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Information on Stainless Steel Doors

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Stainless steel does not stain, corrode or rust as easily as ordinary steel, but it is not stain proof, it is also called corrosion-resistant steel or CRES when the alloy type and grade are not detailed, particularly in the aviation industry.

There are different grades and surface finishes of stainless steel to suit the environment the alloy must endure. Stainless steel is used where both the properties of steel and resistance to corrosion are required.

Stainless steel differs from carbon steel by the amount of chromium present. Carbon steel rusts when exposed to air and moisture. This iron oxide film (the rust) is active and accelerates corrosion by forming more iron oxide.

Stainless Steel contains sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion, blocks corrosion from spreading into the internal structure of the metal.

Stainless steel is widely used in the 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, it is suitable for most classes of food and beverage products processing.

Guidelines of the Materials of Construction for Equipment in Contact with Food have been published by the European Hygiene Engineering and Design Group (EHEDG).

Stainless Steel used in Food Processing

Most containers, pipe work and food contact equipment in stainless steel is manufactured from either grade 304 or 316 type austenitic stainless steel.

Is grade 316 the only Stainless Steel that is classed as Food Safe'?

Grade 316 Stainless Steel (which includes types 1.4401 and 1.4404) is often referred to as'Food Safe.

However, there is no official classification for 'Food Safe' and so, depending on the application, the equally common grade 304 (i.e type1.43010 or grade 430 (i.e. type 1.4016) is suitable for food processing and handling, bearing in mind that in general terms the corrosion resistance ranking of grades can be taken as- 1.4401/1.4404 (Grade 316)>1.4301 (Grade 304).1.4016 (Grade 430)

Grade 316 is more likely to be a requirement in watch making than necessarily for the food Industry.

Grade 304 is suitable for use in Marine Environments due to its high oxidation resistance.

Corrosion Hazards to Stainless Steel in Food Processing

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 steel. Smooth surfaces not only promote good cleanliness but also reduce the risk of corrosion.

An understanding of the types of corrosion to which stainless steel can be susceptible can be useful in identifying problems due to wrong grade selection inappropriate use of equipment.

Pitting and Crevice Corrosion.

Both crevice and 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 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 accurs in the presence of chlorides at temperatures generally above 50 degrees centigrade.

Inter-Granular Corrosion

'IGC' (Known in the past as 'weld decay') is a 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 austenitic grades of stainless steel. The risk of IGC attack is virtually eliminated if the low carbon (0.030% maximum, e.g type 1.4307) or the stabilised' (e.g type 1.4541) grades 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 and so on.

The following cleaning methods are suitable for stainless steel equipment:

  • Water and Steam
  • Mechanical Scrubbing
  • Scouring Powder and Detergent
  • Alkaline Solutions
  • Organic Solvents
  • Nitric Acid

Typical Applications of the Various Stainless Steel Types

Grade Typical Applications
420
(martenstic)
Cooking utensils, professional knives, spatulas etc.
430
(ferritic)
Table surfaces, equipment cladding and panels (i.e. components requiring little forming or welding) Used for moderately corrosive environments (e.g. vegetables, fruits, drinks, dry foods, etc)
304
(austenitic)
Vats, bowls pipe work, machinery parts (i.e.
components requiring little forming or welding) corrosion resistance superior to Grade 430
316
(austenitic)
Components used with more corrosive foods (e.g. meat/blood, food with moderate salt content etc) which are frequently cleaned, with no stationary solids and under excessive stress.
1.4539
(austenitic)
Used with corrosive foods (e.g. hot brine with solids that act as crevice formers, stagnant and slow moving salty foods)
1.4462
(duplex)
Used with hot foods ( e.g. hot brine with solids, stagnant and slow moving salty foods) Higher strength than austenitic grades. Good resistance to stress corrosion cracking in salt solutions at elevated temperatures.
6%MO types
(austenitic)
Used with corrosive foods (e.g hot brine with solids that act as crevice formers, stagnant and slow moving salty foods). Good resistance to stress corrosion cracking in salt solutions at elevated temperatures. Used in steam heating and hot work circuits, hot water boilers, ect.

Ferritic stainless steel generally has better engineering properties than austenitic grades, but has reduced corrosion resistance due to the lower chromium and nickel content. It contains between 10.5% and 27% chromium and very little nickel, if any, but some ferritic types contain lead.