Anodize chipping / flaking after pressure test

"We had some tubes and endcaps (7075) hard anodized and sealed. The next day we sent the parts out for pressure test. The test put the parts under the same pressure as being underwater at 6000 meters (8700 psi). When the parts were taken apart the anodize was missing on the inside of the face o-ring. Almost as if the anodize had flaked off. Any ideas? Did we send the parts to pressure test too soon?"

Let me first preface my remarks by saying I have no idea what I'm talking about.

Are these those ocean bottom seismometer capsules? Just guessing.

Some 6000 meters [8700 psi] is quit a squeeze and maybe compacting the T-6 Aluminum 7075 hard anodized tubes to a point where the hard anodized compresses along with the aluminum tube, lets say, "By quit a bit" whatever that means and when relieved of pressure and allowed to expand, the tube expands and the hard anodized coating stays put. In a nano way of thinking. Therefore 'tearing' the anodized coating away from the anodized surface of the aluminum? Two materials contracting and expanding at different amounts?

Either that or whatever is between the aluminium and the hard anodizing is causing an adhesion problem between the two substrates and is being exacerbated by compacting and expanding from pressure. Also thinking that the threaded aluminium 'near the O-ring area' just might be a tad thinner and distort a bit more. Hence flaking?

Think about double titanium hulled submarines. Tie a line about 18" high port/starboard tightly at the surface and when submerged that line could touch the deck. Likewise tight when submerged will pull the line apart again and again when surfacing. These boats are rubber lined inside, no paint. I'm thinking the same thing is happening with these tubes. Circles can't easily be crushed. That is why submarines are round.

Just talking out loud with no knowledge.

So how much does aluminium contract and expand under 6000 meters of pressure? How much does hard anodizing contract and expand under 6000 meters of pressure? And what makes hard anodize 'stick' to aluminum? And "Did we send the parts to pressure test too soon?". Does hard anodize need to "sit and normalize"?

Henry Ford when casting the Ford Model "T" engine blocks let them sit outdoors for six months to 'normalize' and to limit grey iron castings from cracking when machined. This was before a vanadium steel metalergy change of 1905 used in the Model "T" era. FIFO. Ford ordered 3,000,000 pounds of the stuff in 1908.

My experience with aluminum is we loose 3" in length by the time we weld up a 150 foot boat. The stuff wiggles all over the place. And shrinks. When welded and measured, we cut the boat apart, jack her back to size, and weld her up again. Sometimes we build our same boat three times.

skip.

Last edited by skip (02/08/2010 - 03:58 PM)