No abstract

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 ...

In the present investigation optical microscopy, in combination with sputtered neutral mass spectrometry (SNMS), has been used to examine the conditions for initiation of pitting corrosion in the heat affected zone (HAZ) of two super martensitic (SM) stainless steel weldments. It is shown that the corrosion resistance depends mainly on the nature of the surface oxide, as determined by the oxygen potential in the shielding gas and the HAZ temperature–time relationship, and less strongly on the underlying microstructure. In the absence of H2S the initiation is associated with spalling of the iron enriched chromium oxide within a narrow region approximately 6 mm from the fusion line, where the contaminated oxide layer is thinnest. As H2S is introduced, the region close to the fusion boundary becomes susceptible to localised corrosion because of the more extensive metal oxidation. Thus, the high temperature oxidation and iron oxide scale formation accompanying the welding operation appear to be the main factors affecting pitting corrosion initiation in SM stainless steel weldments.