Molybdenum ferritic stainless steels

Steel is the most common constructional material, and is used wherever corrosion rates are acceptable and product contamination by iron pick-up is not important. For processes at low or high pH, where iron pick-up must be avoided or where corrosive species such as dissolved gases are present, stainless steels are often employed. Stainless steels suffer various forms of corrosion, as described in Section 53.5.2. As the corrosivity of the environment increases, the more alloyed grades of stainless steel can be selected. At temperatures in excess of 60°C, in the presence of chloride ions, stress corrosion cracking presents the most serious threat to austenitic stainless steels. Duplex stainless steels, ferritic stainless steels and nickel alloys are very resistant to this form of attack. For more corrosive environments, titanium and ultimately nickel-molybdenum alloys are used. 

Bond, A. P., Effects of Molybdenum on the Pitting Potentials of 18<7oCr Ferritic Stainless Steels at Various Temperatures , J. Electrochem. Soc., U8, 208c (1971)... 

Schmidt, W. and Jarleborg, O., Ferritic Stainless Steels with 17% Cr, Climax Molybdenum GmbH... 

The H2SO4-CUSO4 test, unlike the Huey test, is specific for susceptibility due to chromium depletion and is unaffected by the presence of submicro-scopic a-phase in stainless steels containing molybdenum or carbide stabilisers. It can be used, therefore, with confidence to test susceptibility in austenic (300 series) and ferritic (400 series) stainless steels and in duplex austeno-ferritic stainless steels such as Types 329 and 326. 

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... 

The corrosion resistance of stainless steels increases with their chromium content. Unfortunately, stainless steels with a high chromium content not only cost more, but they are also more difficult to work with (machining, welding) than ordinary stainless steels that contain 13 to 18% chromium. Among these, ferritic stainless steel with 17% chromium (AISI430), austenitic stainless steel with 18% chromium and 8-10% nickel (AISI 304) and austenitic stainless steel containing 16-18% chromium, 10-14% nickel and 2-3% molybdenum (AISI 316) are most widely used. 

Table 8 provides a summary of the redox potentials of various tests for austenitic and ferritic stainless steels. Because both carbides and molybdenum-rich sigma-phase may result in intergranular attack in highly oxidizing solutions, the nitric acid test should be specified for materials to be used in nitric acid or other highly oxidizing environments. 

Stainless steels are resistant to corrosion by most salts. The exceptions are the halide salts that cause pitting, crevice corrosion, and SCC. Of these salts, those containing chlorides are the most corrosive, followed by fluoride, bromide, and iodide salts. Stainless steels with higher chromium, molybdenum, and nitrogen concentrations will resist pitting and crevice corrosion more effectively. Austenitic stainless steels with higher molybdenum and nickel, ferritic stainless steels with no nickel or copper, and duplex stainless steels will resist SCC. 

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. 

Sulfate Reducing Bacteria SRBs have been implicated in the corrosion of cast iron and steel, ferritic stainless steels, 300 series stainless steels and other highly alloyed stainless steels, copper nickel alloys, and high nickel molybdenum alloys. They are almost always present at corrosion sites because they are in soils, surface water streams and waterside deposits in general. The key s5unptom that usually indicates their involvement in the corrosion process of ferrous alloys is localized corrosion filled with black sulfide corrosion products. 

One of the most effective elements added to austenitic stainless steel, and for that matter even ferritic stainless steel, in order to improve pitting resistance is Mo [8]. Molybdenum, however, is a highly versatile element, existing in the passive film in a number of oxidation states. In the case of the hexavalent state it has been observed in both the cationic and anionic states, namely as molybdenum trioxide and ferrous molybdate. It has most commonly been reported to exist in the quadrivalent state as molybdenum dioxide and oxyhydroxide. 

Residual stresses occur from welding and other fabrication techniques even at very low stress values. Unfortunately, stress relief of equipment is not usually a reliable or practical solution. Careful design of equipment can eliminate crevices or splash zones in which chlorides can concentrate. The use of high-nickel stainless steel alloy 825 (40% nickel, 21% chromium, 3% molybdenum and 2% copper) or the ferritic/austenitic steels would solve this problem. 

Ferrous materials steel, cast iron, iron, stainless steel, high-silicon iron, high-silicon molybdenum iron, high-silicon chromium iron, magnetite, ferrite. 

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