A double zincate pretreatment of Al for plating was investigated in a LiOH-based zincate solution. The zinc deposition rate from the LiOH-based zincate solution was lower than that from NaOH or KOH-based solutions, resulting in decreased deposition of Zn and dissolution of the Al substrate. This phenomenon was interpreted as a large coordination number for water molecules to bond with Li+ ions compared to Na+ and K+ in the zincate solutions. For a LiOH-based zincate solution at a concentration near the solubility limit of water, the concentration of free water molecules decreases due to a strong restriction around Li+ ions. This situation results in a decrease of the activity coefficient of OH- which leads to dissolution of the Al substrate, and decreases the coordination number for water molecules to in relation to Zn(OH)42- ions to accelerate the electron transfer from and Zn deposition on the Al substrate. Addition of Fe(III) ions to the LiOH-based zincate solution increased the rate of Zn deposition, suggesting that the co-deposition of Fe obstruct the growth of Zn deposition and results in imperfect insulation of the Al substrate by Zn deposition.
