Deposition Of Corrosion Products Research Articles

The role of acetic acid in FeCO3 scale deposition on CO2 corrosion of API X65 carbon steel under high temperatures

ABSTRACT The combined effects of high temperatures and different acetic acid (HAc) concentrations on corrosion and deposition of FeCO3 scales on API X65 carbon steel in CO2-saturated brines were investigated. Conditions with the absence and presence of 100 and 600 ppm of HAc were evaluated under high pressure at 90°C and 120°C. Electrochemical impedance spectroscopy, linear polarisation resistance, weight loss and characterisations through scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction were conducted. Conditions with the absence of acid presented a huge deposition of sweet corrosion products and low corrosion rates. The addition of 100 ppm of acid at 90°C did not increase significantly corrosion rates; however, it retarded the precipitation of FeCO3. Higher temperatures favoured iron carbonate nucleation even in the presence of HAc. The highest corrosion rates were observed for the environment with 600 ppm of HAc at 120°C.

Corrosion Evolution of High-Strength Aluminum Alloys in the Simulated Service Environment of Amphibious Aircraft in the Presence of Chloride and Bisulfite

The corrosion evolution of 2024-T351 and 7075-T651 aluminum alloys in the thin electrolyte layer (TEL) and wet–dry alternating cycle (WDAC) environment is studied in this work. The results show that in the TEL environment, the competitive effect between H+ that accelerates corrosion reactions and deposition of aluminum sulfate that impedes corrosion attacks exists during the corrosion exposure. The difference is that with increasing HSO3−, subsurface intergranular corrosion on 2024-T351 is promoted to form exfoliation corrosion eventually and the degree of exfoliation corrosion begins to decrease because the blocking effect of aluminum sulfate exceeds the expediting effect of H+. For 7075-T651, the corrosion area and the corrosion diameter decrease gradually, which is attributed to the HSO3−-enhanced deposition of corrosion products and their blocking effect. In the WDAC environment, the corrosion processes of 2024-T351 and 7075-T651 are the acidic dissolution of the matrix during the soaking phase. When the HSO3− concentration is high enough (0.1 M), the inhibiting effect of aluminum sulfate becomes the dominant factor.

Multi-physics analysis of the galvanic corrosion of Mg-steel couple under the influence of time-dependent anisotropic deposition film

Abstract The anisotropic deposit film formed during the galvanic corrosion can impede the mass transfer of the involved species, thereby affecting the electro-chemical behavior and the evolution of galvanic corrosion. The limitations of experimental studies in the spatial-temporal scales restrict a deeper understanding of the corrosion mechanism, which can be complemented by numerical simulation. A multi-physics coupled model is proposed in this work to systematically investigate the temporal and spatial evolution of galvanic corrosion of the Mg-steel couple with the growing anisotropic deposition layer. By utilizing the multi-physics field coupled technique, various coupled physical-chemical processes underlying the corrosion behavior are built into the model, including chemical reactions, ionic mass transfer in the bulk solution and the deposition layer, interfacial reaction, deposition of corrosion products as well as the morphological transitions caused by metal dissolution and deposition. In particular, the anisotropic deposit film is considered to be a porous layer with a porosity varying in time and space as the corrosion evolves. The predicted corrosion morphology by this model is better than the previous models. The coupled relationship between the electrochemical behavior (e.g., electrode reaction kinetics, current density, surface potential) and the physical processes (e.g., ionic transport, geometric evolution of metal surface and film interface) is revealed. The results indicate that a porous deposition layer with a denser inner layer and a loose outer layer is generated, leading to more significant inhibition of mass transfer in the inner layer than the outer layer. The anisotropism of the deposition layer results in a non-uniform conductivity distribution and a discontinuous current density distribution in the electrolyte. The current density on the electrode surface is inhibited by the deposition layer and the variation in the cathode/anode area ratio during the corrosion process. The competition between the transport process and the electrochemical reaction determines the spatial-temporal evolution of the ion concentration.

