In a Titanium Sheath Heater Subjected to Daily Cleaning with 10% Phosphoric Acid at 80°C, How Does the Rinse Water Quality (Tap vs. Deionized) Influence the Repassivation Kinetics and Subsequent Pitting in Service?

In industrial operations, titanium sheath heaters are put into alternating corrosive and washing conditions with a daily cleaning cycle, using 10% phosphoric acid at 80°C. The quality of the rinse water between cleaning cycles is essential to the repassivation kinetics of the titanium surface. Deionized water with resistivity > 1 MΩ·cm permits quick repassivation and formation of a stable passive film within 10–30 min. Tap water containing chlorides (usually 50–200 ppm) and other ions slows down repassivation, increasing the time to achieve a stable layer to 2–6 h, and leaving adsorbed chlorides on the surface which serve as pitting start sites during later operation. The quality of rinse water chosen directly affects the pitting density and service life of the heater.

The mechanism of influence of rinse water on the kinetics of repassivation

After treatment with 10% phosphoric acid the titanium surface is in active or partly passive condition. The acid is removed by rinsing but it also sets the chemistry of the solution in which repassivation happens. The deionized water provides a clean, low conductivity environment for the uniform growth of the titanium passive film. No hostile ions are present and a dense stoichiometric TiO2 layer can be formed. Chloride ions from tap water are adsorbed on the titanium surface and compete with oxygen for adsorption sites. And, chlorides stop the creation of the passive film, and can become trapped under the oxide as it is forming, causing weak spots that will start pitting when the heater is put back into service. The repassivation time is the time required for the corrosion current density to drop below 1 µA/cm2 after rinsing.

Quantitative repassivation times and pitting rates of different rinse water qualities

Controlled testing of Grade 2 titanium coupons, using a daily 1 h immersion in 10% phosphoric acid at 80 °C, followed by a 15 min rinse and then immersion in 3% NaCl at 60 °C to replicate service, has established the following behavior over 30 cycles. The repassivation time upon rinsing with deionized water with resistivity of 18 MΩ·cm is 10–30 minutes, passive film thickness after 1 hour is 3–5 nm, pitting density in subsequent service is 0–5 pits per cm² and maximum pit depth after 500 hours is 0–20 µm. This performance is considered great. The repassivation time in softened tap water (10,000 Ω·cm resistivity, 20 ppm chloride) increases to 30–90 min, film thickness after 1 h is 2–4 nm, pitting density is 5–20 pits/cm2 and pit depth is 20–80 μm, indicating good performance. normal tap water, 2000 Ohm-cm resistivity, 100 ppm chloride repassivation time 1-3 hours film thickness after 1 hour 1-3 nm pitting density 20-80 pits/cm2 pit depth 80-200 mm mediocre to poor. The hard tap water has a repassivation time of 3–6 h, film thickness of 1–2 nm after 1 h, pitting density of > 80 pits/cm² and pit depth of > 200 µm at 500 Ω·cm resistivity and 200 ppm chloride, which is unacceptable.

Effect of Rinse Duration and Temperature on Quality of Repassivation

The efficiency of the rinse is a function of the water quality and the parameters of the rinse. A 5 min rinse with deionized water resulted in an 80% repassivation compared to a 30 min rinse. A 60-minute rinse with tap water with 100 ppm chloride is not equivalent to a 10-minute rinse with deionized water in terms of repassivation quality. Rinse water temperature is another important factor; warm rinse water (40°C) improves repassivation kinetics by 30–50% compared to cold rinse water (20°C) for both deionized and tap water. But, warm tap water might cause chlorides to concentrate throughout the drying process making the result worse. For all water qualities, an additional advantage is a final rinse with deionized water after the main rinse.

Phosphoric acid cleaning cycles - Guide for choosing the quality of rinse water

Recommendations for the quality of the rinse water for daily cleaning cycles with 10% phosphoric acid at 80°C are given in the following table, depending on the desired pitting resistance and service life in subsequent chloride-containing service.

Desired Pitting Density (pits/cm2) after 500 Hours Maximum Allowable Chloride in Rinse Water (ppm) Minimum Recommended Rinse Water Resistivity (Ω·cm) Recommended Rinse Duration (minutes) Final Rinse Required <5 <10 >100,000 15 Yes (DI spray)
5 - 20 10 - 50 10,000 - 100,000 20 Recommended
20–80 50–150 1,000–10,000 30 Recommended >80 >150 <1,000 60+ Not effective Engineering Beyond Rinse Water Quality Control

The titanium grade impacts the repassivation kinetics. Grade 7 (palladium-stabilized) repassivates 2-3 times faster than Grade 2 in any quality of rinse water, lowering sensitivity to chloride contamination. Wall thickness offers a pitting allowance. A 2.0 mm wall, with moderate pitting from tap water rinse, may last 5–8 years, but a 1.2 mm wall would fail sooner. The technique of drying after rinsing is important. Forced hot air drying at 60°C for 10 minutes eliminates the leftover moisture, avoiding chloride concentration areas. If the volume of deionized water is restricted, a cost-effective compromise is a two-stage rinse (tap water followed by deionized water).

Providing an Informed Specification

For a titanium sheath heater used daily with 10 % phosphoric acid at 80 °C and then exposed to a chloride containing environment, use a two-stage rinse. An initial rinse with tap water for 1 minute followed by a final rinse with deionized water of 18 MΩ·cm resistivity for 10 minutes at 40 °C. Specify final rinse water to be less than 10 ppm chlorides. If deionized water is not available for existing installations, add a deionization cartridge to the rinse line or extend the rinse time to 60 minutes with forced air drying. After the rinse procedure during commissioning, a test coupon was measured for repassivation potential. A value less than -0.1 V vs. Ag/AgCl in 3% NaCl indicates incomplete repassivation and requires longer or higher-quality rinse. The engineer uses deionized washing water rather than tap water for rapid repassivation and no further pitting in service.