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Spark erosion on aluminum frames while anodizing

2003

We are anodizing small bicycle components ,which are individually tied by a 20 gauge aluminum wire and these are in turn tied to an aluminum frame made of 3/4 inch square rod. For the last 30 years this was working fine but suddenly we find that some of the wires get snapped off and there is a spark erosion on the frames.

Interestingly this happen only on few pieces in a lot, and erosion happen at the place where the frame is welded by aluminum. Could this be because of leakage in Diodes in our rectifier which we had replaced? Or a loose shunt on the rectifier?
Or does it have to do anything with the chemistry or contaminations?
Does putting in a frame while current is on could be the reason for this as we get a small spark between the anode rod and the frame?

The Bath is of sulfuric acid .
We do not face this problem when we are anodizing large aluminum extrusions.

Thank you for your time, Baldev

Baldev Singh
job work - LUDHIANA, Punjab, India

This is a wild guess, but is it possible that these pieces did not make contact until the sparking occurred? In other words, they had no current on them early in the process, when the voltage was low? Can you take these "sparked" parts out immediately to see what you have, or must they continue through the process and finish up before you can get a look at them?

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003

Thank You Ted! We tried same pieces in a new bath in a small tank and they came out fine . I'll check on the loose contact but that does not seem to be the problem as we are getting these eroded pits on the frames also.

Baldev Singh
job work - LUDHIANA, Punjab, India
2003

There is no question that problems can be caused by many things, including poor temperature control, insufficient agitation, bad diodes in the rectifier, etc. But when you actually see a spark it can only be from a sudden current surge related to either suddenly making contact and previously having no anodizing, or the current for some reason suddenly breaking through the anodized film that has formed.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003

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