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Is the passive film important? How is it formed?

Yes, it is a very important factor against corrosion. Stainless steel is an iron and chromium alloy (and sometimes contains nickel, molybdenum and other metals). All theses metals have great capacity to react with the environment. Stainless steel does not easily react with the environment due to the features of some of those alloys (particularly chromium) to form a film that is adherent, thin and resistant to oxides and hydroxides on the material surface. The passive film is an iron and chromium oxy-hydroxide that is spontaneously formed in the presence of water (air humidity, for example). There are different passive films for stainless steels. The film depends on the material finishing (more or less rough surface) as well as on the medium it was used to form it. Oxidizing media in general help to form and preserve the passive film. Passive films formed on oxidizable steels by means of nitric baths have better quality, are more homogeneous and have oxy-hydroxides richer in chromium. Frequently the treatment known as passivation consists of a nitric acid bath (15/20%) in which the material is kept for a certain time (normally, not more than 30 minutes).
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