How to Test for Passivity in a Metal’s Surface?

Paperwork is a crucial part of ensuring that a task is completed in the correct manner to demonstrate that a system has been appropriately passivated. This is a crucial part of demonstrating compliance.

One of the main issues then becomes how to demonstrate that the passivating process that was just completed actually did the job at hand. The straightforward solution to this is to test this process.

However, this testing is a complex process. The standards recommended for assuring passivation by ASTM A-380 include the Ferroxyl Test, and others, primarily for welded joints. These tests are performed to determine that the quality of the passive film remains intact while the free iron is removed.

Astro Pak conducts several additional tests to validate success to ensure that passivation has been accomplished and to certify the caliber of the passive layer itself. A decision regarding scheduling upcoming services can also be determined through specialized testing.

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Ferroxyl Test

A passivated surface can become contaminated with a tiny quantity of free iron or metal, which will weaken the stainless steel’s ability to resist corrosion. To evaluate the surface in this manner, procedure 7.3.4 of ASTM A-380 permits spraying it with a potassium ferricyanide solution.

Blue discoloration indicates the presence of free iron. Its high sensitivity means that the test can identify even trace amounts of iron, but it can also provide false positive results that incorrectly indicate the presence of iron.

Depending on the environment where the item is examined, it can be a simple indicator of the presence of iron particles (rust film) in the surrounding air.

It also does not provide quantitative information about the passivation of the surface or the ratio of chromium (Cr) to iron (Fe), with a greater Cr:Fe ratio being desirable for maximum corrosion resistance. This test's results do not indicate any other inclusions, such as aluminum, which could later cause pitting corrosion.

Copper Sulfate

Copper sulfate (CuSO4) testing, like the Ferroxyl Test, gives a clear indicator of the presence of free iron on a surface. Copper sulfate is a diluted mixture of sulfuric acid, water, and copper. It should only be used on ferritic 400 series steels and austenitic 200 and 300 series stainless steels with at least 16% chromium since it is less sensitive than the Ferroxyl Test.

Testing martensitic or ferritic 400 series steels that contain less than 16% chromium will result in false failures since they wrongly indicate the presence of contamination. A coppery sheen on the surface instead indicates the presence of free iron due to the iron’s reaction with sulfuric acid, which causes the copper to precipitate out of the solution.

Salt Spray Testing

Salt spray testing is more sophisticated, time-consuming, and requires specialized equipment than the other two tests. It can be utilized by the owner of stainless-steel equipment to validate the operation or identify a problem.

It is often saved for components or assemblies that operate in chloride environments. ASTM B-117 and other standards offer instructions for conducting these tests.

Depending on the type of stainless steel being tested, the tests could last anywhere from 24 hours to weeks. When exposed to a 3% salt solution for 96 hours, a properly passivated item made of stainless steel 316 will pass the test; however, a part made of stainless-steel types 304, 430, or 431 would fail much faster.

They are instead examined using a 0.3% concentration of the salt solution. The passivation process is successful if there is no rust on the pieces.

The Koslow Passivation Tester

In a more recent development, Koslow Scientific developed a portable testing device that uses electrochemistry rather than chemistry to detect the presence of free iron.

The meter employs the metal’s surface potential under regulated circumstances, including a constant pH. It is currently restricted to usage on 300 and 400 series stainless steels and 17-4PH and 17-7PH stainless steels. Any free iron that makes a momentary circuit detected by the tester can be identified by measuring the voltage potential across a surface.

The Koslow Passivation Tester can inform operators of the need for passivation and subsequently assess the treatment’s efficacy, like the two chemical tests. The tester can be used on vertical or overhead surfaces since it uses electric current, enabling a complete inspection.

Auger Electron Spectroscopy

Auger Electron Spectroscopy (AES), a new technique created toward the end of the 20th century, was one of the tools used to assess the efficacy of passivation procedures. AES bombards the metal surface with electrons while being carried out in a lab.

The difference between the auger electron bombardment and the binding energy of the surface elements has a unique value that the instrument identifies. The concentration by the depth of each element can be determined by observing the intensity of the peak of these values.

The AES can be utilized to perform a deeper analysis of the metal in addition to scanning across the surface; due to this, the relative proportions of chrome and iron at various depths can be calculated.

X-Ray Photoelectron Spectroscopy

X-Ray Photoelectron Spectroscopy (XPS), like AES, gained popularity in the late 1990s. It is often referred to as “Electron Spectroscopy for Chemical Analysis” (ESCA). It is similar to AES in that the metal surface is electromagnetically blasted, with X-Rays employed instead of electrons.

This test is destructive since the metal surface is vaporized across an area of 200 Å (20 nm) at a depth of 50 angstroms (50 Å) or 0.1 nm.

The main objective of the passivation process is to remove all remaining metals, including iron and iron oxides, from the upper 20–50 Å of stainless steel’s surface. The depth of the optimal passive layer that develops on top of this surface is 30 Å.

In addition to calculating the chrome to iron ratios, the Spectroscope can also calculate the chromium oxide to iron oxide ratios, which provides details about each element’s level of oxidation.

Electrochemical Testing

While AES and XPS give chemical composition analyses to show a treated stainless-steel item's relative resistance to corrosion, electrochemical testing is utilized to forecast when pitting corrosion will happen. This process, described in ASTM-G61, was first known as Cyclic Polarization (CP).

It uses a piece of stainless steel as an electrode in a circuit. A current is run through, and the possible density is analyzed to detect when pitting occurs — the lower the current density, the more immune the steel is to pitting corrosion. 

A program for preventative passivation treatments, for example, can be created using this data to calculate the metal’s maintenance cycle.

Testing for Success

Various tests are performed to prove, confirm, and predict stainless steel passivation. No single test is appropriate in every situation. Additionally, while the user can easily complete certain tasks, others call for specific tools, facilities, or tests. When used together, as appropriate, the result is a higher level of confidence in the fact that the parts and equipment will operate to their optimal level.

This information has been sourced, reviewed, and adapted from materials provided by Astro Pak Corporation.

For more information on this source, please visit Astro Pak Corporation.

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