How did it get corroded?

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The ASM manual gives good advice on this:

Traditional pickling solution for zirconium is 25-50% HNO3 at 70 vol% + 2-5% HF at 49 vol% + remainder vol% water. The mixture ratio of HNO3 to HF must be > 10:1 normality. This should also be kept below 150F to prevent hydrogen absorption. After suitable time to remove what you need removed pull this out and rapidly rinse with flowing water to prevent insoluble fluoride surface staining. If this rinse doesn't prevent the staining, you can instead, after the HNO3-HF bath, pull out and immendiately dip into a "stop bath" composed of 70% HNO3 + 30% water.

Metal removal rate is linear with HF concentration, and doubles as bath temperature goes from 110F to 160F.

I'd also recommend a test piece using your exact process including fracture toughness testing afterwards to assure your variables do not introduce hydrogen. This should either be compared to fracture toughness values for this heat prior to pickling, or compared to any design requirements.

To echo cloa, how did this get corroded in the first place?

My hunch is that he has Fl staining on the surface now.

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