Influence of zinc injection on deposition of corrosion products on inner wall of heat transfer tube

The zinc injection process has been widely used in the primary coolant of PWR nuclear power plants. This study explored its effect in the water coolant of fusion reactors. The research uses the International Thermonuclear Experimental Reactor (ITER) water-cooled experimental simulation circuit, with Fe3O4 particles as corrosion products. The deposition thickness of Fe3O4 on the wall of the stainless steel tube at temperatures from 80 °C to 150 °C was measured by a non-destructive online magnetic thickness measurement. The experimental results show that zinc injection has a significant inhibitory effect on the deposition of corrosion products on the tube wall, especially at the elbow. After adjusting the pH to the optimal value of 9, adding zinc can continue to reduce the thickness of the deposition layer. The results of the microscopic analysis indicated that the Zn injection formed a new dense surface oxide film and inhibited the deposition of Fe3O4 particles on the tube wall. The research results indicate that the chemical process of zinc injection water can be applied to the operation of the water-cooled fusion reactor in future.

Galvanic Corrosion Performance of an Al–BN Abradable Seal Coating System in Chloride Solution

In this study, we investigated the galvanic corrosion performance of an Aluminum–Boron Nitride (Al–BN) abradable seal coating system (with a Ni5Al bond layer and a 0Cr17Ni4Cu4Nb substrate) in chloride solution by electrochemical methods. The results indicated a three-stage process occurred during the anodic dissolution of the coupled coating system, consisting of a spontaneous pitting stage I under charge transfer control with a decreasing rate, a corrosion developing stage II under mass transfer control with an increasing rate, and a final steady stage III. Precipitation of Al(OH)3 restricts the oxygen transport process to the cathode and induces localized acidification of the occluded pores of the Al–BN layer, which was the mechanism that could explain the changes of corrosion performance during the three immersion stages of Al–BN coating system. The study suggests that galvanic corrosion of the porous multi-layer Al–BN abradable coating system is mostly influenced by its corrosion product deposition.

