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Cleaning Stainless Steel

2023-01-31

Cleaning Stainless Steel

Protect stainless steel from rust using proven cleaning methods and passivating treatments

Stainless steels are highly corrosion resistant—however, stainless steel applications can still remain at risk to surface damage. Oxidation, corrosion, rusting, or staining can occur over the long-term in harsh environments without routine cleaning and maintenance. Repeat mechanical damage also contributes to a faster degradation of the metal.

All stainless steels contain at least 10.5% chromium by weight. It is this chromium content that creates a shield called the passive layer, which protects stainless steel from corrosion—unlike other steels. The higher the chromium content, the greater the corrosion resistance. Stainless steel rusting occurs when the passive layer is damaged and there is not enough chromium for it to reform.

Cleaning stainless steel bollard
Stainless steel applications require regular cleaning and maintenance to protect from oxidation, corrosion, rusting, and staining.

How does the passive layer work?

The passive layer is created as the chromium content in the stainless steel’s surface reacts to oxygen. The passive layer acts as a protective barrier, preventing further oxidation of the stainless steel. In comparison, ordinary carbon steel surfaces form ferric oxide when exposed to oxygen. Ferric oxide does not form a continuous layer, so it eventually spalls off, leaving raw steel exposed and prone to a destructive rusting cycle.

The passive layer of stainless steel 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.

Causes of stainless steel corrosion

Chromium can protect stainless steel if the localized concentration is 12% or higher. Anything reducing the localized chromium concentration below the 12% threshold will cause staining or rust. Common causes of stainless steel corrosion include chlorides, hydrochloric acids, sulfuric acids, iron or carbon steel contact, and high temperatures. 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, or by the addition of other elements such as molybdenum, nickel, and nitrogen.

Chlorides

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.

Acids

Strong acids destabilize the passive layer. When stainless steel is exposed to hydrochloric and sulfuric acids, general surface corrosion can occur. Hydrochloric acid is extremely acidic and used in plastic production. Diluted hydrochloric acid is found in descaling agents for removing limescale. Sulfuric acid is a corrosive chemical that can cause severe chemical burns and secondary thermal burns. It is used in mineral processing, oil refining, and wastewater processing. More diluted formulas are found in household products such as drain cleaners.

Iron and carbon steel particles

Stainless steel corrosion can also be triggered by contact with iron or carbon steel particles. Trace particles from iron or carbon steel will rust stainless steel surfaces. If left unattended, rust spots may compromise the chromium oxide surface and evolve into localized corrosion such as pitting. Contamination occurs when stainless steel is subject to sparks or particles from nearby welding, cutting, drilling, or grinding of carbon steel.

Iron and carbon steel particles on drill
Contamination occurs when stainless steel is subject to sparks or particles from nearby welding, cutting, drilling, or grinding of carbon steel.

High temperatures

A less common form of rusting occurs after the stainless steel has been exposed to very high temperatures in the 750–1550°F range. This type of corrosion is found in welding applications in which stainless steel is heated and then cooled. Sensitization can occur, where 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 runs the risk of ruining stainless steel forever. However, the damage can sometimes be mitigated with heat treating.

Stainless steel being welded
Sensitization in welding applications causes grain boundaries to lose chromium content and the chromium oxide layer becomes discontinuous.

Handling stainless steel

The service life of stainless steel can be extended by preventing contact with corrosive substances. Follow best practices when working with stainless steel:

The most common mistake people make is using a corrosive product to clean stainless steel surfaces. Never use the following products on stainless steel:

Routine stainless steel cleaning

Stainless steel must be cleaned regularly to preserve the metal’s integrity and maintain a pristine appearance. With proper care, and by selecting an appropriate grade, stainless steel can be saved from corrosion. When stainless steel maintenance is neglected, corrosive substances can accumulate on vulnerable surfaces.

Routine cleaning steps (perform every 6 months)

  1. Use a soft nylon brush to remove accumulated dirt.
  2. Wash with mild soap or detergent.
  3. Follow with a clean-water rinse.
  4. Wipe dry with a soft cloth, disposable wipe, or air blower.

Stainless steel care and maintenance may require the use of harmful chemicals. Follow all safety instructions provided with cleaning or polishing agents. Ensure personal protective equipment is worn in accordance with occupational health and safety guidelines.

Specific stainless steel cleaning

When stainless steel has become extremely dirty with signs of surface discoloration or corrosion, routine cleaning may no longer suffice. Cleaning methods designed to tackle specific types of damage are listed below.

Fingerprints on stainless steel railing
Unsightly fingerprints left on stainless steel applications can be cleaned with soap and warm water.

Fingerprints

Oil and grease marks on stainless steel
Oil and grease marks on stainless steel can be removed with solvent and non-scratching cloth.

Oil and grease marks

Burnt-on stain on stainless steel pot
Use diluted ammonia or detergent for burnt-on stains on stainless steel pots and pans.

Burnt-on stains

Coffee stains on stainless steel
Coffee or tea stains on stainless steel tableware can be cleaned up using a sodium carbonate or bicarbonate solution.

Discoloration, coffee and tea stains

Discoloration

Coffee stains

Tea stains

If the object is too large to soak, use a sponge or a soft cloth.

Rust stain on stainless steel bollard
Localized rust stains can be treated with an oxalic acid solution.

Localized rust stains

Limescale on stainless steel
Try a vinegar solution for limescale build-up on stainless steel.

Limescale, cement and mortar splashes

Limescale

Cement and mortar splashes

Graffiti damage on stainless steel
Graffiti on stainless steel can be removed using biodegradable graffiti-cleaning wipes.

Graffiti

Stainless steel with heat stains
Heat stains on stainless steel surfaces can be removed with a domestic metal polish.

Heat stains

Dent marks on stainless steel
Heavily damaged products should be removed from service until a repair or replacement is made.

Structural damage

Corrosion on stainless steel bollard
Minor and moderate corrosion can be treated using an all-purpose lubricant or phosphoric acid-based stainless steel cleaner.

Corrosion

Minor corrosion

Moderate corrosion

Severe corrosion

Passivation procedure

A clean stainless steel surface can be re-damaged through mechanical means, extreme heat, or chemical damage. When this happens, iron is exposed, and the material is once again subject to rusting. Passivation may need to be performed on a regular basis to prevent rust. Passivation maximizes the inherent corrosion resistance of a stainless alloy.

Ideally, the passive layer will develop immediately after machining or passivation to completely cover the stainless steel surface. In reality, 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 and remove surface contamination.

A two-step passivation procedure provides the best possible corrosion resistance:

1) Cleaning

The stainless steel 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 or mechanical abrasion, or by pickling before passivation can take place.

2) Passivating treatment

The stainless steel part is immersed in a passivating acid bath. Any one of three passivating approaches can be used: nitric acid passivation, nitric acid with sodium dichromate passivation, or 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.

Extending stainless steel’s lifespan

Proper routine care, and specialized cleaning for more serious problems, can extend stainless steel’s lifespan and ensure that applications operate smoothly. Although stainless steel’s built-in chromium content works to guard against corrosion, relying on this alone is insufficient. Exposure to chemical solutions, weathering, salt, or mechanical damage—all contribute to the need for routine passivating treatment. Passivation procedures and correct cleaning methods help stainless steel continue to shine as one of today’s most versatile materials in the commercial and industrial world.

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