Ferritic stainless steels, corrosion carbonate

Because of the greater carbon and nitrogen contents of the intermediate-purity ferritic stainless steels, prevention of susceptibility to intergranular corrosion is more difficult than with the ultrahigh-purity alloys. Small amounts of niobium and/or titanium are added to combine... 

One material that has wide application in the systems of DOE facilities is stainless steel. There are nearly 40 standard types of stainless steel and many other specialized types under various trade names. Through the modification of the kinds and quantities of alloying elements, the steel can be adapted to specific applications. Stainless steels are classified as austenitic or ferritic based on their lattice structure. Austenitic stainless steels, including 304 and 316, have a face-centered cubic structure of iron atoms with the carbon in interstitial solid solution. Ferritic stainless steels, including type 405, have a body-centered cubic iron lattice and contain no nickel. Ferritic steels are easier to weld and fabricate and are less susceptible to stress corrosion cracking than austenitic stainless steels. They have only moderate resistance to other types of chemical attack. 

Corrosion of Stainless Steels in Acids Stainless steels are iron-based alloys with chromium as the main alloying element. The most interesting alloys for technical applications are ferritic stainless steels, austentic stainless steels, and duplex stainless steels. The distinction between the stainless steels comes from their different crystallographic structures. Ferritic-martensitic stainless steels and martensitic stainless steels have less nickel and a higher carbon content and can be hardened by heat treatment. The corrosion behavior of these steels is mainly influenced by the formation of carbides, which generally increase the corrosion rate. 

Table 9.1 summarizes environmental alloy combinations that have been shown to produce see. The test temperature accelerates the See for most of the systems listed in Table 9.1. Electrochemical methods and stress corrosion tests should be performed to evaluate possible corrosion environments for a given alloy. More information on these and additional systems may be found in the ASM Metals Handbook [30]. Other significant alloys include nickel alloys [31], austenitic stainless steel [30], carbon steels [32], copper alloys [33], ferritic, martensitic, duplex [31,32], titanium alloys [33], and aluminum alloys [34].

Ferritic stainless steel has the reputation of being less sensitive to intergranular corrosion than austenitic stainless steel. This type of corrosion can nevertheless take place under certain conditions of thermal treatment [20]. The diffusion coefficients of both carbon and chromium in ferrite are larger than in austenite. Grain boundary precipitation of carbides and nitrides of chromium can therefore occur at temperatures of 540-600 °C already. The behavior differs from that of austenitic stainless steel, which becomes sensitized at higher temperatures only. Because of the larger diffusion... 

The ferritic stainless steels contain 15-30% chromium with a low carbon content (0.1%). Resistance to atmospheric corrosion of the ferritic grades will depend on the chromium content as well as the condition of exposure. 

Field studies (exposure tests) in marine or simulated marine environments demonstrated the much better corrosion resistance of stainless steels in concrete. After 4.5 years in natural marine conditions no cracking and no pitting corrosion occurred on an Fe-11% Cr alloy (Hewitt and Tull-min, 1994). Under accelerated chloride ingress the same alloy showed some pitting corrosion after one year, whereas specimens with plain carbon steel had already cracked. A 9.5 years exposure program on steels embedded in concrete containing up to 3.2% chloride additions with respect to the cement content showed that ferritic stainless steel with 13 % Cr showed corrosion at chloride levels over 1.9% (Treadaway etal., 1989). 

The ferritic stainless steels, such as types 405 and 430, should be considered when the potential exists for SCC. The corrosion resistance of ferritic stainless steels is improved by the increased addition of chromium and molybdenum, whereas ductility, toughness, and weldability are improved by reducing carbon and nitrogen content. 

Stainless steels. When chromium is present in amounts in excess of 12%, the steel becomes highly resistance to corrosion. There are several types of stainless steel which are summarized below. Ferritic stainless steels. Ferritic stainless steels contain between 12 and 25% chromium and less than 0.1% carbon. This type of steel cannot be heat treated, but may be strengthened by work hardening. 

In order to preserve the structural integrity and corrosion performance of the new generation of ferritic stainless steels, it is important to avoid the pickup of the interstitial elements carbon, nitrogen, oxygen, and hydrogen. In this particular case, the vendor used a flow rate intended for a smaller welding torch nozzle. 

Martensitic Stainless Steels. The martensitic stainless steels have somewhat higher carbon contents than the ferritic grades for the equivalent chromium level and are therefore subject to the austenite—martensite transformation on heating and quenching. These steels can be hardened significantly. The higher carbon martensitic types, eg, 420 and 440, are typical cutiery compositions, whereas the lower carbon grades are used for special tools, dies, and machine parts and equipment subject to combined abrasion and mild corrosion. 

Compared with ferritic carbon and low-alloy steels, relatively little information is available in the literature concerning stainless steels or nickel-base alloys. From the preceding section concerning low-alloy steels in high temperature aqueous environments, where environmental effects depend critically on water chemistry and dissolution and repassivation kinetics when protective oxide films are ruptured, it can be anticipated that this factor would be of even more importance for more highly alloyed corrosion-resistant materials. 

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