Aluminium Nitride as a Corrosion Protection Coating for Steel: Selfsealing Porous Electrode Model

Abstract

The use of aluminium nitride (AIN) as a corrosion protection coating for mild steel is discussed. For comparison, the corrosion protection capability of Al and AI–AIN cermet coatings on mild steel is also examined. The coatings, ∼1.0 μm thick, were grown on water cooled steel substrates by sputtering an Al target in 5N₂–95 (Ne_1-x + Ar_x) discharges. Coating crystallography, morphology, surface chemistry, and electrical resistivity were determined prior to corrosion testing. Surface chemistry was also determined after exposure to the electrolyte. Cathodic polarisation electrochemical measurements were used to determine the corrosion rate. Electrolyte pH measurements were carried out to measure 0H^- ion concentration resulting from the interaction of the coating with the electrolyte. Microcrystalline AlN coated steel was found to corrode at a rate of 1.5 orders of magnitude slower than uncoated steel, and 2 orders of magnitude slower than Al coated steel. The selfsealing porous electrode model of the corrosion process is presented, which describes the AlN coating as a barrier to both H⁺ diffusion and electromigration.