How Does the Residual Carbon Contamination from Drawing Lubricant (0.05 mg/cm² vs. 0.5 mg/cm²) Shift the Critical Pitting Temperature of a Titanium Heater in 1% HCl at 70°C?

Residual carbon contamination of titanium heater tubes from chlorinated paraffin drawing lubricants is a common manufacturing problem. When the heater is put into 1% hydrochloric acid at 70°C, the carbon residue decomposes oxidatively at local points, establishing reducing circumstances which lead to the breakdown of the passive film. The critical pitting temperature (CPT), which is the temperature at which stable pitting starts, decreases with the rise of surface carbon contamination. For a clean surface containing 0.05 mg/cm$^2$ carbon the CPT is about 75°C in 1% HCl, giving a safety margin of about 70°C. For a contaminated surface with 0.5 mg/cm2 carbon the CPT decreases to 50–55°C meaning that pitting occurs at normal operating temperatures of 70°C.

Mechanism of Carbon-Induced Depression of Critical Pitting Temperature

At high temperatures, residual carbon combines with trace oxygen or water to produce carbon monoxide and carbon dioxide. This reaction uses oxygen at the titanium surface, producing a locally reducing environment. Degradation of the passive coating from TiO 2 to Ti 2 O 3 or even to metallic titanium diminishes the pitting potential. Carbon particles are also used as crevice sites for chlorides to concentrate. The combined effect causes a 15–25°C decrease of the CPT at moderate pollution levels. The shift scales almost logarithmically with carbon loading, with each decade of increasing carbon pollution decreasing the CPT by 10-15 deg C.

Quantitative Variation of the Critical Pitting Temperature with Carbon Content

Titanium Grade 2 CPT values have been determined by controlled potentiostatic testing in deaerated 1% HCl with varying amounts of surface carbon. CPT is 75-80°C with carbon contamination < 0.05 mg/cm2 (properly cleaned) and safe operation at 70°C with no pitting after 1,000 hours. With carbon at 0.05-0.10 mg/cm2 the CPT drops to 65-70°C; borderline operation at 70°C results in pitting initiation after 200-500 h. At 0.10-0.25 mg/cm² of carbon the CPT is 55-65°C, pitting occurs at 70°C in 50-200 hours and pit depth is 0.1-0.3 mm in 500 hours. At 0.25–0.50 mg/cm2 of carbon the CPT is lowered to 45–55°C and at 70°C fast pitting occurs within 10–50 hours with severe attack. If visible residue is present and the carbon is above 0.50 mg/cm², CPT is below 40°C and pitting occurs immediately at 70°C.

Influence of HCl Concentration on Carbon Induced CPT Shift

The magnitude of the CPT shift is HCl-concentration dependent. For 0.5% HCl, the CPT is 90°C for a clean surface and 70°C for a surface with 0.25 mg/cm2 carbon, a shift of 20°C . At 1.0% HCl, the clean CPT is 78°C and the polluted CPT is 58°C, another 20°C change. At 2.0% HCl, the clean CPT is 65°C and the contaminated CPT is 45°C, a 20°C shift again. The clean CPT is 55 °C at 3.0 % HCl whereas the contaminated CPT is 35 °C, a change of 20 °C. The uniform 20°C shift over the range of concentrations shows that the effect of carbon pollution on the absolute CPT is a constant shift, not a relative shift.

Limits on Carbon Contamination and Cleaning Validation for 70°C HCl Service

The following table shows the maximum amounts of carbon contamination that can be safely operated at 70°C in 1% HCl, with recommended cleaning methods and verification tests.

Target Service Life at 70ºCVerification Test for Required Cleaning Method Maximum Allowable Carbon (mg/cm2)5 yrs <0.05Alkaline clean, ultrasonic, and vapor degreaseSolvent extraction + gravimetric 2-5 years <0.10Vapor degrease + alkaline washWater break test (continuous film)
1–2 years <0.15Visual inspection (no smears) Vapor degreasing solely
<1 year (intermittent) <0.25 Solvent wipe None
Not acceptable >0.25 Reclean N/A
Engineering Beyond Carbon Removal and the Grade Election

The CPT shift from the carbon contamination is affected by the titanium grade. Grade 7 (palladium-stabilized) has a baseline CPT 15–20°C higher than Grade 2, therefore a polluted Grade 7 heater with 0.25 mg/cm2 carbon has a CPT of ~65°C, which is still below 70°C operation. So Grade 7 doesn't eliminate the risk, it gives a margin. CPT is not affected by wall thickness but the duration for pit propagation is affected. A wall thickness of 2.0 mm with pitting at a rate of 0.1 mm per hour would survive for 20 hours, allowing for detection. Carbon contamination can be eliminated by plasma cleaning or vacuum baking at 300 °C for 2h, which lowers carbon to below 0.01 mg/cm² . ASTM F331 solvent extraction is the most robust method for measuring residual carbon.

A knowledgeable specification

Need a maximum residual carbon contamination of 0.05 mg/cm2 (solvent extraction ASTM F331) for a titanium heater in 1% HCl at 70°C. Define a cleaning procedure: vapour degreasing with trichloroethylene or an alternative acceptable solvent, alkaline cleaning at 60 °C for 10 min, ultrasonic cleaning in deionized water and drying in clean and filtered air. For important applications, specify a verification test after cleaning: Immerse a sample coupon in 1% HCl at 70°C for 168 hours and examine for pitting under 50× mag; reject the lot if any pits are observed. If the carbon level is unknown on existing heaters, do a water break test after cleaning. If water does not form a continuous film on the surface, it must be recleaned. The engineer controls residual carbon contamination to 0.05 mg/cm2, such that the critical pitting temperature is above 70°C to avoid premature pitting in dilute hydrochloric acid service.