effect of gap size on flange face corrosion

AbstractBolted flanged joints play a critical role in offshore wind turbine tower structures, serving as integral components that connect various sections of the tower. This research study employs electrochemical techniques to investigate the effect of gap dimensions, which determine the crevice gap thickness and crevice depth, on corrosion behavior of 321 stainless steel flange sample plates in a 3.5 wt% NaCl solution at 50°C. Gaskets are used in this study to create gaps between two flange surfaces. A novel fixture is utilized to simulate the applied stress on the gasket, fluid flow within the fixture, and the geometric aspects of the gasket and flange. The findings reveal that increasing the gap thickness from 1.58 to 6.35 mm results in a rise in the general corrosion rate of the flange surface from 0.09 to 1.03 mm y−1, and crevice corrosion initiation time increases from 0.23 to 3.12 h. Furthermore, reducing the crevice depth (d) from 7.49 to 0 mm leads to a decrease in the general corrosion rate from 0.09 mm y−1 to 0.04 µm y−1, and cases with d = 3.81 and d = 0 mm show no observable crevice corrosion after potentiostatic tests.

Crevice Corrosion Initiation and Growth of Die-Cast ADC12 Aluminum Alloy

Die-cast ADC12 (Al-3Cu-10Si) aluminum alloy is widely used because of mechanical properties and castability. In atmospheric environments, crevice corrosion sometimes occurs on ADC12; therefore, improvement of corrosion resistance and elucidation of corrosion mechanisms are needed.As-cast specimens of ADC12 were used in this study. Pure Al sheets were used for comparison. The microstructure of ADC12 was composed of α-Al and Al-Si eutectic phases. Intermetallic particles (IMPs) containing Cu existed. IMPs containing Fe also existed. The size of IMPs was around 1 μm.Crevice was made between a polycarbonate nut and bolt, and immersion test was performed in 0.1 M NaCl. After immersion, the dissolution of α-Al was observed inside the crevice. To ascertain the effect of pH on α-Al phase dissolution, immersion test was carried out in 0.1 M HCl and 0.1 M NaCl (pH 10.0) In 0.1 M HCl, α-Al dissolved, and the corrosion morphology was similar to that inside the crevice.

Time-Lapse Observation of Crevice Corrosion in Grade 2205 Duplex Stainless Steel.

The objective of this study was to investigate and visualize the initiation and propagation of crevice corrosion in grade 2205 duplex stainless steel by means of time-lapse imaging. Transparent Poly-Methyl-Meth-Acrylate washer and disk were coupled with duplex stainless steel to create an artificial crevice, with electrochemical monitoring applied to obtain information about the nucleation and propagation characteristics. All nucleation sites and corroding areas inside crevices were recorded in situ using a digital microscope set-up. Localized corrosion initiated close to the edge of the washer, where the crevice gap was very tight, with active corrosion sites then propagating underneath the disk into areas with wider gaps, towards the crevice mouth. The growth was associated with a rise in anodic current interlaced with sudden current drops, with parallel hydrogen gas evolution also observed within the crevice. The current drops were associated with a sudden change in growth direction, and once corrosion reached the crevice mouth, the propagation continued circumferentially and in depth. This allowed different corrosion regions to develop, showing selective dissolution of austenite, a region with dissolution of both phases, followed by a region where only ferrite dissolved. The effect of applied electrochemical potential, combined with time-lapse imaging, provides a powerful tool for in situ corrosion studies.

Effect of molybdate on crevice corrosion initiation of hydrogen-charged duplex stainless steel

In this paper, we studied the initiation of crevice corrosion of hydrogen-charged 2507 and investigated the effect of different Na2MoO4 concentrations on crevice corrosion. It was found that Na2MoO4 inhibits the adverse effect of hydrogen and enhances the inhibition of crevice corrosion. In addition, hydrogen-charged 2507 crevice corrosion initiates in the austenite phase. While Na2MoO4 improves the crevice corrosion resistance of both phases, crevice corrosion initiation sites changed in reverse, initiating in the ferrite phase.

