EIS AS A MEANS OF EVALUATING ELECTROLESS NICKEL DEPOSITS

Electroless nickel coatings on mild steel were exposed to quiescent acidic NaCl solutions. Thick coatings exhibit a frequency dispersion in the Bode phase diagram which has been attributed to a self-protective reaction of the coating involving an enriched phosphorus layer and the electrolyte, The open-circuit potential (OCP) starts moving towards more negative potentials as soon as penetration of the coating has taken place, thereby triggering corrosion of the substrate. Non-uniform or porous EN coatings drift towards negative potentials with little or no frequency dispersion. The degradation of coatings was monitored by electrochemical impedance spectroscopy (EIS).