Copper
Copper is generally resistant to reducing acid solutions containing only low levels of oxygen but is readily attacked by oxidizing acids. These include nitric, sulfurous, and concentrated sulfuric acids as well as solutions containing oxidizing salts, such as ferric chloride.
Copper in solution tends to reduce the corrosion rate of stainless alloys in (reducing) sulfuric acid. Those alloys intended for use in environments containing much sulfuric acid invariably have some copper as an alloy addition. These include 904L, 20Cb-3 , 825, Nicrofer 3127 hMo (alloy 31), and the Hastelloy alloys G, G-3, and G-30.
Additions of some 4 or 5% silicon increase resistance to corrosion in oxidizing environments. The silicon is primarily used in alloys meant to withstand concentrated, hence oxidizing, sulfuric acid. Such alloys in current production include A610, A611, Sandvik SX, and Haynes D-205TM. The use of silicon as an alloying element for corrosion resistance dates back to before World War I. Although not a stainless steel, one of the oldest and most generally corrosion resistant alloys ever developed is the 14.5% silicon cast iron, Duriron . A silicon oxide film is believed responsible for this grade’s useful resistance to environments ranging from oxidizing to reducing. This includes seawater, and organic and many inorganic acids, though not halogen acids. A further addition of 3% molybdenum provides resistance to hot hydrochloric acid. Lack of strength and ductility limit the cast iron’s range of use.
STAINLESS STEELS
James Kelly
Rochester, Michigan
Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright 2006 by John Wiley & Sons, Inc.
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