Effect of Surface Pretreatment on the Underpaint Corrosion of AA2024-T3 at Various Temperatures

The effects of surface pretreatment on the rate of scribe-creep caused by underpaint corrosion on coated AA2024-T3 (UNS A92024) were investigated. Scribe-creep experiments were conducted on epoxy polyamide-coated (average coating thickness: ~10 μm) AA2024-T3 in 80% relative humidity at 25°C, 40°C, and 50°C. Scribe-creep was observed to be enhanced by exposure test temperature regardless of surface pretreatment with an activation energy of 30 kJ/mol to 40 kJ/mol. The scribe-creep rate was accelerated at all temperatures especially by pretreatments that increased the concentration of surface Cu or left a high capacity for Cu-replating. Sodium hydroxide (NaOH) etching particularly increased the amount of replated Cu at the coated metal interface compared with an as-received condition and a NaOH etch followed by a nitric acid (HNO3) deoxidation. The effect of each surface pretreatment to enhance or retard scribe-creep is traced either to the initial level of Cu replating prior to coating or to its ability to supply Cu for replating in the scribe-creep filament wake. This Cu replating enhances the rate of cathodic electron transfer reactions, which supports the galvanic corrosion process between scribe-creep head and tail. When Cu was eliminated as an alloying element, or when surface Cu was minimized at the coating-metal interface by HNO3 deoxidation pretreatment, scribe-creep corrosion rates were lowered. This was rationalized to occur as a result of a decrease in the cathodic oxygen reduction reaction rate, which supports anodic undercutting at the head of the corrosion front.