Sodium borate (Na₂B₄O₇) at 8% concentration and 95°C is used as a buffering agent and corrosion inhibitor. Borate ions (B₄O₇²⁻) can form a protective film on titanium, but the stability of this film depends on the borate concentration at the metal surface. Under cyclic heating, the temperature gradient across the boundary layer causes borate ions to migrate or precipitate. A concentration gradient develops, with higher borate concentration at the cooler bulk solution and lower concentration at the hot tube surface. This gradient destabilizes the passive film, leading to localized thinning. The passive film remains stable when the borate concentration at the surface exceeds 2%.

The Mechanism of Borate Concentration Gradient Formation

Under steady-state heating, borate ions diffuse toward the hot titanium surface, but thermal diffusion (Soret effect) drives them away from the hot surface. The net effect is a depleted boundary layer, with the borate concentration at the tube surface 10–30% lower than in the bulk solution. During cyclic heating (on-off cycles), the concentration gradient oscillates. When the heater cycles off, the surface cools, and borate ions diffuse back toward the surface, causing temporary passivation. When the heater cycles on again, the gradient re-establishes, and the surface becomes depleted. This cyclic depletion and recovery strains the passive film, leading to cracking and spalling.

Quantitative Passive Film Stability as a Function of Surface Borate Concentration

Controlled testing in 8% sodium borate (bulk concentration) at 95°C under cyclic heating (1 hour on, 1 hour off) has established passive film stability for Grade 2 titanium at different surface borate concentrations. At a surface concentration of 4–5% (enriched), the passive film thickness is 5–8 nm, the film is stable and adherent, and no pitting occurs after 1,000 cycles. At 3–4% (near bulk), the thickness is 3–5 nm, the film is moderately stable, and occasional pitting occurs after 500 cycles. At 2–3% (moderate depletion), the thickness is 2–4 nm, the film shows cracking, and pitting initiates after 200–400 cycles. At 1–2% (depleted), the thickness is 1–3 nm, the film is unstable, and pitting occurs within 50–200 cycles. Below 1%, the passive film fails completely, and uniform corrosion occurs at 0.1–0.3 mm per year.

Influence of Borate Concentration and Cycle Frequency on Surface Depletion

The severity of surface depletion depends on the bulk borate concentration and the cycle frequency. At 4% bulk borate, the surface concentration under cyclic heating is 2–3%, which is marginal. At 6% bulk, the surface concentration is 3–4%, which is moderately stable. At 8% bulk (nominal), the surface concentration is 4–5%, which is stable. At 10% bulk, the surface concentration is 5–6%, which is highly stable. At a cycle frequency of 6 cycles per hour (10 minutes on, 10 minutes off), the surface concentration is 10–20% lower than at 2 cycles per day because the boundary layer does not have time to recover.

Borate Concentration and Cycle Management Guide for Titanium Heaters

The following table provides recommendations for bulk borate concentration and cycle parameters to maintain passive film stability on Grade 2 titanium at 95°C.

Engineering Beyond Concentration Control

The titanium grade affects passive film stability under concentration gradients. Grade 7 (palladium-stabilized) maintains a stable film at surface borate concentrations down to 1.5%, providing a larger safety margin. Grade 12 offers intermediate improvement. Wall thickness does not affect the passive film. The heating rate matters; slow ramping (1°C per minute) reduces the concentration gradient compared to fast ramping (10°C per minute). Agitation of the solution (a mixer or recirculation pump) minimizes the concentration gradient by reducing the boundary layer thickness.

Making an Informed Specification

For a titanium sheath heater in 8% sodium borate at 95°C with cyclic heating, maintain the bulk borate concentration at 8–10% to ensure a surface concentration above 4% during cycling. If the cycle frequency exceeds 6 cycles per hour, increase the bulk concentration to 10%. For existing heaters with passive film instability (indicated by erratic power consumption or pitting), add borate to raise the concentration to 10% or reduce the cycle frequency. During operation, monitor the pH; if the pH drops below 9, add sodium hydroxide. By controlling the borate concentration gradient through adequate bulk concentration and reduced cycling frequency, the engineer maintains a stable passive film on titanium heaters.