A Surface Analytical Method for Determining the Porosity of Tungsten Coatings as Hydrogen Permeation Barriers

Fast and reliable methods that permit the determination of the through-thickness coating porosity are required to develop optimized coating–substrate systems that act as hydrogen permeation barriers. In this work, we present an analytical approach for determining the porosity of tungsten coatings that are 1.5 and 3 μm thick. The coatings are applied to X60QS API 5L pipeline steel, which has a surface roughness of Ra 0.4 μm, as well as to polished substrates with a roughness of Ra 0.2 μm. The method utilizes X-ray photoelectron spectroscopy (XPS) measurements for the coating analysis. The W-coatings were produced by radio frequency magnetron sputtering. The through-thickness porosity was calculated as the ratio of the Fe 2p to W 4f signals from the XPS survey spectra corrected for the respective sensitivity factors. The results from three different samples, for each of which three different areas were analyzed, showed a high porosity of 3.5 (0.8) % for the 3-μm-thick W-coating on the rough substrate, no pores for the 3-μm-thick W-coating on the smooth substrate, and less than 0.5% porosity on one out of three samples of the 1.5-μm coating on the smooth substrate. These results correlate well with the intensity of the hydrogen permeation current.