The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Etching Super Invar

Recently we etched super Invar with 1 pbw fully concentrated HCl/1 pbw DI water at RT for 8-13 minutes and found that after the etch the super Invar part would not pass water break. The parts had been previously cleaned with Enbond 160SE, an alkaline cleaner and the parts had passed water break before the etch. Any ideas on what might be happening here?

Greg Wasielewski
Aerospace - Palo Alto, California, USA
2006

---

2006

One possibility
Any oil on the surface of the etch solution can re-coat onto the part as it is removed.
Machined parts often have oil introduced into the machined surface (Beilby layer) when this is removed by the etch, the oil is released. Or it could be oil from an overhead hoist etc.
The quick fix is to return the parts to the alkaline cleaner. Check for oil in the etch and if necessary replace it - clean the tank thoroughly first.

Geoff Smith
Hampshire, England

Geoff is right. This effect is caused by the tendency of the alkalies to reduce surface tension of the rinse water. Any alkaline residue on the part surface makes the water blanket over soils and oils giving a false impression of being clean. Subsequent acid neutralizes any such residue and the water breaks.

Guillermo Marrufo
Monterrey, NL, Mexico
2006