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Ask the Experts Metals

We anneal and passivate 300 series stainless tube and sometimes get a “grey” coloration on the ID—can we keep it bright?

Zbigniew Zurecki
Research Associate

Formation of oxide film is a function of hydrogen/water partial pressure ratio, temperature, and time, all of which can change significantly at the furnace exit. An elevated dew point due to air ingress reacting with hydrogen, combined with reduced temperature, will lead to oxidation if the time within the exit area is sufficiently long. Additionally, the ID surface of the tubing may cool more slowly than the OD, causing inconsistent oxidation. Countermeasures include increasing the tubing’s travel speed, increasing the hydrogen flow rate over all tubing surfaces, and applying a nitrogen curtain at the exit to dilute air ingress and assist in cooling. There are technological, safety, and cost considerations associated with each countermeasure. Contact Air Products’ technical specialists for assistance at 800-654-4567.
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