A study of the dissolution and repassivation of an Fe-17.4 a/o Cr alloy in a pencil-type artificial pit simulating one-dimensional diffusion, has been carried out in a chloride-containing bulk solution. The attainment of saturation of dissolution products within the artificial pit was characterized by the formation of a salt layer on the alloy surface. A transport model has been used to calculate the decrease in concentration at the metal interface for currents below that required to maintain the salt layer. The concentration at the metal interface was calculated from the balance of electrochemical dissolution and diffusion in the localized environment. The dissolution rate of the alloy showed a maximum at a particular concentration and, both this concentration, and the rate, increased with potential. Passivation took place below a critical concentration. The potential and critical current for passivation increased with potential. The dissolution kinetics were affected by a remnant film ~hat was present on the metal surface after the salt layer had dissolved. Its effects during active anodic dissolution slowly disappeared with time.To understand the nature of localized corrosion, we must know the dissolution and passivation kinetics of metals in solutions over a range of concentration of their own dissolution products. The electrode kinetics of a metal in this localized environment also provides a basis for understanding the influence of alloying elements and inhibitors on localized corrosion.One method for determining dissolution and passivation behavior of metals is a method described as lead-in-pencil electrode. ~-12 This method has been used to study salt film formation ~-~2 and uniform diffusion-controlled dissolution, ~ The active dissolution kinetics of salt film free surfaces also have been measured. 2-7 The method has been used to determine the dependence of the metal dissolution kinetics on the metal-ion concentration and hence give an estimate of the concentration of soluble dissolution products at which repassivation takes place. 6' 7 The salt films and dissolution products in the lead-in-pencil method may be particularly relevant to localized corrosion at the very high potentials observed in chlorinated seawater. I3When stainless steels are used in the lead-in-pencil method, the metal at first undergoes pitting corrosion until the pits coalesce to form a uniformly dissolving surface. The products diffuse away from the metal and establish a steep concentration gradient of metal ions. The dissolution kinetics of the bare metal can be measured up to diffusioncontrolled rate and are very dependent on potential. Transient measurements where the metal-ion increases above the equilibrium saturation, can be made at these higher concentrations prior to nucleation and precipitation of the salt in the supersaturated solution. Here very high current densities, generally controlled by solution resistance, temporarily flow before precipitation of a thick salt film that drops the current and gives the characteristi...

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In this investigation, the dissolution and repassivation kinetics of a super martensitic ͑SM͒ stainless steel ͑Fe-12.3Cr-6.5Ni-2.6Mo͒ have been characterized using the artificial pit technique. As a part of this study, a diffusion model has been developed and employed for calculation of the pit surface concentration of dissolved species during the potential step experiments. For concentrations close to the saturation level, the dissolution kinetics are adequately described by a Tafel slope of approximately 57 mV/dec and a current density of 0.5 mA/cm 2 at Ϫ300 mV vs. saturated calomel electrode. However, repassivation of the active pit surface occurs when the concentration of the dissolved species drops below 30% of the saturation value. Based on a comparison with relevant literature data, the observed response of the SM stainless steel to localized corrosion is similar to that reported for other high alloyed steels. This result is expected if the dissolution and repassivation kinetics are controlled by the content of Cr, Ni, and Mo in the parent material.Martensitic stainless steel pipes of the 13Cr-4Ni type have been used successfully in the oil industry as Oil Country Tubular Goods material since the 1970s. 1 Later, modified or what are known as super martensitic ͑SM͒ stainless steels with improved weldability and corrosion resistance were developed. These grades are now in commercial use, including use as pipelines. 2-6 Although localized pitting corrosion is observed in most stainless steels, the problem is particularly prominent in SM stainless steel pipes that are employed as flow lines transporting corrosive well stream with H 2 S. Since the main emphasis in the past has been on ''fit for purpose'' testing and qualification of the material for use offshore, 6 a more fundamental understanding of the underlying corrosion mechanisms is lacking. 7 Specifically, there is a need to clarify the dissolution and repassivation kinetics of SM stainless steels and compare the results with those reported for other classes of stainless steel under similar experimental conditions. [8][9][10][11] When a stainless steel is exposed to an electrolyte containing aggressive chloride anions, the passive oxide layer is undermined. Chloride ions penetrate the oxide layer at specific points on the surface and increase the rate at which metal dissolves by the reaction Me → Me nϩ ϩ ne. 8 Generally, the process is distinguished between the initiation stage, involving the breakdown of the passive film and the development of an aggressive solution, and the growth stage, where stable pit growth occurs when the pitting potential E pit is reached. 12,13 During pitting corrosion the anodic dissolution creates a flow of electrons ͑current͒ from the pit surface ͑anode͒ to the surrounding passive metal surface ͑cathode͒. As metal cations are dissolved in the adjacent electrolyte, chloride anions migrate from the bulk solution toward the pit to maintain electric neutrality in the solution. A hydrolysis reaction of the metal ions lowers the ...