HYDROGEN ABSORPTION BY CATHODICALLY PROTECTED UNDERGROUND STEEL PIPING IN THIOSULFATE CONTAINING MEDIA

This paper deals with the influence of the cathodic protection conditi ons on the hydrogen absorption by underground steel pipes for natural gas transport in media containing thiosulfate. Hydrogen electrochemica l permeation tests were performed in soil-like media with different co ncentrations of thiosulphate. The effect of applied potential as well as that of the medium composition on the hydrogen absorption were anal yzed. The deposit formed during cathodic polarization on the protected steel surface was examined by scanning electron microscopy and X-ray diffraction. The results confirmed the catalytic effect of the thiosul phate on the hydrogen absorption. However it was shown that a high neg ative electrode potential as well as a calcium-magnesium deposit forme d during the cathodic polarization can inhibit this catalytic action. The mechanism of the inhibition seemed to be unique and was attributed to a distance effect between the electronegative catalytic species an d the metallic surface due to either an electrical field (created by a high negative potential) or a physical barrier (created by a calcium- magnesium deposit). (C) 1998 Published by Elsevier Science Ltd. All ri ghts reserved.