To have their optimum corrosion resistance, stainless steel surfaces must be clean and have an adequate supply of oxygen to maintain their passive surface layer.
Rust staining can occur and has been reported as anything from a slight brown ‘bloom’ on the surface to severe surface pitting or rusty scour marks on items such as handrails. These effects are usually due to surface contamination from contact with non-stainless steel items.
Iron contamination can be costly to remedy, and is avoidable.
These issues have been well documented and most publications on stainless steels deal with the problem.
Key issues to consider
Avoid contamination during all storage, handling and fabrication stages and also during the service life of the stainless steel item.
If contamination is suspected then test the surface.
Where contamination is detected take steps to remove it all and avoid spreading it around during the removal operation.
Avoiding ‘iron’ contamination
Stainless steel supplied by reputable manufacturers, stockholders or fabricators will normally be clean and contamination free.
These items should not show rust staining, unless contamination is introduced.
The use of non-stainless steel processing and handling equipment is a frequent source of contamination. Work table bearers, lifting ‘dogs’ and chain marks have all been noted as causes. Non-metallic contact materials and vacuum lifting equipment should be used to avoid process contamination.
Handling or fabricating stainless steel on equipment, using tools also used for non-stainless steels should be avoided. Working in ‘mixed-metal’ fabrication shops, without taking segregation and cleaning precautions can result in contamination.
Cutting or grinding debris from non-stainless steels should not be allowed to settle on stainless steel items.
As soon as any of this contamination becomes wet, rust staining will result.
Testing for ‘iron’ contamination
American standards ASTM A380 and A967 outline iron contamination tests.
Some of the tests simply look for rust stains from contact with water or high humidity environments, but for detection of the ’cause’ ie free iron on the surface, rather than the ‘effect’, which is the resulting rust stains, then the ‘ferroxyl test’ is probably the better method. This will detect either free iron or iron oxide and is sensitive enough to detect small levels of contamination.
ASTM A380 outlines the procedure in section 7.3.4.
Nitric acid is added to distilled water, followed by the potassium ferricyanide.
The ‘recipe’ is shown in the table.
|Distilled water||94 weight %||1000 cm3|
|Nitric acid||3 weight %||20 cm3|
|Potassium ferricyanide||3 weight %||30 grams|
Preparation of the solution must be done using equipment where no iron or steel comes into contact with the reagents. It should be applied to the stainless steel surface preferably using an atomizer spray.
A blue stain, appearing in about 15 seconds, indicates the presence of iron. The solution has to be removed from the surface as quickly as possible after testing using either water or 5-20% acetic acid (or vinegar) and scrubbing with a fibre brush, finally rinsing with the solution used, several times.
The standard notes that potassium ferricyanide is not toxic but that the fumes may become toxic if the solution is heated.
Test kits are available commercially from some BSSA member companies.
Removing ‘iron’ contamination
Any cleaning process that can remove embedded iron can be used.
It is important to ensure that all the contamination is removed or not spread to other areas of the stainless steel product surfaces, otherwise rust staining can recur. In this respect, chemical, rather than abrasive cleaning may be advisable.
Cleaning and iron recontamination is well documented in stainless steel cleaning product suppliers literature and literature published by the Nickel Development Institute.
As with cleaning, a stepwise approach, depending on the severity of the staining should be considered.
Nitric acid or nitric / hydrofluoric acid preparations are the most effective but may cause surface etching, which may be unacceptable on the restored item.
Methods for removing ‘iron’ contamination
Mild staining or surface ‘bloom’
Mild-non scratching domestic cleaning creams or polishes can be used.
These usually contain calcium carbonate, with surfactant additions. ‘Jif’ kitchen cream cleaner (Lever Brothers Ltd) is an example of such a product.
Domestic stainless steel cleaners, which may contain citric acid can also be used. Shiny Sinks -(Home Products Ltd) is an example of such a product.
Cleaning methods for stainless steel
Fresh iron / steel grinding grit or dust
A saturated solution of oxalic acid, applied with a soft cloth or cotton wool and allowed to stand for a few minutes, without rubbing or abrading.
This should etch out the iron particles, without leaving scratches or significantly altering the surface texture of the stainless steel.
Moderate rust staining
Phosphoric acid cleaners can be effective if sufficient time and care is taken, with minimal risk of etching the surface.
Alternatively, dilute nitric acid should remove small amounts of embedded iron and will help repassivate the cleaned surface.
More severe rust staining
Nitric / hydrofluoric acid pickling preparations should remove more embedded iron than nitric acid alone.
Surface etching is likely and so complete restoration to the original finish and surface texture may not be possible.
If these preparations are left on stainless steel surface too long, pitting can be caused.
There is a limit to what can be achieved.
Although contamination may be removed, these treatments will not remove any pitting associated with severe staining.
In such cases mechanical grinding may have to be considered to ‘bottom-out’ the pits which means that a complete restoration of the surface will then be needed.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)