There is a widespread misconception that stainless steel should not rust; however, oxidation, corrosion, rusting or “staining” of stainless steel is actually quite common.
Understanding the Passive Layer
When the surface of ordinary carbon steel is exposed to oxygen, it forms ferric oxide (Fe2O3), colloquially known as rust. Ferric oxide doesn’t form a continuous layer on the steel, so it eventually spalls off, leaving raw steel exposed and setting off a destructive rusting cycle.
Stainless steel frequently used in architecture because of its unique look and ease of maintenance.
Stainless steels undergo a very different process because of their high chromium content. When chromium reacts with oxygen, it creates a passive layer of chromium oxide. Unlike ferric oxide, chromium oxide doesn’t spall off. It acts as a shield, preventing further oxidization of the stainless steel.
The passive layer is self-repairing: if it is damaged, chromium in the exposed stainless steel will react with oxygen to form new chromium oxide. As long as there is sufficient chromium present, the chromium oxide layer will continue to reform and protect the stainless steel surface.
All stainless steels contain at least 10.5% chromium by weight. The higher the chromium content, the greater the corrosion resistance of the steel.
Stainless steel rusting occurs when the passive layer is damaged and does not have a high enough chromium concentration to reform.
Causes of Stainless Steel Corrosion
Chromium can protect stainless steel if the localized concentration is 12 percent or higher. Anything that reduces the localized chromium concentration below the 12 percent threshold will cause staining or rust.
Common causes of stainless steel corrosion include:
- Hydrochloric Acids
- Sulphuric Acids
- Contact with Iron
- Contact with Carbon steel
- High temperatures
Chromium oxide is particularly vulnerable to chlorides. Corrosion is accelerated in coastal areas with salt-spray exposure, and in areas where de-icing salts are used during winter. Components for the chemical and food industries have high chromium content to compensate for regular exposure to chlorine, salt, and other corrosive substances.
Strong acids destabilize the passive layer. Exposure to hydrochloric and sulphuric acids causes general surface corrosion.
Stainless steel corrosion can also be triggered by contact with iron or carbon steel particles. Trace particles from iron or carbon steel will rust on stainless steel surfaces. If left unattended, rust spots may compromise the chromium oxide surface and, in the worst cases, it can evolve into "pitting," i.e. localized corrosion. Contamination is common when stainless steel is subject to sparks from nearby welding, cutting, drilling, or grinding of carbon steel.
A less common form of rusting occurs after the stainless has been exposed to very high temperatures, usually in the 750-1550°F range (400-850°C). This type of corrosion can be found in welding applications in which stainless is heated and then cooled. If this happens, sensitization can occur; the carbon and the chromium bond to form carbides. This causes the grain boundaries to become chromium deficient, and the chromium oxide layer becomes discontinuous. Heat sensitization can ruin stainless steel forever; however, the damage can sometimes be mitigated with heat treating.
There are over 150 grades of stainless steel, and some are more prone to corrosion than others. The corrosion resistance and other useful properties of stainless steel are enhanced by increasing the chromium content, and/or by the addition of other elements such as molybdenum, nickel, and nitrogen.
Basic Stainless Steel Care
You can extend the service life of stainless steel by preventing contact with corrosive substances. Follow these best practices when working with stainless steel.
- Always wear clean gloves.
- Always use stainless steel tools and components.
- Avoid any contact with carbon steels or iron.
- Do not weld, cut, drill or grind carbon steel near stainless steel.
- Avoid contact with concrete detergents.
- When power washing nearby surfaces, wrap stainless steel with plastic.
- If chloride solutions or concrete detergents contact stainless steel, rinse immediately.
- Never scrub with steel wool or other abrasive scrubbing pads.
The most common mistake people make is using a corrosive cleaner. Never use the following products to clean stainless steel.
- Oven cleaners
- Chloride bleach
- All-purpose cleaners
- Any cleaner containing chloride
- Abrasive cleaners
How to Clean Stainless Steel
Stainless steel railings are constantly handled, and quickly accumulate fingerprints. Use rubbing alcohol for a clean finish.
Stainless steel needs to be cleaned regularly to preserve corrosion resistance and maintain a pristine appearance. With proper care, and provided an application-appropriate grade was selected, stainless steel will not corrode. That said, corrosive substances accumulate on neglected surfaces. Stainless steel needs to be cleaned regularly with safe products.
For most applications, stainless steel is easy to clean. A quick wash with soap and water followed by a clean-water rinse is usually adequate for domestic and architectural equipment.
For best results, follow these pro tips:
- Clean stainless steel when it is cool to the touch.
- Use a mild detergent, or dilute (1%) ammonia solution in warm water.
- Use clean water. Gritty, dirty, or excessively hard water can leave spots or brownish stains.
- Wipe down with a clean sponge or cloth.
- Use clean rinsing water to avoid water marks.
- Dry with disposable wipes or an air blower.
Stainless steel care and maintenance may require the use of harmful chemicals. Follow all use and safety instructions provided with cleaning or polishing agents. Ensure personal protective equipment is worn in accordance with occupational health and safety guidelines.
Where stainless steel has become extremely dirty with signs of surface discolouration (perhaps following periods of neglect) alternative methods of cleaning can be used, as outlined below.
Use soap or detergent and warm water. Alternatively, a (hydrocarbon) solvent like rubbing alcohol or acetone can be used. Commercially available solutions or polishes are available to clean and minimize fingerprint marking.
Oil and grease marks
To remove oily and greasy traces, you can use isopropyl alcohol (rubbing alcohol), acetone, or methylated spirit. Apply solvent several times with a clean, non-scratching cloth until all oil or grease is removed.
