ABSTRACT

The Italian ministerial decree (D.M. 4 April 2014) foresees that at rail crossings and parallelisms, pipes for liquid and gas transportation shall be encased in a well-sealed, coated steel pipe called casing pipe. Laboratory tests have been carried out to investigate the protection and interference condition of both the carrier pipe and the casing in the case where water is present in the annulus. Cathodic protection was applied to the gas pipe by an impressed current system, cell voltage was increased and the IR-free potential of both pipes was measured. Different configurations were considered for the casing: bare tube with and without a through hole; coated tube with a defect on the internal and external surface; bare tube electrically connected to the internal pipe by a shunt. In the presence of a perfect watertight seal, no interference takes place on the casing pipe, as expected. If an electrolyte is present in the annulus, overprotection conditions on the internal pipe are reached in all tested conditions, except for test performed on the connected pipes. In the presence of a bare casing, interference effects are negligible but overprotection condition on the internal tube is reached at the lowest cell voltage.

INTRODUCTION

Stray currents originating from direct current (d.c.) systems may cause severe material damage by corrosion on buried or immersed metal structures1. To prevent the effects of stray current corrosion caused by direct current, different methods can be used, such as cathodic protection (c.p.), forced drainage bond or insulating joints1,2. The Italian ministerial decree D.M. 4 Aprile 20143 demands that ducts buried in correspondence of rail crossings or parallelisms shall be encased in a coated steel pipe, called casing pipe. To ensure the protection of the inner tubing (carrier pipe), the ends of the casing pipe must guarantee the perfect watertight seal of the annulus while spacers of insulating material keep the two tubes electrically separated (Figure 1). In practice, it is almost impossible to achieve a perfect watertight seal. To avoid corrosion of the carrier pipe, a first possibility is the injection of a suitable filler material into the annular space. The filler material should either inhibit corrosion (e.g. visco-elastic compounds, inhibited wax) or be designed to allow c.p. current to reach the carrier pipe4. For casings that pass c.p. current (i.e. bare or poorly coated steel pipes or uncoated concrete pipes), the external cathodic protection of the carrier pipe can be effective in protecting the carrier pipe provided there is no contact between the carrier pipe and the casing, and that there is enough electrolyte in the annular space. Without any electrolyte in the annular space, atmospheric corrosion can occur at coating defects. Moreover, if c.p. of the casing is required, the casing should be resistively bonded to the carrier pipeline. The aim of this work is to investigate the behavior of the pipes in cases when the insulation of the carrier tube is not granted: this can occur if an electrolyte is present between the two tubes because of a leakage in the watertight system or in the casing. Laboratory tests have been carried out to investigate the protection (and over-protection) of the carrier pipe (from here on called ”gas pipe”) and the interference condition of the casing pipe, in case of water in the annulus. Different conditions were taken into consideration in order to verify the effects of coatings on the casing pipe and eventually the effect of an electrical connection between the two tubes, which aims at avoiding anodic interference of the internal surface of the casing tube. In all conditions, c.p. was applied to the gas pipe and both tubes were submerged in a conductive solution. This way it was possible to analyze the level of protection (or overprotection) on the gas pipe and the conditions of anodic/cathodic interference on the casing pipe.

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