Investigation of Metal Cation Effects on Corrosion Behavior of Aluminum Alloys

Introduction Due to excellent mechanical property and corrosion resistance, aluminum alloys are widely used in airplanes, automobiles, and kitchen utensils. The corrosion resistance of aluminum alloys relies on highly protective and reformative passive films. However, the passive films are destroyed in presence of chloride ions, and the corrosion rate of aluminum alloys depends on the concentration of chloride ions 1 - 3. A few studies have shown the corrosion behavior of aluminum alloys in freshwater; however, the corrosion rates of aluminum alloys were changed in freshwater which sampled different place 4. Because freshwater also contains some metal cations in dilute concentration. The authors expected that the metal cations changed the corrosion rate of aluminum alloy in freshwater. The authors have been investigated the corrosion of A3003 aluminum alloy in freshwater focused on the effects of metal cations5 - 8. The corrosion rate of A3003 aluminum alloy is changed with metal cations in model freshwater despites the concentration of chloride ions, dissolved oxygen and pH are the same. The researches also clarified that Zn2+ is the best metal cation to inhibit the freshwater corrosion of A3003 among the metal cations used6, 7. An Auger spectroscope was used to investigate the corrosion morphology and composition of corrosion products of A3003 formed in model freshwater with different Zn2+ concentration8. From the cross-sectional AES point analysis, it has been clarified that the corrosion products have multi-layer structure.However, it is not fully elucidated the role of the metal cations on the corrosion behavior of aluminum alloys. The purpose of this study is to investigate the effects of metal cations on corrosion behavior of aluminum alloys in aqueous environment. Experimental Sheets of aluminum alloys (A7075, A6061and A2024) were used as the specimens. The sheets were cut into 7 7 mm for immersion corrosion tests, and 10 10 mm for electrochemical measurements. The specimens were grounded with SiC abrasive paper. Before the tests, specimens were cleaned in highly purified water and in ethanol using ultrasonic bath.Model solutions of 1 molm-3 NaCl (NaS), 0.5 molm-3 MgCl2 (MgS), and 0.5 molm-3 ZnCl2 (ZnS) were used in this experiment. The pH of the model solutions was maintained at around neutral.Electrochemical tests were carried out in a three-electrode cell using a potentiostat. An 4 cm2 Pt plate and an Ag/AgCl-saturated KCl electrode were used as the counter and the reference electrodes. Before the measurements, the specimens were immersed in the solutions for 3.6 ks (1 h) at 298 K. Electrochemical impedance spectroscopy (EIS) tests were carried out in the frequency range from 1 mHz to 10 kHz, and the modulation amplitude of the applied sinusoidal wave was 10 mV.Specimens were immersed in the model freshwater for 2.59 Ms (30 d) and 0.6 Ms (7 d) at 25 ºC. The mass of a specimen was measured before and after the test. Surface analysis were carried out by scanning electron microscope (SEM) and X-ray photoelectron spectroscope (XPS). Results From the immersion tests, the mass of specimens increased after immersion in the solutions and corrosion behavior of aluminum alloys was changed with the metal cations present in the solutions. Independent of used alloy, SEM images clearly showed the deposited corrosion products on the surface. Uniform deposition of corrosion products was observed on the specimen immersed in ZnS as compared to the NaS and MgS. EDS results demonstrated the presence of zinc-related products on the specimen immersed in ZnS XPS results suggested that zinc ions were incorporated in the oxide films of aluminum alloy and increased the corrosion resistance property of the oxide films. EIS results showed the highest impedance in ZnS as compared to the other solutions. References 1) T. H. Nguyen and R. T. Foley, Journal of the Electrochemical Society, 127, 2563-2566 (1980).2) R. T. Foley, Corrosion, 42, 277-288 (1986).3) B. Zaid, D. Saidi, A. Benzaid, and S. Hadji, Corrosion Science, 50, 1841-1847 (2008).4) Editorial board of Japan Aluminium Association: "Aluminum Handbook. Tokyo", Japan Aluminium Association, (2007), p 73.5) K. Otani, M. Sakairi, T. Kikuchi, and A. Kaneko, Zairyo-to-Kankyo, 59, 330-331(2010).6) M. Sakairi, K. Otani, A. Kaneko, Y. Seki, and D. Nagasawa, Surface and Interface Analysis, 45, 1517-1521 (2013).7) K. Otani, M. Sakairi, R. Sasaki, A. Kaneko, Y. Seki and D. Nagasawa, Journal of Solid State Electrochemistry, 18, 325-332 (2014).8) K. Otani, Md. Saiful Islam, M. Sakairi, and A. Kaneko, Surface and Interface Analysis, 51, 1207-1213 (2019).

Multiphysics coupling model for the crack repairing process using electrochemical deposition

A multiphysics coupling model is proposed to analyze the transient deposition products on metal surfaces by considering mass transfer, electrochemical reactions, and precipitation reactions. We employ a level set method to capture the growing interface of corrosion product deposition. Comparing the simulations with experimental results shows that the proposed model can effectively describe the growth of filling depth. Under the early stages, we reveal the linear growth mechanism of the filling depth, which mainly contributes to the hydroxide concentration. Besides, the propagating nitrate ionic enrichment zone spreads as t. Detailed finite element simulations in conjunction with existing experimental data corroborate these relations. Furthermore, detailed studies are performed to elucidate the crack repairing. The factors affecting the crack repairing are investigated, including the applied voltage, deposition rate prediction constant, and electrolyte concentration. The presented results provide some important guidance and physical insights into the optimization for crack repairing process applications by electrochemical deposition.

Finite Element Method for Thin Film Corrosion Modelling: Where We Advanced and Where We would like to Advance?

