Hybrid PEO/sol–gel coating systems for aluminum alloys: Mechanisms, processing parameters and corrosion protection performance

In the past few decades, the aviation sector has focused on minimizing the weight and fuel consumption of aircraft by using light materials such as aluminum alloys. Although these materials provide a good strength-to-weight ratio and are economical, the addition of elements such as copper, which improves the mechanical properties, leads to corrosion issues. To overcome this, Plasma Electrolytic Oxidation (PEO) is a promising and environmentally friendly technique for the deposition of hard and wear-resistant ceramic coatings. Although these coatings have numerous benefits, they tend to be porous, which is a limitation in terms of corrosion resistance. To address this issue, sol-gel sealers have been investigated as a secondary process. Sol-gel coatings are versatile, inexpensive, and environmentally friendly, and they can easily penetrate and seal the porous layers created during the PEO process. By providing chemical and mechanical bonds, sol-gel coatings improve the barrier properties of PEO coatings and corrosion resistance. The purpose of this review is to present a comprehensive examination of the PEO and sol-gel processes, the influence of process parameters, the mechanisms at play, and the recent advancements in the addition of corrosion inhibitors to sol-gel matrices. The focus of this review is to optimize the hybrid PEO/sol-gel process for aluminum alloys to produce a robust and multi-functional coating for aerospace applications.