A stone-hard-soil structure was observed on a metallic pipeline under cathodic protection and subject to alternating current corrosion at a coating defect. Using X-ray diffraction, energy dispersive X-ray spectroscopy, ion-chromatography and inductively coupled plasma optical emission spectroscopy, the stone-hard-soil was characterized as being enriched in NaCl. Local alkalization following the cathode reactions caused precipitation of calcite, believed to be partly responsible for the stability of the structure. Very close to the corrosion site at the epicenter, calcite and quartz was depleted, possibly owing to an extremely high pH. Formation of cementlike minerals and corrosion products is observed.
Junker A, Belmonte LJ, Kioupis N, Vendelbo Nielsen L, Møller P.