According the some manufacturers of PEEK, the useful pH range of PEEK tubing and fittings for HPLC is 2 – 14 for use with HPLC solvents and buffers.

However, I would be very careful with pH 0 to 6 for long term exposure. PEEK is manufactured using Phosphoric Acid and some of the granulate may even still contain thisAcid, meaning that this acid should be a strong reagent for PEEK. I suggest a higher pH as starting point for solutions with medium to high ionic strength for long term exposure.

PEEK laboratory tubing information