Aluminum Alloy Corrosion Resistance

An aluminum alloy was coated with Minimox^™ solution and put into service in an oxidizing atmosphere. The assembly was exposed to a hot acidic atmosphere with cyclic corrosion conditions. After long term exposure, the cross sections below were obtained by an outside laboratory.

Untreated Al Alloy	Minimox™ solution-coated Al Alloy

The Minimox^™ nanocrystalline treatment demonstrated corrosion resistance under these hot, acidic conditions. The customer tested over 60 aluminum coating and alloy systems, and Minimox^™ solution and only one other system were deemed to have passed the test requirements.

How can an ultra-thin coating improve corrosion resistance?

The nanocrystalline coating process alters the nucleation and growth of thermal oxides - yielding an oxide surface that is more dense and adherent, even though it is thin. The composition of the surface oxide also changes. This oxidation layer improves the corrosion resistance not only by reducing surface imperfections and voids (as demonstrated with the nickel surfaces shown below) but also by increasing the surface chromium/iron ratio of many alloys. For 410 stainless steel, the surface Cr/Fe ratio of untreated and oxidized alloy is 0.03. After Minimox^™ solution treatment and oxidation, the Cr/Fe ratio is 5.7. The composition of the protective layer is primarily a function of the alloy, not the coating material. There is no chromium in the solution.

	Uncoated Ni 600 alloy; heated in air to 1000°C for over 500 hours. Oxide scale is blistered, flaked, and porous and has multiple protrusions. (original 100X)
	Microvoids under the blistered oxide of uncoated Ni 600 alloy are ideal sites for corrosion. These corrosion nucleation sites are absent in the nanocrystalline-coated material. (original 1000X)
	Nanocrystalline coated Ni 600 alloy; heated in air to 1000°C for over 500 hours. The oxide is dense, thin, adherent, and protective. (original 100X)