Characteristics of Food Contact Surface Materials: Stainless Steel

This technical review article describes the properties and characteristics of stainless steel in sanitary design when used as a food contact surface, particularly when compliance with the requirements of standards promulgated by 3-A Sanitary Standards, Inc. is intended. It discusses the general characteristics of stainless steels, including ferrite, austenite, cementite, and martensite. It discusses stainless steel categories and grades used in industrial applications and their properties, including both non-magnetic types such as superaustenitic and magnetic types such as ferric and martenistic as well as duplex stainless steel and precipitation hardened stainless steel. Specific emphasis is given to those categories and grades used in food contact applications. The article contains a discussion on the general steps in stainless steel manufacturing, including melting/casting, forming, heat treatment/annealing, descaling, cutting, and finishing, and describes common types of acceptable surface modification and finishing of stainless steel. In addition to the topics listed above, the article describes common types and causes of corrosion of stainless steel, including uniform, pitting, crevice, stress cracking, galvanic, contact, and biologically and microbiologically influenced corrosion. Finally the article discusses methods of preventing corrosion, including surface maintenance and cleaning and a process referred to as “passivation.” This article contains micrographs showing, at varying levels of magnification, chemical corrosion and some common surface finishes, including number 4 (150 grit), 2B, and 2D finishes, as well as photographs showing unacceptable product contact surface finishes of welds in stainless steel tubing.