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 diss olved in the solid solution (austenite) does not show detrimental effect on the stability of a passive film. The solubility of copper in ferrite is lo wer than in austenite and therefore the epsilon-phase can be precipitated d uring 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 sensit isation in a temperature range 500-600 degrees C. The presence of the epsil on-phase stimulates pitting corrosion of ferrite. Electrochemical measureme nts 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 incr eases the resistance of alloys to local attack. Our experiments show that t he increase of copper content in ferritic alloys increases also the tendenc y to chromium segregation at grain boundaries and thus the tendency to inte rgranular corrosion of cast alloys. (C) 2000 Elsevier Science S.A. All righ ts reserved.