Stable Pitting Research Articles

In Situ Microscopic Observation of Pitting Corrosion Behavior of A1050-O

Al-Fe alloys are used for containers, cables, and window flashings because of their high formability, high strength/weight ratio, and easy machinability. The mechanical properties of Al-Fe alloys have been improved by controlling the morphology of precipitates, however, there have been reports that some precipitates are the sites for pit initiation. The precipitates are known to have a function as the sites for oxygen reduction reaction which results in local alkalization. Due to the local alkalization, Al-matrix around the precipitates dissolves, and trenches are formed around the precipitates. It is known that pits are readily initiated in the trenches, the corrosion behavior from trench formation to pit initiation is still unclear. In this research, in situ observations on the dissolution behavior around precipitates was performed using a micro-electrochemical system, and the effect of pH and precipitate compositions on pitting were determined. In this study, A1050-O was used as the specimens. The specimen surface was polished down to 0.25 µm with a diamond paste and cleaned ultrasonically in ethanol before electrochemical measurements were taken. Two types of the precipitates, AlxFe and AlxFe2Si, were observed in A1050-O. Anodic polarization curves were measured in 0.1 M NaCl (pH 6.0, naturally aerated). The polarization was started from -0.9 V vs. Ag/AgCl (3.33 M KCl), and the scanning rate was 23 mV / min. A stable pit was initiated at -0.4 V, and it was confirmed that the pit was generated at a precipitate. Using a micro-electrochemical system, potentiodynamic anodic polarization curves for a small area with each type of the precipitate were measured in 0.1 NaCl (pH 6.0, naturally aerated) at 298 K. The electrode area was ca. 5000 µm2. The surface appearance of the precipitate was observed during the anodic polarization using a water immersion objective lens. A deep trench was formed around the AlxFe2Si precipitate in the potential range from -0.86 to -0.65 V. Around the AlxFe precipitate, no trench was formed. It was found that the pit was initiated in the trench around the AlxFe2Si precipitates. In order to analyze the corrosion behavior of A1050-O, open circuit potentials were measured in 0.1 M NaCl (pH 6.0, naturally aerated) and 0.1 M NaCl-containing citrate buffer (pH 6.0, naturally aerated) solutions. Fig. 1a shows the time variation of the open circuit potentials of A1050-O in each electrolyte. In 0.1 M NaCl, the potential increased to ca. -0.65 V. On the other hand, in the 0.1 M NaCl-containing citrate buffer solution, the potentials decreased to ca. -0.83 V. Fig. 1b and 1c show the optical micrographs of the AlxFe2Si precipitate after the immersion. Even though a deep trench was formed around the precipitates in 0.1 M NaCl, no trench was formed around the precipitates in 0.1 M NaCl-containing citrate buffer solution. The depassivation pH of pure Al (99.999%) was measured to elucidate the corrosion behavior from trench formation to pit initiation. Electrolyte was 0.1 M NaCl (naturally aerated), and the pH was adjusted with NaOH solution. The open circuit potential decreased with pH from pH 6 to 13, but the open circuit potential increased at pH 14. After the measurement, the specimen surface was corroded. It is suggested that the surface film on pure Al was stable from pH 6 to 13, and that the open circuit potential decreased due to the pH dependence of the electrode potential of hydrogen generation reaction. At pH 14, the surface film dissolved, and bare Al was exposed to the solution. Because oxygen reduction reaction readily occurs on the bare Al, predominant cathodic reaction changed from hydrogen generation to oxygen reduction. The increase in the open circuit potential in 0.1 M NaCl (Fig. 1a) indicates the exposure of bare Al-matrix. In this research, it was found that the citrate buffer prevent the local alkalization and the trench formation around the precipitates. Figure 1

Effect of Molybdenum on Pit Initiation at Manganese Sulfide Inclusions in Stainless Steel

