Structure and corrosion resistance of electrolytic nickel coatings containing codeposited nonconducting particles

Abstract

The codeposition of nonconducting silicate particles with different grain size during the plating of nickel coatings with preferential orientation along with 〈2 1 1〉 axis is investigated by electron microscopy. The results confirm that small size particles, about 20 nm, are included in the twinning defects, inherent to this orientation, blocking the latter as active dissolution sites. Larger particles, approximately 1.5 μm, are included in the bulk of the coating, and cause a continuous redistribution and decrease of the corrosion rate. On the other hand, when particles of 20 nm size are used, an unusual decoration of the twinning plane {111} perpendicular to the substrate, may lead to the elucidation of some theoretical problems related to the initiation of the texture 〈2 1 1〉. A dissolution mechanism is proposed, based on the mutual effect of both factors: redistribution of the corrosion current, and blocking of the twinning planes which act as active dissolution centres. This mechanism is confirmed by Corrodkote tests of codeposited nickel coatings.