Preparation and characterization of phosphate coatings developed on 316 L by a cathodic treatment

Abstract

The electrochemical formation of phosphate coatings on 316 L stainless steel is investigated in this study, focusing on the influence of cathodic treatment parameters such as temperature, treatment time, and prior activation to achieve efficient and high-quality coatings. The primary aim is to optimise electrochemical process conditions for phosphate coating development, evaluating their effectiveness in terms of crystal formation, coating density, and corrosion resistance. Several experimental conditions were assessed, including temperatures ranging from 40 °C to 70 °C, treatment durations between 500 and 2000 s, and the influence of prior activation using a commercial Ti-phosphate activator. The experimental approaches included both galvanostatic and potentiostatic treatments, with sample characterisation performed via techniques such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-Ray diffraction (XRD) to analyse coating morphology, chemical composition, and crystalline phase. Additionally, corrosion current density measurements were carried out to assess the electrochemical resistance of the coatings. The main conclusions are that higher temperatures (70 °C), though complete coverage is not achieved, enhance coating efficiency, that is lower corrosion rates were recorded. Hopeite predominates at lower temperatures, while scholzite is more common at higher temperatures. Treatment times above 1500 s at 70 °C do not lead to significant improvements, whereas treatment times over 2000 s at 50 °C would be suitable to produce efficient coatings. Prior activation speeds up coating formation but does not notably improve efficiency.