For a Titanium Heat Rod Used to Melt Paraffin Wax at 80°C with Occasional Overheating to 120°C, What Is the Critical Cumulative Overheating Time (hours) Before the Wax Degrades into Carboxylic Acids That Attack Titanium?

Paraffin wax is melted at 80°C for many industrial operations such as candle production, investment casting and coatings. Titanium heat rods are applicable for this service at normal operating temperatures. Sometimes overheating to 120 C occurs owing to failure of the thermostat or poor heat transfer which causes thermal deterioration of the paraffin wax. This degradation gives rise to carboxylic acids, including acetic, formic, and stearic acids, due to oxidation and cracking processes. These acids attack the titanium passive film and cause pitting and uniform corrosion. The crucial cumulative overheating time is 100 hours at 120°C before the acid concentration becomes aggressive. The acid concentration is above 500 ppm and the titanium corrosion rate increases from below 0.01 mm per year to 0.1-0.3 mm per year if the threshold is exceeded.

The mechanism of paraffin decomposition and acid formation at high temperatures

Paraffin wax is stable at 80°C and does not degrade significantly. At 120 °C there is thermal oxidation: paraffin hydrocarbons (C20–C40) react with oxygen dissolved in it with formation of alcohols, aldehydes and finally carboxylic acids. The reaction occurs via a free radical process. The resulting acids are corrosive to titanium since they combine titanium ions and destroy the passive film. The Arrhenius relationship indicates that the deterioration rate is nearly doubled for every 10°C increase in temperature above 100°C. Once acids are generated they remain in the wax and will continue to damage the titanium surface even if the temperature drops back to 80°C.

Quantitative Acid Formation and Corrosion Rate Versus Overheating Time

Controlled tests with paraffin wax (melting point 55°C, oil content <1%) at 120°C have indicated the following acid concentrations and Grade 2 titanium corrosion rates. The TAN is below 0.1 mg KOH/g, the titanium corrosion rate is below 0.01 mm per year and the surface is bright at 0 hours of the entire overheating at 120°C (typical operation at 80°C only). After 25 h overheating, the TAN is 0.3-0.6 mg KOH/g, the corrosion rate is 0.01-0.03 mm/yr and little etching is observed on the surface. At 50 h, TAN increases to 0.6–1.2 mg KOH/g, the corrosion rate increases to 0.03–0.08 mm/year and widespread etching may be observed. At 100 hours (critical threshold), the TAN is 1.2-2.5 mg KOH/g , corrosion rate is 0.08-0.15 mm per year and pitting initiates with a pit density of 5-20 pits per cm2 . After 200 h, the TAN is 2.5-5.0 mg KOH/g. The corrosion rate is 0.15-0.30 mm per year and pitting is serious with a pit depth of 0.1-0.3 mm. Corrosion rate >0.3 mm/yr and perforation of 1.2 mm wall at 500 hrs in 2000–4000 hrs. TAN >5 mg KOH/g.

Influence of wax formulation and oxygen availability on the rate of acid formation

Degradation rate is dependent on the wax composition and oxygen availability. Paraffin wax with more oil content degrades faster. 60 hours to 1.0 mg KOH/g TAN at 120C for 2% oil content vs. 100 hours for low-oil wax. The branching molecular structure of microcrystalline wax makes it slower to decompose than paraffin wax. The critical cumulative overheating time can be extended to 200 h by the free radical-scavenging activity of antioxidants such as butylated hydroxytoluene (BHT) at 500 ppm. Nitrogen blanketing to exclude oxygen will prevent acid generation completely but is generally impractical in melting tanks. Repeated overheating is worse than continuous overheating because when the tank is opened, new oxygen is introduced.

Critical cumulative overtemperature time and mitigation guide

The table below indicates important cumulative times of overheating at various temperatures for paraffin wax, and recommended measures for Grade 2 titanium heaters.

Overheating Temperature (°C) Critical Cumulative Time to 1.0 mg KOH/g TAN (hours)Over-heating at 80°C, resulting corrosion rate (mm/year)Suggested Action
100 500 0.02-0.05 Acceptable; check yearly. 110 200 0.03-0.06 Acceptable; check quarterly. 120 100 0.08-0.15 Replace wax if exceeded. 130 50 0.10-0.20 Install overheat protection. 140 25 0.15-0.30Add antioxidant; Grade 7 Titanium 150 10 0.20–0.40 Redesign heater with temperature limit Engineering Beyond Overheat Time Control

The resistance to acid corrosion is greatly influenced by the grade of titanium. The corrosion rate in acidic wax is lower by a factor of 2–3 for grade 7 (palladium stabilized) and the critical overheating time is increased to 250 hours at 120°C. The wall thickness gives a corrosion allowance: a 2.0 mm wall corroded 0.15 mm per year by deteriorated wax lasts for 13 years. IfTAN is more than 1.5 mg KOH/g then the wax must be changed. The addition of 0.1% sodium carbonate to the wax neutralizes acids and avoids assault of the titanium, but may impair wax characteristics. No overheating . There is a temperature sensor and alert at 100°C and heater switch off at 110°C .

Making a Knowledgeable Specification

- a titanium heat rod melting paraffin wax at 80° C with periodic overheating to 120° C. Total overheating time shall not exceed 100 hours during the life of the heater. Install a temperature sensor with a data recorder to record the whole duration over 100°C. If the total time exceeds 100 hours, change the wax charge and check the heating surface for pitting. For applications where heat build-up is frequent or unavoidable, specify Grade 7 titanium and add 500 ppm BHT antioxidant to the wax. Operating: Measure the total acid number of the wax every 6 months. Replace the wax if the total acid number reaches 1.5 mg KOH/g. The engineer keeps the total time of overheating below a critical limit of 100 hours, therefore avoiding paraffin deterioration and the subsequent acid attack on the titanium heaters.