Acids high-nickel cast iron

HIgh-nIckel austenitic oast Irons. High-nickel austenitic cast irons contain large percentages of nickel and copper and are fairly resistant to acids. When nickel levels exceed 18%, austenitic cast irons are nearly immune to alkalies or caustics, although SCC can occur. High-nickel cast irons can be nodularized to yield ductile irons. 

Corrosion resistance of nickel allovs is superior to that of cast irons but less than that of pure nickel. There is uttle attack from neutral or alkaline solutions. Oxidizing acids such as nitric are highly detrimental. Cold, concentrated sulfuric acid can be handled. 

Copper is added to cast irons in special cases. Copper additions of 0.25 to 1% increase the resistance of cast iron to dilute acetic, sulfuric, and hydrochloric (HCl) acids as well as acid mine water. Small additions of copper are also made to cast irons to enhance atmospheric corrosion resistance. Additions of up to 10% are made to some high nickel-chromium cast irons to increase corrosion resistance. 

Carbon-steel tubes and cast-iron shells are least expensive, but high-sulfur crude oils may justify the use of 5 per cent chromium or even 18-8-chrome nickel tubes" particularly in cracking service. Refer to Chap. 9, pages 276 to 284. For the acid corrosion produced by salt brine, admiralty metal or occasionally cupronickel tubes are justified. Ammonia is often used as a neutralizer for acid corrosion, and it attacks admiralty metal so rapidly that admiralty metal cannot be used for such a situation. The mechanical properties of this alloy are also impaired by temperatures above 500°F, although under favorable conditions it can be used up to tube-wall temperatures of 600 F. Recommendations are as follows ... 

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