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The corrosion environment in the food and beverage industry involves moderately to highly concentrated chlorides, often mixed with significant concentrations of organic acids. The water side of the processing equipment can range from steam heating to brine cooling. Purity and sanitation standards require excellent corrosion resistance to pitting and crevice corrosion. Sulfiting agents producing sulfur dioxide when used to treat foods include sodium sulfite, sodium bisulfite, potassium bisulfite, sodium metabisulfite, and potassium metabisulfite. All are generally corrosive to food-processing equipment. (reference)
The importance of cleanliness in food processing plants is extremely high. To eliminate bacteria and viruses from surfaces and equipment, it is imperative to adopt an effective cleaning and sanitization program. Most of these sanitizers can be quite corrosive to metals.
Underdeposit corrosion is a special type of crevice corrosion that is likely to occur in cooling systems where scales or foulants exist. The presence of general fouling and scales can cause the formation of a differential cell, which begins the process of corrosion. Due to the difference in oxygen concentration at the metal surface beneath the deposit and the oxygen concentration in the water, a differential cell forms, resulting in the corrosion reaction. The food-processing industry uses water for washing, transporting, blanching, cooking, cooling, and cleaning. In particular, heating and cooling processes require large amounts of water. Underdeposit corrosion is caused by using water in boilers, rotary cookers, and hydrostatic sterilizers. Aluminum alloys are susceptible to underdeposit corrosion. Stainless steels are also susceptible to underdeposit corrosion, as well as deep pitting. Anodic, cathodic, and filming inhibitors are used to mitigate corrosion. Biocides such as chlorine dioxide and bromine compounds (oxidizers) are used for sterilization; however, these chemicals can interfere with the performance of the inhibition system. In order to prevent corrosion, the concentration of biocides into the water stream must be controlled.
Galvanic corrosion is an accelerated attack between two dissimilar metals that are coupled in electrical contact and exposed to an electrolyte. For example, in hydrostatic sterilizers, the flight bars are made from aluminum or stainless steel, the transport chain is made from carbon steel, and both are exposed to hot water and steam. The less noble metal of the couple is susceptible to galvanic corrosion. In the case of aluminum and carbon steel, the aluminum is less noble and, therefore, will corrode. Replacement of chains and flight bars can cost up to $250,000 in a typical commercial sterilizer.
Stress corrosion cracking (SCC) of S30400 and S31600 stainless steel piping and tanks is a problem in water lines in brewery applications. A common form of stress corrosion cracking occurs at temperatures higher than ambient in the presence of chlorides. Duplex stainless steels and alloys containing molybdenum alloys are used as alternatives that are more resistant to stress corrosion cracking. Cracking may occur from the process or from the outside, for example under insulation. Reducing the amount of oxygen ingress and lowering the process temperature minimizes the possibility of stress corrosion cracking.
