The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Trouble with salt spray tests on class 1a finish

What is the best way to prepare test panels for mil-5541-class 1a testing? Panels are said to have too many pits ? Lab is thinking its in the cleaning of the test panels.Using alkaline pre-cleaner with a caustic etch. Then deoxidizing in a nitric acid bath.

Douglas A. Pickerd
plating line operator - Blairsville, Pennsylvania, U.S.
2005

---

2005

Douglas,

Not certain what you mean by "too many pits", are any pits acceptable? Were they there to start with or only after processing?

Regardless, a few things you may be able to do to ensure you pass the corrosion resistance test:

1. Check that the panels do not have any corrosion pits before you start processing. If the parts start with corrosion they tend to fail the salt spray test.

2. Look at your etching process, are the parts fully desmutted after the nitric acid? I am not keen on caustic etches for aluminium personally, have always found them a little too aggressive, plus they do create a smut on the surface that needs to be totally removed before any further processing.

3. Look at your conversion tank, are you getting sufficient coating (do a coating weight check to ensure you are getting at least the minimum coating weight recommended by the manufacturer).

4. Make sure you do not dry the panels above 140F (preferably not above 120 °F).

This should give you a start. I would suspect that any pitting would be found before processing or after pickling.

Brian Terry
Aerospace - Yeovil, Somerset, UK