The authoritative public forum
  for Metal Finishing since 1989
  

  -----

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

---

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 ASTM B597 [withdrawn] recommends furnace 'soak' times of 15-25 minutes for salt baths or 25-35 minutes in air; shorter if the actual metal temperature can be measured, with total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 ⇦ this on Amazon [affil link] 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
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.