Improved Phosphate Conversion Coating of Steel for Corrosion Protection

Background: Corrosion is a crucial worldwide problem that strongly affects natural and industrial environments, in particular affecting land, sea and air transportation vehicles: Cars, trucks, buses, merchant ships, and aircraft. This study describes a corrosion protection technology, based on chemical conversion coatings (CCC), to prevent, avoid or minimize corrosion event. Phosphating, a particular CCC was applied. Methods: Carbon steel pieces were phosphate by immersion in a solution containing phosphoric acid (PA, H3PO4), zinc sulphate, hydrogen peroxide as an oxidant and a Zr salt, a sealing agent. The characteristics of the phosphate layer was determined by Scanning Electron Microscope (SEM), Energy Dispersive X-Ray Spectroscopy (EDS) and Raman Spectroscopy. The phosphate steel was painted with an industrial epoxy paint. Its corrosion resistance was determined by potentiodynamic polarization and by exposure in a saline chamber following the practice recommended in American Society for Testing and Materials (ASTM). Results: The steel specimens were weighed before and after immersion in the phosphating solution, after different immersion times. The weight increases resulting from the formation of the phosphate layer were recorded and are expressed in mg/cm2. The corrosion rates were calculated using the Tafel slopes based on the Stern-Geary equation. The current density i in µA/cm2 units were converted into corrosion rate in mm/y units, applying the Faraday equation. Conclusion: Phosphating is a useful pretreatment, based on a phosphoric acid solution containing additional components to improve the protective capabilities of coating systems that are applied to steel in order to improve its corrosion resistance. Keywords: Carbon steel, chemical conversion coatings, corrosion, phosphating, zirconium salts.