Stainless steels sometimes suffer from pitting corrosion in chloride environments. The pit initiation sites for commercial stainless steels have been attributed to sulfide inclusions such as MnS. It is well known that addition of Mo enhances the pitting corrosion resistance of stainless steels, and the role of Mo has been studied extensively. However, it is still debated how Mo inhibits pitting corrosion of stainless steels. In this study, a microelectrochemical technique was applied to clarify the role of Mo addition in the inhibition of pit initiation at MnS inclusions in stainless steels. Two re-sulfurized stainless steels were prepared by vacuum induction melting (50 kg ingot): one was Mo-free (18Cr-10Ni-0.8Mn-0.03S), and the other was Mo-added (18Cr-10Ni-2.3Mo-0.8Mn-0.03S). The ingots were hot-rolled. In these steels, S was added to form MnS inclusions. To reduce the delta-ferrite phase in the steels, they were heat-treated at 1273 K for 96 h, followed by hot-rolling at 1473 K from 20 mm down to about 12 mm in thickness. Additional heat-treatment was conducted at 1273 K for 96 h and at 1323 K at 0.5 h, and the steels were finally water-quenched. After heat-treatment, the steels were cut into specimens with dimensions of 15 mm × 25 mm × 5 mm. The specimens were abraded with a series of SiC papers up to 1500 grit and then were polished by a diamond paste down to 1 µm. Potentiodynamic polarization curves were measured in naturally aerated 0.1 M NaCl (pH 5.5) at 298 K. The electrode area was ca. 100 µm × 100 µm. All the potentials reported in this study refer to the Ag/AgCl (3.33 M KCl) electrode (0.206 V vs. standard hydrogen electrode at 298 K). The potential scan rate was 3.8 × 10–4 V/s (23 mV/min). To analyze the effect of Mo on the pit initiation at the MnS inclusion, the anodic polarization curves were measured in 0.1 M NaCl. Figure 1a shows the polarization curves of a small area with the MnS inclusions in Mo-free and Mo-added specimens. Figures 1b and 1c show the SEM images of the inclusions in Mo-free and Mo-added specimens after polarization. The experiments were started at –0.2 V. The gradual increases in the current densities of both specimens above ca. 0.1 V are due to the anodic dissolution of the MnS inclusions. The large increase in the current density of the Mo-free specimen at 0.4 V was attributed to the stable pit initiation at the inclusion. As shown in Fig. 1b, a pit was initiated at the MnS/steel matrix boundary. On the other hand, the current density of the Mo-added specimen decreased to the level of the passive state of stainless steel above ca. 0.45 V. In Fig. 1c, no pit was observed in the Mo-added specimen even though the bare steel surface was exposed by the dissolution of the MnS inclusion. Mo species were estimated to be released from the exposed bare steel surface. This suggests that Mo species inhibited pit initiation at the MnS/steel boundary. Figure 1

Electrochemical Study of Steel Corrosion in Saturated Calcium Hydroxide Solution with Chloride Ions and Sulfate Ions

The corrosion of Q235 carbon steel in the saturated calcium hydroxide solution containing chloride ions and sulfate ions are studied using electrochemical methods and wire beam electrode (WBE) sensor. The cyclic potentiodynamic polarization measurements showed that localized corrosion was mainly induced by the adsorption of chloride ions on the passive film. When the passive film is intact, sulfate ions are not corrosive to the passive film and it can inhibit the pitting initiation caused by the chloride ions. However, the WBE test results indicate that once a stable pit has already formed, sulfate ions cannot mitigate the pitting corrosion, and it can further promote the propagation of the major anodic area. Through the electrochemical impedance spectroscopy measurements, it can be found that the addition of sulfate ions in the solution containing chloride ions will not result in the rise of the general corrosion rate.

