SOME FORMS OF CORROSION

General Corrosion

This is the most common form of corrosion, accounting for the greatest tonnage loss of metal. It is characterized by relatively uniform attack of the entire area exposed to the corrosive environment. The passive film slowly dissolves but continually re-forms. Since the attack is linear with time, the life of equipment subject to general corrosion is reasonably predictable. If the passive film is locally disrupted, as by chlorides, corrosion modes such as pitting, crevice, and stress corrosion may occur. These are more difficult to predict and tend to cause premature equipment failures. Erosion may also remove the passive film and

contribute to much higher than expected general corrosion rates.

Stainless steel passivates simply by being exposed to air. A metallographic specimen of AL-6XN, for example, must be etched immediately after polishing. Otherwise it will passivate in air so that a uniform etch cannot be achieved. Passivation in acid is not required. But, during normal fabrication practices, enough iron is picked up to cause surface rusting in damp weather. A treatment in nitric–hydrofluoric acid may be used to remove this surface iron contamination.

Uniform corrosion rates may be stated as an average metal thickness loss with time, in mils per year or millimeters per year. A convenient rating for metals subject to uniform attack based on corrosion rates is as follows:

Excellent—rate less than 5 mils/year (0.13 mm/ yr). Metals suitable for making critical parts.

Satisfactory—rate 5–50 mils/yr (0.13–1.3 mm/ yr). Metals generally suitable for noncritical parts where a higher rate of attack can be tolerated

Unsatisfactory—rates over 50 mils/yr (1.3 mm/ yr). Metals usually not acceptable in the environment.
An approximate ranking of a few common alloys by increasing resistance to general corrosion would be 304L, 316L, 2205, 20Cb-3/825, RA333, AL-6XN, 625, and C-276. Alloy selection does depend upon the exact corrosive environment in question. Some specific examples include hot concentrated caustic, where commercially pure nickel or the 76% nickel alloy 600 is used. Alloy 33 may be required for mixtures of caustic with oxidizing chemicals. For sulfuric acid alloys 20Cb-3 or 825 are usually chosen; however, if chlorides are present in the acid, one of the 6% molybdenum grades such as AL-6XN would be preferred. AL- 6XN is used for organic acids, such as napthenic acid in refinery service. For nitric acid service chromium is beneficial, molybdenum not. Alloys commonly selected include 304L or a low-carbon version of 310. RA333 is used when the same piece of equipment must see very high temperatures, in the red heat range, in one zone and aqueous corrosion in another.

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STAINLESS STEELS
James Kelly
Rochester, Michigan

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright  2006 by John Wiley & Sons, Inc.

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