Surface pretreatments of aluminium for electroplating

TLDR: In this paper, a new pre-treatment sequence was designed to improve the coverage rate of zinc alloy films, chemically deposited prior to nickel electroplating, which consists of a double-step nitric + hydrofluoric acid solution immersion, with NaOH immersion treatment between the two acid immersion steps.

Abstract: The effects of concentration and temperature of sodium hydroxide and nitric acid + hydrofluoric acid pretreatment solutions on the morphology of aluminium surfaces have been studied SEM/EDS observations indicated that NaOH solutions produced etch concavities on the surface, but failed to etch second-phase particles, namely FeAl3 The acid solutions, however, succeeded in preferentially removing these particles, while they did not change the surface roughness of the aluminium In an attempt to improve the coverage rate of zinc alloy films, chemically deposited prior to nickel electroplating, a new pretreatment sequence has been designed It consists of a double-step nitric + hydrofluoric acid solution immersion, with NaOH immersion treatment between the two acid immersion steps The results obtained with this new sequence indicate that the first acid step removes most of the biggest FeAl3 particles, as well as the previous oxide layer, while the alkaline step generates a micro-roughness on the surface whereby a net of shallow cellular cavities, as in a honeycomb structure, forms The last acid treatment preferentially etches the small FeAl3 particles and the most protuberant inhomogeneities of the surface This new pretreatment sequence proved to be more effective than those previously reported in the literature, in terms of producing a good surface on which zinc alloy films could develop The adhesion of nickel electrodeposits to such a surface reached values one order of magnitude higher than those for conventional two-step sequences Interpretation of these results is based on the improved chemical homogeneity of the surface resulting from the removal of second-phase particles and on the small (ca 1 μm in diameter) and uniform dimensions of the cells produced by the etch treatment This effectively leads to a good surface coverage during the zincate treatment, as intially intended