What is Stainless Steel and Why do We Use it in Sanitary Process Systems?

June 26, 2014
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Stainless Steel Sanitary Process Piping

Stainless steel is a generic term that refers to a variety of types of steel. But what is it that makes stainless steel “stainless” and why is stainless steel the material of choice for sanitary process components? This post will focus on those questions and provide insight into term that is often oversimplified.

Stainless steel, like all steels, is made primarily from iron and carbon. Steel is make in a two-step process. In the first step, iron ore is smelted in a blast furnace. After the raw “pig iron” is made molten, most modern plants blow high purity oxygen into the molten ore, lowering the carbon content and eliminating other impurities. Steel corrodes because the main metal, iron, is chemically very active and quickly combines with other elements, such as oxygen, causing oxidation, corrosion, and rust.

What makes stainless steel different is the addition of chromium to the soup. When chromium is added to steel, it reacts with the free oxygen to form chromium oxide, which acts give stainless its corrosion resistant properties.  This passive- meaning chemically inert- layer is what we’re trying to create or enhance when we passivate steel. In addition to chromium, other elements are added to stainless to enhance different properties and the relative amounts of these elements is what determines the grade of stainless.

Most of the steels used in sanitary process are austenitic 300 series steels- 304 and 316. Superaustenites, such as AL6XN exhibit even better resistance to the corrosive pitting effects of chlorides through the addition of molybdenum.

The other type of steel we’ll see in sanitary process are precipitation hardened martensitic or semi-austenitic steels. These steels, which include 17-4PH feature the addition of elements like aluminum, copper, and niobium which makes the steel extremely strong while still easily machined (weld-ability does suffer slightly, however).

Continually, stainless steel, particularly austenitic steels, have properties in addition to corrosion resistance that make it an excellent choice for sanitary process equipment. We’ve already touched n a few of them, but stainless is strong, durable, easily fabricated, relatively cheap, aesthetically pleasing, and easy to clean. This means we can machine, weld, and shape steel into almost any shape we want. The finished steel product will be strong, durable and perform over a variety of high and low temperatures.

But to really drive the point home, what makes “stainless” “stainless” is its corrosion resistance. When steel is attacked by the environment, it rusts and pits. These pits create an area where product can be trapped. This isn’t good in a sanitary process. These places are hard to clean and once the passive layer is gone, the steel is compromised and prone to additional attack.

Rusting and corroded steel also dramatically affects the mechanical properties of steel. The first site that is generally attacked are the joints- namely welds. Weld and weld quality have been the subject of previous posts and will be reemphasized in the future. Rust will compromise a system, exposing it to detrimental environmental conditions and comprising system and product integrity.

To conclude the main type of steel used in sanitary process equipment are austenitic 300 series steels. All stainless steels benefit from the addition of chromium which helps create a passive chrome oxide surface layer on the steel that helps reduce chemical attack and corrosion. We’ll elaborate on this topic continually throughout the blog, but don’t let that stop you from contacting a Holland Sales Engineer today.