Evaluation of stable and safe pit wall slope design for underground extraction of mineral reserves under Kamagan open pit mine bottom

Evaluation of stable and safe pit wall slope design for underground extraction of mineral reserves under Kamagan open pit mine bottom

Hot Corrosion Behavior of Inconel 625 Superalloy in Eutectic Molten Nitrate Salts

Corrosion resistance of Inconel 625 Ni-based superalloy was studied in a molten nitrate salt consisting of 40 KNO3–60 NaNO3 (wt%) at 500 and 600 °C. Open-circuit potential, potentiodynamic polarization, electrochemical impedance spectroscopy and gravimetric tests were used to evaluate the degradation mechanism and corrosion behavior of the alloy. Surface morphology and chemical analysis of corrosion products were characterized by means of scanning electron microscopy and energy-dispersive X-ray spectrometry. The weight-loss curves showed that with the increase in temperature, the oxidation rate and mass gain increased; the relationship between the mass gain and time was close to the parabolic oxidation law. The electrochemical corrosion results confirmed that during the exposure of Inconel 625 alloy to the molten salts, nickel dissolves as a result of non-protective NiO layer formed. The formation of a non-protective oxide layer with low barrier property was responsible for observing the weak corrosion resistance of the alloy at high temperatures (500 and 600 °C). Cyclic polarization tests showed a positive hysteresis confirming the nucleation and growth of stable pits on the surface of Inconel 625 at high anodic overpotentials. Sodium nitrite acts as an efficient pitting inhibitor for this case. In this way, the sodium nitrite with the concentration of 0.1 molal was found to have an optimum inhibition effect on pit nucleation at 600 °C.

Improving Pitting Corrosion Resistance at Inclusions and Ductility of a Martensitic Medium-Carbon Steel: Effectiveness of Short-Time Tempering

The optimal tempering conditions for a martensitic medium-carbon steel (0.47 mass% C) were investigated in terms of balancing ductility and pitting corrosion resistance. By tempering the as-quenched martensite, its Vickers hardness drastically decreased within 0.1 h, suggesting that ductility was sufficiently recovered by short-time tempering. Based on the results of micro-scale polarization in boric-borate buffer solutions with NaCl (pH 8.0), stable pits were initiated at non-metallic inclusions in the specimens tempered for at least 1 h; however, no stable pit was generated on the 0.1 h tempered and as-quenched specimens. Short-time tempering of martensite was suggested to be a feasible approach to striking an optimal balance between facilitating pitting corrosion resistance and achieving the desired mechanical properties of martensitic carbon steels.

Modeling of the Critical Pitting Temperature between the Laboratory-Scale Specimen and the Large-Scale Specimen

Critical pitting temperature (CPT) of 316L stainless steel with different specimen areas in 3.5 wt% NaCl solution was investigated by the potentiostatic method. The results showed that the CPT exhibited a decreasing tendency with the increase in specimen areas, which was attributed to the higher probability for large-scale specimens to attain a large pit depth than small specimens. Therefore, stable pitting is easier to form, resulting in a lower CPT. Subsequently, a Gumbel extreme function was applied to establish a model correlating CPT with the specimen area. The CPT was linear with the double logarithm of the specimen area and could be accurately predicted for large-scale specimens.

Localized Corrosion: Passive Film Breakdown vs Pit Growth Stability: Part II. A Model for Critical Pitting Temperature

Potentiostatic temperature-scanned experiments were performed on 316L stainless steel and revealed that both the measured values of critical pitting temperature (CPT) and their distribution ranges increased with the decrease of the aggressiveness of exposure conditions. Previous models for CPT cannot explain these observations, so a model for CPT is proposed based on criteria for pit growth stability with added effects of the passive film breakdown. The criterion for pit stability is assumed to be the maintenance of a critical pit solution concentration at the pit surface, which requires the maximum pit dissolution current density to be equal to or greater than the diffusion current density associated with this critical concentration. The resulting model can explain many observations of nature of the CPT. The CPT is shown not to be a single-valued critical value; instead, it is a statistically distributed value based on the detailed behavior of many pitting events. For potentiostatic measurements, the measured CPT is just the temperature at which the first stable pit forms. The commonly recognized and tightly distributed CPT is determined from the perspective of pit growth stability and it represents the temperature with the highest probability to form the first stable pit.

