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Stainless steel corrosion and tea staining
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Stainless Steel Corrosion and Tea Staining
Stainless steel provides structural integrity and aesthetic appeal when utilised in construction, though its propensity to become tarnished and brown in colour over time – especially in coastal locations – has been identified as a significant issue when using the product in construction.
Much research into the brown discolouration of stainless steel – known as tea staining – has been undertaken by research and industry bodies right across the globe.
Although initially just a cosmetic issue, having no impact whatsoever on the structural integrity of the material, tea staining indicates corrosion, and occurs most predominantly in areas within a five-kilometer radius of the ocean.
Research, however, has indicated that tea staining can occur on stainless steel structures more than twenty kilometers from the ocean should factors including wind, sheltering, high temperatures and pollution also be at play.
Tea staining occurs when atmospheric conditions are too harsh for the grade of stainless steel used, with the effects becoming more apparent the closer the material is to salty water.
The factors listed below each play a large role in encouraging the presence of tea staining, and should be considered when attempting to avoid the damage.
- Corrosive substances – one of the biggest causes of tea staining is the presence of sea salt on the material, as it has the ability to remain moist at very low levels of relative humidity. This means the steel remains wet and proceeds to corrode. Corrosion is aggravated further by the presence of industrial pollutants that may exist in the environment.
- Atmospheric conditions – High humidity and temperatures, like those commonly experienced in coastal areas, create ideal conditions for tea staining. A thin layer of moisture that develops due to high humidity allows salt to diffuse, leaving behind a corrosive compound.
- Design – if drainage systems in the construction are not well designed, e.g. the surface is almost horizontal, or the steel captures pollutants, tea staining will likely become apparent. Severe corrosion beyond tea staining occurs in structures with corners and crevices, including intermittent welds, as these tend to readily trap water and other contaminants.
- Roughness of surface – surfaces featuring grooves or folds are more likely to see tea staining because of their propensity to trap salts and chlorides. As the surface dries the salt becomes more concentrated, aggravating the corrosion and increasing the rate at which tea staining will occur. Surface roughness should be measured at no more than 0.5µm Ra, as cut and abraded surfaces smoother than this are far less likely to corrode.
- Surface characteristics – stainless steel should be clean and free of pollutants in order to avoid corrosion, which can be achieved by conducting acid pickling, acid passivation and electro polishing for satisfactory amounts of time. If stainless steel has been welded, heat input will have destroyed the materials passive layer. Details on how this can be restored are explained later.
- Appropriate grade – the hundreds of stainless steel grades available owe their corrosion resistance to the chromium oxide film on the surface. A material’s corrosion resistance is measured using the Pitting Resistance Equivalent (PRE), formulated by testing the grade’s chromium, molybdenum and nitrogen levels. Increasing a grade’s PRE will ensure it is more corrosion resistant.
- Maintenance – while stainless steel is a relatively low maintenance material, it is not maintenance free, and should be given a regular wash. Oils and waxes can be applied to reduce the accumulation of corrosives, though these need to be renewed often, as they tend to attract debris.
In order to best preempt and subsequently avoid tea staining, several steps can be taken during fabrication.
- Design and fabrication – surfaces should be able to drain freely, with runoff not channeled, and the surface exposed to rain. Abraded surfaces should have their grain running vertically, and be no more coarse that 0.5µm Ra, finished with nitric acid passivation to achieve the best resistance to corrosion.
- Grade selection – stainless steel is graded by the limit of salts it can tolerate before corrosion. A minimum grade 316 should be chosen for areas within five-kilometres of the surf, as lower grades, i.e. 304 and 430, will likely see tea staining and higher rates of corrosion in a coastal environment, grade 2205 Duplex offers the most cost effective protection against surface corrosion designed into the product from manufacture.
- Treatment of welds and hot work – pickling following welding removes the welding oxide and chromium depleted layer from the surface, restoring the passive layer, meaning the steel retains its corrosion resistance.
- Installation and inspection – a completed structure must be inspected for damage and contaminants, with any suspected pollutants treated with repeated cycles of a misting and drying test using tap water.
- Avoid hydrochloric acid – though it is often used to clean cement and mortar, hydrochloric acid will stain a surface and begin serious corrosion if used around stainless steel.
The following design recommendations should be taken into consideration when constructing with stainless steel.
- Planning – forward planning must take place to specify the expectations of the structure, by way of a maintenance program. If maintenance is kept up and corrosion resisted, structures are likely to stay structurally sound for many years.
- Environment – while areas within five kilometres of the surf are most susceptible to tea staining, there is no real rule, as wind and weather conditions can see structures twenty kilometres from the surf experience tea staining.
- A smooth and clean surface finish – when it comes to reducing corrosion levels, the smoother the surface the better. It is recommended that a surface roughness of less than 0.5µm Ra is used. Methods including passivation, electropolishing, and pickling can be utilised to remove contamination from surfaces, though these processes have been known to decrease a surface’s aesthetic appeal. Mirror polish of components offers the best protection.
- Get the right grade – in marine and tropical environments, stainless steel with a corrosion resistance grade of 316 should be used as a minimum. A higher corrosion resistant steel could be used if the structure requires a higher aesthetic appearance using grades such as Duplex 2205.
- Weld treatment – generally in construction a weld should comply with AS/NZS 1554.6 Level 2, Class B, as explained in the relevant international standards. However, this doesn’t ensure minor defects won’t occur. A protruding weld should be ground flat and polished to a 320 grit to provide true corrosion resistance. It must be remembered that passivation takes place in moist air within 24 hours, though a chemical passivation treatment using a nitric acid solution can be used to reduce the time for passivation to occur, create a corrosion avoidance film, deplete iron contamination and rid the surface of exposed manganese sulphide. If the environmental circumstances are especially harsh, the process of chemical passivation requires completion following abrasion. Alternatively, a grade II blasting finish can be undertaken, which sees the surface’s heat tint and chromium depleted layer removed, while ensuring the surface roughness is never higher that 0.55µm Ra. It should also be notes that this will not control tea staining. Provided there is no surface crevices or defects, a grade II pickled finish will protect against tea stain corrosion without polishing down the weld. Pickling should take place as the final process in the weld, to completely remove any contaminants and replace the passive layer, which restores a corrosion resistant finish.
- Regular maintenance – regular maintenance is absolutely necessary in order to ensure tea staining does not occur. Contaminant deposits that inflict corrosion, such as that of salt, can be rain washed clean, however, it is imperative the surface has good drainage. Soap or a mild detergent combined with warm water will produce the best finish on stainless steel surfaces, and it is important that harsh cleaners that contain chlorides or bleach and never used to clean stainless steel surfaces.
Should tea staining occur on a surface, the seven points first mentioned can be addressed, in order to correctly identify where the problem has occurred leading to surface corrosion.
By utilizing 2205 duplex stainless steel for investment castings and CNC machining of glass hardware we can all but eliminate the risk of tea staining corrosion if usage guidelines are adhered to.
Photo (left) is an example of extreme surface tea staining due to NO maintenance .
- Choose the correct grade of stainless steel to suit your environment
- Don’t ever use Hydrochloric Acid on or around stainless steel
- Mirror polished finishes offer superior corrosion protection over satin and brushed finishes
- All welds and hot work on the metal must be treated to reinstate the protective chromium coating
- Duplex stainless steels offer superior corrosion resistance over 304, 316 and 316L
- Maintain the metal by washing with warm soapy water and rinsing to remove surface contaminants .
How do you educate your customers on cleaning stainless steel products?