Tough stains, discoloration, oxidation, water stains
Use mild, non-abrasive cleansers. Apply with a soft cloth or sponge and rinse with clean water and dry. Avoid scouring pastes. You can also use cream detergents containing calcium carbonate or citric acid.
Localized rust stains caused by iron or carbon steel contamination
To remove contaminants from stainless steel surfaces, use a soft cloth to apply a solution of oxalic acid. Leave the solution on the surface for a few minutes to dissolve contaminating particles. Once clean, thoroughly rinse away all residual solution with clean water.
Burnt-on food and fat
Soak in hot water and diluted ammonia or detergent, then remove the grime with a nylon brush. If needed, you can use a fine abrasive powder (be careful; abrasives will scratch polished surfaces). Rinse thoroughly with clean water.
Coffee and tea stains
Soak the stainless steel in hot water and sodium carbonate (washing soda) for tea stains, or sodium bicarbonate (baking soda) for coffee stains. If the object is too large to soak, use a sponge or a soft cloth.
Limescale, cement and mortar splashes
If mortar or cement comes into contact with stainless steel, rinse immediately. Use a 10%/15% phosphoric acid based solution in warm water. Spread cleaner evenly over the surface, wait 30-60 minutes, then neutralize the acid with an alkaline cleaner or diluted ammonia and rinse with clean water. On limescale stains, you can also dilute one part of vinegar in three parts of water and apply with a nylon brush. Specific products are commercially available.
Heating stains or strong discoloration
Use domestic metal polish to improve the appearance of tarnished stainless steel. Chrome polishes for automotive parts are widely available and extremely effective. Treat the entire stainless steel surface to avoid discolored patches.
Badly neglected and corroded surfaces, with accumulated grime deposits
Minor: Use an all-purpose lubricant, such as WD-40™, to wipe affected stainless steel. Domestic stainless steel cleaners containing calcium carbonate or citric acid can also be used. Rinse thoroughly with clean water. If rust remains, treat as moderate. If rust returns in a short time, treat for iron or carbon steel contamination.
Moderate: Use a phosphoric acid-based stainless steel cleaner (like E-NOX CLEAN™, for example). Identify the areas that need to be cleaned. Spray on and spread cleaner evenly over the surface. Leave on for 30-60 minutes. Neutralize the acid with a spray on alkaline cleaner (like UNO S F™). Wipe the surface clean with a paper towel, then thoroughly rinse away all residue with clean water. If rust remains, stainless steel may need to be treated for severe corrosion or be replaced. If rust returns in a short time, treat for iron or carbon steel contamination (see passivation below).
Severe: Due to the highly corrosive nature of serious rust treatments, and the inherent risks to personnel and surrounding environments, a professional service provider is recommended. Severe rust is treated with a pickling bath, typically containing highly corrosive hydrofluoric acid. Once treated, stainless steel can be passivized with mild nitric acid.
* Products are presented as examples of commercially available materials for review and comparison purposes only. Mention of said products does not constitute an endorsement of effectiveness. Vendors noted have no relation or affiliation with Reliance Foundry.
Graffiti (paint and ink) can be removed from stainless steel with a biodegradable graffiti-cleaning spray or wipes. Avoid using knives or hard scraping tools to remove graffiti, as these may damage stainless steel surfaces. Follow manufacturer's directions.
Heavily damaged products
Heavy damage includes structural compromises such as visible dents, cracks, breaks and rust that can undermine the integrity of a product. Heavily damaged products should be removed from service until a repair or replacement can be made. To prevent worsening of damage, any significant rust should be removed as soon as possible.
Conduct a test before cleaning stainless steel with any chemical or abrasive medium. Apply the cleaning solution to a small, unobtrusive hidden or non-critical area of the surface, and check the resulting finish.
A clean stainless steel surface can be re-damaged through mechanical means, extreme heat, or chemical damage. When that happens, iron is exposed, and the item is once again subject to rusting. Passivation may need to be performed on a regular basis to prevent rust.
Stainless steel building panels are difficult to passivate or treat for severe rust because they are in a fixed position. The best way to keep them in shape is regular preventative cleaning.
Exposure to cleaning solutions, dirty conditions, bleach or salt all contribute to the need for routine passivation of the stainless steel.
Passivation maximizes the inherent corrosion resistance of a stainless alloy. It is not a scale removal treatment, nor is it like a coat of paint.
Ideally, the passive layer will develop immediately after machining or passivation to completely cover the stainless steel surface. In the real world, particles and contaminants are likely to adhere to the surface. If they are allowed to remain, these foreign particles reduce the effectiveness of the original protective film.
Passivation is needed to maximize the natural corrosion resistance of stainless steel. The passivation process removes surface contamination.
A two-step passivation procedure can provide the best possible corrosion resistance:
The part must be thoroughly cleaned. Stainless steels cannot be passivated unless the surface is clean and free from contamination and scale. In extreme cases, foreign matter may have to be removed by grinding/mechanical abrasion, or by methods such as “pickling” (involving nitric/hydrofluoric acid mixtures) before passivation can take place.
- Passivating treatment
The stainless steel part is immersed in a passivating acid bath. Any one of three approaches can be used—nitric acid passivation, nitric acid with sodium dichromate passivation, and citric acid passivation. The passivation treatment varies depending on chrome content and machinability characteristics of the stainless steel grade, as well as the prescribed acceptance criteria.
For more information, refer to ASTM A967 “Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts.”
- Stainless Steel Panels: Oleg Savenok, CC 1.0, via Unsplash
- Stainless Steel Architecture: Ashim D’Silva, CC 1.0, via Unsplash