Choosing and Cleaning Stainless Steel
In previous articles we’ve covered what needs to go into your cleanroom, from air filters to cleanroom garments to furniture. We’ve talked about the materials that wearable cleanroom gear like gloves and garments are made of, but what about what your equipment is made out? We understand that in our cleanrooms we need materials that are easily cleaned and corrosion resistant that won’t affect the particle count in the environment, which stainless steel is great for. But just like how there’s a lot of variation between different kinds of cleanrooms, there’s a lot of variation between the different types of stainless steel.
Putting The Stainless in Stainless Steel
Let’s start with a quick chemistry lesson before we begin comparing different types of stainless steel. What exactly makes stainless steel “stainless”? To qualify as stainless a piece of steel must be infused with at least 10.5% of chromium and small amounts of other elements. So how does that 10.5% of chromium make our hypothetical hunk of steel stainless? Think of what happens if you leave an unseasoned cast iron skillet sitting out for an extended period of time. It starts to rust. The rust forms when the iron atoms on the surface of the skillet react to the oxygen in the air to form an oxide. The oxide that initially forms(Fe3O4 or Iron II) has a much larger molecular structure than iron atoms, so large gaps form on the surface of the skillet which allows for other potentially corrosive substances to work their way in. The most common of these substances is a secondary form of iron oxide commonly known as rust(Fe2O3 or Iron III). Once rust gets below the surface it corrodes the iron further flaking it away and causing a number of issues in your cleanroom environment. With the addition of chromium, the oxides that form fit together in a much tighter and more ordered structure. The smaller gaps and bonds form what is called a passivation film. A passivation film is just a chemical layer that protects a material from the damaging effects of oxygen and moisture, in this case by having extremely dense chemical bonds that keep the corrosive elements out.
Types of Stainless Steel
High school chemistry lesson completed, let’s focus on the chemical differences between regular steel and stainless and then break it down further by types of stainless steel. There are three common types of stainless steel: 304, 316, and 430. The most standard and basic form of stainless steel is SAE 304. 304 stainless steel has a mixture of chromium along with nickel, which makes it non-magnetic and resistant to most corrosive substances. The important quality for a cleanroom is that it is resistant to most corrosive substances. If the equipment is for an environment with industrial solvents and chemicals, the surface of the equipment may become pitted or tarnished which allows rust to set in. For environments where harsh chemicals might be used SAE 316 stainless steel is a much better alternative. SAE 316 includes a molybdenum additive to increase the corrosive resistance of the material. The molybdenum allows 316 to stand up to industrial solvents such as chloride solutions or phosphoric and sulfuric acids. The last type of stainless steel is SAE 430. Unlike 304 and 316, 430 is a magnetic steel with good corrosion resistance, but only in environments where concerns over corrosion are small. Intended for use in environments with minimal corrosion but potentially wide temperature swings, SAE 430 sees lots of use in stainless steel kitchen and automotive equipment. Due to its reduced corrosion resistance 430 is the cheapest of the stainless steel options. SAE 304 and 316 are ideal materials for any cleanroom where harsh chemicals will be used, due to their corrosion resistance, where equipment made using 430 steel is a cost effective solution for cleanroom environments that won’t be working with many caustic chemicals.
Cleaning Your Stainless Steel
One of the everyday interactions you’ll have with your equipment is cleaning it, so make sure you do it correctly. We explained earlier that the steel is protected by its layer of chromium oxide, which forms a barrier that keeps out any corrosive elements. If there’s any dirt or other contaminants on the surface, this chromium oxide layer becomes less effective because oxygen isn’t able to get to the chromium atoms to rebuild that layer when it’s weakened. So stainless surfaces need to be cleaned to be effective. Instead of having to worry about scrubbing off a patina or a ‘seasoning’ to remember our cast iron skillet example, the more you clean stainless steel the more oxygen can get to the surface to reform that protective oxide coating. The biggest thing to watch out for when cleaning stainless steel is scratching the surface. A scratch on the surface removes that oxide layer and allows for corrosive elements to get into the steel to form rust. The best way to clean your stainless steel surfaces is to use a certified cleanroom wipe and non-chlorine cleaning agent. To diverge into another quick chemistry lesson, chlorine has the maximum number of leftover electrons it can without being stable at an atomic level, which is an all or nothing proposition in chemistry. This means that chlorine has a lot of energy that it can use to break down or corrode materials like our stainless steel to try and achieve chemical stability. Some safe alternatives to chlorine based cleaners are acetone and isopropyl alcohol. We’ve explained what exactly makes stainless steel stainless, 10.5% chromium, the most common types of stainless steel, SAE 304, 316, and 430, and the differences between them, additional additives. Hopefully with this information you can make a thoroughly informed decision when it comes to you next equipment purchase. Once you have your next piece of equipment, you can properly clean it without worrying about damaging its protective qualities.