The authoritative public forum
  for Metal Finishing since 1989
-----

Pressure Washer Deanodizing

2002

In our maintenance facility, we use a spray cabinet washer to wash some vehicle engine parts. Recently, the washer discolored a number of anodized parts -- turning the parts into a spotty yellowish white color, from the original greenish yellow color. One of the parts became black, instead of white. Using an eddy current thickness gauge, we found coating thickness was reduced when we see the spotty white color; the black color indicates that the thickness have reached zero. Can anyone explain to us what happened? What can we do to prevent that? Can hot tap water de-anodize?

The spray cabinet washers we use involve pressure-spraying detergent-water solution onto the parts. The solution is heated to 160degF, and re-used. We filter and chemically treat the solution every 40hrs of washer run time.

We wash 16 different anodized components in the washer, but only 2 changed color. One of the discolored parts is AL 7075-T7352 coated IAW ASTM B580. The other one is AL 6061-T6 coated i.a.w. ASTM B580 Type A. Both are supposed to have dichromate sealed.

Stanley Poon
- Newport, Rhode Island

---

Since only a few parts discolored, it could be a sealing problem, but that would probably not explain the black part. When aluminum is etched with caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] in preparation for anodizing or plating, it will often exit the etch tank gray-black because of copper or other alloying materials left behind. This sounds a lot like the color you are describing. Caustic soda is also used to quickly and easily strip anodized coatings. You need to be very careful about the alkalinity of the detergents you use, and I think this would be my first suspect.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002

---

2002

I would agree with comments thus far; look at seal and alkalinity (pH) of cleaners. Also, chlorides in tap water can be high enough to cause aluminum fits, especially at elevated temperature.

Secondly, if parts are heated above a few hundred degrees, it's likely the seal has been dehydrated and no longer protective. If that's the case, the anodize is very prone to attack by your washer.

Lastly, 7075 is a very corrosion prone alloy. We leave 6061 parts racked over night in the anodize department without much concern, however, leaving 7075 out over night in the acidic environs of the anodize department, they form a white corrosion product. I believe this is related to the magnesium content in 7xxx series alloys.

Hope this helps...

Milt Stevenson, Jr.
Syracuse, New York