The effect of copper on passivity and corrosion behaviour of ferritic and ferritic-austenitic stainless steels

The paper presents investigations of the influence of copper as an alloying element on the structure and on the passive behaviour of ferritic Fe-18%Cr and duplex ferritic-austenitic Fe-24%Cr-6%Ni-3%Mo cast alloys. Copper dissolved in the solid solution (austenite) does not show detrimental effect on the stability of a passive film. The solubility of copper in ferrite is lower than in austenite and therefore the epsilon-phase can be precipitated during thermal treatment (500-600 degrees C). Ferritic alloy containing high copper content (1.58%Cu) shows precipitation of the epsilon-phase already in cast form (without sensitisation). In duplex ferritic-austenitic steels, the precipitation of the epsilon-phase in ferrite occurs during the sensitisation in a temperature range 500-600 degrees C. The presence of the epsilon-phase stimulates pitting corrosion of ferrite. Electrochemical measurements of the passive behaviour of investigated materials show that the alloys containing precipitates of the the epsilon-phase undergo pitting corrosion in 1 M H2SO4-1 M NaCl. Thermal treatment dissolving the epsilon-phase increases the resistance of alloys to local attack. Our experiments show that the increase of copper content in ferritic alloys increases also the tendency to chromium segregation at grain boundaries and thus the tendency to intergranular corrosion of cast alloys. (C) 2000 Elsevier Science S.A. All rights reserved.