Corrosion-Resistant, Chromate-Free Talc Coatings for Aluminum

Aluminum (Al) and Al alloys exhibit unusual passivity when immersed in alkaline lithium (Li) salt solutions. This passivity is a result of precipitation of a continuous, polycrystalline talc coating. Talc coatings persist when removed from the Li salt solution and offer increased corrosion protection during exposure to aggressive environments. Efforts to develop a low-cost, low-toxicity, easily applied corrosion-resistant coating for Al are ongoing. Surface analytical and electrochemical experiments aimed at characterizing the structure, composition, and barrier properties of talc coatings were conducted. The solution chemistry involved in talc formation was studied, and the conditions under which coatings form readily were established. Results showed coatings formed by immersion in an alkaline Li salt bath followed by a low-temperature (≤ 100°C) heat treatment in air or water provided barrier properties that compared favorably with those of traditional inorganic corrosion-resistant coatings, such as those produced by chromate conversion.