Stainless steels are common in the food and beverage industry for manufacture, bulk storage and transportation, preparation and presentation applications. Depending on the grade of stainless steel selected, they are suitable for most classes of food and beverage products. The 316 and 304 grades are often referred to as the food grade stainless steel. There is no known official classification for this and so, depending on the application, the equally common 1.4301 and 1.4016 grades may be suitable for food processing and handling.
Most containers, pipework and food contact equipment is manufactured from either 304 or 316 type austenitic stainless steels. The 17% chromium ferritic stainless steel (430 type) is often used for splashbacks, housings and equipment enclosures, where corrosion resistance requirements are not so demanding. In addition to these non-hardenable austenitic and ferritic types higher strength ‘duplex’ types, such as grades 1.4362 and 1.4462 are useful for ‘warm’ conditions (i.e. over 122°F) where stress corrosion cracking be a risk, such as in brewery sparge tanks. Hardenable “martensitic” type stainless steels are widely used for cutting & grinding applications, especially where knives are needed.
If the grade of stainless steel is correctly specified for the application, corrosion should not be encountered. The types of corrosion to which stainless steels can be susceptible can be useful in identifying problems due to wrong grade selection or inappropriate use of equipment.
- Crevice and pitting corrosion (local deep pits on free surfaces) occur under washers, flanges and soil deposits or growths on the stainless steel surface.
- Stress corrosion cracking is a localized form of corrosion characterized by the appearance of cracks in materials subject to both stress and a corrosive environment.
- Intergranular corrosion formerly known as weld decay is the result of localized attack, generally in a narrow band around heat affected zones of welds. More likely to occur in the standard carbon austenitics.
Effective cleaning is essential in maintaining the integrity of the process and in prevention of corrosion and maximizing safety in the food processing industry. The cleaning method selected and the frequency of its application depends on the nature of the process, the food being processed, the deposits formed, hygiene requirements etc. The cleaning methods suitable for stainless steel equipment are:
- Water and Steam
- Mechanical Scrubbing
- Scouring Powder and Detergents
- Alkaline Solutions
- Organic Solvents
- Nitric Acid
Chemical disinfectants are often more corrosive than cleaning agents and care must be exercised in their use.
- Hypochlorites, chloramine and other disinfectants can liberate free chlorine, which can make equipment susceptible to pitting.
- Sodium hypochlorite or potassium hypochlorites are commonly found in commercial sterilizing agents. If these substances are used with stainless steel, the duration of the treatment should be minimal and followed by thorough rinsing with water.
- Chloride-containing sterilizing agents should not be used with stainless steel at higher temperatures.
- Milton solutions (hypochlorite & chloride) can be very aggressive to stainless steels and should be avoided.
- Tetravalent ammonium salts are much less corrosive than hypochlorites, even when halogens are present in their formulation.
- Iodine Compounds may be used for the disinfection of stainless steel.
- Nitric acid has strong bactericidal action, even in low concentrations and can be a low cost disinfectant for stainless steel equipment, especially in dairies and pasteurizing equipment.
Operation equipment often contains gaskets or other components that can absorb or retain fluids. These liquids may be become concentrated by evaporation and corrosion may develop as a result. Equipment should be disassembled occasionally for thorough cleaning to prevent corrosion and unsanitary conditions from emerging. If the disassembled equipment exhibits corrosion (crevice corrosion usually), then the corroded surfaces should be cleaned and a maintenance schedule with should be written up and observed frequently.