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White Corrosion on 6061 Alodine 1500cc, Salt Spray

2003

I appreciate the search function for this website - this allowed me to read all there is on this topic in general. There was much on 2024 anodize and salt spray. We are seeing what I suppose ought to be called white corrosion (white splotchy areas) on 6061 aluminum that has been treated with Alodine 1500 clear coat. There don't appear to be pits - or at least they are not naked eye - apparent, w/o magnification. The de-ox has a 24.5 ml titraton per the manufacturer's test procedure (which stipulates a 70 ml titration max -- this is where you dump the solution). Expressed as "aluminum," this is 6 g/L out of a 17.2 g/L max. The de-ox is protected by a TDS spec we hold on alkaline final rinse of 175 ppm max (the water is cycled through act carbon and mixed bed exchange resin to remove detergents and TDS and not all that often dumped). The recycle water from carbon and resin bottles is tested very often to make sure the bottles are working well. Thus, I don't think we've put alkaline junk into the de-ox. The Alodine started this mis-behavior when we did an add, moving the concentration from 1.22 oz./gal. up to 1.31. This may be a coincidence (?). About 65 to 70% of the chrome in the 1500 cc is hex. I think we are headed in the wrong direction, but not a disaster yet. This is a little different than earlier letters related to salt spray.

Any thoughts? Would much appreciate!

David H. Duncan
- Ponderay, Idaho, USA

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Not quite enough info ˜ have you tested for the presence of a chem film coating? Re concentration: Are units fluid oz./gal? When titrating for hex. chrome, should be at 1.0 vol.% Alodine 1500. What is the pH? (should be about 2.4). The usual poor chem film suspects: caustic etch (little or none is best), deox. (some leave immersion-plated Fe; try 50% nitric), pre-chem film rinsing (DI water is best), post-chem film rinsing (too hot, final rinse should be DI), drying (too hot, keep < 140 F).

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.
2003

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Thanks for the reply! Yes, I could give a little more info. The coating weight is on the low end of the range we find OK -- it is 4 mg/sq. ft. -- and we have a number of times seen it this low before without the problems we see now, and it has been as high as 10 mg/sq ft at times. Yes, I am speaking of fluid oz/gal, and the volume % Alodine 1500 is 1.23% (this is 1.31 oz/gal per the manufacturers test method titration, iodometric). The pH is 2.6, and I guess we could trim it down to 2.4 with a little nitric acid... We do not do alkaline etch prior to the Alodine. Our De-Ox is 100 g/L nitric acid, and the hexchrome is 7 g/L, about where the manufacturer recommends. We do several rinses after De-Ox, and the final is high quality DI. The final post-Alodine rinse is DI ambient temp. We do air drying of the parts. I hope this further info gives more insight.

Thanks again.

David H. Duncan
- Ponderay, Idaho, USA
2003

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2003

David -

We have done designed experiments where we have found that the deox Al level is the main culprit for salt spray failures on Alodine type conversion coatings. Often we saw pitting by the time we got to 5 or 6 g/L aluminum. Unless you are going to regenerate your deox tank I suggest you schedule a dump for it. The Alodine tank should last a long time.

Hope this helps.

Terry Tomt
- Auburn, Washington, USA

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Do not dump anything until you check the "deI water" for chlorides. 50 ppm is OK for ferricyanide accelerated chromates, but you need to ask the Alodine folks what the limits are for non-ferricyanide 1500. Just splash a drop of 0.1N Silver Nitrate into the water. If it clouds you are above 50 ppm Chloride.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2003

I just wanted to say thank you to the folks who responded. I've got a hunch that the aluminum content of the deoxidizer was it. We're OK on the quality of the DI (chlorides). If others have found corrosion problems with aluminum getting above 5 or 6 g/L in the de-ox, then our tank at 6 g/L must be it. Goes to show that you have to know more than the specs tell! Thanks again, and thanks, too, for the website!.

David H. Duncan
- Ponderay, Idaho, USA
2003