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Inconel 718 turns black during pickling only if heat treated

Q. We pickle Inconel 718 before welding in HF/Nitric solution in preparation for welding. We measure material removal as a way to control the process (soak time).

Lately, I was asked to pickle some parts that had been heat treated already. The metal turned dark grey/black in the exact same pickling solution that leaves non-heat treated Inconel bright with no pitting.

Can someone explain what happens to this metal in heat treatment that would cause it to be sensitive to the pickling process and turn black?

Thank you for any info.

Mickie Davis
- Cookeville, Tennessee
July 28, 2021

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