Food Grade Stainless Steel – Austral Wright Metals

BRITISH STAINLESS STEEL ASSOCIATION Making the Most of Stainless Steel

Selection of Food Grade Stainless Steel for the Food Processing Industries

Introduction

Stainless steels are widely used in food and beverage manufacturing and processing industries 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.

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Food Grade Stainless Steel Used in Food Processing

Most containers, pipework and food contact equipment in stainless steels are manufactured from either 304 or 316 type austenitic stainless steels.

The 17% chromium ferritic stainless steel 1.4016 (430 type) is also used widely for such applications as 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 (SAF2304) and 1.4462 (2205) are useful for ‘warm’ conditions (i.e. over 50°C) where stress corrosion cracking (SCC) can be a corrosion risk, such as in brewery sparge tanks.

Hardenable “martensitic” type stainless steels are widely used for cutting & grinding applications, especially as knives.

Is 316 type the only Stainless Steel that is classed as Food Grade Stainless Steel?

The ‘316’ grades (1.4401 / 1.4404) are often referred to as the ‘food’ grades.

There is no known official classification for this and so, depending on the application, the equally common 1.4301 (304 type) and 1.4016 grades (430 type) may be suitable for food processing and handling, bearing in mind that in general terms the corrosion resistance ranking of grades can be taken as: –

Corrosion Hazards to Food Grade Stainless Steel

If the grade of stainless steel is correctly specified for the application, corrosion should not be encountered.

Surface finish and condition is very important to the successful application of stainless steels. Smooth surfaces not only promote good cleanability but also reduce the risk of corrosion.

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.

• Pitting and Crevice Corrosion

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.

Pitting corrosion is characterised by local deep pits on free surfaces.

Crevice corrosion occurs in narrow, solution-containing crevices or sharp re-entrant features in a structure. Examples of potential sites for crevice corrosion are under washers, flanges and soil deposits or growths on the stainless steel surface.

• Stress Corrosion Cracking

‘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.

• Intergranular Corrosion

‘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 Food Grade Stainless Steel Equipment

Effective cleaning of food grade stainless steel 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.

The cleaning methods listed are suitable for food grade stainless steel equipment.

Disinfection of Stainless Steel Equipment

Chemical disinfectants are often more corrosive than cleaning agents and care must be exercised in their use.

• Hypochlorites

Hypochlorites, chloramine and other disinfectants can liberate free chlorine, which can cause pitting.

Sodium hypochlorite or potassium hypochlorites are often used in commercial sterilising agents. If these substances are used with stainless steel, the duration of the treatment should be kept to minimum and followed by thorough rinsing with water.

At higher temperatures, chloride-containing sterilising agents should not be used with stainless steel.

Milton solutions (hypochlorite & chloride) can be very aggressive to stainless steels.

• Tetravalent ammonium salts

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

Iodine compounds may be used for the disinfection of stainless steel.

• Nitric acid

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

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.