Effect of Surface Pretreatment on the Underpaint Corrosion of AA2024-T3 at Various Temperatures
VIRGINIA UNIV CHARLOTTESVILLE
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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 micrometers AA2024-T3 in 80 relative humidity at 25 deg C, 40 deg C, and 50 deg C. Scribe-creep was observed to be enhanced by exposure test temperature regardless of surface pretreatment with an activation energy of 30 kJmol to 40 kJmol. 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.
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