Everyone knows that stainless steel resists corrosion, but beyond that, an amazing range of half-truths and exaggerations have evolved - often misleading and sometimes simply wrong. This article examines some of the more common myths, explains why they are wrong, and more to the point, provides correct information.
When comparing hot and cold formed stainless steel, the first question you would ask yourself is: are there any chemical differences between the two? ASSDA has previously published articles on the various surface finishes including the few hot and multiple cold finished processes, however this article concentrates on the differences.
When used properly, stainless steel enjoys a strong and enduring reputation for visual appeal and structural integrity in a wide range of applications and environments. But, like all materials, stainless steel may become stained or discoloured over time, impairing the overall look. This brown discolouration - tea staining - has been identified in coastal applications in Australia and overseas.
The aim of AS 1528: Stainless steel tubes and tube fittings for food processing and hygienic applications is to standardise hygienic tube and fittings for use in dairy, food and beverage manufacturing. It has been successful in maintaining the required food safety standards in Australia and New Zealand.
Humanity’s use of materials has progressed over the millennia from natural resources such as plants and stone to manufactured materials such as ceramics, metals and plastics with a corresponding increase in consumption of energy and materials – and increasing waste production. In parallel, the world’s consumers have grown exponentially from about 1 billion in 1800, to 7.6 billion in 2018 and a predicted 9.8 billion in 2050 – all demanding more infrastructure, facilities and resources to support the expectations of higher standards of living. This has led to an increasing realisation that green production, recycling, waste reduction and more efficient use of resources are essential.
Connections are vital
Any visit to a dairy, beverage or food processing plant will drive home the critical importance of the connections between the tanks, mixers, driers, pumps, etc. The image above (courtesy of TFG Group) showing an image of a brewery is a typical example. These tubes and/or pipes carry the process materials, the heating or cooling or wash water, gases, and also dispose of the wastes.
The normal state for stainless
Stainless steels resist corrosion because they have a self-repairing “passive” oxide film on the surface. As long as there is sufficient oxygen to maintain this film and provided that the level of corrosives is below the steel’s capacity of the particular material to repair itself, no corrosion occurs. If there is too high a level of (say) chlorides, pitting occurs. As an example, 316 works well in tap water (<250 ppm) all over Australia, but will rapidly corrode in seawater because seawater has very high chloride levels (20,000 ppm).
Almost all of the stainless steels in use have 16% chromium or more and have nickel or other additions to make them austenitic and hence formable, tough and readily weldable. However, the formal definition of a stainless steel is that it is an iron- and carbon-based alloy with more than 10.5% chromium. Historically, the corrosion mitigation industry regarded alloys with more than 12% chromium as stainless steels mainly because those alloys did not corrode in mild environments. Because of the perceived problem of high initial price when using stainless steels, alloys that are ‘barely’ stainless (and with low nickel to boot) are more competitive with painted or galvanised carbon steel than higher alloys.