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Test to reveal oxides on steel surface

We are a looking for a process test that will indicate presence of Iron oxides on the surface of low carbon steel. It is obvious when tubing is discolored yellow but lab verification shows presence of oxides on non discolored tube.

Have tried Bleach test and Copper sulphate, They can be subjective ? Are there any other tests out there.

Thanks,

Richard Samuel
Engineer: Automotive - Archbold, Ohio
2004

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Iron is not a noble metal (and steel even less so), so a surprise would be the absence of oxide from mild steel. Usually, the oxide is invisible, but moisture can cause red rust. Has the tube been heated in air? Heating in air to an oxide thickness of about 300 nanometers (0.3 microns) produces a yellow optical interference coloring. Is the tube interior the problem; if so, what fluid and what temperature are involved?

Many physical methods can measure the oxide film thickness or color, as your lab knows (what did they report, and how?). The questions are: what oxide thickness is tolerable, what measurement sensitivity and speed are required, and how much are you willing to spend? The usual acidified copper sulphate ⇦ this on eBay or Amazon [affil links] test solution reacts more rapidly with a freshly cleaned steel surface (maybe a 10nm oxide) than a more heavily oxidized surface. Using less sulfuric acid should reduce the dissolution of oxide, thereby increasing the selectivity toward iron metal. I am unfamiliar with the bleach test, but hypochlorite will rapidly convert a green Fe+2 solution to a reddish Fe+3 solution. Perhaps, use a volt-ohm-meter to check the surface electrical resistance. Compare results with probes flat against the surface with sharp contacts through the oxide. If dealing with a moving tubing production process, a copper wheel could be used in checking conductivity. Also, a laser beam reflected off the surface at a wide grazing angle could give the oxide thickness.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2004