Pitting Behavior of Duplex 308L Stainless Steel in Methanol/Water/HCl Solutions

Passivity breakdown experiments have been performed on a duplex stainless steel in 0.5N HCl, H₂O/methanol solutions as a function of δ-ferrite content of the alloy and water/methanol ratios. Ferrite contents in a nominally 308L stainless steel were controlled by heat treatment at different temperatures in the two phase ferrite-austenite region followed by quenching. Microstructures consisting of 0 to 30 Vol% ferrite were produced. The corrosion behavior of the steel 308L is adversely influenced by the formation of δ-ferrite and by a decrease of the water content of the water/methanol corroding media: the passivation potential and the critical current density for passivation increases, the pitting potential and the repassivation range decreases. Increasing ferrite content from 0 to 1 Vol%, a pronounced shift of all these values occurs. The pitting resistance of duplex steel in strongly passivating (aqueous) solutions is primarily a function of the composition of δlγ interface, relatively independent of the interfacial area or of the ferrite content higher than 1 Vol%. The integrity of the passive film is thought to be associated with segregation of sulfur to the δlγ interface. The pits initiate in duplex alloy at the δlγ phase boundaries, in the single phase alloy at the sulfide inclusions. In weakly passivating solutions (less than 20 Vol% water), the integrity of passive film is reduced. General weakening of the passive film eliminates the preference of pit initiation sites to phase boundaries or sulfide inclusions and results in general pitting of the surface. Accordingly, the mechanism of pit formation in duplex and single phase alloy becomes identical.