Enhancing the corrosion resistance of magnesium alloy AZ91D in 3.5 per cent NaCl solution by cerate conversion coatings

• Published in: Anti-corrosion methods and materials 2006-11, Vol.53 (6), p.367-373
• Main Author: Salam Hamdy, Abdel
• Format: Article
• Language: English
• Subjects: Alkaline cleaning; Alloys; Analytical methods; Cerium; Cerium oxides; Chemical tests; Chloride; Chloride ions; Chromates; Chromating; Cleaning; Coating effects; Coatings; Coatings technology; Conversion coatings; Corrosion; Corrosion effects; Corrosion inhibitors; Corrosion prevention; Corrosion resistance; Corrosion resistant alloys; Diffusion barriers; Electrodes; Electron microscopy; Etching; Hydroxides; Inhibitors; Localized corrosion; Magnesium; Magnesium alloys; Magnesium base alloys; Metal surfaces; Metals; Morphology; Oxide coatings; Pitting (corrosion); Polarization; Potassium; Potassium hydroxides; Protection; Protective coatings; Scanning electron microscopy; Scientific imaging; Sodium chloride; Studies; Surface preparation; Surface treatment
• ISSN: 0003-5599; 1758-4221
• DOI: 10.1108/00035590610711714

Purpose - To develop new eco-environmentally friendly surface treatments based on cerate compounds as alternatives to the process involving toxic chromates for the corrosion protection of magnesium alloys.Design methodology approach - A treatment process in which a surface was alkaline-etched prior to ceria treatment is proposed. The process involves cleaning, etching in potassium hydroxide followed by treatment in ceria conversion coatings. The effect of surface preparation prior to ceria treatment on the corrosion resistance of AZ91D in 3.5 per cent NaCl solution was measured using AC impedance spectroscopy and DC polarization techniques. Surface examination was performed by scanning electron microscopy, energy dispersive X-ray.Findings - It was shown that the ceria treatment can be used as a localized corrosion inhibitor for alloy AZ91D in NaCl solution. The level of inhibition strongly depended on the cerium concentration. Moreover, ceria treatments improved the pitting corrosion resistance due to the formation of protective oxide films which act as a barrier to oxygen diffusion to the metal surface. According to the EIS and polarization measurements, alkaline etching in KOH is more effective in reducing the pitting corrosion of AZ91D than was HCl. It was shown that surface treatment in alkaline solution (KOH) prior to ceria treatments played an important role in inhibiting the active surface sites, rejecting the chloride ions from the surface and forming uniformly distributed oxide film.Originality value - Ceria conversion coatings seem very promising as alternatives to toxic chromating for the corrosion protection of magnesium alloys in NaCl solution.