Effect of crevice gap on crevice corrosion initiation and development of 2205 duplex stainless steel in NaCl solution

The crevice geometry has an important influence on the initiation and development of crevice corrosion. The crevice gap size determines the volume of the crevice solution and the cross-sectional area of the material exchange channel between the crevice solution and bulk solution, and it is the dominant factor of the initiation and development mechanism of crevice corrosion. In this paper, the effect of crevice gap size on the crevice corrosion behavior of 2205 duplex stainless steel (DSS) in NaCl solution was investigated by electrochemical method combined with modeling. The quantitative contribution of the volume effect of crevice solution and the diffusion effect of H+ on the crevice corrosion initiation period, as well as the corrosion characteristics and mechanism at initiation and development periods of crevice corrosion, were discussed. Results show that the initiation process of crevice corrosion of 2205 DSS in 3.5 wt% NaCl solution follows the critical crevice solution mechanism. The crevice gap size affects the acidification rate of the crevice solution and the initiation period time of crevice corrosion through the volume effect and the diffusion effect. The influence of diffusion effect on the initiation period time was more significant than volume effect. The corrosion development process is mainly determined by the potential and current density distribution within the crevice, and thus follows the IR drop mechanism (I refers to the current and R refers to the resistance). The crevice gap size is the dominant factor for the development mechanism of crevice corrosion.

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).

Sudden pH and Cl− Concentration Changes during the Crevice Corrosion of Type 430 Stainless Steel

To determine whether large sudden pH and Cl − concentration changes occur during the crevice corrosion of Fe-16Cr, as in the case of Fe-18Cr-10Ni-5.4Mn, simultaneous measurement of the pH and Cl − concentration was performed in 0.01 M NaCl (pH 3.0). The corrosivity of the crevice solution was different before and after crevice corrosion initiation, and the sudden changes in the pH and Cl − concentration inside the crevice were the same as those for Fe-18Cr-10Ni-5.4Mn. The incubation period was characterized by weak acidification (pH 1.6−2.0) and a small accumulation of Cl − (0.1−0.3 M). The growth period was characterized by strong acidification (pH ≤ 1.0) and a large accumulation of Cl − (≥1.0 M). At the crevice corrosion initiation site, a metastable pit was observed and a S signal was detected from the residual in the pit. It can be concluded that the crevice corrosion of Type 430 stainless steels is initiated by Cl − , which generates metastable pitting at sulfide inclusions. The effect of the electrode potential on the pH and Cl − concentration was investigated in 0.01 M NaCl from −0.05 to 0.1 V. No effect of potential was observed during both the crevice corrosion incubation and growth periods.

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.

Hydrogen effect on the passivation and crevice corrosion initiation of AISI 304L using Scanning Kelvin Probe

Scanning Kelvin Probe was applied to study passivation of AISI 304L stainless steel after cathodic polarisation. The rate of passivation in air decreased as a function of duration and current density. X-ray Photoelectron Spectroscopy showed enrichment of the surface film by hydroxides of Fe (II) that was the result of hydrogen effusion from the bulk. SKP measured a decreased potential drop in the passive film. Pre-polarisation accelerates the crevice corrosion of steel in presence of chlorides. Using SKP mapping, increased hydrogen sub-surface concentration and lower level of passivity was observed in anodic zones of the crevice.

A One Dimensional Crevice Experiment for Determining the Critical Factors Contributing to Crevice Corrosion Repassivation

