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Surface contamination and the formation of deposits are critical factors that may lead to drastically reduced life. These contaminants may be minute particles of iron or rust from other non-stainless steels used in nearby construction and not subsequently removed. Industrial, commercial and even domestic and naturally occurring atmospheric conditions can result in deposits that can be quite corrosive. An example is salt deposits from marine conditions.
Working environments can also create more aggressive conditions, such as the warm, high humidity atmosphere above indoor swimming pools. These environments can increase the speed of corrosion and therefore require more frequent maintenance. Modern processes use many cleaners, sterilizers, and bleaches for hygienic purposes. All these proprietary solutions, when used in accordance with their makers' instructions are safe, but if used incorrectly (e.g. warm or concentrated) can cause discoloration and corrosion on the surface of stainless steels. Strong acid solutions (e.g. hydrochloric acid or "spirits of salts") are sometimes used to clean masonry and tiling of buildings but they should never be permitted to come into contact with metals, including stainless steel. If this should happen the acid solution must be removed immediately by copious water flushing.
Maintenance During Installation
Cleaning of new fabrications should present no special problems, although more attention may be required if the installation period has been prolonged. Where surface contamination is suspected, immediate attention to cleaning will promote a trouble-free service life. Food handling, pharmaceutical, and aerospace applications may require extremely high levels of cleanliness.
On-Going Maintenance
Advice is often sought concerning the frequency of cleaning of products made of stainless steel, and the answer is quite simply "clean the metal when it is dirty in order to restore its original appearance". This may vary from once to four times a year for external applications or it maybe once a day for an item in hygienic or aggressive situations. In many applications, the cleaning frequency is after each use.
Good Housekeeping During Manufacturing
Stainless steel can be contaminated by pick-up of carbon steel ("free iron") and this is likely to lead to rapid localized corrosion. The ideal is to have workshops and machinery dedicated to only stainless steelwork, but in a workshop also processing other steels avoid pick-up from:
- Tooling used with other metals
- Grinding wheels, wire brushes, linishing belts
- Steel storage racks
- Contamination by grinding or welding sparks
- Handling Equipment
- Adjacent carbon steel fabrication
Cleaning Methods
Stainless steel is easy to clean. Washing with soap or a mild detergent and warm water followed by a clean water rinse is usually quite adequate for domestic and architectural equipment. An enhanced appearance will be achieved if the cleaned surface is finally wiped dry. Specific methods of cleaning areas in Table 1.
Sections below give passivation treatments for removal of free iron and other contamination resulting from the handling, fabrication, or exposure to contaminated atmospheres, and pickling treatments for removal of high-temperature scale from heat treatment or welding operations.
Passivation Treatments
- Grades with at least 16% chromium (except free machining grade such as 303), 20-50% nitric acid, at room temperature to 40oC for 30-60 minutes.
- Grades with less than 16% chromium (except free machining grades such as 416), 20-50% nitric acid, at room temperature to 40oC for 60 minutes.
- Free machining grades such as 303, 416, and 430F, 20-50% nitric acid + 2-6% sodium dichromate, at room temperature to 50oC for 25-40 minutes.
Pickling Treatments
- All stainless steels (except free machining grades), 8-11% sulphuric acid, at 65 to 80oC for 5-45 minutes.
- Grades with at least 16% chromium (except free machining grades), 15-25% nitric acid + 1-8% hydrofluoric acid, at 20 to 60oC for 5-30 minutes.
- Free machining grades and grades with less than 16% chromium such as 303, 410, and 416, 10-15% nitric acid + 0.5-1.5% hydrofluoric acid, at 20 to 60oC for 5-30 minutes.
"Pickling Paste" is a commercial product of hydrofluoric and nitric acids in a thickener - this is useful for pickling welds and spot contamination, even on vertical and overhanging surfaces.
Table1. Methods of Cleaning Stainless Steel
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Routine Cleaning
All finishes
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Soap or mild detergent and water (Preferably warm)
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Sponge, rinse with clean water, wipe dry if necessary. Follow polish lines.
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Fingerprints
All finishes
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Soap and warm water or organic solvent (eg acetone, alcohol, methylated spirits)
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Rinse with clean water and wipe dry. Follow polish lines.
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Stubborn Stains and Discolouration.
All finishes.
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Mild cleaning solutions, eg. Jif, specialty stainless steel cleaners.
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Use rag, sponge or fibre brush (soft nylon or natural bristle. An old toothbrush can be useful). Rinse well with clean water and wipe dry. Follow polish lines.
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Lime Deposits from Hard Water.
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Solution of one part vinegar to three parts water.
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Soak in solution then brush to loosen. Rinse well with clean water.
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Oil or Grease Marks.
All finishes.
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Organic solvents (eg. acetone, alcohol, methylated spirits, proprietary "safety solvents"). Baked-on grease can be softened beforehand with ammonia.
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Clean after with soap and water, rinse with clean water and dry. Follow polish lines.
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Rust and other Corrosion Products.
Embedded or Adhering "Free Iron".
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Rust stains can be removed by adding one part of nitric acid to nine parts of warm water. Leave for 30 to 60 minutes, then wash off with plenty of water, and flush any drains thoroughly. See also previous section on Passivating.
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Rinse well with clean water. Wear rubber gloves, mix the solution in a glass container, and be very careful with the acid. (see Precautions for acid cleaners)
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Routine Cleaning of Boat Fittings.
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Frequent washing down with fresh water.
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Washing is recommended after each time the boat is used in salt water.
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Cooking Pot Boiled Dry.
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Remove burnt food by soaking in hot water with detergent, baking soda or ammonia.
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Afterwards clean and polish, with a mild abrasive if necessary. See comments re steel wool.
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Dark Oxide From Welding or Heat Treatment.
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"Pickling Paste" or pickling solutions given on previous page.
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Must be carefully rinsed, and use care in handling (see Precautions for acid cleaners).
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Scratches on Polished (Satin) Finish.
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Slight scratches - use impregnated nylon pads. Polish with scurfs dressed with iron-free abrasives for deeper scratches. Follow polish lines. Then clean with soap or detergent as for routine cleaning.
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Do not use ordinary steel wool - iron particles can become embedded in stainless steel and cause further surface problems. Stainless steel and "Scotch-brite" scouring pads are satisfactory.
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Precautions
Acids
Acids should only be handled using gloves and safety glasses. Care must be taken that acids are not spilled over adjacent areas. All residues must be flushed to a treated waste stream. Always dilute by adding acid to water, not water to acid. Use acid-resistant containers, such as glass or plastics. If no dulling of the surface can be tolerated a trial treatment should be carried out; especially for pickling operations. All treatments must be followed by thorough rinsing.
Solvents
Solvents should not be used in confined spaces. Smoking must be avoided when using solvents.
Chlorides
Chlorides are present in many cleaning agents. If a cleaner containing chlorides, bleaches, or hypochlorites is used it must be afterward promptly and thoroughly cleaned off.
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