Effect of Surface Pretreatment on the Underpaint Corrosion of AA2024-T3 at Various Temperatures
2006; Volume: 62; Issue: 4 Linguagem: Inglês
10.5006/1.3280663
ISSN1938-159X
AutoresDaryl A. Little, Márta A. Jakab, John R. Scully,
Tópico(s)Advanced ceramic materials synthesis
ResumoThe 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.
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