From its inception more than 40+ years ago, the one-dimensional pencil electrode experiment (1-D pit) has allowed investigators to study factors on pitting corrosion such as ohmic drop, concentration of species as a function of pit depth and the stability of salt films.[1,2] In recent work, Srinivasan and Kelly have developed a 1-D transport model of repassivation for pencil electrodes made of SS 316L.[3] Unfortunately, for the phenomenon of crevice corrosion, the understanding of these critical factors (that no doubt control crevice propagation and repassivation) are far less understood. The primary reason for this has been the inability to simultaneously measure the location of the active crevice front (e.g. its depth) and the active area (which fixes the current density). These two parameters are easily measured in the 1-D pit experiment.Previously, we introduced a method for measuring both the depth of the active front in crevice corrosion and the active area.[4] The method, similar to that which Pickering introduced years ago,[5] records video of a propagating crevice in the optical microscope. The crevice (RCA) is formed by an alloy 625 washer covered by an acrylic washer that acts as a crevice former. The potential of the assembly is controlled potentiostatically using a traditional three electrode set up and the current is recorded as a function of time. A stereo microscope equipped with a digital camera is used to record images of the initiation and propagation of crevice corrosion. Image processing software is used to quantify the area of the active front. We calculate the actively corroding area, crevice current density and wall potential (Eapp-IR) all as a function of time.In this presentation, we will present the results from our crevice repassivation study on alloy 625. We use the THE method (ASTM G192) to analyze the transition to repassivation in the 625 RCA. Once initiated, the crevice was allowed to propagate for various times, equivalent to increasing charge passed. After a fixed time, the applied potential was decreased in a stepwise fashion. At a critical potential, forward movement of the active corrosion front towards the mouth (e.g. new damage area) stopped, though crevice propagation continued. Transport calculations reveal that this potential may be associated with salt film dissolution, consistent with the transition potential (ET / Esat) reported for one dimensional pitting experiments. The results are discussed in terms of traditional crevice corrosion models (such as Oldfield-Sutton[6] and IR*[5]) and more recent models for pit growth stability (Li Scully Frankel[7]). A mechanism for crevice growth and the transition to repassivation that is similar to that believed to controlled pit propagation and repassivation is proposed.AcknowledgmentsWork on this project was made possible by a grant from the US DoD TCC #1000000149, program directors Gregory A. Shoales, USAFA, and Daniel Dunmire, Corrosion Policy Oversight.References JW Tester, HS Isaacs, JES, 122, 1438, 1975.RC Newman, HS Isaacs, JES, 130, 1621, 1983.J Srinivasan, RG Kelly, JES, 163, C768, 2016.RS Lillard, S Mehrazi, DM Miller, JES, 16 7, 2020.5. K Cho, HW Pickering, JES, 138, L56, 1991.6. JW Oldfield, WH Sutton, Br. Corr. J., 13, 1978.7. T Li, JR Scully, GS Frankel, JES, 166, 2019.

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.

Fretting initiated crevice corrosion of 316LVM stainless steel in physiological phosphate buffered saline: Potential and cycles to initiation

Mechanically assisted crevice corrosion (MACC) has been associated with implant failure in vivo and is a serious concern in numerous metallic implant systems. Stainless steel medical devices may be subjected to fretting and crevice corrosion in the human body as are titanium and CoCrMo alloys due to the presence of a passive oxide film on their surface. One mechanism of MACC that has not been clearly identified and studied is fretting-initiated crevice corrosion (FICC) of stainless steel where an initial fretting event can initiate a rapid propagating crevice corrosion process even when fretting has ceased. FICC pin-on-disk experiments were performed at varying potential conditions and duration of fretting to explore the role of potential and fretting duration on the initiation of crevice corrosion. Triggering of a propagating crevice corrosion reaction on stainless steel at 250 mV vs Ag/AgCl/KCl (saturated) in PBS solution required only 2 s (2 cycles at 1 Hz) of fretting. Crevice corrosion continued to propagate under a 1.8 mm diameter pin with only 100 μm of direct contact, dissolving in both the depth and width dimension away from the fretting contact while the currents rose from 0.2 μA to 15 μA within 5 min. Three different potential-dependent FICC regions were identified that included unstable crevice corrosion (50 mV and above), metastable crevice corrosion (−100 mV to 0 mV) and stable fretting corrosion (between −500 mV and −150 mV). Crevice corrosion can be induced by fretting at potentials as low as −100 mV. Below −100 mV, there was no FICC, but rather fretting corrosion stopped immediately after fretting ceased and returned to a stable baseline current. Metastable FICC was shown at potentials between −100 mV and 0 mV, when the crevice corrosion current gradually decreased over several seconds or longer after fretting ceased. Self-sustained, unstable crevice corrosion started at 50 mV, where prior to fretting the currents were low, and after just a few cycles of fretting the crevice current rose rapidly and continued to increase after fretting stopped. Increase of potential increased the susceptibility of stainless steel to FICC. Scanning electron microscopy and digital optical microscopy revealed pitting and crevice corrosion on samples at −100 mV and higher potentials, where FICC was developing. By removing the oxide film, fretting motion significantly facilitates the critical crevice solution development, lowering the critical crevice potential and decreasing the initiation time for crevice corrosion. These results indicate that fretting initiated crevice corrosion may affect the performance of stainless steel in vivo. Statement of SignificanceAISI 316L stainless steel has been widely used as a metallic biomaterial for orthopaedic, spinal, dental and cardiovascular implants. Crevice corrosion has been a serious concern for stainless steel implants. For the first time we demonstrated and systematically studied the process of fretting-initiated crevice corrosion (FICC) in 316L stainless steel in simulated physiological solution of phosphate buffered saline. By removing the oxide film, fretting motion significantly facilitates the critical crevice solution development, lowering the critical crevice potential and decreasing the initiation time for crevice corrosion. Our findings indicate fundamental differences between the FICC mechanism and conventional crevice corrosion theory, showing that fretting can play a significant role in the initiation of crevice corrosion of stainless steel.

