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Hard coat coil problem

 

Please, help!

We have hard coat anodizing tank. H2SO4 conc. 300 g/l. Max rectifier capacity 2000 amp. T = 32 F. We were using 316 SS platecoil as a cathode (and cooling coil) for 10 years. We had to buy a new one 10 month ago. It started to etch recently. What is causing the problem? (Current? Acid?) We know that Zr is the best choice, but we would like to know what is the " best buy". Is it possible to use SS as a cooling coil & use some other material as a cathode (ex. 6063 AL)?

Thank you,

Marina Metter
- Baltimore, Maryland

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300 gm/L is too high, causes very soft coating and/or rub-off. Get down to 165 gm/L or if copper alloy in the 2000 series use 200 or 225 gm/L. Stainless Steel is theoretically OK, however, in the presence of any galvanic relationships it will dissolve, find the galvanic voltage and reverse it with a small battery charger ⇦ this on eBay or Amazon [affil links] . You waste heat and power using stainless steel cathodes - instead the current standard is Aluminum alloy 6063 T5 or T6. There is controversy as to whether to weld or bolt. People who sell ready to use aluminum cathodes recommend bolting (easier to ship). I am still old fashioned and prefer welding.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina