Letter 31036

Corroding AL A380 with resin impreg and Iridite  

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We have manufactured several thousand Al A380 Die casted parts in Asia that are starting to show signs of corrosion.

The parts were cast, machined, then resin impregnated with Ultraseal PC504/66, then clear Iridited to Mil-C-5541 [link is to spec at TechStreet]-class-3.

The customer has tested some units in a salt spray chamber as per Mil-810F [link is to spec at TechStreet] Method 509.4 Procedure I. They failed miserably.

We have built similar parts in this grade with these post treatments, with great success. But that customer did not test them the same way.

I'm looking for comments and opinions on if these 2 post treatments should stand up to the test/environment.

Please note that I may use your responses in dealing with this customer. If you are not comfortable with that, please do not respond.

John Jamael
Contract Manufacturer - Ottawa, Ontario, Canada

Clear chromate on aluminum castings usually do not do well in salt spray. MIL C5541 specifies ASTM B-117 [link is to spec at TechStreet] 5% salt spray. With vacuum sealing and everything proper using Iridite 14-2 clear (class three) can pass. However, yellow chromate stands a much better chance of passing salt spray and other corrosion test methods. The difficulty in passing corrosion tests lies in the fact that the alloying constituents in the aluminum form corrosion cells. Silicon is one of the bad ones. Fluoride-Nitric acid treatment prior to chromating helps some in minimizing the silicon left on the surface to cause trouble. Porosity and a rough surface works against good corrosion protection.

Don Baudrand Don Baudrand, Consultant   Poulsbo, Washington

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