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.

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

(Invited) A One Dimensional Crevice Experiment for Determining the Critical Factors Contributing to Crevice Corrosion Stability and Repassivation

From its inception more than 40+ years ago, the one-dimensional pencil electrode experiment (1-D pit) has allowed investigators to investigate factors on pitting corrosion such as ohmic drop, concentration of species as a function of pit depth and the stability of salt films.[1,2] In fact, in recent work, Srinivasan and Kelly have developed a 1-D transport model of repassivation for pencil electrodes made of SS 316L.[3] Unfortunately, for the phenomena of crevice corrosion, the understanding of these critical factors (that no doubt control crevice propagation and repassivation) are far less understood. The primary reason for this has been the inability to simultaneously measure the location of the active crevice front (e.g. its depth) and the area of the front (which fixes the current density). These two parameters are easily measured in the 1-D pit experiment. In this presentation, we will introduce a method for measuring both the depth of the active front in crevice corrosion and the active area. The method is similar to that which Pickering introduced years ago,[4] we record video of a propagating crevice in the optical microscope. In this technique the crevice is formed by a set of washers, a piece of acrylic that acts as a crevice former and the metal specimen, nickel alloy 625. The potential of the assembly is controlled potentiostatically using a traditional three electrode set up and the current recorded as afunciton of time. A stereo microscope equipped with a digital camera is used to record images of the initiation and propagation of crevice corrosion. Image processing software is used to quantify the area of the active front. A typical result is presented in Figure 1 which shows the curent density of the active crevice front as a function of crevice depth. As seen in this figure, the crevice propagates from the deepest regions of the crevice towards the crevice mouth. this initial propagation direction is a result of an increase in the concentration of metal salts ahead of the active front and a corresponding decrease in pH. It can also be seen in this figure that at distances greater than 1 cm from the mouth, the crevice is able to propagate at current densities less than 1 mA/cm2. Once the active front reaches a distance of 1cm, an exponential increase in curent density is reorded. This increase is attributed to the IR* mechanism.[4] With the knowledge of the current density and distance from the crevice mouth we are able to use a simplification of the Nernst-Plank equation to calculate the concenrtation of species in solution. As might be anticipated from Figure 1, the concetration of metal salts increases with increasing current density. As such, concentration near the mouth is greater than at the tip, the saturation limit being reached at a distance of approximately 0.4cm from the mouth. Acknowledgments Work on this project was made possible by a grant from the US DoD TCC #1000000149, program directors Gregory A. Shoales, USAFA, and Daniel Dunmire, Corrosion Policy Oversight. References JW Tester, HS Isaacs, JES, 122, 1438, 1975.RC Newman, HS Isaacs, JES, 130, 1621, 1983.J Srinivasan, RG Kelly, JES, 163, C768, 2016.K Cho, HW Pickering, JES, 138, L56, 1991. Figure 1 Current density as a function of distance inside an alloy 625 crevice. Figure 1

