Fatigue Response of Common Conversion Coatings on Al7075

Available data is lacking for predicting failure of USAF outer moldline coatings.  To address this and to aid modeling of the degradation of coatings, a study was undertaken in which specimens were coated, damaged by milling, then given a Prohesion exposure.  Four point bending fatigue was performed until cracks formed in the coating.  Two coatings were investigated including a chromium conversion coating and a rare-earth conversion coating (RECC).  Identical topcoats were employed for both systems.  The specimens were 1.6 mm thick Al7075-T6.  Damage was introduced using a flat endmill and ranged from barely removing topcoat to revealing the substrate.  Damage sites had combinations of exposed substrate, primer, and/or topcoat.  Damage depth was estimated to be between 20 and 120 μm.  Prohesion exposure was 28 days.  Bending fatigue resulted in one or more cracks in the coating initiating at the damage sites.  Testing was halted when crack growth in the coatings was evident.  Light microscopy revealed that the RECC system performed more poorly than the chromate-containing system.  The RECC coating was elevated and its size surrounding the damage site was greater than the chrome-bearing coating.  A slight difference in fatigue life has been observed with the RECC system performing more poorly.  Additional testing is ongoing.