Corrosion Resistance of Aluminum Coatings Deposited by Warm Spraying on AZ91E Magnesium Alloy

The corrosion resistance of aluminum coatings on Mg alloy (AZ91E) substrate was investigated in 3.5% NaCl solution. The Al coatings were deposited using a warm spraying (WS) method under three different nitrogen flow rates (NFR): 1.0 m³/min, 1.5 m³/min, and 2.0 m³/min. The decrease of NFR during WS led to a decrease in the porosity of WS coatings from about 2.0% down to 0.5% and an increase in oxygen content from 0.24% up to 0.85%. The Al coating with the lowest porosity and the highest oxygen content exhibited the lowest current density in the passive range, highest breakdown potential during anodic polarization, and repassivation behavior. The impedance measurements revealed that this Al coating effectively protected the Mg substrate during 14 d of immersion in the test solution as its resistance remained four orders of magnitude higher (11,266 Ω·cm²) when compared to bare substrate (∼70 Ω·cm²). The microscopic observations confirmed that the coating remained consistent and well bonded to the substrate, and no corrosion products layer on coating/substrate interface was formed, indicating that the electrolyte did not penetrate through the coating. The higher porosity leads to faster degradation of the coatings formed under higher NFR.