Which stainless steel grade is best for food manufacturing environments?

APC Technology

Tuesday, 16 May, 2017

Which stainless steel grade is best for food manufacturing environments?

Stainless steel is a superior material to use in manufacturing equipment and it does actually stain less easily than other iron-based metals — but it is not completely stainless. Inevitably it will be marked by fingerprints and grease, develop discolouration and eventually rust. However, stainless steel can withstand more abuse before showing wear and tear than other build materials.

The benefit of using stainless steel over other metals is its innate ability to form a passive layer that prevents corrosion. The chromium which is found in stainless steel reacts with oxygen environments much the same as iron, which results in rust. However, only a very fine layer of chromium will oxidise and the chromium oxide is highly durable and non-reactive. It adheres to stainless steel surfaces and won’t transfer. It is also self-renewing; if it’s removed or damaged, more chromium will react with oxygen and replenish the barrier. Therefore the higher the chromium content, the faster the barrier repairs itself.

Additional benefits of using stainless steel include:

  • superior resistance to chloride;
  • high and low temperature resistance — resists scaling and maintains high strength at high temperatures while also showing exceptional toughness at cryogenic temperatures;
  • improved resistance to pitting and crevice corrosion;
  • ease of fabrication;
  • strength — the cold work hardening properties of many stainless steels can be used in design to reduce material thickness and therefore reduce weight and costs;
  • aesthetically appealing;
  • life cycle characteristics — stainless steel is a durable, low-maintenance material and is often the least expensive choice in a life cycle comparison.

Like all metals there are varying grades of stainless steel, each with a different alloy composition, which results in different physical characteristics.

To qualify as stainless steel there must be at least 10.5% chromium. Depending on the grade it may contain higher levels and additional alloying ingredients. The two most common grades are 304 and 316. The main difference is the addition of molybdenum, an alloy which dramatically enhances corrosion resistance, especially for more saline or chloride-exposed environments.

Understanding the difference between stainless steel grades 304 and 316

304 stainless steel

304 grade is the most common form used around the world because it still offers excellent corrosion resistance at an affordable price. It typically contains between 16 and 24% chromium and up to 35% nickel. The fact that it can withstand corrosion from most oxidising acids means its durability makes it easy to sanitise and it is suitable for kitchens and food applications. However, it is susceptible to corrosion from chloride solutions, which means if industrial cleaning agents are being used, for example in a factory or for clean down, then the longevity will be reduced.

316 stainless steel

316 grade has almost identical physical and mechanical properties as 304 stainless steel but the key difference is 2–3% of molybdenum. This addition increases resistance, particularly against chlorides, corrosive chemicals and industrial solvents. There are also improvements in steel resistance to pitting and crevice corrosion.

316 stainless steel is suitable for industrial applications involving processing chemicals, highly sterile factory environments, medical and pharmaceutical production as well as high saline environments such as coastal regions and outdoor areas.

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