The Effects of Copper and Copper-Tin Additions on the Corrosion of Molybdenum-Containing Austenitic Stainless Steel in Dilute Sulphuric Acid

Abstract

Effects of less than 0.2 percent copper alone and in combination with tin on the active corrosion behaviour of Fe-18 Cr-20 Ni-2 Mo in boiling five percent sulphuric acid were evaluated by measuring the weight loss at various potentials and at its rest potential.
The addition of 0.2 percent copper decreased the critical current density for passivation to about one half of that for the alloy without copper. Copper suppressed the metal dissolution at relatively noble potentials in the active state, increased the cathodic reaction rate, and shifted the corrosion potential in the noble direction.
The addition of copper in combination with tin was particularly beneficial to decreasing the corrosion rate in the above medium and in boiling 0.5 percent hydrochloric acid. The corrosion ratepotential curve in sulphuric acid showed that the corrosion rate of the copper-tin-containing alloy was lower than that of the alloy containing either copper or tin alone at relatively noble potentials in the active state (about-200- +50mV, E_h). Its corrosion rate was particularly low between -80 and -30mV. The corrosion potential of this system stays in this potential range. As to the corrosion rate at less noble potentials in the active state, the copper-tin-containing alloy ranks between the tin-containing alloy and the copper-containing alloy.
The relatively low corrosion rate of the commercially produced molybdenum-containing austenitic stainless steel in boiling five percent sulphuric acid, compared with that of the steel melted in the laboratory, was attributed to the presence of both copper and tin as residual elements in the former.