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?

The eutectic salt bath is operated at 300°C and a nitrogen blanket protects the titanium immersion heaters from oxidation. But oxygen may leak out of the nitrogen blanket into the titanium surface and form an oxygen-rich alpha-case layer. The thin layer is brittle and lowers both fatigue life and ductility. The growth rate of the alpha-case is a parabolic law, the depth being proportional to the square root of the oxygen partial pressure times time. The highest safe oxygen partial pressure to prevent accelerated alpha-case growth, defined as less than 10 µm of alpha-case after 10,000 hours of operation, is 0.001 atm (0.1% oxygen by volume). Above this level the growth of alpha-case is greatly accelerated resulting in embrittlement and premature failure.

Mechanism of the Oxygen-Induced Formation of Alpha-Case at 300°C

So at 300°C oxygen interstitially enters into the titanium lattice and stabilises the alpha phase. The alpha-case layer is hard, often 300-500 HV vs. 180-220 HV for the base metal, and is essentially non-ductile . The alpha-case growth follows the equation d² = k_p × t, where d is the depth of the alpha-case, k_p is the parabolic rate constant, and t is the period. The rate constant kp is proportional to the square root of the partial pressure of oxygen. For practical service times the alpha-case development rate is insignificant below a certain oxygen partial pressure. If the growth rate is above this threshold it can be significant and may cause embrittlement of the thin walled heating tube within a few of months.

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

The depth of alpha-case formation for Grade 2 titanium has been established through controlled testing in nitrogen-oxygen combinations at 300°C for 10,000 hours. At an oxygen partial pressure of 0.0001 atm (0.01% oxygen), the depth of the alpha-case is 2–5 µm and is minimal with no influence on mechanical properties. At 0.0005 atm (0.05% oxygen) the depth is 5–10 µm and is acceptable for most of the applications with a safety margin. The critical threshold is 0.001 atm (0.1% oxygen) where the alpha-case depth is 8-15 m after 10,000 hours which is the maximum allowable limit. At 0.002 atm (0.2% oxygen) the depth increases to 15–25 µm and a measurable drop in ductility is observed. At 0.005 atm (0.5% oxygen) the depth is 25–40 mm, with substantial embrittlement seen. At 0.01 atm (1% oxygen) the alpha-case depth is 35–60 µm that leads to cracking during thermal cycling. At the air typical 0.1 atm (10% oxygen) the depth is greater than 100 µm after 10,000 hours causing significant embrittlement.

The Influence of Temperature and Duration of Exposure on Tolerance to Oxygen

The safe partial pressure of oxygen drops significantly with increasing temperature. After 10,000 hours at 300°C and 0.001 atm O₂ the alpha-case depth is 8-15 µm. At 350°C with the same 0.001 atm O2 the depth increases to 20–35 µm, which is above the permissible limit. Depth: 15-25 mm @ 300°C and 0.002 atm O2, also above limit. At 250°C and 0.005 atm O 2 the depth after 10,000 hours is just 5-10 µm, indicating that lower temperature allows higher oxygen tolerance. For exposure lengths less than 10,000 hours, oxygen levels higher than 10,000 hours may be tolerated. For a 1,000 hr exposure at 300oC the maximum acceptable oxygen partial pressure climbs to 0.005 atm (0.5% oxygen) with an alpha-case depth of 5-10 μm.

Guide to Controlling Oxygen Partial Pressure for Nitrogen Blankets

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

Desired Service Life (hours) Maximum Allowable Alpha-Case Depth (µm) Maximum Safe Oxygen Partial Pressure (atm) Equivalent Oxygen Concentration (%)Nitrogen Purity Recommended 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 titanium grade influences the kinetics of alpha-case formation. The alpha-case for Grade 7 is similar to Grade 2 at the same oxygen partial pressure due to the little effect of palladium on oxygen diffusion. Wall thickness offers a material allowance so that 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 only has 0.98 mm of ductile core which may be marginal. The nitrogen blanket should be monitored continually using an oxygen analyser with an alarm set at 0.05% oxygen (0.0005 atm) to offer a safety buffer below the 0.1% limit. Alpha-case growth can also be affected by the salt bath chemistry. Chloride impurities in the salt accelerate diffusion of oxygen.

Informed Judgement Specification

10,000-hour service life, titanium immersion heater, 300°C, eutectic salt bath, nitrogen blanket, specify a maximum oxygen partial pressure of 0.001 atm (0.1% oxygen). Continuous oxygen monitoring in the nitrogen blanket with alarm at 0.05% oxygen and interlock to shut off heater if oxygen surpasses 0.2%. Specify nitrogen purity minimum 99.9% with dew point below -60°C for dryness. For service life over 20,000 hours specify 99.95% nitrogen (0.05% O2) or install oxygen getter system. When commissioning, check the oxygen level on the heater surface and not just at the gas supply intake, as air might leak in through the seals. The oxygen partial pressure is controlled to less than 0.001 atm in order to prevent increased alpha-case formation and embrittlement of titanium heaters in high-temperature salt bath service.