Effect of protective casing on the cathodic protection condition of a buried pipeline at rail crossing

The Italian Ministerial Decree (D.M. 4 April 2014) foresees that at rail crossing and parallelism, buried pipes for liquid and gas transportation shall be encased in a coated steel pipe called casing pipe. According to the decree, the ends of the casing pipe should ensure the perfect watertight seal of the annulus while spacers of insulating material should avoid electrical contact between the two pipes. Nevertheless, on-field experience proved that in most of the cases water from soil is present into the annulus in between pipe and casing, causing possible corrosion. Based on that evidence, laboratory tests have been performed to investigate the protection level on the gas transport pipe and the interference condition of the casing pipe. Different configurations were considered for the casing: bare pipe with a passing hole; coated pipe with a coating defect on the internal and external surface; bare casing electrically connected to the internal pipe with a shunt. Results showed that in the presence of a perfect watertight seal, no interference takes place on the casing pipe, as expected. If an electrolyte in present into the annulus, the external surface of the casing and the internal surface of the pipe are cathodically polarized, while internal surface of the casing is anodically interfered. Increasing the cell voltage, overprotection conditions on the internal pipe are reached in all tested configuration, with the exception of tests performed on the pipes electrically connected.