Electrodeposition and corrosion behavior of Zn coatings formed using as brighteners arene additives of different structure

This study examined the effect of the molecular structure of the aliphatic chain in arenes (phenyl + aliphatic chain) used as brightener additives on the reduction kinetics, morphology and corrosion resistance of Zn coatings formed by reduction of Zn(II) ions in a chloride electrolytic bath. Three such brighteners were tested—benzylideneacetone (BDA), benzylacetone (BA) and butylbenzene (BB)—dissolved in polyethyleneglycol (MW200) PEG200, in order to determine the influence of the phenyl, carbonyl and/or C = C double bond present in these compounds on the zinc reduction process. The results showed that, in the presence of PEG200 alone, the reduction rate (j0) of Zn(II) ions was higher than in the absence of additives. By contrast, in the presence of the brightener-PEG200 mixtures, the values of j0 were lower, leading to a reduction in the size of the Zn clusters. Among the brighteners tested, no significant differences were observed in the values of j0 as a function of the molecular structure of the arene aliphatic chain. SEM analysis revealed a change in the Zn coating morphology from hexagonal crystals with a (101) preferred orientation in the absence of additives, to flakes grouped in hemispherical clusters with (002), (100) and (101) crystallographic orientations when PEG200 or brightener-PEG200 mixtures were present in the electrolytic bath. In addition, the cluster size decreased in accordance with the j0 values. The comparison of the values of corrosion current density indicates that there are no significant differences between them.

Research highlights
► The reduction rate of Zn(II) was higher with PEG200 than without additives.
► The presence of arene-PEG200 associates decreases the size of the Zn clusters.
► The structure of the aliphatic chain does not influence onto morphology of the coatings.
► The Zn coatings obtained with brightener-PEG200 associates had lower values of Jcorr.