Determining the Passivation

Question:

What test methods are available in determining the passivation of stainless steel? Please give me the “down and dirty” methods. What certified labs are available for performing passivation tests on stainless steel? D.P.

Answer:

The primary ASTM standard concerning this subject is A 380, Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment and Systems. It details the classic methods for passivating stainless steel (typically a 20-50% HNO3 solution at 120-160F, followed by thorough rinsing and drying) and also provides several tests to detect the presence of free iron on the surface of the stainless steel.

These test methods must presume that the passivation treatment can be considered adequate if it has removed all of the free iron from the surface. The standard lists three methods under section 7.2.5, “Tests for Free Iron.” None is very difficult, and it depends on the equipment you have available and the chemical expertise you may have.

The first test (7.2.5.1, Water-Wetting and Drying) subjects the stainless part to repeated water wetting and drying for 24 hours, but does not define the cycle, only the total dry time of 8 hours in a 24-hour test. One example conforming to this could be 2 hours wet and 1 hour dry. There should be no rust stain or corrosion product after the exposure.

Another test (7.2.5.2, High-Humidity Test) subjects the part to 100% humidity and 100-115F temperature for 24-26 hours. Again, there should be no rust stain or corrosion product after the exposure.

The last test (7.2.5.3, Copper Sulfate Test) is more involved, but quicker. An acidified copper sulfate solution is placed on the part (or immersed in it) for 6 minutes. Deposited copper will indicate the presence of free iron.

These are all easy tests to perform. I would look in your area for Metallurgical Testing Laboratories and ask them to quote a price on whatever method they think is the least expensive. Alternately, ASTM has a website that may allow you to find a lab quickly. They can be found at www.astm.org.