Newly developed nanocrystalline Al alloy coatings for the performance enhancement of Mg or Al-Li alloys

Aerospace applications have a thirst for lightweight materials.  Alternative alloys, such as aluminum lithium and magnesium are emerging as favorites due to their unique property sets. However corrosion continues to be a limiting factor in the implementation.  Al-Li alloys are prone to corrosion, but also increased fatigue crack growth rate which can be exacerbated by surface pits and corrosion defects. Magnesium is highly reactive and multi-material couples can lead to rapid galvanic corrosion.

Recent scientific advances in the electroplating of aluminum from ionic liquids has enabled the commercialization of protective coatings, which can enhance the performance of Al-Li or Mg.  We report here on the development and testing of new Al-X alloys (where X = Mn, Cr, Mo, Ni Co, Zr) made from nanocrystalline alloy coatings.  These alloys have been engineered to leverage the property advantages of nanocrystalline metals.  In particular, the stable thermodynamic nature of the alloys produces a uniform solid solution strengthened alloy devoid of second phase particles, which could otherwise lead to micro-galvanic cells and pitting corrosion.  In-situ activation of active substrate materials, like Al-Li or Mg, allows for an intimate and direct, metal-to-metal interface which boosts adhesion.  Further, the nanostructured grains of these new alloys provide greater strength and hardness to the coating, which improves durability and handling.

We report here on the corrosion properties of these alloys in industrially relevant environments: ASTM B117 salt spray and acetic acid vapor.  Performance of the coatings will be related to the structure and composition of the alloys.