Applications of High-Alloy Steels

Intergranular corrosion depends on the length of time the steel is exposed to the sensitizing temperature (500-750°C), even if made from low-carbon or titanium-or niobium-stabilized steel. 

Equipment fabricated from such a steel may undergo corrosion by condensation of even mild corrosives unless it is possible to keep it above the dew-point or to neutralize acidic condensates. This kind of corrosion can be prevented by a preliminary heat-treating at temperatures of 815-915°C. The niobium-stabilized steels respond best to this treatment. 

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. 

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Unalloyed steels can be used in air up to 550°C and low-alloy steels up to approximately 600°C. The applicability of high-alloy steels is determined by the alloy contents, with special importance attached to Cr, Si, and Al, as demonstrated in Figures 20.48,20.55, and 20.56. Water vapor and carbon dioxide in air generally worsen the scaling behavior of steels. The resistance of steels in water vapor is of particular importance in steam boilers and heat exchangers. It has been investigated in the literature at temperatures up to 800°C. 

The density of sulfur is close to that of lithium chloride. The volume of solid sulfur formed in reaction (11.9) is less than 16% of the overall volume of solid products. Sulfur dioxide formed in reaction (11.9) is rather soluble in electrolyte, so that the intrinsic pressure in the cells is enhanced negligibly. Despite this, the cases of thionyl chloride-lithium cells are made to be rather strong and the extreme corrosion activity of thionyl chloride enforces application of high-alloy steels or nickel. Sintered glass-metal pressure seals are used in thionyl chloride-lithium cells, same as in cells with some other systems. 

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