Stainless steel is a generic term for a family of corrosion-resistant alloy steels, which contains a different percentage of chromium, and nickel for each grade of stainless steel; 304 and 316 have extra molybdenum. It requires low maintenance but is not maintenance-free. Stainless steel mesh and wire grades should be appropriately chosen for the application required, with the correct fabrication, the installation procedures should be adhered to, and if cleaned regularly the aesthetic appeal can be maintained.
WHAT IS TEA STAINING?
Tea staining is discolouration of the surface of stainless steel by corrosion.
It is a cosmetic issue that does not affect the structural integrity or the lifetime of the material. It is wise to remove the staining as soon as possible after it has appeared. Tea staining occurs most commonly within about 5 km of the shoreline and becomes progressively worse closer to the marine source. Also, when fabricating steel and stainless steel by using the same equipment can lead to tea staining.
However, wind exposure, pollution levels, local sheltering and higher temperatures can create environments where tea staining might occur 20 kilometres or more from the shoreline. The effect is much less severe around sheltered bays. These same factors also increase the corrosion rates of alternative materials.
Tea staining can be avoided through the selection of a suitable grade, surface finish, fabrication and installation practices and washing regime for the corrosion conditions.
When tea staining does occur, it is usually visible within a month or two – older installations are unlikely to be affected unless corrosion conditions change. It is wise to remove the staining within a few months after it appears, to prevent it from progressing to more serious corrosion.
Maintaining Stainless Steel
The cleaning frequency of stainless steel depends on some points:
- The environment
- Position of the stainless steel structure
- Stainless steel surface finish and structure design
- Client’s expectations
A rule of thumb for cleaning external stainless steel is when you clean the surrounding windows also clean the stainless steel.
A Guide to Specifying and Cleaning External Stainless Steel
|Environment||Stainless steel 304||Stainless steel 316|
|Clean inland||3 – 6 months||6 -12 months|
|Polluted urban/industrial||Not suitable||6 -12 months|
|Coastal/marine (not splashed)||Not suitable||3 – 6 months|
Cleaning Note: Rainwater is an effective natural cleaner, stainless steel that does not get exposed to rain will require more frequent cleaning.
Ensure the surface, to be treated, is clean and free of grease and oil deposits, and is be evenly exposed to the treatment chemicals. Simple hosing with fresh water may be adequate, but for dirty or greasy surfaces wash down with an alkaline cleaner and rinse with clean fresh water.
The ASSDA Technical Bulletin – Preventing Coastal Corrosion has more information – download it from http://www.assda.asn.au