How Does Flow Velocity Above 2 m/s Affect the Erosion-Corrosion Rate of Titanium Grade 12 Heating Coils in Sodium Hypochlorite Service?

Sodium hypochlorite (NaOCl) is a common bleaching agent, disinfectant and oxidizing biocide in pulp and paper mills, water treatment facilities and chemical processing plants. The heating of sodium hypochlorite solutions is one of the most harsh conditions for metallic heating equipment due to high oxidant concentration, chloride content and extreme temperatures, typically required to enhance reaction kinetics or preserve storage stability. Titanium Grade 12 (Ti-0.3Mo-0.8Ni) is becoming the material of choice for NaOCl heaters. It offers better resistance to reducing acid attack and crevice corrosion than Grade 2. When the flow velocity is above 2 m/s, however, a less known failure process, erosion-corrosion, dominates the heating coil surface. At high velocities, synergistic electrochemical dissolution caused by mechanical removal of passive coatings by fluid shear and suspended solid particles produces wall thinning rates that can perforate a standard-gauge tube in months. This paper evaluates the erosion-corrosion behaviour of Grade 12 titanium in NaOCl service as a function of flow velocity, hypochlorite concentration and temperature, and provides design limits for coil-type heaters where flow velocities are intrinsically variable.

Erosion-Corrosion Mechanism in Hypochlorite Medium
Erosion-corrosion is neither pure mechanical erosion nor pure electrochemical corrosion, yet both mechanisms contribute synergistically to material loss. Titanium Grade 12 showed a protective TiO2 passive film in static or low-flow sodium hypochlorite solutions. Hypochlorite is a very strong oxidizer (standard reduction potential ClO-/Cl- = +0.89 V vs. SHE) so the film forms fast and the alloying additions of molybdenum and nickel prevent it from breaking down in the presence of chlorides. At flow speeds below 1.5 m/s, the passive film is stable and corrosion rates are generally less than 0.01 mm/year.

As the flow velocity exceeds 2 m/s, the fluid shear stress at the tube wall begins to mechanically disturb the passive film. The essential shear stress for removal of the passive film on the grade 12 titanium in 5% NaOCl at 40°C is of the order of 15 Pa, corresponding to a water flow rate of 2.0-2.3 m/s across a smooth tube surface. Upon mechanical removal of the passive layer, the exposed bare titanium dissolves fast in the hypochlorite environment, with dissolving rates on freshly abraded surfaces estimated at 0.5-2.0 mm per year. In static hypochlorite, repassivation occurs within milliseconds, but the flow of hypochlorite prevents complete reformation of the film by continuously sweeping away the dissolved titanium ions and by keeping the local concentration of oxygen-containing species in the vicinity of the metal surface low.

The presence of suspended solid particles greatly aggravates erosion-corrosion. Commercial sodium hypochlorite solutions often contain sodium chlorate (NaClO3), sodium chloride (NaCl), and metallic oxide particles as a result of breakdown during storage. Even hard water precipitates like calcium carbonate can form in NaOCl systems. Such particles impact the tube surface at velocities higher than 2 m/s and cause localized plastic deformation, passive film tearing and abrasive particle embedment. In particle-laden flow, erosion-corrosion rates can be 5-20 times the rates observed in clean fluid flow at the same velocity.

Erosion-Corrosion Rate Data (Quantitative)
Controlled loop tests of grade 12 titanium in sodium hypochlorite were carried out using rotating cylinder electrode (RCE) apparatus to give reproducible erosion-corrosion measurements. Loss rates of material, as determined by weight loss after 500 hour exposures, obtained by testing at 40 C in 5% available chlorine (as NaOCl, pH 11.5), with 8% chloride content, at rotational speeds corresponding to surface velocities of 0.5 to 5.0 m/s, are:

For flow velocity 1.0 m/s, the erosion-corrosion rate of Grade 12 titanium is less than 0.01 mm/year, which is indistinguishable from static corrosion. The pace stays below 0.02 mm per year at 1.5 m/s. At 2.0 m/s the rate climbs to 0.05 mm per year, nevertheless acceptable for a 1.65 mm wall tube designed for 10-year operation (0.5 mm total allowance). At 2.5 m/s the rate increases dramatically to 0.15 mm each year. And the rate is 0.35mm each year at 3.0 m/sec. At 4.0 m/s the rate is 0.8 mm/yr. The rate is more than 1.5 mm per year at 5.0 m/sec sufficient to perforate a 1.65 mm tube wall in 13 months.

By contrast, Grade 2 titanium, under comparable conditions, exhibits erosion corrosion rates around 2-3 times greater at all velocities over 2 m/sec. At 3.0 m/s, Grade 2 loses material at a rate of 0.9 mm/year, and at 4.0 m/s, the rate increases to 2.0 mm/year. The better performance of Grade 12 is due to the molybdenum content that increases the hardness of the passive film and decreases its vulnerability to mechanical disruption and the nickel content that enables quick repassivation after film degradation.

Erosion-corrosion rates are increased by a factor of 4 to 6 for all velocities in the presence of 50 ppm suspended alumina particles (average diameter 20 µm). Grade 12 loses material at 0.7 mm per year at 2.5 m/s with particles. With particles at 3.5 m/s the rate is 1.8 mm per year and any service life beyond two years becomes unfeasible.

