Acids high-chromium cast irons

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 ... 

Corrosion can have very detrimental effects on the wear resistance of certain white irons. If the matrix wears out and exposes the carbides to a corrosive environment, they may quickly deteriorate, causing a further loss of wear resistance. Manufacturers of slurry pumps have gone beyond the specifications of ASTM A532-87 to increase the chromium contents to 30% with 50% chromium carbides in a martensitic matrix. These materials have found applications in phosphate matrix and phosphoric acid pumping. These new alloys are intermediary between white irons and superalloys. Walker (1990) reported that the use of these alloys leads to a substantial increase of the wear life over conventional white iron castings (Ni-hard, 27% high chrome). 

---