Stainless Steel Composition
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Stainless Steel Composition Stainless steel is an alloy that contains at least 50% iron and 10% chromium, the chromium slows down the corrosion process and plays a key part in defining stainless steel, generally the higher the chromium percentage (up to 30%), the more corrosion resistance. Other elements are also added to enhance the properties of a particular grade and type of stainless steel.
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Stainless Steel Grades
Stainless steel alloys are grouped according to the structure of their crystals. Adding nickel creates the structure called austenitic. Austenitic stainless steels are identified by their "300" series designation. Grade 304 is made up of 18% chromium and 8% nickel offering good corrosion resistance. Type 304 is the most common stainless steel produced, and satisfies a broad demand for adequate performance at affordable prices. Type 304 is inadequate for most external applications. |
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Corrosion Definition When stainless steel is produced, the chromium forms an oxide layer. As long as this layer remains intact, the stainless steel remains passive. Once the oxide layer begins to break down, the stainless steel becomes active and its corrosion resistance is reduced. Rust is the obvious visible evidence of corrosive activity. When used properly, stainless steel enjoys a strong and enduring reputation for visual appeal and structural integrity in a wide range of applications and environments. |
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Stainless Steel "Tea Staining"
Incorrect specification, fabrication or subsequent handling may cause stainless steel to stain or discolour, impairing the overall look. This brown discolouration is referred to as "tea staining" and has been identified predominantly on external applications in the UK and overseas. Tea staining mainly occurs as a result of environmental factors such as wind exposure, industrial pollution and elevated temperatures, and is worse closer to the sea, a main cause of salt water pollution. Tea staining can be controlled and does not affect the structural integrity or the longevity of the material. |
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Design, Fabrication and Handling Poor design and fabrication can lead to tea staining or more serious corrosion of stainless steels. In the design stage, it is advisable to avoid crevices such as intermittent welds and areas where water can collect, it is also imperative to avoid carbon steel contamination. Rough surfaces promote tea staining. The main cause of this problem is the salt deposited on the stainless steel surface, the smoother the finish the better the corrosion resistance. To reduce the risks of tea staining, choosing or creating a smooth surface are ideal combatants. Smooth surface finishes stay cleaner between washes and don't have deep surface grooves where chlorides and other contaminants can collect and concentrate. A process called pickling may be used to remove surface contaminants, but may dull the surface. Alternatively, electro-polishing may be used which has the added benefit of brightening the surface. |
Example of crevice corrosion in stainless steel |
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Specify Grade 316 Stainless Steel As outlined in previous section, Grade 316 is ideal for external or highly corrosive environments. Regular maintenance and use of lanolin based protective products will ensure the best resistance against tea staining or corrosive effects. Maintenance and Protection Even smooth stainless steel finishes in external applications may show tea staining if not washed regularly. Rain washing the stainless steel surface can help reduce tea staining, and should therefore be an important project design consideration. If this is not possible then best results are achieved by washing with soap or mild detergent and warm water followed by ringsing with clean cold water. Surface appearance may be further enhanced by wiping dry the washed surface and treating the stainless steel with an approved polish and protective coating spray. |
Various treatments are available for cleaning and general maintenance of stainless steel |