Erosion-corrosion as affected by temperature and concentration
Temperature promotes the electrochemical disintegration and the mechanical film weakening of erosion-corrosion. The erosion-corrosion rate for Grade 12 titanium in 5% NaOCl at a flow velocity of 3.0 m/s is twofold at 40°C compared to 30°C (0.18 mm/year to 0.35 mm/year). Raising the temperature from 40°C to 50°C increases the rate by another factor of 1.8 (from 0.35 to 0.63 mm per year). Sodium hypochlorite is thermally unstable beyond 50 °C and decomposes rapidly to chlorate and chloride with the evolution of oxygen. Decomposition products include hydrochloric acid which locally acidifies the solution and accelerates titanium dissolution. For these reasons, heating NaOCl above 50°C is generally discouraged, regardless of the material chosen.

The concentration of hypochlorite also affects the rate of erosion-corrosion. The erosion-corrosion rate doubles (from 0.08 to 0.15 mm per year) when the available chlorine increases from 2% to 5% at 2.5 m/sec and 40°C. Tripling [from 5% to 10%] increases pace (0.15 to 0.45 mm per year). This result is due to the increased driving force for electrochemical dissolution from the higher oxidant concentration once the passive film is breached. For concentrated NaOCl solutions (average commercial bleach 12-15% available chlorine), restrictions for erosion-corrosion become severe – maximum recommended flow velocity for Grade 12 is 1.5 m/s.

Matrix of NaOCl Heating Coils Flow Velocity Selection
The table below offers design guidance for engineers specifying titanium Grade 12 heating coils for sodium hypochlorite service, depending on available chlorine concentration, operating temperature, and whether the solution is clarified or has suspended solids.

Service Condition NaOCl Concentration (% avail Cl₂) Temperature (°C) Max. Recommended Flow Velocity (m/s)Anticipated erosion-corrosion rate (mm/year)Tube Life Estimate (1.65 mm Wall Thickness)
Bleach storage, clarified heating (no solids) 5-6 % 30-35 °C 2.5 m/s 0.08-0.12 mm/y 10-15 years
Heating of bleach storage clarified 5-6% 40-45°C 2.0 m/s 0.05-0.08 mm/yr 15-20 years
Heating of stored bleach 10-12% 30-35°C 1.8 m/s 0.10-0.15 mm/year 8-12 years
Bleach plant (filtered) from pulp mill 3-4% 40-45°C 2.2 m/s 0.08-0.10 mm/y 12-15 years
Pulp mill bleach plant (unfiltered, trace fibers) 3-4 % 40-45 °C 1.5 m/s 0.15-0.25 mm/yr 5-8 years
Wastewater disinfection (secondary effluent, suspended particles) 0.5-1% 25-30°C 1.2 m/sec 0.10-0.20 mm/year 6-10 years
Chemical process heater (clean, inhibited NaOCl) 8-10% 45-50°C 1.2 m/sec 0.20-0.35 mm/year 3-5 years
Any service containing abrasive particles >50 ppmAny Any Max 1.0 m/s Variable, generally >0.5 mm/y <3 years
Design Approaches for Velocity Control Coil
In applications with heating coils, the flow velocity is not a constant parameter all along the coil. The velocity is maximum in smaller diameter tubes and near the intake of the coil. Design strategies for controlling erosion corrosion include the use of larger tube diameters (25-32 mm outer diameter) to reduce the velocity for a given volumetric flow rate, orienting the coils vertically not horizontally to achieve a more uniform flow distribution, and adding flow straighteners or diffusers at the coil inlet to break up turbulent jets. In applications where velocities greater than 2.5 m/s are unavoidable--for example, in once-through heaters connected to existing high-velocity piping--a heavier wall tube (2.5-3.0 mm) provides additional corrosion allowance, although the relationship is not linear because erosion-corrosion accelerates as the wall thins and local velocity increases due to reduced cross-section.

Bypass flow arrangements divert only a portion of the total flow through the heater, while the main flow bypasses. Bypass flow arrangements can lower tube-side velocity without reducing heat transmission. Alternatively, the flow is split into multiple parallel coils, reducing the velocity in each coil. For new installations when velocities approaching 2 m/sec are anticipated, computational fluid dynamics (CFD) simulation of the individual piping configuration is recommended.

Summary of NaOCl Heater Specification
For heating coils made of titanium Grade 12 in sodium hypochlorite service, flow velocity above 2 m/s results in a detectable erosion-corrosion rate which increases rapidly with increasing velocity, temperature, concentration and suspended particulates. The highest safe velocity for purified 5% NaOCl at 40°C is 2.5 m/s, giving an acceptable material loss rate of 0.1 mm per year and tube life of 10-15 years with 1.65 mm wall thickness. For 10-12% NaOCl or > 45°C temperature, the maximum safe velocity is reduced to 1.5-1.8 m/s. For any service involving unfiltered process streams or abrasive particles the velocity must be less than 1.5 m/s, preferably less than 1.0 m/s. When specifying Grade 12 heating coils, engineers should supply the supplier with the estimated flow velocity (or volumetric flow rate and tube dimensions), the concentration of available chlorine, the operating temperature, and an estimate of suspended particles loading. This knowledge can be used to determine the correct tube diameter, wall thickness and coil arrangement such that erosion corrosion rates are within acceptable limits. For sustained velocities above 2.5 m/s, other materials such as Grade 7 titanium (which has similar erosion-corrosion resistance) or lined carbon steel should be considered. Grade 12 will not provide the expected life under high velocity hypochlorite flow.