Features of Local Corrosion of AISI316L Steel Manufactured by Selective Laser Melting

Regularities of corrosion damage in the case of pitting and gap corrosion are studied on samples of austenitic stainless steel (AINSI 316L) produced using the powder selective laser-melting (SLM) technique. It is found that the corrosion rate and the appearance of corrosion damage considerably depend on the surface state and heat treatment. Heat treatment (austenitization) results in a decrease in the local corrosion rate by 20–25%, a decrease in the number of pittings, and smoothing of the crevice corrosion profile as compared to the initial SLM state. The opening of the subsurface pores in the course of machining somewhat decreases corrosion resistance. It is shown that the initiation and development of pitting and crevice corrosion are related to the presence of surface and subsurface pores in the SLM material.

Predicting the risk of pitting corrosion initiation of stainless steels using a Markov chain model

A probabilistic approach based on Markov chains for the assessment of pitting and crevice corrosion initiation is proposed. A Markov chain is a stochastic process that undergoes transitions from one state to another through a finite number of possible states, until a so‐called “absorbing state” from which the system has no tendency to evolve is attained. The proposed model calculates the probability to have pitting or to maintain a stable passive condition involving a large number of operating parameters related to both metal and environment: steel chemical composition (namely, the PREN index), chlorides content ([Cl−]), chloride critical threshold ([Cl−]cr), temperature, pH, fluid velocity, electrochemical steel potential, E, presence of crevices. Model and algorithms are mainly based on corrosion data collected from literature. Although the proposed model needs experimental confirmation, the simulations here presented seem acceptable, considering the engineering application of stainless steels.

(Invited) Characterizing Crevice Corrosion Processes Electrochemically, Microscopically, and Spectroscopically

Due to the occluded nature of the process, crevice corrosion is difficult to investigate electrochemically, microscopically and spectroscopically. The electrochemistry/chemistry occurring within propagating crevices can be highly localized and difficult to characterize. With an emphasis on Ti and Ni alloys, we have developed a range of methodologies to determine the electrochemical/chemical details of crevice corrosion processes and how they are influenced by the compositional and microstructural properties of the materials. On both alloy categories crevice propagation can be supported by both external and internal cathodic processes. The kinetics of the external process (O2 reduction) can be followed electrochemically using a galvanic coupling technique, while the internal process (H+ reduction), although detectable by its influence on the O2 reduction kinetics, cannot be directly followed in real time. Which of these two reactions is dominant is very dependent on alloy composition and microstructure. For Ti alloys, the microstructure is very dependent on the added alloying elements (e.g., Ni, Pd, Ru) and the content of unavoidable impurities such as Fe, and perhaps also Mo and Cr. Their distribution in the uncorroded alloy can be determined using atomic probe tomography and their redistribution, as a consequence of a period of crevice propagation, by dynamic secondary ion mass spectrometry. By comparison to Ti alloys, Ni-based alloys (in particular Ni-Cr-Mo alloys) are free of segregated secondary phases, and possess high quality, low energy grain boundaries. These two qualities can be demonstrated using electron backscatter diffraction, transmission electron microscopy and electron energy loss spectroscopy. However, while the quality of the grain boundaries is a key feature in controlling the initiation of crevice corrosion, it is the alloy composition which exerts the dominant influence on the accumulation and distribution of corrosion damage by depositing Mo oxides at propagating sites, a process that can be demonstrated by correlating the damage distribution, determined by confocal laser scanning microscopy, with the accumulation of molybdates, confirmed by microRaman spectroscopy. Using this combination of electrochemical, microscopic and spectroscopic techniques we are presently determining the influence of alloy composition on the relative importance of internally and externally supported crevice propagation. Also, we are developing the technique of X-ray photoelectron spectroscopic imaging to demonstrate the self-healing capability conferred on these alloys by the inclusion of Mo.