Stainless steels, specifically the 300 series stainless steel alloys, will develop a protective passive layer and resist corrosion in all normal ambient conditions. However, there are contaminants that, if allowed to stay on the metal surface, can alter the protective layer and render it ineffective. For this reason, it is a good practice to maintain the surface with preventive cleaning, and it is also important to protect the metal during installation to avoid exposure and adherence of adverse impurities.
Note: This guide describes general cleaning and maintenance practices for stainless steel in both interior and exterior environments. For a detailed review of interior cleaning, please find the guide for cleaning interior stainless steels as part of the Zahner resource guide.
Most exterior uses of stainless steel will perform well with normal rainfall. However, it is recommended to clean the surface once or twice a year particularly after the winter road de-icing period. Environments where de-icing salts are common must be cleaned at minimum once a year. Seaside environments must also receive yearly cleansing. Use mild detergent and warm water. Rinse thoroughly and repeat. Performing this simple task will keep the stainless steel performing well for years.
Precautions in the field
Extra precautions should be taken when installing stainless steel in a construction environment. Cleaning acids for concrete and stones can cause particular harm to stainless steel and cause corrosion. Likewise precautions for grinding and welding iron-based metals nearby stainless steel. Iron particles that become embedded in stainless steel will oxidize, causing the appearance of rust.
CLEANING STAINLESS STEEL
To properly clean stainless steel it is necessary to first determine what has to be removed and then select the appropriate cleaning procedure. Approaches to cleaning vary considerably; from those jobs that involve simple removal of dirt or smudges that collect on surfaces, to the most complex operations for removing free iron contamination.
Commercial metal cleaners may also be used, but it is important to make certain that they can be used on stainless steel and will not harm any of the surrounding environments. There are numerous cleaning compounds that are available. It is important to test the material that is being cleaned first to make sure that the cleaning compound does not adversely affect the stainless steel finish.
Below is a list of common mild contaminants, and methods for how to clean stainless steel for inks, oils, adhesives, and water scale:
- Dirt deposits on stainless steel including dust, dirt, fingermarks, and identification markings are easily removed. Frequently, warm or hot water with or without detergent is sufficient. Do not use carbon steel brushes or steel wool as they will leave particles embedded on the surface that will rust and stain the surface. For slightly more aggressive cleaning, use scouring powder with a small amount of vinegar. Rinsing in clean hot water should always follow cleaning. When water is known to contain mineral solids, which leave water spots, the surface must be dried with soft towels. Caution must be used so the towels do not pick up abrasives and scratch the surface. There are some cleaners and oil impregnated cloths that can also help avoid water spotting.
- Inks are typically removed by applying a solvent such as xylene or alcohol. Often the selection of the solvent is based on the type of ink and experimentation for removing and should be tested in an unexposed area if possible. Lastly, the cleaning should be followed by a thorough warm or hot water rinse.
- Oils can typically be removed using isopropyl alcohol or xylene. In cases where these solvents don't work, acetone, methyl or ethyl alcohol, toluene, and mineral spirits will be more effective, but require some precautions. These solvents are flammable and may leave behind a residue. It is important to remove all of the contaminant and the residue by using a warm or hot water rinse or pressure spray. Furthermore, there are many over-the-counter cleaners that contain some of these solvents. It is important to try them in a limited and preferably unexposed area prior to any extensive cleaning.
- Adhesives are best removed using alcohol, xylene, or mineral spirits. It will often leave streaking; therefore, should be followed with a glass cleaner or similar cleaner that utilizes ammonia. If the adhesive is dry and not softening up with the mineral spirits, there are many cleaners in the marketplace that remove stubborn adhesives. It is important to try them in a limited and preferably unexposed area prior to any extensive cleaning.
- Water scale is best removed using vinegar followed with an ample amount of warm water. If that does not remove the scale, a mild abrasive may be required. Caution needs to be taken so that the material is not scratched contrary to the original finish on the stainless steel. Lastly, if the scale is persistent, a cleaner that contains a phosphoric acid or citric acid should remove the scale. It is important to follow the precautions for the cleaner and to flush the surface with warm water.
Each of the above contaminants are mild, and can be cleaned with these simple methods. Below are a listing of harsher contaminants which may require replacement if not mitigated.
SIGNS OF CORROSION
When corrosion occurs on stainless steel it is due to free iron particles on the surface or chlorine attack. Corrosion manifests itself as small brownish spots, often covering large areas of the surface. Corrosion can also occur on the surface when small steel particles are transferred to the surface from improper handling or tooling, or exposure to chlorides. These corrosion conditions can lead to pitting on the surface. Intergranular corrosion develops in severe environments. It occurs when carbide precipitation has weakened the passivity of the surface. The corrosion attack occurs at the grain boundaries of the stainless steel usually in regions around welds.
Crevice corrosion occurs at the joints with nonmetal materials such as gaskets and seals. The passive film is inhibited by the nonmetal materials and when foreign matter (in particular chloride or sulfide salts) migrates to the crevice and lead to the formation of electrolytic cells. If moisture is prevented from collecting in the crevice, corrosion will cease or proceed at a lessened rate. This condition primarily occurs when the gasketing is porous or the sealants cannot react properly to movement. Oftentimes the exposed surface shows little indication of the corrosion. Sometimes it will show itself as rust leaching out of a gasket or sealed condition.
These problems are on the more severe end of the need to clean spectrum and obviously take a more aggressive cleaning process. When iron particles get embedded into the surface and start to oxidize they show up on stainless steel as a dark spot. If left unchecked, they can form pits in the surface of the stainless steel that cannot be removed easily. In order to try to remove the surface contaminant, you should utilize a cleaner that includes a phosphoric, oxalic, or sulfamic acid. Oftentimes the acid must maintain contact with the surface in order to completely remove the rust. It is important to flush the surface with copious amounts warm water to insure that the acid is completely removed. Furthermore, care must be taken to control the flow of the effluent so it does not stain or harm any surfaces below. It is important to follow the precautions regarding any of these cleaners that contain acid.
For a situation where a weld joint exposure is unavoidable, the first attempt should be to try a mild abrasive cleaner with phosphoric acid. If this does not completely remove the discoloration, an electrolytic weld cleaning process can be used, but that should be performed by someone familiar with using that specific piece of equipment. After the discoloration is removed, it is important to flush the area with warm water.
In the case of severe corrosion, it is often necessary to dis-assemble the stainless steel parts so a more thorough cleaning can occur via acid immersion. These cleaning processes often require a re-passivation of the surface so it is recommended that this process be performed by an expert with stainless steel. Remember crevice corrosion occurs around improper bolts or washers being used that can trap contami-nants and moisture. Proper gasketing or the change in the fastener type can improve the chances to avoid further corrosion significantly.
CHLORIDE CONTAINING SOLUTIONS
Chloride containing solutions, including hydrochloric acid based cleaning agents and hypochlorite bleaches can cause unacceptable surface staining and pitting, and should not be used in contact with stainless steels. Under no circumstances should concentrated bleaches contact decorative stainless steel surfaces. Hydrochloric acid based solutions, such as silver cleaners, or building mortar removal solutions must not be used in contact with stainless steels. Hypochlorite containing bleaches must be used in the solutions suggested in the manufacturer’s instructions and contact times kept to a minimum. Thorough rinsing after use is very important.
There are often simple procedures and over–the-counter cleaners to keep stainless steel surfaces clean and like new. The keys to maintaining the stainless steel finish are:
- Clean the contaminants from the surface immediately.
- Avoid abrasives where possible.
- Test clean in less exposed areas.
- Flush with warm or hot water after cleaning.