Thin film corrosion is a serious issue in almost every sector. Thus, simulation of corrosion under thin electrolyte films has always been of high interest as experimental studies are often challenging. Thus far, progress has been made to model the effect of several important factors on thin film corrosion rates. Some of these parameters are electrolyte thickness, electrolyte composition, chemical reactions in the electrolyte, electrode size and change in electrode size, environmental parameters, and corrosion products deposition. However, these parameters are mainly drawn from different studies and have not been modelled concurrently in a single simulation study, making the thin film corrosion model far from being complete yet. Therefore, despite the many efforts so far, thin film corrosion modelers still strive to push the modelling edges further. This paper takes into account some of the highlighted recent advances in thin film corrosion modelling based on the mentioned parameters to provide a perspective on not only how far the field has come, but also how far it still is from a complete thin film corrosion model. Discussions have also been made on future needs and prospects to advance the thin film corrosion models further.

Corrosion on silvered-glass solar reflectors exposed to accelerated aging tests with polluting gases: A microscopic study

Silvered-glass solar reflectors consist of a reflective layer (RL), composed of silver and copper thin-films, back-protected by paint layers on a glass plate. The RL is prone to corrosion, especially in industrial atmospheres with polluting gases. After applying accelerated aging tests with gaseous pollutants (SO2, H2S, NO2), SG-R samples are studied by SEM and EDS, including FIB preparation. As expected the copper thin-film is firstly corroded, followed by the silver corrosion. Deposits of different corrosion products were found on the reflector’s surface, depending on the polluting gas used. The rear protective paint layers also play an active role.

Pore colmatation in case of liquid corrosion of concrete

Pore colmatation plays a positive role, since a decrease in the permeability of cement concrete due to the deposition of insoluble corrosion products in the pores leads to a slowdown in corrosion processes. To test a mathematical model of pore colmatation of cement concretes under liquid corrosion, a study of mass transfer processes occurring when samples from Portland cement are exposed to a liquid aggressive medium was conducted. The article presents equations that allow us to determine the rate of pore colmatation and the thickness of the layer of colmatant formed in the pores during liquid corrosion of cement concretes. Based on the results of experimental studies of liquid corrosion of cement stone, the mass transfer parameters were determined in the investigated system «cement concrete – 2 % MgCl2 solution». Based on the obtained data, the values of the colmatation rate and the thickness of the layer of colmatant of cement concrete were calculated during liquid corrosion in a 2 % MgCl2 solution. It has been established that over time, the rate of mass transfer processes occurring in concrete decreases due to the difficulty of penetration of aggressive medium deep into concrete due to the deposition of corrosion products in the pores.

Balancing the corrosion resistance and through-plane electrical conductivity of Cr coating via oxygen plasma treatment

Developing an electrically conductive and corrosion-resistant coating is essential for metal bipolar plates of polymer electrolyte membrane fuel cells (PEMFCs). Although enhanced corrosion resistance was seen for Cr coated stainless steel (Cr/SS) bipolar plates, they experience a quick decrease of through-plane electrical conductivity due to the formation of a porous and low-conductive corrosion product layer at the plate surface, thus leading to an increase in interfacial contact resistance (ICR). To tackle this issue, the multilayer Cr coatings were deposited using the magnetron sputtering with a remote inductively coupled oxygen plasma (O-ICP) in the present study. After the O-ICP treatment, a Cr oxide layer (CrO*) is formed on the specimen surface. The CrO*/Cr/SS has a remarkably lower stable corrosion rate (iss) than that of the native Cr oxides (CrOn/Cr/SS). Compared with CrOn/Cr/SS, the excellent performance of CrO*/Cr/SS is attributed to a denser and thicker surface layer of CrO* with Cr being oxidized to its highest valence state, Cr (VI). More importantly, the through-plane electrical conductivity of the specimens treated by the optimized O-ICP decreases much slowly than CrOn/Cr/SS and thus, the increament of ICR of CrO*/Cr/SS after the potentiostatic polarization test is considerably smaller than that of CrOn/Cr/SS, which is benefited from the reduced iss that mitigates the deposition of corrosion products and hinders further oxidation of Cr coating. Therefore, CrO*/Cr/SS proves to be a well balanced trade-off between corrosion resistance and through-plane electrical conductivity. The results of this study demonstrate that O-ICP treatment on a conductive metal coating is an effective strategy to improve the corrosion resistance and suppress the increase of ICR over the long-term polarization. The technique reported herein exhibits its promising potential application in preparing corrosion resistant and electrically conductive coatings on metal bipolar plates to be used in PEMFCs.

