Effect of materials and geometry on hydrogen accumulation inside a hydrogen effusion probe

Abstract

This research focuses on a further development of the Hydrogen Effusion Probe (HEP), a device shaped like a cylindrical cup, for monitoring corrosion of carbon steels. A HEP (called HEPro™) installed at the Point Lepreau Generating Station (PLGS) was simulated using a computational fluid dynamics (CFD) program to solve some issues observed with HEPro; the curving pressure trend and the “kink” appeared in the curve. The model of hydrogen accumulation inside this HEP was also developed and solved by MATLAB software. The simulation result predicted the hydrogen diffusion path and hydrogen accumulation inside the cup. The simulation and modelling results are in good agreement with the plant data. The HEPro at the PLGS works well. However, as it was manufactured from silver, a vacuum pump is necessary for its operation. A cup that is manufactured from a material that has higher hydrogen permeability than silver would work as a diffusing membrane and lead to the simpler system. A piece of carbon steel 1045 was machined into two cylindrical membranes with two different lengths to determine hydrogen permeability and diffusivity in the material at several temperatures. The resulting expressions for increasing hydrogen permeability and diffusivity with increasing temperature were 4.832 × 10^[-4] exp (-8.622 × 10^[4]/RT) mol/m∙s∙Pa^[½] and 6.229 × 10^[-3] exp (-6.508 × 10^[4]/RT) m^[2]/s, respectively. These results were used for modelling hydrogen accumulation inside the carbon steel cups, and also demonstrated that a carbon steel cup would work as a diffusing membrane. Eight cups with different geometries were manufactured from carbon steel and stainless steel, and welded onto a carbon steel pipe containing hydrogen gas. Two cups indicated leak. Therefore, six of them were tested to investigate the effect of material, size, and wall thickness on hydrogen accumulation inside the cups. The results indicated that the hydrogen accumulation inside the cups depends significantly on the cup material. The plateau pressure inside the stainless steel cup was significantly higher than in the carbon steel cup. The change in size of the cups has a moderate effect on hydrogen pressure at steady state inside the cups. The difference in wall thicknesses does not have a significant effect on plateau pressure inside the cups. The hydrogen accumulation inside the cups raises the possibility of a self-moderating-pressure design. A computational fluid dynamics (CFD) program was used to indicate the hydrogen diffusion paths around the cups and hydrogen accumulation inside the cups at steady state. The steady state pressures inside the stainless steel cups did not agree with the experimental results, but the plateau pressures inside the carbon steel cups concur well with the experimental results. A mathematical model was developed which was capable of predicting plateau pressure inside the carbon steel and stainless steel cups.