Abstract
The initiation of localized corrosion of types 430 and 443J1 ferritic stainless steels was evaluated in 0.15 mol dm−3 Na2SO4 solution. A liquid-phase ion gun (LPIG), a silver microelectrode covered with a silver chloride layer, was cathodically polarized to generate Cl− in the vicinity of the stainless steel polarized at 0.4 VSSE. Contact of the stainless steel surface with the Cl−-concentrated environment by the LPIG operation induced a rapid increase in anodic current flow through the stainless steel electrode after the induction period td and consumption of the cathodic electric charge Qd by the LPIG microelectrode. Numerical modeling using the LPIG microelectrode current for td gave a critical Cl− concentration [Cl−]d needed for the initiation of localized corrosion. These parameters obtained by LPIG tests showed that type 443J1 stainless steel was superior to type 430 stainless steel in localized corrosion resistance. AES depth profiling of the stainless steel surfaces after the LPIG operation revealed enrichment of Cl in the outermost oxide film as well as decreased film thickness. The mechanism of degradation of the stainless steel surface due to contact with the Cl−-concentrated solution is discussed. The adsorption of Cl− on the oxide surface is thought to be a trigger of the oxide degradation.
Highlights
The initiation of localized corrosion of types 430 and 443J1 ferritic stainless steels was evaluated in 0.15 mol dm−3 Na2 SO4 solution
Contact of the stainless steel surface with the Cl− -concentrated environment by the liquid-phase ion gun (LPIG) operation induced a rapid increase in anodic current flow through the stainless steel electrode after the induction period td and consumption of the cathodic electric charge Qd by the LPIG microelectrode
The diameter of the silver microelectrode used in this study was 500 μm, which is 2.8-times larger than that used previously,[11] anodic polarization of the LPIG microelectrode charged by the cathodic polarization with a charge of 10 mC could generate and discharge Cl− with an anodic electric charge of 10 mC
Summary
The initiation of localized corrosion of types 430 and 443J1 ferritic stainless steels was evaluated in 0.15 mol dm−3 Na2 SO4 solution. Contact of the stainless steel surface with the Cl− -concentrated environment by the LPIG operation induced a rapid increase in anodic current flow through the stainless steel electrode after the induction period td and consumption of the cathodic electric charge Qd by the LPIG microelectrode. Numerical modeling using the LPIG microelectrode current for td gave a critical Cl− concentration [Cl− ]d needed for the initiation of localized corrosion. These parameters obtained by LPIG tests showed that type 443J1 stainless steel was superior to type 430 stainless steel in localized corrosion resistance. The mechanism of degradation of the stainless steel surface due to contact with the Cl− -concentrated solution is discussed.
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