Challenges in the electrochemical coating of high-strength steel screws by aluminum in an acidic ionic liquid composed of 1-Ethyl-3-methylimidazolium chloride and AlCl3

In this paper we show that screws made of high-strength steel (12.9 grade) can, in principle, be electrochemically coated with up to 40-mu m-thick Al layers in an ionic liquid composed of 1-Ethyl-3-methylimidazolium chloride and AlCl3 (40/60 mol%). The main challenge for the adhesive deposition of Al is a thorough purification and pretreatment of the screws prior to Al deposition. It is shown that a combination of degreasing, etching, plasma treatment, and electrochemical etching in the ionic liquid is a prerequisite for an adhesive and well protecting Al layer. Even then it can happen that at some sites Al does not adhere sufficiently well. A proper posttreatment is essential, too, and we show that the ionic liquid must be removed with another ionic liquid prior to a chemical passivation in boiling distilled water to get a high layer quality.