Contrary to popular belief, stainless steel is susceptible to rusting if it is not properly maintained. It's stainless steel; not stain free. This Buyer’s Guide will discuss the common types of stainless steel used in food service and the best way to protect your investment.
To be classified as stainless steel, the metal alloy has to contain iron and must contain at least 10.5% chromium. Other alloying elements are added to increase the corrosion resistance, improve the fabrication properties, increase the high temperature performance, improve the machining characteristics, etc., which is why there are many varying grades.
Common Metallurgic Structures of Stainless Steel
Before diving into cleaning, polishing, or maintence of stainless steel in your commercial kitchen, review the types of stainless steel that you will find on your equipment.
Each type of stainless steel has different characteristics and has can be used for a number of different applications. To differentiate between the types, they are classified in different series by their crystalline structure - the unique arrangement of the atoms or molecules that create the stainless steel's structure.
The main series of stainless steel used in foodservice equipment include:
- Austentic Stainless Steel: 200-300 Series
- Austentic stainless steel is used in over 70% of stainless steel production. It has a face-centered cubic crystalline structure which means its molecules have multiple points of attaching to one another which gives it structural strength but also flexibility. This type of stainless steel is created with chromium and nickel in varying degrees which further divides the type into common types such as 304 (18% chromium & 8% nickel), 316 or marine stainless steel (one of the most common in foodservice), and 18/10 (18% chromium & 10% nickel).
- Austentic stainless steel can be polished to mirror finish, is non-magnetic, maintains structure at high and low temperatures, and is extremely corrosion resistant.
- Flatware is typical made of 18/10 austentic stainless steel. Foodservice Work Tables are often made of 316 series stainless steel.
- Generally more expensive due to its corrosive resistance.
- Ferritic Stainless Steel: 400 series
- Ferritic stainless steel has a much lower chromium and nickel content than austentic. It can be classified in grades such as 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni, based upon the alloys of chromium in its composition.
- Ferritic stainless steel does have some corrosive resistance and can be polished, but has limited temperature range before its structure is weakened. It is also magnetic properties thanks to its body-centered cubic structure.
- Martensitic Stainless Steel
- Martensitic stainless steel is extremely strong and tough especially after tempering or heat treatment. You can see martensitic stainless steel in items such as pocket knives. It contains 12-14% chromium, less than 2% nickel, and less that 1% carbon.
- Martensitic stainless steel is magnetic and is often used for medical instruments or lightweight precision tools.
Why Corrosion or Rust Won't Form on Stainless Steel
The molecular composition of stainless steel is what gives it that "stainless" quality. All stainless steel contains some level of chromium, usually between 12-30% chromium. This allows the formation of a rough, invisible, corrosion-resisting chromium oxide film to form on the steel surface as the chromium reacts with oxygen in the air. As long as the film is intact and not broken or contaminated, the metal remains protected and will resist rust or corrosion.
If damaged mechanically or chemically, this film is self-healing, providing that oxygen, even in very small amounts, is present. The corrosion resistance and other useful properties of the steel are enhanced by increased chromium content and the addition of other elements such as molybdenum, nickel, and nitrogen. For this reason stainless steel is often the preferred metal for restaurant equipment.
Enemies of Stainless Steel
There are many chances for the chromium oxide protective film to break down or be disturbed. Here are some of the most common reasons:
- Mechanical abrasion caused by steel pads, wire brushes and scrapers. Use these with care.
- Hard Water and Mineral Deposits may leave spots and, when heated, leave deposits behind that can begin to break down the passive layer and rust stainless steel. Other deposits from food preparation and service (such as salt deposits) must be properly removed or they can also encourage corrosion.
- Chlorides, particularly household and industrial cleaners, should not be used to clean stainless steel.
- Metal-to-metal contact between dissimilar metals, such as zinc or iron, can also cause rust or corrosion.
Reducing Chance of Damaging Stainless Steel
- Use non-abrasive tools when cleaning stainless steel products. Soft cloths and plastic scouring pads will not harm steel’s passive layer. Stainless steel pads also can be used but the scrubbing motion must be in the direction of the manufacturers’ polishing marks.
- Clean with the polish lines or “grain.” When visible lines are present, always scrub in a motion parallel to the lines. When the grain cannot 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 choice of non-chloride cleaners. If you are not sure of chloride content in the cleaner used, contact your cleaner supplier. If your present cleaner contains chlorides, ask your supplier if they have an alternative. Avoid cleaners containing quaternary salts; it also can attack stainless steel and cause pitting and rusting.
- Treat your water. Though this is not always practical, softening hard water can reduce deposits. There are filters that can be installed in faucets to remove distasteful and corrosive elements. To insure proper water treatment, call a treatment specialist.
- Keep your food equipment clean. Use alkaline, alkaline chlorinated or non-chloride cleaners at recommended strength. Clean frequently to avoid build-up of hard, stubborn stains. If you boil water in stainless steel equipment, remember the single most likely cause of damage is chlorides in the water. Heating cleaners that contain chlorides have a similar effect.
- Rinse and Wipe. If chlorinated cleaners are used, rinse and wipe equipment and supplies before drying immediately. The sooner you wipe off standing water, especially when it contains cleaning agents, the better. Avoid the use of oily rags or greasy cloths when wiping the surface. After wiping equipment down, allow it to air dry; oxygen helps maintain the stainless steel’s protective film.
- Never use hydrochloric acid (muriatic acid) on stainless steel.
Stainless steel needs to be cleaned for aesthetic considerations and to preserve corrosion resistance. Stainless steel actually thrives with frequent cleaning, and, unlike some other materials, it is impossible to “wear out” stainless steel by excessive cleaning. The effect of surface/pattern roughness, grain/pattern orientation, and designs that allow for maximum grain cleaning (exterior applications) should be considered when selected from the methods below.
|Job or Use
||Warm Water, Soap, Ammonia, Detergent
||Apply with soft cloth or sponge. Can be used on all finishes.
|Remove Fingerprints or Smears
||3M Stainless Steel Cleaner and Polish, Arcal 20, Lac-O-Nu, Lumin Wash, O’Cedar Cream Polish, Stainless Shine
||Provides barrier film to minimize fingerprints. Can be used on all finishes.
|Stubborn Stains & Discoloration
||3M Stainless Steel Cleaner and Polish, Allchem Concentrated Cleaner, Samae, Twinkle, Cameo Copper Cleaner, Liquid Nu Steel, Copper’s or Revere Stainless Steel Cleaner, Household Cleaners, Lumin Cleaner, Zud Restoro, Sta-Clean, Highlite, Allen Polish, Penny-Brite, Copper-Brite
||Rub lightly using damp cloth in direction of polish lines or grain.
|Grease & Fatty Acids, Blood, Burnt-on or Baked-on Foods
||Scotch-Brite Power Pad 2001, Easy-Off, De-Grease-It, 4% to 6% hot solution of agents such as tri-sodium polyphosphate, 5% to 15% caustic soda solution
||Excellent removal on all finishes. Particularly useful where rubbing is not practical.
|Grease and Oil
||Any good commercial detergent or caustic cleaner.
||Apply with sponge or soft cloth in direction of polish lines.