C. F. Dong

Stress corrosion cracking of 2205 duplex stainless steel in H2S-CO2 environment

Stress corrosion cracking (SCC) behavior of 2205 duplex stainless steel (DSS) in H2S–CO2 environment was investigated by electrochemical measurements, slow strain rate test (SSRT), and scanning electron microscopy (SEM) characterization. Results demonstrated that the passive current density of steel increases with the decrease of solution pH and the presence of CO2. When solutions pH was 2.7, the steel SCC in the absence and presence of CO2 is expected to be a hydrogen-based process, i.e., hydrogen-induced cracking (HIC) dominates the SCC of the steel. The presence of CO2 in solution does not affect the fracture mechanism. However, the SCC susceptibility is enhanced when the solution is saturated simultaneously with H2S and CO2. With elevation of solution pH to 4.5, the hydrogen evolution is inhibited, and dissolution is involved in cracking process. Even in the presence of CO2, the additional cathodic reduction of H2CO3 would enhance the anodic reaction rate. Therefore, in addition to the hydrogen effect, anodic dissolution plays an important role in SCC of duplex stainless steel at solution pH of 4.5.

In Situ Characterization of Pitting Corrosion of Stainless Steel by a Scanning Electrochemical Microscopy

In this work, a scanning electrochemical microscopy (SECM) was used to characterize in situ the metastable and stable pitting processes occurring on a stainless steel in the chloride solution. It was found that metastable pitting would occur on the steel that was at corrosion potential and passive potential. The positive shift of potential would enhance the metastable pitting current. On application of a potential exceeding pitting potential, the pit became stabilized and maintained a continuous growth. The SECM is capable of detecting the microdissolution event and provides a “visual” observation of the pitting processes.

Electrochemical Behavior of 304 Stainless Steel in Marine Atmosphere and Its Simulated Solution

The electrochemical behavior of field-exposed 304 stainless steel in a marine atmospheric environment and its simulated solutions were studied by corrosion rates test, cyclic anodic polarization (CAP), electrochemical impedance spectroscopy (EIS), and scanning Kelvin probe (SKP). The morphology, composition, and structure of the rust were analyzed by different methods: SEM, EDS, and Raman spectroscopy. The electrochemical experiments showed that the exposure samples in the marine atmosphere-simulated solution have a characteristic of lower pitting potential, and the stability of passive film became weak with exposure time extension. The corrosion products of 304 stainless steel in Xisha marine atmosphere are mainly the β-FeOOH, γ-Fe2O3, and Fe3O4. The Kelvin potential of the samples’ surface declined after 12 months exposure and tends to be uneven.

Study on the Passive Film Formed on 2205 Stainless Steel in Acetic Acid by AAS and XPS

The properties of the passive film formed on 2205 stainless steel in acetic acid at high temperature that contained chloride ions were studied by atomic absorption spectrometry (AAS), X-ray photoelectron spectroscopy (XPS), and electrochemical polarization measurements. AAS results show that molybdenum is enriched on the surface as the passive film is dissolved. This enrichment decreases the corrosion resistance because it hinders chloride adsorption and Fe ion dissolution, and acts as a local pH buffer because it consumes protons. The dissolution ratio of Fe/Cr is approximately 10 during the active dissolution of the passive film. XPS results indicate that when the potential is in the passivation region, Cr comprises about 50% of the metal cations in the near-surface region of the passive film and is the main metal constituent in this region. When the polarization potential is much greater than the transpas-sivation potential, the Mo content accounts for approximately 45% of the metal cations in the near-surface region; Fe and Ni have no obvious influence on the formation, dissolution, or puncture of the passive film.

Atmospheric corrosion behaviour of pure Al 1060 in tropical marine environment

Abstract In this study, atmospheric corrosion behaviour of pure Al 1060 after 1–48 months of exposure in a tropical marine environment was studied. The corrosion kinetics was evaluated by weight loss measurement and pit depth calculation. The rate controlling steps as well as the effect of the native oxide film and the corrosion product layer on the corrosion process were analysed via electrochemical impendence spectroscopy. The results showed that synergetic effect of the deposition rate of chloride ions and time of wetness resulted in an abnormal increase in weight loss and an obvious fluctuation in corrosion rate. During the initial exposure period, corrosion was controlled by charge transfer process attributed to the limited corrosion area. With the increasing exposure duration, the rate controlling step was changed to diffusion due to the barrier effect of the corrosion products. Simultaneously, initiation of the new corrosion areas and the growth of the stable pits dominated the corrosion process during this period, resulting in a lower charge transfer resistance, a larger pit depth and a bigger corrosion area.

