Acid Corrosion Inhibitors

[muth]

Can anyone explain in laymans terms how corrosion inhibitors work in acid pickling processes? Why does mill scale dissolve while the non-oxide base metal is not affected?

 

[kenvlach]

Inhibitors are substances that attach strongly to the freshly cleaned metal surface; the adsorbed layer then protects the metal from the acid. High strength steels are also protected from hydrogen embrittlement (by suitable inhibitors).

Inhibitors can be inorganic (chromates, nitrates, phosphates, silicates, etc.) or organic (amines, sulfur compounds, proteins). All of these have either an anion or a polar functional group that strongly coordinates with the metal surface, essentially forming a monolayered compound. The selection depends upon the metal being cleaned and the acid. When pickling prior to plating, the inhibitor must also be easily removed so as to not prevent plating or contaminate the plating solution. Some brand name products include Pickle-Pal® from Columbia Chemicals and the Rodine line of products from Henkel Surface Technologies.

See pages 524-525 plus Index listings under ‘Inhibitors’ in ASM Handbook, Volume 13, Corrosion for more information and other applications of inhibitors.

NACE has a lot of info on inhibitors: A search for Inhibitors at

http://www.nacestore.com:8765/query.html?

yields 465 results.