Electrochemistry of Stainless Steel in Sulphuric Acid

Abstract

IN a study of the electrochemical behaviour of stainless steels, the electrode potential has been continuously recorded during the transformation of an air-stable oxide film to the surface state that exists in dilute, aerated sulphuric acid at temperatures between 25° and 90° C. Several definite inflexions or stages in the process have been observed which are similar in certain respects to the oscillograms obtained by Bonhoeffer and Vetter¹ in the activation and re-passivation of carbon steel in nitric acid. With stainless steel, the processes are much slower than those observed with carbon steel, so that the details of the several stages may be more readily studied. Thus, Fig. 1 shows the results obtained at 85° C. when a niobium-stabilized steel (type 347) was heated in air at 445° C. for 2 hr. and then placed in an aerated 0.1 N mixture of sulphuric acid and sodium sulphate which had a pH of 1.46. A vibrating-reed electrometer measured the potential between the steel electrode and a reference electrode kept at 24° C, so that all potentials to be given include a small potential due to the thermal gradient in the bridge. Dissolved iron was determined at intervals spectro-photometrieally by use of thioglycollic acid².