Understanding the effect of Li and flow velocity on corrosion deposition at 230 °C hydrogenated water

The effect of flow velocity, pH and lithium concentration on corrosion product deposition in accelerated flow regions was investigated in simulated nuclear reactor primary water using discs containing micro-orifices. The results also showed that a well adhered crystalline deposit formed between 2 and 2.5 ppm lithium, where the zeta potential of stainless-steel specimens is negative and the electrokinetic effects are expected to dominate. Conversely, a loosely adherent deposit driven by the deposition of particulates was formed for lower lithium concentrations.

SVET and ToF-SIMS Studies on the Galvanic Corrosion of β-phase/Aluminum Couple in Aqueous Solutions as a Function of pH

The galvanic corrosion of the β-phase (Al3Mg2)/Al couple has been studied using electrochemical methods, scanning vibrating electrode technique (SVET) and Time of flight-secondary ion spectroscopy (ToF-SIMS). Polarization tests reveal that Al3Mg2 is anodic relative to pure Al at pH ≤ 10. Visualization of the current density distribution over the Al3Mg2/Al galvanic couple by SVET reveals that at pH 2, an intense and stochastic anodic current density is evolved from Al3Mg2 while a heavy cathodic current flux is sustained on the surrounding Al matrix. At pH 6 and 10, localized both cathodic and anodic regions are seen on Al3Mg2 indicating its self-dissolution while the surrounding Al matrix remained relatively inert. However, at pH 13, a reversal of polarity occurs and Al3Mg2 becomes cathodic relative to Al. The severity of the galvanic coupling of Al3Mg2 to Al is in the order pH 13 > pH 2 > pH 6 > pH 10. ToF-SIMS analysis reveals that the dealloying of Al3Mg2 in the galvanic couple is by selective dissolution of Mg leading to the deposition of corrosion products dominated by Mg compounds in an acidic and neutral environment. In an alkaline environment, a relatively thin hydroxide rich film is formed.

Real-Time Corrosion Monitoring of Aluminum Alloy Using Scanning Kelvin Probe Force Microscopy

The evolution of the Volta potential of grade AA6063-T5 aluminum alloy microstructure with various intermetallic phases (IMP) during corrosion exposure to chloride-laden thin-film electrolytes at ambient temperature has been investigated, in situ and in real-time, using scanning Kelvin probe force microscopy (SKPFM), to gain insight into local corrosion processes. Trenching around micrometer-sized IMPs, de-alloying, oxidation, and the deposition of corrosion products were observed, which led to an inversion of the electrochemical nobility of the IMPs relative to the alloy matrix. Most of the IMPs showed cathodic Volta potentials (high nobility) relative to the matrix at 40%–80% relative humidity (RH), which became anodic (low nobility) when the surface was moistened with aqueous sodium chloride at elevated moisture (76%–87% RH) producing concentrations up to 5–6 M of chloride. The change of the Volta potential correlated with the onset and progress of corrosion and a nobility inversion of IMPs relative to the alloy matrix was observed. The corrosion behavior and the reasons for galvanic activities among microstructural constituents were elucidated and brought into a broader context to understanding localized corrosion.