Relationships between Pitting Corrosion Potentials and MnS Dissolution of 5–18 Mass% Cr Steels

With the increase in the Cr content in the steels, the Cr and Fe concentrations in the inclusions increased and decreased, respectively. The MnS inclusions were found to dissolve in 0.1 M Na2SO4 around 0 V vs. Ag/AgCl, (3.33 M KCl). Even though the dissolution potential was independent of Cr concentration, the dissolution morphology was remarkable in that the inclusions in Fe-5Cr dissolved and disappeared. For Fe-12Cr, the Cr-enriched portions remained. In the case of Fe-18Cr, Cr-oxides formed on the inclusion surfaces. The micro-scale polarization was performed in 0.1 M NaCl. At approximately 0.05 V, a large pit was generated at the inclusion of Fe-5Cr, and the inclusion disappeared after polarization. For Fe-12Cr, a stable pit was initiated at 0.13 V, and most of the inclusion remained. No stable pit occurred on Fe-18Cr; however, a meta-stable pitting event was observed in the dissolution potential region of the MnS. Cr and O were detected on the entire surface on the inclusion after polarization. It was suggested that the pitting corrosion resistance at the MnS inclusions of Fe-Cr alloys was related to the inclusion dissolution morphology originating from the difference in the Cr concentration of the inclusions.

A Methodology for Fabrication of Highly Pitting Corrosion-Resistant Type 304 Stainless Steel by Plasma Carburizing and Post-Pickling Treatment

Low temperature plasma carburizing was performed to improve the pitting corrosion resistance of Type 304 stainless steel. The optimal composition of the plasma gas and post-treatment were then examined. Carburizing was carried out for 18 ks at 470°C in H2-CH4, Ar-H2-CH4 or Ar-CH4 plasma, and carburized layers ca. 9 μm in thickness with an average ca. 1.3 mass% carbon concentration were obtained. However, it was found that Cr23C6 was precipitated in the carburized layer and intergranular corrosion occurred under anodic polarization in 0.1 M NaCl. The precipitation behavior of Cr23C6 differed according to the plasma gas composition: H2 gas was found to suppress the size of Cr23C6. No Cr23C6 was precipitated in the inner part of the carburized layer formed in the H2-CH4 plasma, and no stable pit was initiated on the inner parts of the carburized layer under anodic polarization in 0.1 M NaCl. Pickling treatment using HF-HNO3 was applied to remove the Cr-depleted zones in the outer part of the plasma carburized layer. The Type 304 stainless steel after the plasma carburizing in the H2-CH4 atmosphere and subsequent the HF-HNO3 pickling exhibited no pitting corrosion under anodic polarization even in 3 M NaCl at 25°C.

Localized Liquation and Resultant Pitting Corrosion Behavior of Welding Coarse-Grained Heat-Affected Zone in Niobium-Stabilized Austenitic Stainless Steel

Effect of localized liquation on pitting corrosion behavior of Nb-stabilized austenitic stainless steels was investigated by potentiodynamic and potentiostatic polarization tests on the welding coarse-grained heat-affected zone (CGHAZ) samples simulated using finite-element software SYSWELD and Gleeble simulator. Results indicated that localized liquation gave rise to Nb-rich liquation bands along grain boundary, inhibiting metastable pit initiation and growth, but resulted in liquation cracks that facilitated the occurrence of stable pitting. Moreover, the large scatter in pitting potential and passive current density observed in CGHAZ samples was attributed to the heterogeneities in the distribution, geometry and dimensions of liquation cracks.

Computational Evaluation of the Influence of Various Uniaxial Load Levels on Pit Growth of Stainless Steel under Mechanoelectrochemical Interactions

Numerical modelling of the effects of different applied load levels on the growth of metastable single pit in stainless steel is the subject of the present work. A cellular automata model integrated with finite element analysis is used, taking into account both the electrochemical reactions during metastable pitting growth process and the real-time influence of local stress/strain distribution at pit surface on anodic dissolution. Specifically, in the current work the impacts of uniaxial load levels on pitting corrosion for various conditions of passive layer degradation extents and hydrogen ions diffusion coefficients are quantitatively assessed in terms of current transient. For a general condition with uniaxial applied load level increases from 0 MPa to 250 MPa, the ascending trend of current transient and current density transient is observed due to the increase of plastic strain accumulation at pit bottom, revealing the enhancement of metastable pit growth rate. Meanwhile, the critical magnitude of load level enabling metastable pitting to alter into the development of stable pit for each condition is discussed and predicted.

