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Will this stainless steel weld rust?

Passivation testing helps ensure stainless welds are protected from the elements

Fabricators who specialize in stainless steel may know this story: A customer calls and complains of rust. A supervisor talks to the welders: Didn’t they clean it properly? The welder says he certainly did—he made sure to grind and polish his work completely, with no trace of blue near the weld. Yet the rust is there. What happened?

It all had to do with what happened to the stainless’s chromium oxide layer during and especially after welding. This is where weld passivation testing can help, and here technology has made it practical for fabricators to conduct such testing in-house.

That’s according to Chester Collier, senior vice president of global distribution at Windsor, Conn.-based Walter Surface Technologies. “Stainless does not react well to a lot of heat,” he said, “so when you’re welding thin material, even TIG welding, you get a lot of bluing and discoloration of the stainless. That’s cosmetic, but here’s something even worse. [Welding] has burned off a layer of chromium oxide, the element that prevents stainless steel from rusting.”

He added that when stainless steel is cleaned properly, its chromium exhibits a kind of creeping action. “In a sense, [the stainless] heals itself,” Collier said. “It’s a little like cutting your hand. If you let the cut get dirty, it will take a long time for the cut to heal, and it won’t heal very well. But if you clean the cut and protect it, the cut will heal itself and not leave too much of a mark. Stainless steel heals itself the same way.”

If bluing or other debris left by the welding process remains, the chromium layer won’t creep and seal the weld. If the oxide layer hasn’t sealed the weld, you get oxidation and rust.

To achieve this passivation, fabricators clean stainless welds one of three ways. They pickle it using a pickling paste; they use abrasives and wire brushes; or they clean it chemically using an acid- or salt-based solution and electricity. After the surface is cleaned, the chromium oxide layer should creep over and seal the weld within 24 to 48 hours, depending on the grade of stainless and specific welding application.

“If a product is cleaned properly, it has annealed itself, and that product can be used outside, even in a saltwater or salty air environment; that material should not experience any discoloration or rusting,” Collier said.

Still, human error can complicate matters. An operator may have used a wire brush, and the weld may look perfectly clean, but remaining traces of contamination still can cause problems. The same could be said for pickling and even chemical cleaning. An operator may miss a spot or not clean it well enough to passivate it.

Passivation testing mitigates this problem. Shops typically send parts out to third parties for such testing. But as Collier explained, fabricators now are starting to use small testing units—one being the SURFOX Smart Passivation Tester—to conduct passivation tests in-house. Some operations have been using the tester on actual welds, while other companies are using the tester to verify that a product (like the tank pictured here) is indeed made out of stainless steel.

The tester basically measures the conductivity of two points, with current passing through a liquid in the sensor that helps make the measurement as accurate as possible. From this measurement, the unit gives either a positive or negative reading. A reading in the positive territory shows that chromium creep (passivation) has occurred and that the material is sealed and protected against oxidation and rust.

“If I get a negative reading, it tells me there’s no chromium creeping over the weld,” Collier said, “and there may be something that’s blocking it, be it a burned discoloration or anything else.”

The testing technology isn’t entirely new, but the way the system records the results is. Traditionally, the person doing the testing would need to record the results on a spreadsheet manually. This wasn’t too arduous, but it opened the door for errors. Now the tester automatically transmits the results to a secure site online.

Here’s how it works. Online, the person doing the testing can enter the job number and grade of stainless. After this is set up, the Bluetooth®-connected testing device sends readings to the host site, which automatically populates a chart showing each passivation reading (ranging from -500 to +150), when those readings were taken, and at what interval (e.g., 25 readings, 8 inches apart). The tester can take a picture of the test points and upload those to the cloud as well. If a person tests a weld several times during a certain period, “each time he takes another reading, it will show that he’s moving closer to the [desired] passivation level,” Collier said.

Collier added that this technology can help provide traceability. The goal is that when a product leaves a fabricator’s facility, it has proof that the stainless is indeed passivated and ready for use.

About the Author
The Fabricator

Tim Heston

Senior Editor

2135 Point Blvd

Elgin, IL 60123

815-381-1314

Tim Heston, The Fabricator's senior editor, has covered the metal fabrication industry since 1998, starting his career at the American Welding Society's Welding Journal. Since then he has covered the full range of metal fabrication processes, from stamping, bending, and cutting to grinding and polishing. He joined The Fabricator's staff in October 2007.