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 steel is repeatedly wetted/dried in order to speed up the corrosion of any ferrous debris that may have been left on the surface after an ineffective chemical passivation process. Pass/fail is determined by the visible presence of any rust spots or staining.
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
A chemical product is applied to the stainless steel surface and reacts with any free-iron that has not been fully removed. 
Ferroxyl indicator is a yellow liquid that turns blue when it reacts with iron.
Copper sulphate solution forms copper deposits during a positive (fail) reaction.
Both tests are very sensitive and very quick. Results can be reviewed in just a few minutes. However, both tests use hazardous products and are not recommended for use on equipment used in food processing or related industries. It is vital that the test reagents are cleaned thoroughly afterwards, as they can damage the surface if not. 

3. Passivity Testing

  • Palladium chloride
  • Passivity meter
The palladium chloride test is a positive-indication test. However, instead of reacting with iron, it determines the passivity of the steel via a reaction with hydrogen. Passive surfaces will not liberate hydrogen from the test solution – and the colour will not change. If the surface is not passive, the test solution will react, and turn black. The test is quite subjective and it is not a clear “pass/fail”. There are degrees of reaction (colour change) which are deemed acceptable. It also uses hazardous products which should be handled with care and thoroughly washed away afterwards. 

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.