Use of Nickel based super alloys for heat pipes

There are various applications that use heat pipes in the moderate temperatures of 450K to 750K such as space nuclear power system radiators, fuel cells, geothermal power, waste heat recovery units and high temperature electronics cooling. Various life tests have been conducted at temperatures of 673K using Hastelloy B3, C22 and Hastelloy C2000, Monel 400 and Monel K500. The test fluids are evaluated and regions of the heat pipes are analyzed to determine the magnitude and type of corrosion in the heat pipes. It is found that Monel heat pipes are suitable for service up to 550K. Copper depleted zones and copper surface nodules developed on the Monel 400 screen wick however not on the Monel K500. Hastelloy B3, C22 and C2000 at 673K were suitable. Alloy C2000 received nominal corrosion when used with titanium tetrachloride. On the other hand, Hastelloy C22 attained 5 to 10 micro-m thick double corrosion layer when evaluated with AlBr3 service fluid. The outcomes show that the tested envelope materials and service fluids can develop feasible material- service fluid combinations.

There is no commonly accepted service fluid over the whole intermediate temperature zone. Likely service fluids comprise elemental working fluids like sulfur, organic compounds and halides. Intermediate temperature based heat pipe life analyses were conducted for 40 years to find suitable working fluid and envelope combinations.

Monel water heat pipes

Various water working fluid heat pipes made from Monel K500 and Monel 400 are tested at different temperatures for different durations. While Monel 400 heat pipes offered suitable service, there were also some surprises found with these tests. Wide changes such as the development of dark subsurface layer and bright nodules were found in the alloy. Although the variation was the most extensive for the Monel- water heat pipes, Monel 400 wire mesh’s morphology was also typical. On the other hand, Monel K500 does not show wide change. Close test of the envelope exhibits a nominal corrosion. Inclusion of various elements to Monel K500 seemed to stabilize the alpha phase and inhibit this degradation. While no large magnitudes of oxygen were noticed, preferential oxidation of Nickel may also have played significant role in the development of the observed morphology and phases. Both Monel 400 and K500 are widely used in steam plants.

Hastelloy C alloys for heat pipes

Hastelloy C2000 and C22 are used with TiCl4, AlBr3, SnCl4 and other heating fluids. Alloy C2000 made a wide reaction with SnCl4. On the other hand, Hastelloy C22 provided significant performance in the heating fluid media and took a considerably long time to build up the corrosion layers.

Hence these Nickel based alloys are found to be suitable for the heating pipe materials for providing outstanding performance in the vigorous conditions. They prevent corrosion in the significantly long time and found to be consistent with the working fluids.