Abstract
The AL 29-4C® alloy (S44735) is a ferritic stainless steel containing 29 percent chromium and 4 percent molybdenum. With this chromium and molybdenum level, the alloy shows excellent resistance to chloride ion pitting and crevice corrosion. This resistance has led to the installation of about 3 million meters of thin-walled condenser tubing in sea water cooled utility heat exchangers. In addition, a variety of brine applications are in operation including service in geothermal fluids.
The S44735 alloy is used only in thin section thickness (1.7 mm - 0.065" maximum) because of ductile to brittle impact transition considerations and most of the applications are in thinner sections than 1 mm. (0.039"). Applications in petroleum refining and chemical processing are reviewed that accomodate the gauge limitations while utilizing the high level of corrosion resistance of the material. Typical are 1 mm (0.039") and 0.7mm (0.028") wall tubing and 0.5mm (0.020") tower packing applications. Organic acids and refinery streams in which some chloride ion is present are typical environments.
The material class can be embrittled by exposure to temperature of 315 to 565C (600 to 1050F) or by exposure to hydrogen charging as from over-enthusiastic cathodic protection. The special condition of Dissimilar Metal Crevice attack which can occur when a pitting resistant alloy is in contact with a non-resistant alloy is also discussed.
