Thickness measurement of Sn-Ag hot dip coatings on Large Hadron Collider Superconducting strands by coulometry

Amperostatic coulometry is applied for the thickness measurement of Sn-Ag hot dip coatings, which comprise an extended Sn-Cu interdiffusion layer. Complementary measurements, notably weight loss, X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDS), X-ray fluorescence (XRF) and dynamic secondary ion mass spectroscopy (DSIMS) have been performed in order to obtain a better interpretation of the coulometry results. Based on the experimental results presented in this article the three potential changes that are observed during coulometry measurements are ascribed to (1) the entire dissolution of pure Sn, (2) the formation of a CuCl salt layer and (3) the surface passivation. The measurement of the pure Sn mass is well reproducible despite of strong coating thickness variations that are detected by XRF. Several experimental problems, in particular a coating undercutting, hamper the determination of the Sn mass in the intermetallic Sn-Cu layer.