Why Does a Titanium Immersion Heater in a 50/50 Ethylene Glycol/Water Mixture with 500 ppm Chloride at 90°C Fail Faster by Crevice Corrosion Than by Uniform Thinning?

Ethylene glycol/water combinations (50/50 vol%) are often employed as heat transfer fluids in solar thermal systems, HVAC and engine cooling. At 90°C, 500 ppm chloride from deteriorated glycol or makeup water is hostile to titanium. Although titanium is very resistant to uniform corrosion in this mixture, with corrosion rates below 0.02 mm per year, failure is primarily due to crevice corrosion at tube supports, flanges and gaskets. The crevice geometry leads to an increase in chloride concentration and a decrease in oxygen concentration, while the products of glycol breakdown (glycolic acid, formic acid) decrease the local pH. Crevice corrosion propagation rates are 0.3-1.0 mm/year, 15-50 times faster than uniform thinning. The failure is quite limited and perforation occurs while 95% of the tube surface is left intact.

Mechanism of Crevice Dominance in Glycol/Water Mixtures

In the bulk solution, ethylene glycol increases viscosity, which decreases oxygen diffusion. Stability of the passive film is guaranteed by sufficient oxygen and moderate chloride concentrations. In a crevice (e.g. under a PTFE support or at a gasket) oxygen is quickly depleted by cathodic processes and is not supplied. Chlorides diffuse into the crevice to preserve electrical neutrality and reach levels of 2,000 to 10,000 ppm. Glycol thermally decomposes at 90°C to organic acids (glycolic, formic, oxalic). These acids lower the local pH of the crevice to 3-4. Chloride, low pH and oxygen depletion together degrade the passive film. Corrosion at the crevice anode is active, driven by the large external cathode of the rest of the tube surface. If the galvanic current between the crevice ( anode ) and the outside ( cathode ) is sustained , the dissolving rate is significant . Eventually , perforation occurs .

Quantitative Comparison of Uniform and Crevice Corrosion Rates

Controlled tests in 50/50 ethylene glycol/water with 500 ppm chloride (starting pH 6.5) at 90°C for 2,000 hours have revealed radically differing corrosion rates. The general corrosion rate of a smooth tube without fissure is only 0.008 to 0.015 mm/year. The surface is brilliant and free of pitting, indicating a stable passive film. With this rate, a wall thickness of 1.2 mm would provide a service life of more than 50 years. However the crevice corrosion rate was up to 0.30–0.70 mm/year under a PTFE support with a 0.2 mm crevice gap. The initiation stage lasts between 200 and 500 h, and a groove with a depth of 0.3–0.6 mm develops within 1,000 h. With a rubber gasket that has a gap of 0.1 mm, the crevice corrosion rate increases to 0.50 to 1.00 mm/year, starting in 100 to 300 hours and perforating a 1.2 mm wall in 1200 to 2400 hours. In a tube to tubesheet rolled joint the rate of crevice corrosion is 0.20 to 0.50 mm per year with initiation in 300 to 800 hours.

Impact of Chloride and Glycol Degradation on Crevice Severity

Severity of crevice attack is a strong function of glycol condition and chloride concentration. Fresh glycol (with inhibitors) at 100 ppm chloride gives a crevice pH of 5.5 after 500 h with a corrosion rate of 0.10-0.25 mm per year. Fresh uninhibited glycol at 500 ppm chloride falls to pH 4.5 at the rate of 0.30-0.60 mm per year. Thermally decomposed glycol (500 hours at 90°C) with 500 ppm chloride gives pH 3.5. Rate is 0.60-1.00 mm/year. Severely deteriorated glycol with more than 1,000 ppm organic acids with 1,000 ppm chloride, pH 3.0, crevice corrosion rate 1.00-1.80 mm per year. Under these conditions, the uniform corrosion rate is < 0.025 mm/year, indicating crevice corrosion as the dominant failure mechanism.

Glycol/Water Heater Crevice Corrosion Prevention Measures

Table below gives guidelines for reducing crevice corrosion in ethylene glycol/water combinations at 90°C, based on heater configuration and crevice risk assessment.

Crevice Hazard Heater ArrangementProposed Design ChangeExpected Crevice Corrosion Rate (mm/yr)
PTFE tube spacers (0.2-0.5mm spacing)High Titanium welded supports replacement <0.02
Flanges with rubber gasketsVery high Use welded or braze connectors <0.02
Roller expanded tubes in tubesheet Moderate Weld tube-to-tubesheet after expansion 0.05–0.10
High compression fittings Eliminate; use welded fittings <0.02
Redesign existing crevices Unable to redesign Moderate Use PTFE tape to patch gap 0.15–0.30
Engineering More than Crack Elimination

The crevice corrosion resistance is strongly influenced by the titanium grade. Grade 7 (palladium stabilized) has 3-5 times longer commencement time and 50-70% lower propagation rate than Grade 2 in glycol/water crevices. The wall thickness gives a material allowance. A 2.0 mm wall with crevice corrosion of 0.5 mm per year lasts 4 years, but a 1.2 mm wall lasts just 2.4 years. The glycol inhibitor package matters; nitrite-based inhibitors promote titanium crevice corrosion, whereas molybdate or azole-based inhibitors are benign. Regular glycol sample for pH, chloride and organic acid concentration makes predictive maintenance possible. Adding monoethanolamine to keep glycol pH above 8.0 greatly decreases crevice attack.

Making a specification with insight

Design a titanium immersion heater for 50/50 ethylene glycol / water with 500 ppm chloride at 90C. Make it crevice-free: use welded tube supports, welded flanges, and welded tube-to-tubesheet junctions. If no fissures are eliminated, use Grade 7 titanium, and check all crevice sites every quarter with a borescope or dye penetrant. For heaters already in service which are experiencing crevice corrosion, refit with PTFE tape or silicone sealant to cover the gaps and add sodium molybdate to the glycol at 200 ppm as a corrosion inhibitor. Check glycol for pH drop below 6.0 during operation; if pH drop is found, change glycol charge immediately. Crevice corrosion can cause heater failures 15-50 times faster than uniform thinning. Knowing this, the engineer emphasizes crevice reduction over material grade selection in ethylene glycol/water service.