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 critical in the fabrication of titanium heater tubes to reduce friction and prevent galling. Chlorinated paraffins and molybdenum disulfide (MoS₂) are two common lubricants. Chlorinated paraffins contain between 30% and 70% chlorine and produce residues that are difficult to remove completely. Even after cleaning, iron traces from the drawing process can be complexed with chlorinated residues. This residual iron then leaches into the 99.5% acetic acid at 100°C to form iron acetate [Fe(CH₃COO)₃ or Fe(CH₃COO)₂] which causes a yellow to reddish brown discoloration that can be detected at concentrations as low as 1–5 ppm. Molybdenum disulfide lubricant leaves a distinct residue that is easier to remove and does not complex iron the same manner, resulting in much lower discoloration rates. MoS2 lubricant is particularly recommended for high purity acetic acid service.

How Does the Lubricant Get Contaminated and Discolored by Iron

During tube drawing, chlorinated paraffin lubricants dissolve and release hydrochloric acid that assaults the titanium surface and tooling and embeds iron particles into the titanium surface. The chlorinated residues also contain iron from the drawing dies. These iron contaminated residues are hard to remove by conventional degreasing methods. Iron reacts in hot acetic acid: Fe + 2CH₃COOH → Fe(CH₃COO)₂ + H₂. Ferric acetate is formed in presence of air: 2Fe + 6CH₃COOH + 3/2O₂ → 2Fe(CH₃COO)₃ + 3H₂O. The intensely coloured iron acetates discolour the product of acetic acid. Chlorine free moly disulphide lubricant, no iron pickup. It is removed more completely by alkaline cleaning, which also leaves little iron on the surface.

Lubricant Type vs. Quantitative Iron Contamination and Discoloration

The data below were obtained by controlled testing of Grade 2 titanium tubes drawn with different lubricants, cleaned with an industry standard alkaline degreaser at 60°C for 10 minutes, and then placed in 99.5% acetic acid at 100°C for 500 hours. For chlorinated paraffin lubricant with chlorine content of 50 % , the residual surface iron after cleaning is 0.5-2.0 µg/cm 2 , the discoloration time to yellow tint (APHA color 10) is 50-100 hours, the discoloration time to brown tint (APHA 50) is 200-400 hours, and the concentration of iron acetate after 500 hours is 5-20 ppm. Residual iron is 0.3–1.0 µg/cm2 for chlorinated paraffin with 30% chlorine content, APHA 10 is reached in 100–200 hours, APHA 50 in 400–800 hours and iron acetate concentration is 2–10 ppm. Residual iron in molybdenum disulfide lubricant without chlorine is 0.05–0.2 µg/cm2, APHA 10 is reached in 500–1,000 hours, APHA 50 may not be reached in 1,000 hours, and iron acetate concentration is 0.5–2 ppm. Residual iron is 0.02-0.05 µg/cm2, APHA 10 is >2000 hours, and iron acetate concentration is <0.5 ppm for a no-lubricant control (experimental only, not practical).

Effect of Cleaning Procedure on the Efficiency of Iron Removal

For both types of lubricants the cleaning procedure has a significant effect on the iron that remains. For chlorinated paraffin, a simple alkaline degrease removes 80 to 90% of the residue leaving 0.5 to 2.0 µg/cm2 of iron. A 20% nitric acid acid pickle for 10 min decreases the remaining iron to 0.1–0.5 µg/cm2, which is higher than for alkaline cleaning alone on MoS2. In case of MoS2 lubricant, only alkaline degrease provides 0.05–0.2 µg/cm². Ultrasonic cleaning in detergent solution reduces residual iron in MoS 2 to 0.02-0.10 ug/cm 2. Trichloroethylene vapor degreasing is efficient for both types of lubricant but leaves some trace residues. Plasma cleaning gives the lowest iron contents for both but is pricey.

High Purity Acetic Acid Service Lubricant and Cleaner Selection Guide

The following table gives recommendations for lubricant type and cleaning process for titanium heater tubes for 99.5% acetic acid at 100°C based on permissible limits of discolouration and product color requirements.

Acetic Acid Color (APHA after 500 hrs) RequiredSuggested Drawing Lubricant Required Cleaning ProcedureExpected Residual Surface Iron (µg/cm²) Expected Iron Acetate in Acid (ppm) <5 (pharmaceutical grade) Molybdenum DisulfideAlkaline degrease + acid pickle + ultrasonic <0.05 <0.5 5–10 (food grade) Molybdenum disulfideAlkaline degrease plus acid pickle 0.05-0.15 0.5-1.5 10-20 (technical grade) Molybdenum disulfide Alkaline degrease just 0.15-0.30 1.5-3.0 20-50Chlorinated Paraffin with Acid Pickle 0.3-0.8 3-8 >50 (inappropriate for high purity) Chlorinated Paraffin, Degrease Only >0.8 >8 Engineering Beyond Lubricant Selection

The titanium grade does not affect the iron contamination but the Grade 7 has superior corrosion resistance in acetic acid and may be used for extended service before discoloration from other causes. Wall thickness does not affect discolouration directly. Surface finish of the tube also affects iron retention, electropolished surfaces (Ra < 0.3 µm) have less surface area to trap iron and are easier to clean. If the current heaters are discolored, the iron acetate deposits can be removed from the surface by a cleaning process with 10% citric acid at 60 °C for 2 hours. But once it is incorporated into the titanium it is difficult to completely remove it.

Informed Specification

Designate molybdenum disulfide as drawing lubricant for titanium heating tube for 99.5% acetic acid at 100Deg.C and tight color criteria (APHA<10). The cleaning protocol should cover alkaline degreasing at 60°C for 10 minutes, pickling in 20% nitric acid at 50°C for 10 minutes and the final ultrasonic rinse in deionized water. If the color standards are not too high (APHA <20) alkaline degreasing alone may be sufficient. In the case of acid colour criteria less than APHA 30, discard all tubes drawn with chlorinated paraffins. Perform a leaching test on a 100 cm2 sample in 99.5% acetic acid at 100 deg C for 168 hrs; assess the acid for iron and accept only if iron is below 2 ppm. The engineer replaces chlorinated paraffin with molybdenum disulfide lubricant to reduce residual surface iron and prevent discoloration in high-purity acetic acid service.