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Passivity vs. corrosion rate

We are testing the rate of corrosion of relatively pure iron with nitric acid at very low pHs. The problem is incorporating for passivation, the corrosion rate becomes complicated. Meanwhile, mechanics need an easy way of predicting how soon the iron sheets can dissolve under the acid over time. A chemist advised the usage of less concentrated nitric acid to avoid passivity altogether. I assumed passivity was a factor of concentration of reactants? But our experiments showed the degree of passivation for metals was nearly the same under many different circumstances. Is there any way passivity can be controlled?

Elena
machine shop safety inspector - Toronto, Ontario, Canada
2004

Besides dilution you may also try adding a bit of chloride (1-2%) to increase and stabilize the speed of reaction and eliminate passivation.

Guillermo Marrufo
Monterrey, NL, Mexico
2004

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Why repeat nearly 200 years of research? Michael Faraday didn't have modern equipment, but he attributed the behavior of iron in concentrated nitric acid to the formation of a protective iron oxide film. Once formed, the slow dissolution of this film determines the corrosion rate. � from Stainless Steel �77, Climax Molybdenum Co. It is quite difficult to explain passivity without diagrams. Get a book, e.g., Corrosion Handbook, Corrosion Engineering, ASM Metals Handbook Vol. 13 'CORROSION' [affil link on Amazon] or 13A, etc.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2004