STAINLESS STEEL CORROSION DATA Stainless Steel

The ability of stainless steels to resist corrosion is related to the chromium content, for chromium is the alloying element that endows stainless steels with their corrosion resistance. By combining with oxygen, chromium forms a thin but transparent chromium-oxide surface protective film. A steel must contain not less than 10% chromium to be considered stainless. As a general rule, the higher the alloy content, the more resistant it will be to corrosion because of the passive oxide film. Because of the important role of this film, care must be observed in manufacturing or in the operation and use of stainless steel components to avoid destroying or disturbing its passivity. Normally the destruction or disturbance of the protective film can be reformed or healed in the presence of oxygen to again provide maximum protection.

The stable protective nature in atmospheric or mild aqueous environments can be enhanced by higher chromium, nickel, molybdenum and other alloying elements. Chromium improves film stability; molybdenum and chloride penetration; and nickel acid environments. For example, Type 316 contains about the same amount of chromium as Type 304, more nickel, and 2 to 3% molybdenum. Type 316 is more resistant to corrosion that Type 304, especially chloride's pitting attack, and it is widely used in chemical processing because of its increased resistance to aggressive agents.

In rural atmospheres, virtually all stainless steels will give completely satisfactory service in terms of atmospheric corrosion resistance. In industrial environments, the choice to which stainless steel to sue is fairly broad, except many users tend to prefer the 300 Series stainless steels, such as Type 304, because of their all-around good corrosion performance and ease of fabrication. While there is little concern about the sulphur content of the atmosphere, which has been known to accelerate corrosion in other metals, the primary concern with stainless steel is the presence of chlorides. In industrial area, dust and flying ash containing chlorides accumulate on flat surfaces. And if there is insufficient rainfall to wash away the accumulation, the possibility of some staining or pitting attack on the stainless steels with lower alloy content must be considered. Higher alloy grades such as Type 316 may be advisable.

As the chloride content of the industrial environment increases, such as near chemical plants producing chlorine or hydrochloric acid, corrosive stainless steels exposed in New York City for periods of up to 26 years were essentially unaffected. The same materials exposed to a chemical environment near Niagara Falls were attacked to varying degrees in much shorter periods. In such areas, the selection of materials is more critical, but the choice can still be made from among the range of stainless steel alloys.

Marine environments are also high in chlorides and are notoriously aggressive to metals, but stainless steels are preferred for construction in many coastal applications. A thin rust film may develop on the surface of those stainless steel types, but in most instances this is more a visual impairment than a functional failure from corrosion. Type 304 is used for coastal atmospheres and does very nicely especially where the metal is exposed to regular rainfall or is washed occasionally. Where staining is objectionable as with architectural applications, Type 316 is the preferred choice which has greater resistance to staining. Conditions below the surface of the ground can be fair more aggressive than those in the atmosphere, because materials are frequently submerged for long periods in soils containing water, salts, or chemicals, all of which increase the potential corrosivity of the environment. These soils are subject to a complex set of conditions that may vary from time to time, depending upon the weather and other circumstances.

Nevertheless, stainless steels are often preferred for sub-surface environments. To provide more complete data on the performance of stainless steels in underground applications, including data on many different soil conditions, the Committee of Stainless Steel Producers initiated an extended test program in cooperation with the National Bureau of Standards. The results of this program, which continue, are available from Washington Steel Corporation.

Laboratory Corrosion Resistance Tables The data contained in the following tables, while indicative of what can be expected in service, cannot be considered an absolute reference in predicting service results. Whenever possible, samples should be exposed to actual operating conditions before drawing any conclusions.