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corrosion of duplex chrome-nickel coating

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rob768

Mechanical
Aug 3, 2005
440
Hello all, We have encountered a very serious pitting corrosion in the chromium layer of a Ni/Cr coated steel rod of a hydraulic piston. The nickel layer seems to be fully intact, but the chrome is seriously and very agressively pitted. The corrosion appears, position wise, to be related to a metal scraper ring in the cover of the cylinder, so galvanic corrosion may be an option. Anyone has any experience with similar damages?

some photo's:



]
 
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I am not the client, so it is not my decision. We are performing a failure analysis in order to solve this problem. The thing is, this island worked ok for several years, and all of a sudden these problems were discovered. We now traced it back to a major overhaul with extensive welding on the legs connected to the cylinders.
 
Looking at the third picture, I would agree with you that it could be galvanic. Does the ring of corrosion on the cylinder correspond with the scraper ring position. What is the scraper made from, and lastly, has there been any change in enviroment. (looks like an oxide layer to me, but ive been wrong many times).
 
There's a bronze ring on one type of cylinder, and a steel rim in another type. There has been no change in working environment, and the only thing suspect is extensive welding that has been done prior to the first discovery of the damage. The layer is black, but it looks burned rather than an oxide layer. There is nothing to be seen on microscopic sections other than clean edges, with no burning or oxide/corrosion products at all.
 
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