Some observations on environmental corrosion behaviour and electrochemical passivation characteristic of silver-cadmium contact alloys in neutral - saline and alkaline - sour media

Abstract

The atmospheric/environmental corrosion tests conducted on two different contact-alloys, Ag-Cd and Ag-CdO, revealed the following order with regard to variation in corrosion rate. Ag-CdO in M.A. > Ag-Cd in M.A> Ag-CdO in LA> Ag-Cd in LA. where, M.A. = Marine Atmosphere LA = Industrial Atmosphere The electrochemical polarisation measurements conducted on these materials (Ag-Cd & Ag-CdO) seperately in 3.5% NaCI and in 1% Na,S solutions, under fully immersed condition, exhibited the following order in respect of variation in corrosion current density. Ag-Cd in N.S.> Ag-CdO in N.S.> Ag-CdO in AS.> Ag-Cd in A.S. where, N.S. = Neutral-saline solution (3.5% NaCI solution). A.S. = Alkaline-sour solution (I%Na2S solution). The electrochemical passivation characteristic of these alloys in 3.5% NaCI and in 1% Na,S solutions whre also investigated. The present investigation reflects a similar variation trend for both iQ and icril' as given below: Ag-Cd in N.S.> Ag-CdO in N.S.> Ag-CdO in A.S.> Ag-Cd in A.S. where, iQ = Anodic current density corresponding to the initiation (point "Q" in the polarisation diagram) of passive-film damage. icrit = Critical current density corresponding to the nose (point "P" of the polarisation curve) of the primary - passivation peak. The experimental findings have also been interpreted in accordance with the electrochemical redox e.m.f., considerations.