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 ε-phase can be precipitated during thermal treatment (500–600°C). Ferritic alloy containing high copper content (1.58%Cu) shows precipitation of the ε-phase already in cast form (without sensitisation). In duplex ferritic–austenitic steels, the precipitation of the ε-phase in ferrite occurs during the sensitisation in a temperature range 500–600°C. The presence of the ε-phase stimulates pitting corrosion of ferrite. Electrochemical measurements of the passive behaviour of investigated materials show that the alloys containing precipitates of the the ε-phase undergo pitting corrosion in 1 M H2SO4–1 M NaCl. Thermal treatment dissolving the ε-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.