Despite what you’ve heard, stainless steel IS susceptible to rusting. Certainly, metal corrosion is common. In fact, it is easily recognizable on iron and steel as yellow or orange rust. Such metals are active. They actively corrode in a natural environment. Consequently, when their atoms combine with oxygen, it forms rust.
In contrast, stainless steels are passive metals. They contain other metals like chromium, nickel, and manganese that stabilize the atoms. As a result, they don’t react to the environment.
- 400 series stainless steels are ferritic. They contain chromium and are magnetic.
- 300 series stainless steels are austenitic. They contain chromium and nickel.
- 200 series stainless, also austenitic. It contains manganese, nitrogen, and carbon.
- Austenitic types of stainless are not magnetic. Consequently, they provide greater resistance to corrosion than ferritic types.
Specifically, with 12-30 percent chromium content, an invisible passive film covers the steel’s surface. Critically, this acts as a shield against corrosion. Therefore as long as the film is not broken or contaminated, the metal is passive and stainless. However, if the passive film is broken, the metal can corrode. As a result, if the corrosion continues, the steel rusts.
Enemies of Stainless Steel
Significantly, there are three things that break down stainless steel’s passivity layer and allow corrosion:
- Mechanical abrasion. Mechanical abrasion means scratching the steel surface. Steel pads, wire brushes, and scrapers are prime culprits.
- Deposits and water. Tap water has varying degrees of hardness. Depending on what part of the country you live in, you may have hard water. Certainly, hard water contains high mineral concentrations. For this reason, it may leave spots. Likewise, when heated, it can leave deposits. Significantly, if these deposits are allowed to remain, they can break down the passive layer and rust stainless steel. Consequently, deposits from food preparation and service must be properly removed.
- Chlorides. Chlorides are found nearly everywhere. Notably, they are in water, food, and table salt. Above all, some of the worst chloride perpetrators are household and industrial cleaners.
Here are some best practices to prevent stainless steel rust.
- Use the proper tools, and clean with the polish lines. When cleaning stainless steel, use non-abrasive tools. Soft cloths and plastic scouring pads will not harm steel’s passive layer. Stainless steel pads can be used, but the scrubbing motion must be in the direction of the manufacturer’s polishing marks. Some stainless steel comes with visible polishing lines or “grain.” When grain is present, always scrub in a motion parallel to the lines. If the grain can’t be seen, play it safe and use a soft cloth or plastic scouring pad.
- Use alkaline, alkaline chlorinated, or non-chloride-containing cleaners. While many traditional cleaners are loaded with chlorides, the industry is providing an ever-increasing selection of non-chloride cleaners. If unsure of chloride content in the cleaner, contact your cleaner supplier. If your present cleaner contains chlorides, ask your supplier for alternatives. Definitely avoid cleaners containing quaternary salts. In fact, they can attack stainless steel and cause pitting and rusting.
- Treat your water. Softening water reduces deposits. Notably, there are filters that remove distasteful and corrosive elements. With this in mind, contact a treatment specialist to ensure correct water treatment.
- Keep your equipment clean. Use alkaline, alkaline chlorinated, or non-chloride cleaners, at recommended strength. Certainly, clean frequently to avoid the build-up of hard, stubborn stains. Remember, the most likely cause of damage is chlorides in the water. Likewise, heating cleaners that contain chlorides can accelerate damage.
- Rinse, rinse, rinse. If chlorinated cleaners are used, rinse and wipe equipment and supplies immediately. Critically, the sooner you wipe off standing water, the better. This is especially true when it contains cleaning agents. After wiping equipment down, allow it to air dry. Oxygen helps maintain stainless steel’s passivity film.
- Never use hydrochloric acid (muriatic acid) on stainless steel. Hydrochloric acid can cause cracking, corrosion, and pitting on stainless steel.
- Regularly restore/passivate stainless steel.