Degradation of austenite stainless steel by atmospheric exposure in tropical marine environment

Abstract The atmospheric corrosion kinetics of AISI 304 and AISI 316 stainless steel in tropical marine environments is observed by weight loss. The corrosion products are identified by laser Raman spectroscopy, and surface morphology is observed by SEM. The results show that the corrosion rates tend to decrease as the exposure time increases. The mass loss obeys the empirical kinetics equation. The value of n can be used to predict the corrosion resistance of stainless steel, indirectly. The rust layer composition of the two stainless steels is different, and the percentage of Fe3O4 and β-FeOOH are also different.

Corrosion behavior of aluminum alloys in Na2SO4 solution using the scanning electrochemical microscopy technique

Abstract The corrosion behavior of aluminum alloys 1060 and 2A12 in a 10 mM Na2SO4+5 mM KI solution was investigated by scanning electrochemical microscopy (SECM) and scanning electron microscopy (SEM). The potential topography and corrosion morphology results show that the potential of the sample surface over the same area changes with the increase of immersion time. The corrosion area becomes large, and the potential becomes more negative. The corrosion potential of the 2A12 alloy surface is lower than that of 1060 aluminum, and 2A12 alloy becomes easily corrosive. This is the reason that preferential dissolution in the boundary region of some intermetallic particles (IMPs) occurs and different dissolution behaviors are associated with different types of IMPs because of different potentials.

Simulating Intergranular Stress Corrosion Cracking in AZ31 Using Three-Dimensional Cohesive Elements for Grain Structure

AbstractIn this study, a grain boundary model with three-dimensional (3D) cohesive elements for analyzing the intergranular stress corrosion cracking (IGSCC) on the crystal level in polycrystalline materials is presented. The objectives are to characterize the grain boundary microstructure and the fracture mechanism of IGSCC in AZ31 Mg alloy. In order to investigate the development of the microcrack and its effects on macrocrack evolution, a novel model of IGSCC propagation has been developed, in which the 3D Voronoi tessellations geometry is employed to model polycrystalline grain structures. And the 3D cohesive elements with zero constitutive thickness are directly inserted on the faces of two adjacent grains. The effect of the embrittlement due to the presence of hydrogen has also been included in the cohesive model. To validate the model, an IGSCC process of AZ31 Mg alloy in NaCl solution has been simulated, with the influence of hydrogen concentration being taken into account. It is found that damage develops at the triple lines between the grains and the combinations of grains can lead to high stresses at the grains boundary, especially those that are normal to the direction of the applied strain. In this paper, the effects of damage due to hydrogen and the grain sizes in microstructure are considered. The simulation results have a good consistency with the experimental phenomenon.

Pitting corrosion behaviour of AZ31 magnesium in tropical marine atmosphere

Abstract Corrosion behaviour of AZ31 magnesium during the initial six exposure periods in a tropical marine atmosphere is investigated. The results reveal that corrosion process of magnesium is dominated by pitting corrosion which consists of initiation of new pits, propagation of small scale pits and coalescence of neighbouring pits. There exists a critical depth above which the pits cease to grow down, resulting in the fluctuation of the mean pit depth. Different exposure conditions are found to be crucial for the different pit characters. Pits on the skyward surface are in deep-hole shape, while the groundward surface is covered with shallow dish pits.

Cellular automaton model for simulation of metastable pitting

Abstract As a new simulation method, cellular automata (CA) approach had been used in a number of areas, including electrochemistry and corrosion science research. In this work, a probability CA model was developed to simulate the growth of metastable pits. The simulation imaged the morphology of the pit and calculated the time dependences of pitting current and pitting current density. Results demonstrated that the CA approach is capable of simulating corrosion pit in terms of the pit morphology and pit growth kinetics. During growth of a hemispherical metastable pit controlled by diffusion, the time dependence of pitting current follows I = f(t2) law. Furthermore, it is determined that, for the simulation results to be consistent with the experimental results, an optimal range for variable d should be 0·1–0·25.