A ‘tortuous path’ and ‘protective oxide layer’ work in tandem in unique corrosion-resistant polyetherimide coatings

Galvannealed steels are mainly surface treated via chromating and phosphating processes to make their surface more corrosion-resistant and enhance paint adhesion. However, environmental regulations have put a control on the usage of these pretreatments, and these need to be replaced by environment-friendly pretreatment chemicals having similar range of properties. In this regard, a unique polyetherimide-based coating system is proposed herein containing a di-anhydride molecule and two di-amine molecules, which can avoid harsh chromating, phosphating and silane-based pre-treatments and offers properties that are not achieved using conventional routes. CNTs are added to a base pre-polymer (polyamic acid) to offer a tortuous path, whereas polyaniline-coated ceria (PANI@CeO₂) is added, prior to imidization, to offer a protective oxide layer, which worked in tandem to offer a corrosion-resistant coating. The base coating shows the lowest corrosion resistance owing to the higher porosity, presence of microcracks and a combination of both the nanoparticles offering higher noble open circuit potentials (OCPs) at all immersion times, indicating prolonged coating stability and less tendency to corrosion. In addition, electrochemical impedance spectroscopy (EIS) study of both CNT and PANI@CeO₂ reinforced composite coating shows the highest corrosion resistance and low water uptake with respect to other coating systems evaluated here. The highest corrosion resistance in the composite coatings may be due to the low coating porosity, absence of microcracks, tortuous pathways for corrosive ion movement and ennobling effect due to the presence of PANI@CeO₂ and CNTs.