On the AA2198-T851 alloy microstructure and its correlation with localized corrosion behaviour

The corrosion behaviour of AA2198-T851 alloy has been investigated using a combination of immersion tests and different microscopy techniques. Results showed that severe localized corrosion initiated within 1h of corrosion immersion test in 0.01molL−1 NaCl solution. The corrosion mechanism in this alloy is intragranular and it propagates crystallographically. The propagation occurs through grain orientation dependent bands and it can be likened to the movement of dislocation through slip planes. Corrosion rings around stable pits result from pH variations. Secondary pits form around primary pits within corrosion rings and are preceded by hydrogen evolution.

Microstructure-sensitive modeling of pitting corrosion: Effect of the crystallographic orientation

This work presents an innovative framework for the simulation of stable pit growth that accounts for the material microstructure in the computational model to investigate its effect on pit growth behavior. While many microstructural attributes can affect corrosion kinetics, the current work focuses on the effect of the crystallographic plane orientation at the corrosion front by utilizing variations in polarization behavior with orientation in a single phase material. The current modeling results show that crystallographic orientation significantly affects local pit growth rates (even with corrosion potential variations as little as 5–10% across orientations) and the resultant tortuous shapes are similar to pits observed in experimental studies.

Differences in pitting growth kinetics between Zr60Ni25Al15 and Zr60Cu25Al15 metallic glasses exposed to a 0.6 M NaCl aqueous solution

By the artificial pit electrode approach, growth behaviours of stable pits for melt-spun Zr60Ni25Al15 and Zr60Cu25Al15 metallic glasses (MGs) exposed to a 0.6M NaCl solution are investigated. By characterising the current transient events and growth kinetics parameters, the effect of selective dissolution caused by the late transition metals on the pitting growth kinetics is revealed. The pit solution acidity is estimated with local acidification model. It is shown that the stable pitting growth for Zr60Cu25Al15 MG is considerably more favourable compared to Zr60Ni25Al15, because of its more rapid pit growth rate and more aggressive pit solution.

Micro-Electrochemical in Situ Observation of Pit Initiation at Precipitates in AA5182 Al-Mg Alloy in 0.1 M NaCl

