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?

Heaters with titanium sheath are exposed to alternating corrosive and rinse conditions in industrial processes with daily cleaning cycles using 10% phosphoric acid at 80°C. The quality of rinse water between cleaning cycles is important for determining the kinetics of repassivation of the titanium surface. Deionised water of resistivity higher than 1 MΩ·cm is used for quick repassivation and a stable passive film is formed in 10–30 minutes. The presence of chlorides (average concentrations in tap water are 50-200 ppm) and other ions in the water retards repassivation, resulting in an increased time to development of a stable layer, in the range of 2 to 6 hours. Moreover, the adsorbed chlorides remain on the surface and act as sites for pitting start in subsequent service. The character of the rinse water has a direct bearing on the density of pits and the life of the heater.

Effect of Rinse Water on Repassivation Kinetics

Exposure to 10% phosphoric acid leaves the titanium surface in an active or semi-passive condition. Rinsing eliminates the acid, but also controls the chemistry of the solution in which repassivation takes place. The clean, low-conductivity environment of deionised water allows uniform growth of the titanium passive layer. The absence of hostile ions enables the creation of a dense stoichiometric TiO₂ layer. Chloride ions are added to tap water which attach to the titanium surface and compete with oxygen for adsorption sites. Chlorides retard the growth of the passive layer and may themselves become entrapped underneath the developing oxide, leaving weak spots that will be the starting places for pitting on restoration to service of the heater. The repassivation time is the time needed for the corrosion current density to drop below 1 µA/cm² after rinsing.

Quantitative Repassivation Times and Pitting Rates for Different Qualities of Rinse Water

Controlled testing of Grade 2 titanium coupons, immersed for 1 hr daily 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 behaviour over 30 cycles. Repassivation time: 10-30 min with deionised water rinse of 18 MΩ·cm resistivity. Passive film thickness: 3-5 nm after 1 hour. Pitting density in future service: 0-5 pits per cm². Maximum pit depth after 500 hours: 0-20 μm. This is considered a good performance. The repassivation period is 30-90 min, the film thickness is 2-4 nm after 1 hr, pitting density is 5-20 pits/cm2 and pit depth is 20-80 um with softened tap water of 10,000 Omega.cm resistivity and 20 ppm chloride. This is good performance. For ordinary tap water (2,000 Ω .cm resistivity, 100 ppm chloride), repassivation duration is 1–3 h, film thickness is 1–3 nm after 1 h, pitting density is 20–80 pits per cm2, and pit depth is 80–200 µm, moderate to poor. Using hard tap water (resistivity 500 Ω.cm, chloride 200 ppm) the repassivation duration is 3-6 hours, film thickness 1-2 nm after 1 hour, pitting density >80 pits/cm2 and pit depth >200 µm, which is an unacceptable performance.

Influence of Rinse Time and Temperature on Repassivation Quality

The efficiency of the rinse is dependent on the rinse parameters and on the quality of the water. For instance, a 5-min rinse with deionised water produces 80% of the repassivation achieved by a 30-min rinse. Even using tap water with 100 ppm chloride, a 60-minute rinse does not provide the same level of repassivation as a 10-minute deionised water rinse. The temperature of the rinse is also important. For both deionised and tap water, a warm rinse at 40 °C enhances the repassivation kinetics by 30–50% compared with a cold rinse at 20 °C. However, warm tap water might concentrate the chlorides when it dries and make the situation worse. A final spray of deionised water after the primary rinse is beneficial for all water qualities.

Phosphoric Acid Cleaning Operation Rinse Water Quality Selection Guide

The following table gives recommendations for rinse water quality in daily cleaning cycles with 10% phosphoric acid at 80°C based on 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 (Ohm-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 grade of titanium impacts the kinetics of repassivation. Grade 7 (palladium stabilised) re-passivates 2-3 times faster than Grade 2 regardless of rinse water quality, therefore decreasing the sensitivity to chloride contamination. Wall thickness gives a pitting allowance. A 2.0 mm wall with moderate pitting due to tap water washing may survive 5-8 years, 1.2 mm would fail sooner. The drying procedure after rinsing is important, the leftover water was removed by forced hot air at 60°C for 10 minutes to avoid chloride concentration areas. When deionised water volume is restricted, a two-stage rinse (tap water followed by deionised water) is a cost effective compromise.

Making an Informed Speculation

For a titanium sheath heater exposed to a daily cleaning with 10% phosphoric acid at 80°C and then used in a chloride environment, specify a two-stage rinse, consisting of an initial rinse with tap water of 1 min, followed by a final rinse with deionised water of 18 MΩ.cm resistivity for 10 min at 40°C. Final rinse water must be < 10 ppm chloride. If there is no de-ionized water for existing installations, add a de-ionization cartridge to the rinse line or increase the rinse time to 60 minutes with forced air drying. During commissioning, evaluate the repassivation potential of a test coupon following the rinse procedure. A value less than -0.1 V vs. Ag/AgCl in 3% NaCl indicates partial repassivation and a longer or better grade rinse. The engineer uses deionised rinse water, not tap water, so that rapid repassivation occurs, and there is no further pitting in service.