The Corrosion Problem in Pharmaceutical Processing
Pharmaceutical and fine chemical plants handle aggressive media at every step: hydrochloric, nitric, sulphuric and phosphoric acids, organic acids, chloride brines from neutralisation and salting out steps, and solvents. When these streams contact stainless steel, two failures appear. General corrosion thins the wall, and selective attack at welds, crevices and heat affected zones releases iron, chromium and nickel ions into the process stream. In a pharmaceutical context that is a quality failure, not just a maintenance cost, because the released metal ions contaminate the active ingredient and the finished batch can fail specification.
The classic example is thiamine hydrochloride, vitamin B1, which contains a high concentration of free chloride ions. In stainless steel equipment the chloride attack is severe enough that out of specification material can appear within a few production campaigns. Titanium does not suffer this form of attack, so titanium contact parts are used to keep the chloride bearing product within specification.
Corrosion Data and the Passive Film
| Material and medium | Corrosion behaviour |
|---|---|
| Industrial pure titanium TA3 in chloride bearing process media | Typical corrosion rate around 0.00088 mm per year, in the passive range |
| Type 304 and 316 stainless steel in the same chloride duty | Localised pitting and crevice attack, with iron ion release into the product |
| Titanium in nitric acid, oxidising media | Passive film stable and reinforced; low uniform corrosion rate |
| Titanium in hydrochloric and sulphuric acid at higher concentration or temperature | Passive film can break down; Gr7, Gr12 or Gr16 with palladium or molybdenum additions are selected |
Titanium owes its behaviour to a passive titanium dioxide film of a few nanometres that forms instantly in air or water and repairs itself after scratching. The film is stable in neutral and oxidising media, which is why titanium handles chlorides, hypochlorite, nitric acid and seawater so well. In strongly reducing acids the film is destabilised, so the corrosion engineer adds a small amount of palladium (Gr7, Gr16, Gr17) or molybdenum and nickel (Gr12) to shift the alloy into the passive state.
Typical Titanium Equipment
Tubular and plate heat exchangers for heating and cooling aggressive media, with tubes to ASTM B338 and shells and heads clad or solid titanium.
Rising film evaporators, nitration reactors and reaction vessels, where the wetted surface must be inert to the reaction mass.
Centrifuges and filter dryers, where chloride bearing mother liquor would otherwise attack the basket and the casing.
Solids handling equipment: screw feeders, cyclone separators, vortex bodies, hoppers and discharge pipes.
Exhaust hoods and ducting lined with titanium sheet to prevent chloride condensate attack.
Piping and fittings to ASTM B861, ASTM B862 and GB/T 3624, with flanges and fasteners in matching grade.
Grade Selection and Surface Finish
For most pharmaceutical chloride duty, Gr2 or its GB equivalent TA2 is specified, with Gr1 or TA1 where maximum formability is needed for linings and expanded parts. Where reducing acids appear, Gr7 with a palladium content of 0.12 to 0.25 percent, Gr12 with molybdenum 0.2 to 0.4 percent and nickel 0.6 to 0.9 percent, or Gr16 for crevice resistant duty are the correct answers. For hygienic service the surface finish matters as much as the grade: internal surfaces are normally mechanically polished to a roughness of 0.4 micrometre Ra or better, welds are ground and blended, and components are passivated in nitric or citric acid to restore the oxide film after welding and pickling.
Inspection, Certification and Cleanliness Control
Pharmaceutical clients typically require, for each item, a chemical composition report, mechanical test report and certificate to EN 10204 3.1, positive material identification of the wetted parts, dye penetrant or radiographic examination of critical welds, eddy current or hydrostatic testing of exchangers and piping, and surface roughness measurement records. Cleanliness requirements cover freedom from embedded iron contamination, tested with a ferroxyl or acid spot test, and freedom from discolouration or oxide scale on welds, since a blue or grey weld indicates oxygen pickup above 450 °C and a hardened, less corrosion resistant joint.
Frequently Asked Questions
Q: Why is titanium used instead of stainless steel for vitamin B1 production?
A: The product contains free chloride ions that cause localised corrosion of stainless steel and release iron ions into the drug; titanium resists that attack, and titanium contact parts allow the batch quality rate to be maintained at the required level.
Q: Does titanium ever corrode in pharmaceutical service?
A: Yes, in strongly reducing acids such as higher concentration hydrochloric or sulphuric acid; in those duties a palladium bearing grade such as Gr7 or a molybdenum bearing grade such as Gr12 is selected.
Q: What surface finish is required for pharmaceutical titanium equipment?
A: Internal wetted surfaces are normally polished to about 0.4 micrometre Ra or smoother, with welds ground, blended and passivated to restore the passive film.
Q: Can titanium equipment be cleaned in place with sodium hydroxide and nitric acid?
A: Yes, both are compatible with titanium and are the usual cleaning and passivation agents; hot concentrated reducing acid mixtures should be reviewed separately.
Q: What documentation is supplied with titanium process equipment?
A: A mill test certificate to EN 10204 3.1, chemical and mechanical reports, positive material identification, weld examination records, pressure and eddy current test reports, and surface roughness measurements.