Al-Mg alloys such as AA5182 offer the advantages of superior mechanical properties and corrosion resistance. They are widely used in transportation industry and architectural applications. Understanding their precise corrosion mechanisms is required to assess the durability and ensure the safety. It is known that the precipitates (e.g. Fe3Al, FeAl, Al2Cu, and so on) in aluminum alloys act as initiation sites for pitting. Oxygen reduction reaction proceeds on the precipitates preferentially, which causes the alkalization. Trenches (micro-crevices) are formed at the boundary between the precipitates and the aluminum matrix because aluminum readily dissolves in alkaline solutions. Pits are thought to be initiated in the trenches 1, but the successive processes from the alkalization to pit initiation is not fully understood. In this study, in situobservation of pit initiation at the precipitates in Al-Mg alloy was performed in 0.1 M NaCl. A commercial AA5182-O sheet was used as specimens, and the surfaces were polished down to 0.25 μm with a diamond paste. The morphology and composition of precipitates in the alloy were analyzed by a field emission scanning electron microscope equipped with an X-ray energy-dispersive spectrometer, and it was concluded that Al-Fe-Mn, Al-Fe-Mn-Si, and Mg2Si intermetallics existed as the precipitates in the alloy. Macro-scale potentiodynamic polarization was carried out to ascertain the pitting potential of AA5182-O in naturally aerated 0.1 M NaCl. The size of the electrode area was 10 mm × 10 mm. It was confirmed that pitting potentials were located around -0.6 V (vs. Ag/AgCl, 3.33 M KCl). In order to make clear the pit initiation site, a micro-electrochemical in situ observation system similar to that developed by Chiba et al. was used. 2 Figure 1a shows the micro-scale anodic polarization curves for a small area with one or several precipitates of Al-Fe-Mn, Al-Fe-Mn-Si, or Mg2Si in naturally aerated 0.1 M NaCl solution at 298 K. The size of the electrode area was around 50 µm × 50 µm. In the case of the area with Al-Fe-Mn-Si or Mg2Si, crevice corrosion was generated at the periphery of the electrode area, but no pit was initiated at the precipitates. On the other hand, a stable pit was initiation at Al-Fe-Mn. Figure 1b shows the surface appearances of the precipitates after the polarization shown in Fig. 1a. The trench formation was clearly observed around Al-Fe-Mn and Al-Fe-Mn-Si precipitates. During the anodic polarization, the video was recorded, and digital image processing was applied to estimate the change in the area of the corroded region (trench) with electrode potential. Figure 1c shows the area ratio of the trench (corroded region) to the precipitate. The corrosion damage around Al-Fe-Mn was severe compared to that around Al-Fe-Mn-Si. The reason for this is related to the difference in the activity of the oxygen reduction reaction on the precipitates. One possible explanation is that silicon in the precipitates suppresses the oxygen reduction rate and the alkalization around the precipitates. The order of pitting corrosion resistance is as follows: Mg2Si > Al-Fe-Mn-Si > Al-Fe-Mn. 1) J. O. Park, C. H. Paik, Y. H. Huang, and R. C. Alkire, J. Electrochem. Soc., 146(1999), 517. 2) A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 159 (2012), C341. Figure 1

Role of Cerium Ions for Improving Pitting Corrosion Resistance of Sulfide Inclusions in Stainless Steels

Stainless steels sometimes suffer from pitting corrosion in chloride environments. Sulfide inclusions such as MnS are known to be preferential sites for pit initiation, and the chemical composition of the inclusions has a significant influence on pitting corrosion behavior. 1–3 CeS inclusions have better pitting corrosion resistance than MnS inclusions.4 In our previous work, 5 microelectrochemical measurements and thermodynamic calculations were conducted to elucidate the electrochemical properties of CeS inclusions, and the relationship between those properties and pitting corrosion resistance was discussed. The Ce3+ ions released from CeS inclusions was thought to improve pitting corrosion resistance. In this study, we analyzed the role of Ce3+ ions for improving pitting corrosion resistance of sulfide inclusions in stainless steels. A type 304 re-sulfurized stainless steel (0.05%C, 0.39%Si, 1.51%Mn, 0.04%P, 0.02%S, 0.35%Cu, 8.3%Ni, 18.3%Cr, 0.21%Mo, 0.002%Al, 0.080%N, 0.002%O) was prepared by vacuum induction melting and then was hot-rolled. The specimens were heat-treated at 1373 K for 30 min and quenched in water. After heat-treatment, the specimens were polished with a diamond paste down to 1 µm. Potentiodynamic anodic polarization curves were measured in naturally aerated 3 M NaCl (pH 5.0) and 2.97 M NaCl-10 mM CeCl3 (pH 5.0) at 298 K. The electrode area was ca. 100 µm × 100 µm. All the potentials reported in this study refer to an Ag/AgCl (3.33 M KCl) electrode. The potential scan rate was 3.8 × 10–4 V/s (23 mV/min). A scanning electron microscope (SEM) and a focused ion beam system (FIB) were used to observe the electrode surfaces and the cross-sections of the MnS inclusions after polarization. Chiba et al. demonstrated that the trench formation at MnS/steel boundaries caused by the MnS dissolution products and Cl– ions act as a precursor of pits initiation. 6 To analyze the effect of Ce3+ on the trench formation at MnS/steel boundaries, the anodic polarization curves of a small area with the MnS inclusion were measured in 3 M NaCl and 2.97 M NaCl-10 mM CeCl3 (Fig. 1). The experiments were started at –0.2 V and stopped at the same potential. A stable pit occurred in the Ce3+-free solution, and no stable pit was initiated in the Ce3+-containing solution. After polarization, the electrode surfaces and the cross-sections of the MnS inclusions were observed. Figure 2 shows the SEM images of the MnS inclusion after polarization measured in Ce3+-free solution. The boundaries between MnS and steel matrix dissolved selectively, and the deep trenches are formed. On the other hand, in the case of Ce3+-containing solution (Fig. 3), little trench formation was observed. This indicates that Ce3+ ions inhibited the trench formation at the MnS/steel boundaries. References; 1. I. Muto, Y. Izumiyama, and N. Hara, J. Electrochem. Soc., 154, C439 (2007). 2. I. Muto, S. Kurokawa, and N. Hara, J. Electrochem. Soc., 156, C395 (2009). 3. N. Shimahashi, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 160, C262 (2013). 4. B. Baroux, in Corrosion Mechanisms in Theory and Practice (Third Edition), P. Marcus, Editor, p. 443, CRC Press, Boca Raton (2012). 5. M. Nishimoto, I. Muto, Y. Sugawara, and N. Hara, ECS Trans., 75 (52), 27 (2017). 6. A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 160, C511 (2013). Figure 1

