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Low-Alloy Steels Low-alloy steels with 1-2% alloying elements, for example, Cr, Ni, Cu, and Mn, offer improved mechanical strength with limited improvement in corrosion resistance. Increases in the Cr content have some marginal effects on the general corrosion resistance. Improvements in pitting resistance have been observed when Cu additions (0.2-0.3%) are made. However, where steels are used in indoor environments no significant effect of Cr and Cu additions have been noted when compared to the corrosion rates of a nonalloyed steel [58].

Stainless Steels Stainless steels vary widely in composition and are classified according to the metallurgical phase produced on solidification of the metal. Grades are designated as austenitic, ferritic, martensitic, and duplex (austenite and ferrite). In addition a further type of stainless steel is that known as precipitation hardening. The high corrosion resistance of these steels is derived from the ability of the metal to form a protective, self-repairing oxide film. This "self-repairing" ability is, however, subject to the availability of oxygen in the environments surrounding the steel; hence stainless steels are susceptible to corrosion under deaerated conditions, where upon localized corrosion (pitting and crevice corrosion) can occur. The corrosion resistance of stainless steel is the result of the addition of Cr, 11% (minimum). Chromium contents below this level provide limited improvement in corrosion resistance over that of the low-alloy counterparts. In addition, steels with the minimum Cr content and too high a carbon content (>0.03%) are susceptible to sensitization when the metal is welded or heat treated in the temperature range 500-800°C. In this case precipitation of chromium carbide (Cr23 C6) takes place close to the austenite grain boundaries, resulting in a zone highly susceptible to corrosion. This type of corrosion has also been named weld corrosion: see Section 11.5.7. Resistance to weld corrosion is improved by lowering the carbon content, rapid cooling through the 500-800°C range, or adding carbide stabilizing elements such as Ti and Nb.

Other elements that have a major effect on the mechanical and corrosion properties of stainless steel include Ni, Mo, and N. Nickel is added in varying amounts and leads to the stabilization of the austenite phase. Molybdenum and nitrogen are important alloying additions as they lead to improved strength and resistance to localized corrosion, for example, stress corrosion cracking, pitting, and crevice corrosion. The pitting resistance of a stainless steel may be "indexed" based on the popular pitting resistance equivalent number (PREN) and is obtained from the following formula:

From this formula it can quickly be seen that increasing the Cr, Mo, and N content of a stainless steel will lead to a higher, more pitting-resistant, alloy.

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