Dissolution and Repassivation Kinetics of a 12.3Cr-2.6Mo-6.5Ni Super Martensitic Stainless Steel

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 ...

Synchrotron X-ray radiography studies of pitting corrosion of stainless steel: Extraction of pit propagation parameters

“…The values of D eff ΔC determined experimentally by a number of researchers are shown in Table 1. [20] is commonly used as the concentration of metal ions at the pit bottom in the interpretation of 1D artificial pit measurements [8,21,26,31,32].…”

Further Modeling of Chloride Concentration and Temperature Effects on 1D Pit Growth

“…10 More recent study on 1D pit growth of SS 316L, however, has suggested that the length of additional diffusion layer (L') is approximately 40% of pit diameter, and δ should be much larger than L' to apply the L ≈ δ condition for the calculation of i lim .…”

“…It is also usually supposed that L is equal to the pit depth (δ), [9][10][11][12] although the diffusion of metal ions may extend out of pit, forming additional layer of hemispherical diffusion and causing L to be longer than δ. 11 The effect of this additional diffusion layer on i lim was found to diminish if δ of 1D pit with 1 mm diameter was greater than 0.4 mm (aspect ratio of pit depth/mouth: 0.4) for SS 304 11 and super martensitic stainless steel.…”

Microstructure and pitting corrosion of similar and dissimilar stainless steel welds

“…In friction welds, it is observed that similar metal welds exhibit symmetrical and identical flash, while in dissimilar F-A and F-D stainless steels metal welds, the flash was observed to be from ferrite stainless steel side and austenite and duplex stainless steels did not participate in the flash formation, suggesting that deformation is 5 Microstructure of ferritic-duplex stainless steel electron beam weld (cross-section) 6 Microstructure of austenitic-duplex stainless steel electron beam weld (cross-section) Reddy mainly limited to ferritic stainless steel side. The same phenomenon has been reported during friction welding of dissimilar welds, namely, Al to Cu, Ti to steel, Al to steel, etc.…”

“…4,5 Corrosion resistance of martensitic stainless steel mainly depends on the nature of surface oxide, as determined by the oxygen potential in the shielding gas and the HAZ temperature-time relationship. 6 In general, the corrosion resistance of fusion welds is inferior to that of the base metals because of the inhomogeneous, dendritic microstructure produced due to welding. 7 The preferential corrosive attack on the weld metals has been reported to take place either at the alloy depleted regions or at the austenite/delta ferrite interfaces.…”