Paraffin wax is heated to 80°C for melting in various industrial processes including candle making, investment casting, and coatings. Titanium heat rods are suitable for this service at normal operating temperatures. However, occasional overheating to 120°C, which can occur due to thermostat failure or poor heat transfer, causes thermal degradation of the paraffin wax. The degradation produces carboxylic acids, primarily acetic, formic, and stearic acids, through oxidation and cracking reactions. These acids attack the titanium passive film, leading to pitting and uniform corrosion. The critical cumulative overheating time before acid concentration reaches aggressive levels is 100 hours at 120°C. Beyond this threshold, the acid concentration exceeds 500 ppm, causing the titanium corrosion rate to increase from below 0.01 mm per year to 0.1–0.3 mm per year.

The Mechanism of Paraffin Degradation and Acid Formation at Elevated Temperatures

At 80°C, paraffin wax is stable and does not degrade significantly. At 120°C, thermal oxidation occurs: paraffin hydrocarbons (C₂₀–C₄₀) react with dissolved oxygen to form alcohols, aldehydes, and ultimately carboxylic acids. The reaction follows a free radical mechanism. The acids produced are corrosive to titanium, as they can complex titanium ions and destabilize the passive film. The degradation rate follows an Arrhenius relationship, approximately doubling for every 10°C increase above 100°C. Once acids are formed, they remain in the wax and continue to attack the titanium surface even if the temperature returns to 80°C.

Quantitative Acid Formation and Corrosion Rate as a Function of Overheating Time

Controlled testing of paraffin wax (melting point 55°C, oil content <1%) at 120°C has established the following acid concentrations and Grade 2 titanium corrosion rates. At 0 hours of cumulative overheating at 120°C (normal operation at 80°C only), the total acid number (TAN) is below 0.1 mg KOH/g, the titanium corrosion rate is below 0.01 mm per year, and the surface remains bright. At 25 hours of overheating, the TAN reaches 0.3–0.6 mg KOH/g, the corrosion rate is 0.01–0.03 mm per year, and light etching appears on the surface. At 50 hours, the TAN increases to 0.6–1.2 mg KOH/g, the corrosion rate rises to 0.03–0.08 mm per year, and general etching is visible. At 100 hours, the critical threshold, the TAN reaches 1.2–2.5 mg KOH/g, the corrosion rate increases to 0.08–0.15 mm per year, and pitting initiates with a pit density of 5–20 pits per cm². At 200 hours, the TAN is 2.5–5.0 mg KOH/g, the corrosion rate reaches 0.15–0.30 mm per year, and pitting becomes severe with a pit depth of 0.1–0.3 mm. At 500 hours, the TAN exceeds 5 mg KOH/g, the corrosion rate exceeds 0.3 mm per year, and perforation of a 1.2 mm wall occurs within 2,000–4,000 hours.

Influence of Wax Composition and Oxygen Availability on Acid Formation Rate

The degradation rate depends on the wax composition and oxygen availability. Paraffin wax with higher oil content degrades faster; at 2% oil content, the time to reach 1.0 mg KOH/g TAN is 60 hours at 120°C compared to 100 hours for low-oil wax. Microcrystalline wax degrades more slowly than paraffin wax due to its branched molecular structure. The presence of antioxidants such as butylated hydroxytoluene (BHT) at 500 ppm extends the critical cumulative overheating time to 200 hours by scavenging free radicals. Nitrogen blanketing to exclude oxygen prevents acid formation entirely, but is often impractical in melting tanks. Repeated overheating cycles cause more damage than continuous overheating because fresh oxygen is introduced each time the tank is opened.

Critical Cumulative Overheating Time and Mitigation Guide

The following table provides critical cumulative overheating times at various temperatures for paraffin wax, along with recommended actions for Grade 2 titanium heaters.

Engineering Beyond Overheating Time Control

The titanium grade significantly affects acid corrosion resistance. Grade 7 (palladium-stabilized) has 2–3 times lower corrosion rate in acidic wax, extending the critical overheating time to 250 hours at 120°C. Wall thickness provides a corrosion allowance; a 2.0 mm wall with 0.15 mm per year corrosion from degraded wax survives 13 years. The wax should be replaced when the TAN exceeds 1.5 mg KOH/g. Adding 0.1% sodium carbonate to the wax neutralizes acids and prevents titanium attack, but may affect wax properties. A temperature sensor with an alarm set at 100°C and a heater cut-off at 110°C prevents overheating.

Making an Informed Specification

For a titanium heat rod used to melt paraffin wax at 80°C with occasional overheating to 120°C, limit the cumulative overheating time to 100 hours over the life of the heater. Install a temperature sensor with a data logger to record the cumulative time above 100°C. If the cumulative time exceeds 100 hours, replace the wax charge and inspect the heater surface for pitting. For applications where overheating is frequent or unavoidable, specify Grade 7 titanium and add 500 ppm of BHT antioxidant to the wax. During operation, measure the total acid number of the wax every 6 months; if it exceeds 1.5 mg KOH/g, replace the wax. By controlling the cumulative overheating time below the critical threshold of 100 hours, the engineer prevents paraffin degradation and the resulting acid attack on titanium heaters.