ELECTROCHEMICAL POROSITY MEASUREMENT OF ELECTROLESS NICKEL COATINGS ON FERROUS SUBSTRATES - INFORMATION RESULTING FROM AESF RESEARCH-PROJECT

A non-destructive electrochemical method is used to measure the porosi ty of electroless nickel (EN) coatings on steel and gray cast iron. Th e method is based on determining the mixed potential of a duplex metal surface consisting of an EN coating and the ferrous substrate exposed by the pores in a corrosive electrolyte; the measured porosity is qua ntitatively expressed in terms of the fraction of pore area on the coa ting surface. The accuracy of this method is compared to the conventio nal ferroxyl and salt fog spray tests. The electrochemical technique i s found to be more sensitive than the ferroxyl test, and provides the porosity information comparable to that of the salt fog test in a much shorter time without destroying the appearance of test samples. The e lectrochemical porosity measurement is subsequently used to examine th e effect of substrate preparation, including surface polishing and an electrolytic nickel strike, on the porosity of EN coating on carbon st eel and gray cast iron substrates. It is found that the porosity of EN coatings decrease with decreasing roughness of the substrate surface. Mechanically polishing the substrates to 1-mu m smoothness prior to E N plating eliminates nodular EN deposits and reduces the porosity of t he EN coating. Electropolishing of the substrate surface also results in EN deposits with low porosity. An electrolytic nickel strike on car bon steel and gray cast iron prior to EN plating decreases the porosit y of the EN coating by covering exposed graphite and other noncatalyti c inclusions on the substrate surface with a catalytic nickel layer. T he porosity of an EN coating decreases with increasing thickness of th e nickel strike. A nickel strike of 1 mu m in thickness virtually elim inates nodular EN deposits and reduces the coating porosity by a facto r of 100 to an area fraction of pores less than 10(5).