As the axiom goes, “the job’s not over until you finish the paperwork” and getting the paperwork completed in the proper fashion to show that a system has been properly passivated is indeed a crucial part of demonstrating compliance.
But how exactly do you prove that the passivating process that was just completed actually did the job? The straightforward answer is that it gets tested.
However, the straightforward answer is not a simple matter. The standards for ensuring passivation ASTM A-380, suggests the ferroxyl test (and others) – primarily for welded joints – along with a couple of other tests. These tests are used to determine that the free iron is removed and not the quality of the passive film. To properly ensure that passivation has been achieved, as well as to verify the quality of the passive layer itself, Astro Pak performs several other tests to confirm success. Additionally, it is through specialized testing that a determination can be made for the timing of future services.
Even the smallest amount of free iron or metal contamination on a passivated surface is sufficient to reduce the corrosion resistance of stainless steel. To that end, procedure 7.3.4 of ASTM A-380 allows testing the surface by spraying a solution of potassium ferricyanide upon it. If free iron is present, the area will turn blue. Being extremely sensitive, the test will detect even the smallest amount of iron, but it can also generate false positives, indicating the presence of iron where there is none. Depending on the nature of the location where the item is being tested, it may simply indicate the presence of iron dust (rust film) in the atmosphere of the immediate area. It also doesn’t offer quantitative data regarding the passivation of the surface or the ratio of chromium (Cr) to iron (Fe) in the surface with a higher Cr:Fe ratio being desired maximum corrosion resistance. Additionally, this test doesn’t show the presence of other inclusions, such as aluminum, which can be a source of future pitting corrosion.
Like the Ferroxyl Test, testing with copper sulfate (CuSO4) provides a readily visible indication of the presence of free iron on a surface. The copper sulfate is a dilute solution of water, copper, and sulfuric acid. It is not as sensitive as the ferroxyl test and should only be used on austenitic 200 and 300 series stainless steels in addition to the ferritic 400 series steels that contain at least 16% chromium. Performing the test on the ferritic 400 series steels containing less than 16% chromium or the martensitic steels of that series will generate false failures by incorrectly indicating the presence of contamination. Instead of a blue indicator, the presence of free iron is revealed by a coppery sheen on the surface caused by the iron reacting to the sulfuric acid resulting in the copper to fall out of solution.
Salt Spray Test
Unlike the previous two tests, which can be used by the owner of the stainless steel equipment to either verify the work or detect a problem, salt spray testing is more involved, time consuming and requires specialized equipment. It is generally reserved for parts or assemblies that will be expected to encounter chloride environments while in service. Guidance for conducting these tests is provided by ASTM B-117 and other standards. The tests can take between 24 hours to weeks depending upon the type of stainless steel being tested. For example, where a properly passivated part made of 316 stainless can be expected to pass a test involving being sprayed with a 3% salt solution for 96 hours, a similar part made of 304, 430 or 431 types of stainless steel would fail much more quickly. Instead, they are tested with a salt solution at 0.3% concentration. The parts are deemed to be successfully passivated if there is no rust on the parts.
Koslow Passivation Tester
A relatively recent development, Koslow Scientific created a hand-held testing unit that does not rely on chemistry to determine the presence of free iron, but rather by using electrochemistry. Currently limited for use on 300 and 400 series as well as 17-4PH and 17-7PH stainless steels, the meter uses the metal’s surface potential under controlled conditions, including a constant pH. By measuring the voltage potential across a surface, the presence of any free iron is revealed when it forms a momentary circuit that is detected by the tester. Like the two chemical tests, the Koslow Passivation Tester can be used to alert operators of the need for passivation as well as to test the effectiveness of the treatment afterwards. Because it uses electric current, the tester can be used on vertical or overhead surfaces, allowing for more thorough inspection.
Auger Electron Spectroscopy
Near the end of the 20th century, Auger Electron Spectroscopy (AES) was one of the new methods developed to examine the effectiveness of passivation treatments. Performed in a lab setting, AES bombards the metal surface with electrons. The difference between the auger electron bombardment and the binding energy of the elements on the surface has a unique value which identifies it to the instrument. By noting the intensity of the peak of these values, the concentration by depth of each element can be gauged. In addition to being able to scan across the surface, the AES can also be used to analyze deeper into the metal. This allows determination of the relative amounts of chrome and iron at various depths.
X-Ray Photoelectron Spectroscopy
Like AES, X-Ray Photoelectron Spectroscopy (XPS) came to the fore in the late 1990s. Also known as Electron Spectroscopy for Chemical Analysis (ESCA), it is similar to AES in that the surface of the metal is electromagnetically bombarded, but instead of electrons, X-rays are used. This is a destructive test in that the metal surface is vaporized to a depth of 50 angstroms (50Å) or 0.1nm across an area of 200Å (20nm). It is important to remember that the goal of the passivation process is to remove iron, iron oxides and other residual metals from the upper 20-50Å of the stainless steel surface. The ideal passive layer that is formed on top of this surface measures 30Å in depth. The Spectroscope can not only determine the chrome to iron ratios, but also the chromium oxide to iron oxide ratios, providing information about the oxidation state of each.
While AES and XPS provide analysis of chemical composition to indicate the relative resistance to corrosion of a treated stainless steel item, electrochemical testing is used to predict when pitting corrosion will occur. Originally called Cyclic Polarization (CP), it is a procedure laid out in ASTM-G61. It uses a test piece of stainless steel as an electrode in a circuit. A current, measured in micro-amps per centimeter, is run through it and the potential density is analyzed to detect when pitting occurs. The lower the current density is, the more resistant the steel is to pitting corrosion. From this data, the maintenance cycle for the metal can be determined, allowing such things as a schedule of preventative passivation treatments.
Testing for Success
Different tests are used to demonstrate, ensure and predict passivation of stainless steel. No one test is right for all circumstances. And, while some can be easily performed by the user, others require, specialized equipment, facilities, or test . When used together, as appropriate, the result is a higher level of confidence that the parts and equipment will operate to their optimal level.