Metal finishing Q&As since 1989

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Anodizing pitting problem

The problem is material anodizing is pitting. Not all the time mind you. It has been stated that when both the welder (high frequency) and the rectifier are in use the problem occurs. But not all the time. Does this have to do with shielding or lack of. The solution. Or something else?

Richard Ilnisky
- Saskatoon, Saskatchewan, Canada
2003

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I don't think enough info has been provided for anybody to do anything but take a guess. But if the problem happens while welding, and not at any other time, it certainly does sound like the electronic noise of the welder, or simply the current draw, may be upsetting the SCRs on the rectifier.

Aluminum requires careful voltage control, but especially early on to midway through the anodizing sequence. Here's why: bare aluminum is highly conductive, anodizing is quite resistive. So the voltage/current must be very low while it's still bare or almost bare, or it will burn and pit. But once you have substantial anodizing on it, no current will flow unless you have ramped up the voltage as the anodizing thickness ramps up. If random factors cause the voltage to go too high too fast, you can burn through the fledgling anodizing and it will continue to burn and pit because the current will be too high in that area.

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