FIB/SEM Study of Pitting and Intergranular Corrosion in an Al-Cu Alloy

Scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDX) was used to study the formation of stable pits on an aluminum-copper casting alloy in artificial seawater solution at 10°C. The polished specimens were immersed for various times and the development of corrosion around intermetallic particles was monitored. Focused ion beam (FIB)/SEM was used to determine the subsurface microstructure at selected local attack sites within the corroded area. The earliest stages of attack started with pit initiation at the surface of intermetallic particles resulting in dealloying. Cu redistribution occurred during corrosion over the entire surface and its concentration differed from one site to another. FIB imaging revealed that localized corrosion was propagated where continuous particles were buried beneath the surface. Dealloying of the particle and matrix/particle interfacial attack produced a sponge-like remnant and cracks at the interface, respectively. Propagating away from the initia...

Relationship between Microstructure and Corrosion Behavior of Martensitic High Nitrogen Stainless Steel 30Cr15Mo1N at Different Austenitizing Temperatures.

The relationship between microstructure and corrosion behavior of martensitic high nitrogen stainless steel 30Cr15Mo1N at different austenitizing temperatures was investigated by microscopy observation, electrochemical measurement, X-ray photoelectron spectroscopy analysis and immersion testing. The results indicated that finer Cr-rich M2N dispersed more homogeneously than coarse M23C6, and the fractions of M23C6 and M2N both decreased with increasing austenitizing temperature. The Cr-depleted zone around M23C6 was wider and its minimum Cr concentration was lower than M2N. The metastable pits initiated preferentially around coarse M23C6 which induced severer Cr-depletion, and the pit growth followed the power law. The increasing of austenitizing temperature induced fewer metastable pit initiation sites, more uniform element distribution and higher contents of Cr, Mo and N in the matrix. In addition, the passive film thickened and Cr2O3, Cr3+ and CrN enriched with increasing austenitizing temperature, which enhanced the stability of the passive film and repassivation ability of pits. Therefore, as austenitizing temperature increased, the metastable and stable pitting potentials increased and pit growth rate decreased, revealing less susceptible metastable pit initiation, larger repassivation tendency and higher corrosion resistance. The determining factor of pitting potentials could be divided into three stages: dissolution of M23C6 (below 1000 °C), dissolution of M2N (from 1000 to 1050 °C) and existence of a few undissolved precipitates and non-metallic inclusions (above 1050 °C).

Effect of Temperature on Initiation of Metastable Pits and Geometric Features of Stable Pits for 304 Stainless Steel

Effect of Temperature on Initiation of Metastable Pits and Geometric Features of Stable Pits for 304 Stainless Steel