letter 35082

Acid based electrolytic cleaning  

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I'm in the process of learning more about surface cleaning, finishing, and coating. I'm using the ASM Volume 5 for reference. I was wondering if anyone ever used acid for cathodic electro cleaning of carbon steel. Cathodic cleaning uses hydrogen ions for the scrubbing action instead of hydroxyl ions, so I was wondering why a basic solution is recommended for both anodic and cathodic cleaning.

Keith Davis
fuel cells - Danbury, CT, USA

Bases or alkalies are preferred because they are effective in dissolving greases and oils. Acids are not, also some unwanted metallic species may be present dissolved in the cleaning baths. Cleaning in forward polarity may result in their reduction and consequent deposition over the surface of the part instead of leaving the bare metal clean. For that and other reasons, like the brittleness of high strength steels, acid cleaning is preferred as a mid or final step after alkaline cleaning and mostly used on reverse. G. Marrufo-Mexico

Guillermo Marrufo
Monterrey, NL, Mexico

In addition to the scrubbing action of the generated gasses, alkaline cleaners are able to saponify oils and greases. Acids generally have no effect on O&G.

Jeffrey Holmes, CEF - Spartanburg, SC, USA

The scrubbing action is only one aspect of the cleaning process. All things being equal, it's a lot easier to remove organic soils with alkaline cleaner formulations than with acidic products. An acidic product also has the potential of attacking the steel surface, while you can pack the caustic into an alkaline cleaner and not do any harm to the surface.

George Gorecki
- Naperville, Illinois

My particular application for electrolytic cleaning is to remove rust and scale, therefore the breaking down of soils with an alkaline solution isn't paramount. Acid dipping is another method for attacking the oxides and I was wondering if acid in either an anodic or cathodic environment would be most effective.

Keith Davis
fuel cells - Danbury, CT, USA

US patent 6294072 seems to be relevant to your enquiry.

But one must ask - if you wish to use acid, why add the complexity of an external electrical system? - just let the acid dissolve the oxides, as is done in countless thousands of pickling tanks around the world. Electrolytic descaling has the advantage of not needing anything as aggressive as the usual pickling acids, but it swaps one set of operating problems for a different set. To superimpose an electrolytic system on an acid system seems to force on you the difficulties of both, for no overall benefit.

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

Not enough information and far too many variables for a finite answer. Anodic acid might work great for a while and then the amount of iron or copper or chloride ions reach a magic point and the parts would be pitted beyond use. Temperature is a factor as is the voltage applied. In acid cathodic, you reach a point where crud will plate out on the part which can be difficult to get off.
For heavy rust and mill scale, abrasive blasting is probably the best option.
Approach free advice with great caution.

James Watts
- Navarre, Florida

The reason I was contemplating using electrolytic cleaning in conjunction with the acid is to get the benefit of the scrubbing action of hydrogen/oxygen. I can't use abrasive blasting because it will not reach internal cavities. These internal cavities have rust (cast iron manifolds, pump housings). Normal passivation does not actively remove the scale, and I've had good luck doing both acid cleaning, and alkaline electrolytic cleaning, and I was just wondering if the combination would add some benefit. Maybe I'll check out that patent.

Keith Davis
- Danbury, CT USA

In Cathodic Electrolytic Cleaning a current impressed on the cathode. This impressed current reduces the hydrogen cations to H2 gas, which then causes a mechanical scrubbing action on the surface of a material. This mechanical scrubbing has the potential to remove scale, but has an assortment of problems associated with it. The biggest problem is contamination from other metal cations. These cations will plate on the surface of the cathode. Make sure your anode is anodically inert. Graphite should be used. Because of contamination issues many references recommend an alternating current. However, my company has used cathodic electrolytic treatment successfully on the pilot scale. Hope this helps.

Luke Autry
electroplating - Ellington, CT

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