Investigation of Corrosion Resistant Coatings Applied by the Cold Spray Process for Use in Aerospace Applications

The Cold Spray process is an emerging technology that is being evaluated as a method to apply corrosion resistant coatings.  Unlike thermal spray processes such as flame, arc, and plasma where a high energy source is used to heat metal powders to a molten or semi-molten state for deposition, the Cold Spray process uses pressurized gas and unique nozzle designs to accelerate the particles to a critical velocity to achieve a solid state deposition.  In this study, the Cold Spray process was used to apply corrosion resistant coatings to aluminum alloys used in the aerospace industry.  These high-strength alloys have excellent fracture toughness and strength-to-weight ratios.  Based on their specific application, these alloys are subject to environmental conditions that can cause corrosion effects leading to a reduction in performance and an increase in lifecycle costs.  Commercially pure aluminum powder was applied to Al-2024-T3 and Al-7075-T6 substrates by means of the Cold Spray process.  The corrosion resistance of these coating/substrate systems was evaluated and compared to the corrosion resistance of the bare and chromate coated substrates. Specific DC electrochemical tests were chosen that would identify the susceptibility of the coatings to pitting and other corrosive breakdown mechanisms in chloride solutions. These tests included (1) open circuit potential, (2) linear polarization (3) potentiodynamic polarization (4) galvanic corrosion and (5) cyclic polarization.  It was observed that each of the coating/substrate systems provided corrosion protection to the bare substrate material. The microstructures and hardness of the coatings were also evaluated.  The results of these evaluations are presented.