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Mild corrosion and stain in 7075 alloy

We have solution treated 7075 clad sheet (0.032 inch) aluminum alloy at 925 F and quenched in 22% glycol for 5 minutes. After washing and drying, it was found that most of the sheets had got blackish (light to dark) stains/corrosion. Recently we found the Na content in our water is 320 mg/l. What may be the reason and suggestion to avoid stain/corrosion? What may be the effect of Sodium?

R. K. Dass
Alumatherms - Stanton, California, USA
2006

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2006

Common causes for staining are an unclean surface prior to heating and a contaminated furnace atmosphere. Also, for alclad 7075 sheet, the solutionizing time must be limited to avoid surface enrichment of zinc. For 0.032" sheet, the 7072 cladding is only a nominal 1.28 mils thick. ASTM B597 [withdrawn] recommends furnace 'soak' times of 15-25 minutes for salt baths or 25-35 minutes in air; shorter if the actual metal temperature can be measured, with total time above 860 F not to exceed 30 minutes. If the sheet was previously heat treated, I would limit this time much further, e.g., to 5 minutes.

Sodium (Na+) would not cause staining, but accompanying anions could. The sodium is perhaps from Sodium Nitrate [affil links] used as a corrosion inhibitor in polyalkene glycol (PAG) solutions or hard water make-up (use DI). At minimum, monitor the PAG for pH, conductivity (for dissolved salt, up to 1% NaCl) and biological degradation. Follow any additional supplier instructions.

Add a final hot DI water rinse (dip or spray) to avoid further staining and to aid drying.

Ken Vlach [deceased]
- Goleta, California
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