How Does the Tube Drawing Lubricant Type (Chlorinated Paraffin vs. Molybdenum Disulfide) Affect the Post-Cleaning Surface Iron Contamination and the Discoloration Rate in 99.5% Acetic Acid at 100°C?

Drawing lubricants are used to reduce friction and avoid galling in the production of titanium heater tubes. Common lubricants include chlorinated paraffins and molybdenum disulphide (MoS2). Chlorinated paraffins have 30-70% chlorine and create residues that are hard to remove completely. Trace iron from the drawing operation may remain complexed with chlorinated residues even after washing. This residual iron leaches in 99.5% acetic acid at 100°C to generate iron acetate [Fe(CH₃COO)₃ or Fe(CH₃COO)₂] with accompanying yellow to reddish-brown discolouration at levels as low as 1–5 ppm. Molybdenum disulphide lubricant creates a distinct residue that is easier to remove and does not complex iron in the same way, resulting in much reduced discolouration rates. MoS₂ lubricant is well preferred for high purity acetic acid service.

Lubricant-Dependent Mechanism of Iron Contamination and Discolouration

Chlorinated paraffin lubricants degrade in the tube drawing process and release hydrochloric acid which damages the titanium surface and tooling and embeds iron particles in the titanium surface. The chlorinated residues also tie up iron from the drawing dies. Such residues including iron are difficult to remove by traditional degreasing. The iron reacts in heated acetic acid . Fe + 2CH_3COOH _=>_ Fe(CH_3COO)_2 + H_2 Ferric acetate in presence of air . 2Fe + 6CH_3COOH + 3/2O_2 _=>_ 2Fe(CH_3COO)_3 + 3H_2O The iron acetates are of vivid colour and disfigure the acetic acid product. The molybdenum disulphide lubricant has no chlorine and will not cause iron pickup. Alkaline cleaning removes it more completely, leaving minimal surface iron.

Quantifying Discolouration and Iron Contamination as a Function of Lubricant Type

The following findings were obtained from controlled tests on Grade 2 titanium tubes drawn with various lubricants: All tubes were cleaned with an industry-standard alkaline degreaser at 60°C for 10 minutes, and then immersed in 99.5% acetic acid at 100°C for 500 hours. For chlorinated paraffin lubricant with 50% chlorine content, residual surface iron after cleaning is 0.5–2.0 µg/cm², discolouration time to reach a yellow tint (APHA colour 10) is 50–100 hours, discolouration time to reach a brown tint (APHA 50) is 200–400 hours, and iron acetate concentration after 500 hours is 5–20 ppm. For chlorinated paraffins with chlorine content 30%, residual iron is 0.3–1.0 µg/cm2. APHA 10 is attained in 100–200 hours, APHA 50 is reached in 400–800 hours. The concentration of iron acetate is 2–10 ppm. Chlorine-free molybdenum disulphide has a residual iron content of 0.05–0.2 µg/cm2. After 500–1,000 hours APHA 10 is achieved, however APHA 50 may not be achieved within 1,000 hours. Iron acetate concentration is 0.5–2 ppm. In the absence of lubricant (for experimental purposes only, not for practical use), the iron residue is 0.02-0.05 µg/cm², the APHA 10 is >2,000 hours, and the iron acetate concentration is <0.5 ppm.

Influence of cleaning procedure on iron removal

The cleaning technique has a strong influence on the residual iron for both type of lubricant. 80-90% of the residue of chlorinated paraffin is removed by simple alkaline degreasing, leaving 0.5-2.0 µg/cm^2 of iron. Adding an acid pickle of 20% nitric acid for 10 min results in residual iron of 0.1–0.5 µg/cm², which is still higher than the level obtained from MoS₂ with alkaline cleaning alone. Alkaline degrease only for MoS2 lubricant 0.05-0.2 µg/cm2. Ultrasonic washing in detergent solution decreases the remaining iron in MoS₂ to 0.02–0.10 µg/cm2. Vapour degreasing with trichloroethylene is efficient for both types of lubricant and leaves trace residues. Plasma cleaning yields the lowest iron levels for both, but is costly.

Lubricant and Cleaning Selection Guide for High Purity Acetic Acid Service

Table 1. Recommendations for lubricant type and cleaning technique for titanium heater tubes for 99.5% acetic acid at 100°C based on the allowable discolouration limits and product colour requirements.

Colour Required (APHA after 500 hrs.) Acetic AcidSuggested Drawing Lubricant Required Cleaning ProcedureExpected Residual Surface Iron (µg/cm²) Expected Iron Acetate in Acid (ppm) <5 (pharmaceutical grade) Molybdenum disulphideAlkaline degreasing + acid pickling + ultrasonic <0.05 <0.5 5-10 (food grade) Molybdenum disulphideAlkaline degrease + acid pickle 0.05–0.15 0.5–1.5 10–20 (technical grade) Molybdenum disulphide Alkaline degrease only 0.15–0.30 1.5–3.0 20–50Chlorinated paraffin with acid pickle 0.3–0.8 3–8 >50 (unacceptable for high purity) Chlorinated paraffin, degrease just >0.8 >8 Engineering Beyond Lubricant Selection

The titanium grades do not indicate a difference in iron contamination. Grade 7 has a greater resistance to corrosion in acetic acid which translates to a longer service life before discolouration due to other sources. The thickness of the wall has little influence on discolouration. Iron retention is affected by the surface finish of the tube; electropolished surfaces (Ra < 0.3 µm) have less surface area to retain iron and are easier to clean. For the heaters in service showing discolouration, a cleaning cycle using 10% citric acid at 60°C for 2 hours will remove the iron acetate deposits from the surface. However, once iron is incorporated in the titanium it is impossible to extract it completely.

Specifying Informed

Specify molybdenum disulphide as the drawing lubricant for a titanium heating tube to be used for 99.5% acetic acid at 100°C with very strict colour criteria (APHA <10). Cleaning technique of alkaline degreasing at 60°C, 10 min, followed by pickling in 20% nitric acid at 50°C, 10 min and final ultrasonic washing in deionised water is required. If colour standards are less rigorous (APHA <20), alkaline degreasing may be sufficient. Any tubes drawn with chlorinated paraffin should be rejected if the acid colour criteria falls below APHA 30. Incoming Inspection: Leaching Test 100 cm2 sample in 99.5% acetic acid at 100°C for 168 hours, analyse acid for iron, allow only if iron is below 2 ppm. The engineer specifies molybdenum disulphide lubricant in lieu of chlorinated paraffin to reduce residual surface iron and prevent discolouration in high purity acetic acid service.