Corrosion behaviour of electrochemically joined aluminum and stainless steel

Two dissimilar metals viz., aluminum and stainless steel (SS) were joined electrochemically by heavy nickel deposition. Potential-time behavior, Tafel extrapolation method and galvanic coupling experiments were conducted to study the corrosion behavior of the three individual metals in 5% aqueous sodium chloride acidified with acetic acid, alone and in combination. The systems behaved differently in simple immersion test and on coupling with each other. In simple immersion test, nickel and SS were nobler to aluminum. Coupling of aluminum and nickel (bi-metallic) resulted in dissolution of both metals, the rate of nickel being lower. Potentiodynamic polarization experiments showed that the aluminum-SS bi-metallic system has the highest corrosion rate than aluminum-Ni or SS-nickel. Aluminum exhibits pitting type corrosion when coupled to nickel. Experiments with electroplated specimens and SEM analysis confirmed that in aluminum-nickel couple both metals corrode, the corrosion of nickel being considerably lower. Formation of elemental nickel on aluminum surface also confirmed the above result