Three Ways to Test the Surface Cleanliness of your Stainless Steel
Stainless steel gets its corrosion-resistant qualities from the chrome-oxide (passive) layer that forms spontaneously when it comes into contact with atmospheric oxygen. This protective layer is extremely thin and in order to fully form, the stainless surface must be thoroughly cleaned and then passivated.
Chemical passivation is effectively a cleaning operation. A comprehensive degreasing phase is performed first to remove oils, grease and other hydrocarbons. Then, the passivating solution (usually an acid) is applied to remove loosely adhered ferrous debris, and other surface contaminants. Carried out correctly, this process will leave the surface of the alloy in optimal condition, with an enriched chromium-iron ration, to form a dense passive layer.
The passivation process does not change the physical appearance of the metal. So, how can you inspect the work to be confident it has been carried out properly?
Industry specification ASTM A967 ‘Chemical Passivation Treatments for Stainless Steel Parts’ lists various tests. They all work on a similar principle in that they will detect the presence of free-iron contamination. Free-iron could potentially prevent the passive layer from fully forming, and/or lead to corrosion down the line.
The suggested tests can be loosely grouped together into two categories. Accelerated corrosion testing and positive-indication testing. There is also a third category (not included in ASTM A967) that physically detects the presence of a passive-layer.
1. Accelerated Corrosion Testing
- Water immersion
- Salt spray
The tests may take many hours (often days) to carry out and are best suited to small items or test-coupons.
2. Positive-Indication Testing
- Potassium Ferricyanide (Ferroxyl Testing)
- Copper Sulphate
3. Passivity Testing
- Palladium chloride
- Passivity meter
Passivity meters work by measuring the rest-potential of the stainless steel surface. The meter utilises an algorithm and converts the measurement into a 0-100 scale, indicating the quality of the passive-film. The higher the number, the better. The test uses a non-toxic acid as an electrolyte and provides instantaneous results.
Also, as the equipment is very sensitive to free-iron contamination, (presence of free-iron will almost certainly result in a ‘fail’ result) the test can be used to determine both cleanliness and passivity.
Our core business and area of expertise is the passivation of stainless steel.
If you have a requirement for any of the tests listed above, INOX Passivation Ltd are able to help. Contact us now for assistance or advice.