What are we testing for?
The purpose of testing passivation processes is to verify the presence of a passive layer indicating successful treatment. Although stainless steels already have a high resistance to corrosion, some uses demand even more resistance. Some people consider passivation to be a cleaning treatment rather than a protective treatment itself because it works by removing surface iron. The purpose of the test is to look for any signs of free (reactive iron) remaining on the surface. For reference an image detailing the differences between a passivated and unpassivated article can be seen below.
What methods of testing do we offer?
- High humidity test
- Copper sulphate test
Copper Sulphate Testing
The copper sulphate test is intended to determine any presence of free iron. Iron can be introduced to the material during fabrication which can lead to corrosion sites if not removed. The copper sulphate test gives a clear visual indication in the presence of free iron in a matter of seconds making it a cost-effective and fast method of testing.
The mechanism that this test uses is a redox reaction. Copper (II) from the test solution is reduced to copper metal by any free iron and deposited on the surface of the part, producing the visual indication.
High Humidity Testing
High humidity testing requires the parts, or representative test pieces, to be placed into the unit at specified humidity and temperature parameters generally for no less than 24 hours. The conditions are set according to the specification being worked to.
Parts tested then go through a visual inspection to ensure no signs of corrosion are present, this may require using magnification. Using this test method can substantially increase the lead times for stainless steel passivation, due to the time parts must be in the humidity chamber to perform this test prior to release.
- Treatments we offer for passivating stainless steel
- Information on mechanical finishing and when to perform these operations
- Guidance about these tests and others can be found in technical specifications such as ASTM A967.