Effect of Anisotropically-etched Silicon Electrode on Electrolytic Products Flow in Micro ECM

Stray corrosion and electrolytic products deposition in the machining gap are major influence factors seriously restricting the application of micro electrochemical machining (ECM). Currently, one technical bottleneck is preparing thin and durable insulating films on electrode sidewall to restrain the stray corrosion. In previous work, we proposed an anisotropically-etched silicon electrode for micro ECM, on which silicon dioxide and silicon nitride are deposited as insulating films. The heavily doped silicon electrode body has good electrical conductivity, and the silicon-based films have excellent insulating property. The anisotropically-etched silicon electrode on (1 0 0) crystal plane has an isosceles trapezoid cross section. To get better machining performance compared with cylindrical electrode, the silicon electrode is rapidly rotated on a spindle in micro ECM process. The periodically varying lateral gap has a special effect on the electrolyte flow characteristics. A simulation model of electrolyte flow and discrete phase distribution is established by Fluent software. Simulation results indicate that the normal flow rate of electrolyte with silicon electrode is 50 times higher than that with cylindrical electrode, which is benefit for removing solid electrolytic products smoothly. The maximum concentration and residual ratio of solid electrolytic products are decreased with the rotating speed increasing. When the rotating speed is higher than 1200 rpm, the residual ratio is reduced to 13%. ECM experimental results indicate that electrolytic products on the workpiece machined by a silicon electrode are much fewer than that by cylindrical electrode. Silicon electrodes with trapezoidal cross section have a distinct advantage for reducing electrolytic products deposition.

Evaluating Deposit Formation of Iron-Corrosion Products in High- and Low-Pressure Evaporative Circuits When Starting a Combined-Cycle Plant

One of the main causes of equipment damage at TPPs is corrosion of structural materials and deposit formation on the heat-transfer surfaces of the steam-water tract. This process occurs during stationary operation of the equipment and during start-ups and shutdowns of the power unit. In this paper, we assess the deposit formation of corrosion products in the evaporative circuits during the start-up of a combined cycle plant with the subsequent prediction of the number of starts after which it is necessary to carry out a chemical washing of the boiler-utilizer. The changes in the product concentration of iron corrosion in water and steam when starting a 450-MW power unit from a cold state are shown. The probability for the deposit formation of iron-corrosion products is estimated based on the experimental data obtained at start-up, with the subsequent calculation of the material balance for iron-corrosion products for low- and high-pressure circuits. A comparative analysis of the probability for deposit formation of iron-corrosion products in the high- and low-pressure circuits during the start-up period and at nominal operating parameters is performed. As a result of studies, it was found that the iron concentration in the feed and boiler water and saturated steam of the high- and low-pressure circuit significantly exceeds the normalized values by the time when the boiler-utilizer reaches its operating parameters. The specific amount of iron-corrosion products that can be deposited on heat-transfer surfaces in one start for the low- and high-pressure circulation circuit is determined. To reduce the probability for deposits of corrosion products during start-up, it is proposed to use feed water with a lower iron concentration or to increase the purge of the boiler-utilizer. Recommendations on the shutdown of the boiler-utilizer for washing when the maximal specific deposit amount of 200 g/m2 is reached taking into account the number of permissible starts for the low-pressure circuit are given.

CRUD deposition in accelerated high-temperature water: Investigation on the effect of substrate material and water chemistry

Deposition of corrosion products (CRUD) on micro-orifice specimens under accelerated flow conditions was investigated in high-temperature water on several substrate materials (SS 304L, Titanium, Alloy 690, Zirlo and magnesia stabilised Zirconia) and water chemistries (0–2 ppm of Li, 0–2.5 ppm of H2). Regardless of the substrate material, no significant differences in radial CRUD build-up were observed in high purity water (0 ppm of Li and either 0 or 2.5 ppm of H2). Conversely, in lithiated water (2 ppm of lithium and 2.5 ppm of H2), the CRUD surface build-up was dependent on the oxide electrical conductivity, which in turn depends on the substrate material. In fact, no build-up occurred on Zirconia and only a minor deposition on Zirlo. The deposition mechanism was ascribed to be dominated by particulate deposition at neutral pH where magnetite solubility is the highest. Conversely, crystalline build-up induced by electrokinetic activity played an important role at alkaline pH where oxide solubility was the lowest.

