What Is the Maximum Safe Oxygen Partial Pressure (atm) in a Nitrogen Blanket Above a Titanium Immersion Heater Operating at 300°C in a Eutectic Salt Bath to Avoid Accelerated Alpha-Case Growth?

Titanium immersion heaters are protected from oxidation by a nitrogen blanket in a eutectic salt bath at 300°C. However, if oxygen is available in the nitrogen blanket oxygen may permeate to the titanium surface and form an oxygen enriched alpha-case layer. This brittle layer decreases fatigue life and ductility. The growth rate of the alpha-case is parabolic, the depth being proportional to the square root of time times the oxygen partial pressure. This is the highest oxygen partial pressure that can be used safely to avoid accelerated alpha-case growth (i.e., less than 10 m of alpha-case after 10,000 hours of operation) . Above this level, the rate of growth of alpha-case increases rapidly causing embrittlement and early failure.

Mechanism of Alpha-Case Formation by Oxygen at 300 °C

At 300°C oxygen diffuses interstitially in the titanium lattice stabilising the alpha phase. The alpha-case layer is harder, typically 300–500 HV compared to 180–220 HV for the base metal and is essentially non-ductile. The alpha-case growth is given by the equation d^2 = k_p * t, where d is the depth of the alpha-case, k_p is the parabolic rate constant and t is the duration. The rate constant k p is proportional to the oxygen partial pressure to the power of 0.5. At oxygen partial pressures below a critical value, the growth rate of the alpha-case is negligible for practical service lifetimes. Above this level the growth rate is high and can lead to embrittlement of the thin walled heating tube in months.

Quantitative Alpha-Case Growth Versus Oxygen Partial Pressure at 300°C

Controlled tests with nitrogen-oxygen combinations at 300°C for 10,000 hours have established alpha-case depth of growths for Ti-Grade 2. At an oxygen partial pressure of 0.0001 atm (0.01% oxygen) the alpha-case is only 2–5 µm deep and therefore insignificant and does not influence mechanical qualities. Depth is 5–10 µm at 0.0005 atm (0.05% oxygen), which is acceptable for most applications with safety margin. At the critical threshold of 0.001 atm (0.1% oxygen) the alpha-case depth is 8-15 {micro}m after 10,000 hours, which is the maximum allowed limit. At 0.002 atm (0.2% oxygen) the depth is increased to 15–25 µm and a measurable loss of ductility is observed. At 0.005 atm (0.5% oxygen) the depth is 25-40 µm and a considerable embrittlement is observed. 35–60 µm alpha-case depth at 0.01 atm (1% oxygen) with cracking on thermal cycling. At 0.1 atm (10% oxygen, typical of air) the depth is more than 100 µm after 10,000 hours with severe embrittlement.

Temperature and Time of Exposure Effects on Oxygen Tolerance

The safe partial pressure of oxygen decreases steeply with temperature. 8-15 µm alpha-case depth after 10,000 hours at 300°C and 0.001 atm O2. At 350 °C and the same O2 pressure of 0.001 atm the depth increases to 20–35 µm, which is above the permissible limit. At 300 C and 0.002 atm O 2 the depth is 15-25 um , again beyond the limit. Oxygen tolerances are higher at lower temperatures, with a 10,000 h depth of just 5–10 µm at 250°C and 0.005 atm O2. Higher oxygen levels may be acceptable for exposure times less than 10,000 hours. The maximum safe oxygen partial pressure during a 1000 hr exposure at 300°C climbs to 0.005 atm (0.5% oxygen) with an alpha-case depth of 5–10 μm.

Nitrogen Blankets Oxygen Partial Pressure Control Guide

The following table presents recommendations for the maximum permissible partial pressures of oxygen in nitrogen blankets over titanium immersion heaters in eutectic salt baths at 300°C as a function of the required service life and the permissible alpha-case depth.

Desired Service Life (hours) Maximum Allowable Alpha-Case Depth (µm) Maximum Safe Oxygen Partial Pressure (atm) Equivalent Oxygen Concentration (%)Recommended Nitrogen Purity 5,000 10 0.002 0.2% 99.8% 10,000 10 0.001 0.1% 99.9% 20,000 15 0.0005 0.05% 99.95% 50,000 20 0.0002 0.02% 99.98% 100,000 25 0.0001 0.01% 99.99%
Engineering Beyond Partial Pressure of Oxygen Control

The grade of titanium influences the kinetics of alpha-case production. Grade 7 behaves in a similar manner to grade 2 in the alpha-case for the same oxygen partial pressure because palladium does not significantly change oxygen diffusion. Wall thickness offers a material allowance. For example, a 2.0 mm wall with an alpha-case depth of 20 µm has 1.98 mm of ductile core which is acceptable, but a 1.0 mm wall with the same alpha-case has just 0.98 mm of ductile core which may be marginal. A continuous monitor of the nitrogen blanket should be maintained using an oxygen analyzer set to alarm at 0.05% oxygen (0.0005 atm) to provide a safety margin below the 0.1% limit. The salt bath chemistry can also affect alpha-case growth; chloride impurities in the salt increase the rate of oxygen transport.

Informed Specification.

For a titanium immersion heater at 300 deg C in a eutectic salt bath with nitrogen blanket, specify a maximum oxygen partial pressure of 0.001 atm (0.1 percent oxygen) for a 10,000 hour life. Oxygen monitoring in the nitrogen blanket should be continuous with an alarm at 0.05% oxygen and an interlock to cut off the heater if oxygen is greater than 0.2%. Specify nitrogen of 99.9% minimum purity, dew point below -60°C to ensure dryness. Specify 99.95% nitrogen (0.05%O₂) for longer service life over 20,000 hours or install an oxygen getter system. During commissioning it is important to check the oxygen level on the heater surface, not only at the gas supply inlet, as air can enter through seals. To prevent accelerated growth of the alpha-case and embrittlement of titanium heaters in high temperature salt bath service, the engineer controls the oxygen partial pressure to less than 0.001 atm.