Pickling & Alpha-Case Removal
Every time titanium is heated in air — forging, hot forming, annealing, hot rolling — oxygen and nitrogen diffuse into the surface and form alpha case: a hard, oxygen-enriched layer that is brittle and has drastically reduced fatigue strength.
It cannot be seen. A part with alpha case looks like a part without it. It shows up later, as cracks during forming, cracks at the weld, or a fatigue failure well below the design life.
This is the process that removes it — and the checks that prove it is gone.
What Gets Removed, and How
| Stage | Purpose |
|---|---|
| Mechanical or alkaline pre-treatment | Break down heavy scale so the acid works on metal rather than oxide |
| Nitric–hydrofluoric pickling | Controlled dissolution of the contaminated surface layer |
| Multi-stage rinse and neutralisation | Remove acid residue from surfaces and any blind features |
| Deionised final rinse and drying | Prevent staining and residual halides |
| Clean packing | Prevent re-contamination in transit |
Typical removal is in the tens of microns per pass. Heavy scale or a thick alpha case is pre-treated mechanically or by alkaline descaling first, rather than trying to dissolve everything in acid — which wastes material and risks over-etching.
How Removal Is Verified
This is the part that separates a controlled process from a cosmetic one.
Metallographic section. A witness coupon processed with the batch is sectioned, mounted, etched and measured under the microscope. The report states the sampling location, magnification, and both the maximum and average residual layer depth. Microhardness gradient traverses can supplement it where the case is shallow.
If a supplier tells you alpha case removal is verified “by appearance” or “by colour”, the process is not being controlled. Surface appearance is exactly the thing alpha case does not change.
The Hydrogen Problem
Pickling has a second failure mode that is less discussed than alpha case and equally invisible.
In the acid bath, hydrogen ions can be reduced to atomic and then molecular hydrogen, which diffuses into the titanium lattice. The result is hydrogen embrittlement — cracking during subsequent bending, forming or welding, on a part that passed visual and dimensional inspection.
Nitric acid is what prevents it. It oxidises hydrogen as it forms:
2 HNO₃ + H₂ → 2 H₂O + 2 NO₂Which is why the nitric-to-hydrofluoric ratio matters more than the total acid concentration. Industry practice is to keep the HNO₃:HF ratio above roughly 5:1; below that, hydrogen pickup rises sharply. Bath ratio, temperature, immersion time and analysis frequency belong in the process card with records, not in a verbal assurance.
Hydrogen content is tested after pickling and reported as a release condition — by inert gas fusion, not inferred from the incoming material certificate. If hydrogen is out of specification, the correction is vacuum degassing and re-test, never longer pickling or grinding the surface.
The same applies to zirconium, with its own bath chemistry and its own witness coupons — zirconium is not run on titanium parameters.
Contamination Control
Titanium picks up iron readily, and embedded iron becomes a corrosion initiation site. Dedicated fixtures and lifting gear, non-ferrous contact surfaces, batch separation, deionised final rinse, and chloride-free packing are part of the process rather than optional extras.
When You Need It
- After forging, hot forming or hot rolling
- After heat treatment in air or in a non-protective atmosphere
- Before welding, where a contaminated surface will crack the weld
- Before forming operations that put the surface in tension
- On aerospace and medical parts, where alpha case is a specification-level requirement rather than a preference
If the part is going on to welding and fabrication or surface finishing, say so — mechanical finishing changes how a surface looks without changing what is underneath it.
What to Send With Your Enquiry
Grade and product form, the prior thermal history (what heated the part and in what atmosphere), the applicable specification or acceptance standard, maximum permissible residual alpha case, and hydrogen limit. If those last two are not specified on your drawing, tell us the application and we will propose values before quoting.