SECM and EIS Studies of Galvanic Corrosion of Copper/Iron Connected to Automotive Parts in Seawater

The electrochemical behavior of galvanic coupling between copper and iron was investigated in natural seawater by SECM and EIS. The accumulation of metal complexes on the sample prevents the steep decrease in the resistance of film (Rf) and charge transfer (Rct). It was shown that the film resistance, Rf of sample is decreased from 1.37 × 104 Ω cm2 at 1 day to 7.51 × 103 Ω cm2 at 15 days. Similarly, there is a decrease in Rct from 2.95 × 104 Ω cm2 at 1 day to 1.57 × 104 Ω cm2 at 15 days with increase in test time. On the other hand, the double layer capacitance is increased from 157.4 µF at 1day to 977.8 µF at 15 days. Similarly, the film capacitance is also increased from 40.4 µF at 1 day to 123.7 µF at 15 days. The Fe2+ ions at + 0.60 V Cu+ ions + 0.34 V are detected. The tip current at iron is 14.8 nA at 1 day (light pink) and reached 6.0 nA at 15 days (greenish yellow). This indicates that the deposition of metal complexes at the sample slows down the iron oxidation further. After corrosion testing, the enrichment of corrosion products of Cu and Fe at the sample was analyzed by SEM/EDX. FIB-TEM analysis confirms the rust layer containing Cu and Fe in the rust. The corrosion protection performance has been enhanced by nanoscale corrosion products deposition on the sample.

Multi-Physics Coupling Analysis on the Time-Dependent Localized Corrosion Behavior of Carbon Steel in CO2-H2O Environment

Localized corrosion of carbon steel in CO2-H2O environment is a long-standing challenge faced by the oil and gas industry, because of its unfeasible detection and high propagation rate. Numerical modelling can overcome the limitations of the spatial and temporal scales in the experimental studies, thus becoming a valuable complement. A multi-physics coupling model is established to investigate the evolution of localized corrosion of carbon steel in CO2 aqueous environment. The complex interactions among the kinetics of electrode reactions, multicomponent reactions, mass transfer and the deposition of corrosion products are coupled into the model, achieving a comprehensive and physically realistic description of the actual corrosion process. The arbitrary Lagrangian-Eulerian method is implemented to track the moving metal/solution interface. Special emphasis is put on the coupling mechanism among the underlying processes at different time and length scales. This study characterizes quantitatively the time-dependent corrosion behavior, including the distributions of potential and species concentration within the corroding pit, corrosion current density and pit morphology. The inherent relationship between the corrosion behavior and the local corrosive environment within the pit is revealed. The results indicate that the competition between the chemical effect and electrical effect determines the trend and distribution of corrosion current density. The pit shape and cathode/anode area ratio have a great influence on the corrosion behavior due to the coupled role of local solution chemistry and electrical field.

(Invited) The Complementarity of Cr and Mo in Reinforcing Oxide Protectiveness on Ni-Cr-Mo Alloys.

Ni-Cr-Mo alloys are extremely corrosion resistant as a result of the passive oxide film that spontaneously develops on the alloy surface upon exposure to the environment. We report on investigations of the influence of the oxide film composition on the susceptibility of the alloy to crevice corrosion, the rate, morphology and extent of crevice corrosion damage, and the tendency to stifle or repassivate the crevice. Chromium oxides are well known to provide a strong barrier to corrosion in solutions of moderate pH, over a wide range of oxidizing conditions. The corrosion susceptibility, however, can be decreased by alloying additions of Mo, which permit the deposition of Mo-rich corrosion products in response to the failure or partial failure of the Cr oxide barrier in acidic solution or crevice corrosion -a kind of “back-up plan” for the workhorse Cr. Mo can also help to defend against transpassive dissolution of Ni-based alloys, although this influence seems to be a transient effect that is dependent on the local pH. Here we report the results of galvanostatically driven crevice corrosion experiments, online analysis of dissolved corrosion products by atomic emission spectroelectrochemistry (AESEC) during transpassive polarization, and immersion testing in strong acid solution.