aluminum corrosion leading to leaky valve?

[djk]

I am working on refrigeration system valves. I am currently developing a new valve housing machined out of uncoated 7075-T651 material. This housing is a tube about 2 inches long and 3/8" diameter containing a valve seat with a 50 degree conical shape into which an 1/8" ball is seated. I am finding high leakage on the seat when compared to a similar seat out of anodized 6061. After the seat is honed with lapping compound the leakage is acceptable. Additionally, after sitting on the bench a few days, a thin film of what I assume is aluminum oxide forms. This coating is white and has a fine powdery appearance and is fixed to the aluminum but forms only on the inside of the part. The outside of the part appears shiny and freshly machined. <br>
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Questions: <br>
What is the physical mechanism by which aluminum oxide forms? <br>
Is this coating hard enough to prevent a steel ball from forming a seal? <br>
Inexpensive ways to retard the formation of this layer? <br>
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I appreciate any information on these topics. Thank you.

 

[Frederick]

At first guess, I would say you have electrolitic corrosion created by the difference between the two metals. In short, you are creating a small battery. You can probably stop the problem by annodizing the aluminum.

 

[Naren]

Frederick is correct about the galvanic cell created. The white coating is aluminium oxide. The lapping action is what actually removes it. It starts rebuilding itself. This is what gives it corrosion protection, similar to stainless steel passive layer. <br>
The corrosion mechanism: the Al acts as the sacrificial anode when in contact with the steel ball, in liquid. You have to select the ball material to be compatible to Al. See a galvanic series in a corrosion/metallurgical book or consult a specialist<br>
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Naren<br>
naren.sukaih@sapref.com