Qualitative Testing – Stainless Steel Passivation
These passivation tests detect the presence of free iron on the surface, but are not able to provide results related to the thickness and the actual quality of the stainless passivation, nor to the ratio of chromium and iron in the surface layers.
These are the real parameters that govern the passivation of stainless steel and therefore the resistance to corrosion of the metal. Chemical passivation tests can be considered valid as a pass/fail check or to detect the surface pollution, moreover only under certain conditions.
In addition, most of the chemical passivation tests are not able to recognize possible contaminants such as aluminum oxides or carbon compounds whose presence can lead to a possible triggering of corrosive phenomena.
Despite these limitations, some chemical passivation tests are normed in the international standards ASTM and EN-ISO. In particular, the ASTM A967 includes the following valid tests in order to evaluate the passivation of stainless steel:
- Water immersion test: for a minimum of 12 cycles, the passivated workpiece must be immersed in distilled water for 1 hour and subsequently left on open air to dry for one hour. The analysis is visual and there must be, at the end of the cycles, the presence of rust or red pitting, as a result of ferrous contamination.
- High Humidity Test: This passivation test should be performed for a minimum of 24 hours in a climatic chamber at 38 ? C with a HR of 97%. The passivated piece must be beforehand cleaned with acetone. As the previous passivation test, the piece at the end of the 24 hours must show no signs of rusting or contamination.
- Salt Spray Test: this corrosion resistance test detects the superficial Iron and all other contaminants. The workpiece has to be treated with a solution 5% NaCl. Although the salt spray test is well known and widely used, it is not developed to measure the passivation, nor even to the corrosion resistance of the stainless steel alloys, but rather with other purposes, in particular for testing of painted components.
- Copper Sulfate Test: The test performed with the solution of Copper Sulphate is once again a type of test that leads to visual result. It can be performed on pieces that are not used in the food or pharmaceutical industry. This passivation test is based on the reaction of the solution of copper sulphate with the free iron on the surface of stainless: if there is any, it will be literally coppered, so in case of missing or passivation of ferrous impurities, will be clearly visible to the outcome.
- Potassium ferrocyanide / Nitric Acid: This passivation test, as above, is not suitable when the component is used in food environment, pharmaceutical or biomedical. It ‘also recommended only on the series of stainless steel AISI 200 and AISI 300. The visible result, in the case of ferrous contamination or lack of passivation, it will be given by traces of blue color. The test is relatively fast (about 1 minute); it is performed using dangerous chemical agents.
- Palladium Chloride: This test is not considered by ASTM nor is commonly used on industrial scale; although it is on the market, this passivation test is misunderstood by most of the scientific and productive since often the result is distorted by external agents and, especially when the degree of passivation of stainless is low, by the surface finish; also, the result is still visual and questionable, therefore it is cited here only for completeness.