The Influence of Pre-Plating Processes on the Porosity of Electrodeposits on Steel

SYNOPSISThe electrochemical method of obtaining a numerical index of the porosity of electrodeposits, previously described by the authors, has been applied to a study of the effects of pre-plating preparation of steel. Five different rolled steels and coatings of nickel, tin-nickel, tin and speculum were examined. The validity of the test method has been verified by comparison with laboratory and exposure tests yielding visual indications of porosity. It was found that the depth of steel removed by any method of pre-plating preparation had an influence on the porosity of the coating sufficiently large to obscure, if it were not controlled, effects of difference in preparation method. The removal of the extreme surface layer of steel (about 0·00001 in) influences the porosity of the applied coating for better or worse according to the metal deposited and the conditions of deposition. Evidence was obtained that this effect depends on the form of crystal growth of the deposit. With deeper uniform penetration into the steel, zones were reached which were apparently features of the steel and influenced porosity almost independently of the character of the deposit. These zones are attributed to bands of nonmetallic inclusions. The observed effect of depth of penetration helps to explain the divergent results previously obtained in investigations of surface preparation. It suggests that, in practice, when controlled penetration cannot be expected, no method of surface preparation will give consistent porosity of deposits on a range of steels. The response of an undercoating to the condition of the steel surface was found to govern the porosity of a composite coating. Well-chosen undercoatings may therefore offer the best method of obtaining consistent results and of reducing porosity.