We study a model of corrosion and passivation of a metalic surface in contact with a solution using scaling arguments and simulation. The passive layer is porous so that the metal surface is in contact with the solution. The volume excess of the products may suppress the access of the solution to the metal surface, but it is then restored by a diffusion mechanism. A metalic site in contact with the solution or with the porous layer can be passivated with rate p and volume excess diffuses with rate D. At small times, the corrosion front linearly grows in time, but the growth velocity shows a t^{-1/2} decrease after a crossover time of order t_c ~ D/p^2, where the average front height is of order h_c ~ D/p. A universal scaling relation between h/h_c and t/t_c is proposed and confirmed by simulation for 0.00005 <= p <= 0.5 in square lattices. The roughness of the corrosion front shows a crossover from Kardar-Parisi-Zhang scaling to Laplacian growth (diffusion-limited erosion - DLE) at t_c. The amplitudes of roughness scaling are obtained by the same kind of arguments as previously applied to other competitive growth models. The simulation results confirm their validity. Since the proposed model captures the essential ingredients of different corrosion processes, we also expect these universal features to appear in real systems.

Pitting And Crevice Corrosion

Crossover of interface growth dynamics during corrosion and passivation

Atmospheric localised corrosion of stainless steel has been investigated under salt droplets containing a mixture of MgCl2 and NaCl between the deliquescence relative humidity of the two salts where there was precipitation of NaCl crystals. Dish-shaped pits and crevice-like attack could be observed. 
Effects of the change of relative humidity (RH) have been studied. A pit that has grown at 33% RH for 1 day will tend to repassivate when the RH is increased to 85% while pits grown at 33% RH for 3 weeks may not repassivate at 85% RH and can continue to grow when the RH is returned to 33%. A pit that has grown at 33% RH can continue to grow after 1 day at 12% RH if the RH is returned to 33%. RH fluctuations, either to low or high RH, lead to greater number of pits. 
Using the 1D artificial pit method, a potential sweep method was developed to investigate the repassivation process of pits in concentrated solutions, representative of atmospheric conditions. The onset of repassivation is defined as the point where there is no increase in current with an increase in potential. It has been found that repassivation takes place when the concentration of MgCl2 is lower than 3.2 M (equivalent to 64% RH) while there is no repassivation for 3.5 M and 4 M MgCl2 (equivalent to 59% RH and 50% RH respectively). A range of the repassivation potential and the critical metal ion concentration to prevent pits from repassivation was determined.

Atmospheric localised corrosion of type 304 austenitic stainless steels

Nickel alloys are broadly used in upstream oil and gas applications as a result of an advantageous set of properties including high mechanical strength accompanied by good ductility and toughness, ...

Assessing the Pitting Corrosion Resistance of Oilfield Nickel Alloys at Elevated Temperatures by Electrochemical Methods

High strength aluminium alloy AA2024 is susceptible to localized corrosion in the form of pitting and intergranular corrosion (IGC). The corrosion behaviour of this alloy is strongly influenced by the presence of different intermetallic particles. In this study, the effect of applied stress and the role of intermetallic particle removal by surface treatment on the initiation and propagation of localized corrosion are investigated. It was found that applying a stress to as polished AA2024 leads to a drop in breakdown potential. The development of micro/nano crevices adjacent to the intermetallic particles is thought to act as initiation sites for localized corrosion, lowering the breakdown potential of stressed AA2024. The electrochemical behaviour of specific intermetallic particles and the particle free matrix of AA2024 have been studied as a function of stress state using the micro-capillary electrochemical cell in combination with an in situ stressing stage. These experiments emphasized the role of intermetallic particles as the possible key contributing factor in determining the corrosion properties of the alloy under stressed conditions. In situ X-ray synchrotron microtomography experiments were used to observe the evolution of corrosion attack as a function of continuous exposure time in both unstressed and stressed AA2024. Quantitative growth rate measurements indicates that application of stresses of 70% yield strength or above has a significant effect on the localized corrosion propagation.

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Effect of stress on initiation and propagation of localized corrosion in alumninium alloys

The effects of chloride and phosphoric acid (H3PO4) on the corrosion of Alloy C-276 (UNS N10276), UNS N08028, and UNS N08367 have been studied using electrochemical techniques. All of these alloys are widely used in the chemical process industries because of their excellent corrosion resistance in the severe service conditions involving high temperature and stresses. Tests were performed in deaerated solutions, at two different concentrations of sodium chloride (NaCl) and H3PO4 at a fixed temperature of 95°C. Potentiodynamic polarization scans in NaCl solution have been carried out to determine the pitting and the repassivation potentials for UNS N08028 and UNS N08367 and the range of passivity of Alloy C-276. These tests help rank the materials based on resistance to localized corrosion. All the alloys tested show a range of passivity. Alloy C-276 showed the best corrosion resistance under all test conditions. UNS N08367 performed better than UNS N08028. Alloy C-276 had the highest range of pass...

Pitting And Crevice Corrosion

Citations

Crossover of interface growth dynamics during corrosion and passivation

Atmospheric localised corrosion of type 304 austenitic stainless steels

Assessing the Pitting Corrosion Resistance of Oilfield Nickel Alloys at Elevated Temperatures by Electrochemical Methods

Effect of stress on initiation and propagation of localized corrosion in alumninium alloys

Effect of Chloride and Phosphoric Acid on the Corrosion of Alloy C-276, UNS N08028, and UNS N08367

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