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Chromic Acid Anodize line switch to Sulfuric Acid Thin Film Anodize

Q. We are currently reviewing a switch from chromic acid anodize to thin film sulfuric and hoping to use the same line, rectifier, etc., and switch out solutions to the 15-20%/wt sulfuric from the 10% chromic. The program times and ramp rate can be changed fairly easily but there are compatibility issues involved in fixture materials (I believe Hastelloy is good for both) and tank materials (we will have to either swap the tank out or install a carbon steel liner to act as the cathode?) and the chilling coil (mild steel currently) and the electric heater (unsure of material currently and probes) on the tank. Can someone please confirm the material choice as one of the following Carbon Steels:
ASTM A131 Gr. B, ASTM A516 Gr. 70, ASTM A573, ASTM A662? or is there something different out there that is commonly used for these tanks? Also the welds, has anyone experienced failure on tank welds? Any preferred material for chiller coils or are the carbon steel grades specified above OK?
Has anyone else out there ever made this change to a current line?
I would be so happy for any information/advice one might have.
Thank you!

Sheila Diamond
Chemical Support - Halifax, Nova Scotia
March 24, 2011

A. I would use a vinyl drop in liner as the acid will slowly attack the welds. For the least problems, use aluminum for cathodes. You can find the best alloy for that at this site. Too much cathode is not a good idea. The mild steel cooling coil will not last as long as you might think. Stainless will live fairly well as long as you do not exceed 72F, even over the weekend or shutdowns. Your current heater is certainly passivated from its life in the chromic anodize solution. Make sure that the electrical heater does not leak current. You will have a significant excess of voltage which will make current adjustments a bit coarse. No problem if you have auto current control. James Watts - Navarre, Florida A. Swap the tank out, you don't want a "liner" to be your cathode in a sulfuric acid anodizing line, You'll end up with uneven coating thicknesses on your work. Typically you would want a 3-1 anode (work surface area) to cathode ratio. Ideally 6063 Al cathode strips running down the sides of the tanks with a surface area approximate to the aforementioned ration of your common loads. Polypro would be my preference for a tank material. Or, at the very least, line your existing tank with polypro. Marc Green anodizer - Boise, Idaho