Corrosion of Ferritic Stainless Steels

Susceptibility to intergranular corrosion also can occur in ferritic stainless steels (Ref 86-90). As with the austenitic stainless steels, the extent of the susceptibility is a function of the chemical composition and the thermal history of the steel. Also, the mechanism of intergranular attack is essentially the same for both classes of stainless steels, specifically, attack of lowered-chromium-content regions adjacent to precipitated chromium-rich carbides and nitrides. However, there are 

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 

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A more detailed treatment of sensitisation of austenitic stainless steels, of intergranular corrosion of austenitic stainless steels without sensitisation, and of sensitisation and intergranular corrosion of ferritic stainless steels and high-nickel alloys, is given by Cowan and Tedmon . 

Bond, A. P. and Lizlovs, E. A., Intergranular Corrosion of Ferritic Stainless Steel , J. Elec-irochem. Soc., 115, 233C (1968)... 

Prevention of intergranular corrosion of ferritic stainless steels is done by the same methods as for austenitic steels, with the differences in annealing temperature and... 

The stress-corrosion cracking (SCC) mode of failure was later observed even in the case of ferritic stainless steels. The only clear message from this is that the exact mechanism of failure by this mode is not well established. Alloys containing >34% Ni were found to prolong the time of SCC failure. Ferritic type alloys 430 and 434 are resistant to SCC both in MgCl2 and NaCl environments in the mill-annealed condition, but not in welded conditions. Also, welding impairs the ductility and their resistance to SCC. 

High chromium contents increase the corrosion resistance of ferritic stainless steels in highly concentrated sulfuric acid. For example, the ferritic steel 1.4575 (main components 28% Cr, 4% Ni, 2% Mo) is successfully used for applications in this field. At temperatures above 120 °C, the alloy can be used at concentrations between... 

Manufacturing and welding of ferritic stainless steels has to be done carefully to avoid the precipitation of carbides and other phases, which increase the susceptibility to localized forms of corrosion. 

K. Hashimoto, K. Asami, K. Teramoto, An X-ray photo-electron spectroscopic study on the role of molybdenum in increasing the corrosion resistance of ferritic stainless steels in HCl, Corros. Sci. 19 (1979) 3-14. 

Molybdenum in combination with chromium increases the corrosion-resistant properties of ferritic stainless steel in chloride electrolytes and is effective in increasing the resistance to pitting and crevice corrosion. Cr-Ni-Mo-Cu alloys increase the passivity in sulfuric acid concentrations with concentrations between 20% and 70%. Nickel... 

Dundas HJ, Bond AP. Niobium and titanium requirements for stabilization of ferritic stainless steels. In Steigerwald RF, editor. Intergranular Corrosion of Stainless Steel. STP 656. Philadephia ASTM, 1978. 

In general, the corrosion resistance of ferritic stainless steels is 5-10 times greater than that of austenitic steels in Pb-Li[2], For a corrosion limit of 20 um/yr, the peak operating temperature is 500°C for ferritic steels and 410°C for austenitic steels[3]. Consequently, the use of austenitic stainless steels is not recommended [6]. For temperatures in excess of 450°C, ferritic steels require Zr or Ti corrosion inhibitor additives, so their suitability for temperatures as high as 750°C is questionable. 

Fe—Cr. The Fe—Cr phase diagram. Fig. 3.1-106, is the prototype of the case of an iron-based system with an a-phase stabilizing component. Chromium is the most important alloying element of corrosion resistant, ferritic stainless steels and ferritic heat-resistant steels. If a-Fe—Cr alloys are quenched from above 1105 K and subsequently annealed, they decompose according to a metastable miscibility gap shown in Fig. 3.1-107. This decomposition reaction can cause severe embrittlement which is called 475 C-embrittlement in ferritic chromium steels. Embrittlement can also occur upon formation of the a phase. 

Xulin, S., Ito, A., Tateishi, T., et al., "Fretting Corrosion Resistance and Fretting Corrosion Product Cytocompatibility of Ferritic Stainless Steel, Journal of Biomedical Materials Research, Vol. 34, 1997, pp. 9-14. 

Relationship Between Pre-Oxidised Film Structure and Corrosion Resistance of Ferritic Stainless Steels in High Temperature Pure Water... 

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. 

Austenitic stainless steels appear to have significantly greater potential for aqueous corrosion resistance than their ferritic counterparts. This is because the three most commonly used austenite stabilizers, Ni, Mn, andN, all contribute to passivity. As in the case of ferritic stainless steel. Mo, one of the most potent alloying additions for improving corrosion resistance, can also be added to austenitic stainless steels in order to improve the stability of the passive film, especially in the presence of Cl ions. The passive film formed on austenitic stainless steels is often reported to be duplex, consisting of an inner barrier oxide film and outer deposit hydroxide or salt film. 

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. 

Xulin S., Ito A., Tateishi T. and Hoshino A. (1997), Fretting corrosion resistance and fretting corrosion product cytocompatibility of ferritic stainless steel , J. Biomed. Mater. Res., 34,9-14. (doi 10.1002/(SICI)1097-4636(199701)... 

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