ALLOYING FOR CORROSION RESISTANCE STAINLESS STEELS

The development of alloys for controlling corrosion in specific aggressive environments is certainly one of the great metallurgical developments of the twentieth century. The basis upon which alloys resist corrosion and the causes of corrosion susceptibihty of alloys are explored in this and subsequent chapters. In general, the corrosion behavior of alloys depends on the interaction of  

The alloy of specific chemical composition and metallurgical structure. 

The environment, whether it is sufficiently aggressive to break down the protectiveness of the surface film, thereby initiating localized corrosion. 

The alloy/environment combination, controlling whether the film selfrepairs after breakdown and, if not, the type and rate of corrosion that propagates after initiation has occurred. 

The beneficial effect of alloyed chromium or aluminum on the oxidation resistance of iron has already been described (Section 11.11), in addition to the 

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Modification of the metal itself, by alloying for corrosion resistance, or substitution of a more corrosion-resistant metal, is often worth the increased capital cost. Titanium has excellent corrosion resistance, even when not alloyed, because of its tough natural oxide film, but it is presently rather expensive for routine use (e.g., in chemical process equipment), unless the increased capital cost is a secondary consideration. Iron is almost twice as dense as titanium, which may influence the choice of metal on structural grounds, but it can be alloyed with 11% or more chromium for corrosion resistance (stainless steels, Section 16.8) or, for resistance to acid attack, with an element such as silicon or molybdenum that will give a film of an acidic oxide (SiC>2 and M0O3, the anhydrides of silicic and molybdic acids) on the metal surface. Silicon, however, tends to make steel brittle. Nevertheless, the proprietary alloys Duriron (14.5% Si, 0.95% C) and Durichlor (14.5% Si, 3% Mo) are very serviceable for chemical engineering operations involving acids. Molybdenum also confers special acid and chloride resistant properties on type 316 stainless steel. Metals that rely on oxide films for corrosion resistance should, of course, be used only in Eh conditions under which passivity can be maintained. 

ACI. Alloy Castings Institute produced a system for corrosion resistant and heat resistant castings. The letter C indicates the corrosion series and the letter H indicates the heat series. For example, CF-8 is a corrosion resistant stainless steel and HK-40 is a heat resistant stainless steel. 

Aberle D., Agarwal D. (2008), High Performance Corrosion Resistant Stainless Steels and Nickel Alloys for Oil and Gas Apphcations (Paper No. 08085), Houston, TX NACE International. 

Aberle, D., Agarwal, DC. 2008. High performance corrosion resistant stainless steels and nickel alloys for oil and gas applications. NACE—International Corrosion Conference Series, Corrosion 2008 New Orleans, LO March 16-20, 2008 Code 77054, pp. 080851-0808517. 

Like the steam generator shell, tubesheets are fabricated from low-aUoy steels for their strength and economy. In some designs, the tubesheets are clad with stainless steel or nickel-base alloys for corrosion resistance. 

The corrosion resistance of steel is significantly improved by alloying with other metals for example, highly corrosion-resistant stainless steel is an alloy containing about 15% Cr by mass. 

Ferritic stainless steels depend on chromium for high temperature corrosion resistance. A Cr202 scale may form on an alloy above 600°C when the chromium content is ca 13 wt % (36,37). This scale has excellent protective properties and occurs iu the form of a very thin layer containing up to 2 wt % iron. At chromium contents above 19 wt % the metal loss owiag to oxidation at 950°C is quite small. Such alloys also are quite resistant to attack by water vapor at 600°C (38). Isothermal oxidation resistance for some ferritic stainless steels has been reported after 10,000 h at 815°C (39). Grades 410 and 430, with 11.5—13.5 wt % Cr and 14—18 wt % Cr, respectively, behaved significandy better than type 409 which has a chromium content of 11 wt %. 

Materials for metal tanks and installations include plain carbon steel, hot-dipped galvanized steel, stainless steel [e.g., steel No. 1.4571 (AISI 316Ti)], copper and its alloys. The corrosion resistance of these materials in water is very variable and can... 

In general, stainless steels are used for corrosion resistance when oxidising conditions exist. Special types, or other high nickel alloys, should be specified if reducing conditions are likely to occur. The properties, corrosion resistance, and uses of the various grades of stainless steel are discussed fully by Peckner and Bernstein (1977). A comprehensive discussion of the corrosion resistance of stainless steels is given in Sedriks (1979). 

A common chemical laboratory test for corrosion resistance is a simple exposure test using metal coupons. The ASTM standard G48 —Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution—describes a simple exposure test. The material coupons (e.g., 60 x 60 mm) are placed on a glass cradle and immersed in the solution in such a way that the coupons are evenly exposed. 

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