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topic 61101

Electrolytic acid works on outside of welded steel tanks, but inside rusts

May 6, 2020

Q. We are setting up an industrial scale electrolytic cleaning process for welded steel tanks. The post weld scale removal/oil removal on the outside is great. The inside of the tanks are getting an excessive amount of rust forming as soon as air hits the surface. It seems the conversion coating that forms on the outside isn't forming on the inside. Tanks are the cathode, we have used steel and stainless anodes with the same result. 15-20% citric acid to RO water solution, with sodium citrate for conductivity. We have tried a secondary anode inside the tank, as well as running a extra supply line of fluid to the inside of the tank, with no net gain. I suspect that free oxygen is building up on the inside of the tank with nowhere to go, but thought a supply line of fluid would mitigate this. Any advise appreciated

Caleb Cordova
- Hartford, Wisconsin, USA

May 8, 2020

A. Caleb, it sounds to me like you have a bipolar effect - charge on the outside of your welded tank is cathodic, but the inside is anodic. I suggest you use a conforming anode that goes inside the tank and polarise it to the same current density or maybe a bit more, as the outside of your tank, but with a different power supply.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

May 8, 2020

Q. Thanks for you reply! I had considered the possibility that we had a bipole forming at the tank skin. I tried a secondary anode, but run off of the same power supply. It had no effect at all. Why do you suggest a different power supply? Thanks again for your reply!

Caleb Cordova [returning]
- Hartford, Wisconsin, USA

June 15, 2020

A. I would not run a second rectifier for the same process cell as your control of overall current may not be as precise as you need it. Running off of the same rectifier should not cause an issue unless there is an electrical loss somewhere along the bussing or wires.

I agree this is most likely a bi-polar effect because there is close to zero current going to the ID of a tank if the current is only applied from the outside. Remember, electricity goes towards the path of least resistance and in this application is limited to "line of sight" between cathode and anode.

i Brown
- Cleveland, Ohio

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