Stainless Steels for Food Processing – Austral Wright Metals
Application Data Sheet – Stainless Steel
Selection of Stainless Steels for the Food Procession Industries
Selection of Stainless Steels for the Food Processing Industries
Stainless steels are widely used in food and beverage manufacturing and processing industries for manufacture, bulk storage and transportation, preparation and presentation applications.
Stainless Steels Used in Foor Processing
Most containers, pipework and food contact equipment in stainless steels are manufactured from either 304 or 316 type austenitic stainless steels.
Is 316 type the only Stainless Steel that is classes as the ‘Food’ Grade
The ‘316’ grades (1.4401 / 1.4404) are often referred to as the ‘food’ grades.
Austral Wright Metals offer two new generation ferritic stainless steels, which are excellent alternatives to 316 (445M2) or 304 (AWM 404GP®). They have corrosion resistance at least as good as the grade they can replace, and the corrosion resistance ranking is:
445M2 > 316 > AWM 404GP® = 304 > 430
Corrosion Hazards to Stainless Steels in Food Processing
If the grade of stainless steel is correctly specified for the application, corrosion should not be encountered.
The most challenging conditions encountered in food processing equipment are often from chemicals used for cleaning & sterilisation, and the chemicals and steel grades must be chosen to be compatible.
The types of corrosion to which stainless steels can be susceptible are summarised below. This can be useful in identifying problems due to wrong grade selection or inappropriate use of equipment.
Both crevice and pitting corrosion occur most readily in aqueous chloride-containing solutions. Although attack can occur in neutral conditions, acidic conditions and increases in temperature promote pitting and crevice corrosion.
‘SCC’ is a localised form of corrosion characterised by the appearance of cracks in materials subject to both stress and a corrosive environment. It usually occurs in the presence of chlorides at temperatures generally above 50°C. It may occur in austenitic grades, but ferritic and duplex grades are not susceptible to SCC in the conditions encountered in food processing.
‘IGC’ or ‘ICC’ (known in the past as ‘weld decay’) is the result of localised attack, generally in a narrow band around heat affected zones of welds. This is more likely to occur in the ‘standard’ carbon austenitics. The risk of IGC attack is virtually eliminated if the low carbon (0.030% maximum, eg 1.4307, 304L) or the ‘stabilised’ (eg 1.4541, 321) types are selected.
Cleaning of Stainless Steel Equipment
Effective cleaning is essential in maintaining the integrity of the process and in prevention of corrosion. The choice of cleaning method and the frequency of its application depends on the nature of the process, the food being processed, the deposits formed, hygiene requirements etc.
Disinfection of Stainless Steel Equipment
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 cause pitting.
Tetravalent ammonium salts are much less corrosive than hypochlorites, even when halogens are present in their formulation. Laboratory tests have shown that tetravalent (including quaternary) ammonium compounds are at least as effective as hypochlorites as sterilisers.
Iodine compounds may be used for the disinfection of stainless steel.
Even at low concentrations, nitric acid has a strong bactericidal action and can be a low cost disinfectant for stainless steel equipment, especially in dairies and pasteurising equipment.
Maintenance of Food Process Equipment
Stainless steel equipment often contains gaskets or other components that can absorb or retain fluids. These liquids may be become concentrated by evaporation and corrosion may ensue. Equipment should be disassembled occasionally for thorough cleaning. If the disassembled equipment exhibits corrosion (crevice corrosion usually), then the corroded surfaces should be cleaned.
Gaskets should be made of materials which are inert in the service conditions – processing, cleaning and sterilising. In addition, they should be non-porous, and should be made of materials which do not contain chloride.
Typical Applications of the Various Stainless Steel Types
Comprehensive testing has shown that the new generation ferritic stainless steel grades offered by Austral Wright Metals have at least the corrosion resistance of the older austenitic grades in food processing and service environments, including cleaning and sterilisation chemicals.
Austenitic grades 304 and 316 may be susceptible to stress corrosion cracking in the presence of more than about 20 ppm of chloride (salt) at temperatures of 50°C and above. The new generation ferritic grades AWM 404GP® and 445M2 are not susceptible to stress corrosion cracking in the conditions used for food processing.
Kitchen equipment in AWM 404GP®
Food processing factory with 445M2 walls and ceilings.