CORROSION SCIENCE SECTION CORROSION—Vol. 63, No. 9 857 Submitted for publication April 2007; in revised form, July 2007. ‡ Corresponding author. E-mail: kemaln@material.ntnu.no. * MainTech AS, Fossegrenda 7, N-7038 Trondheim, Norway. ** Acergy, Tangen 7, N-4070 Trondheim, Norway. *** Department of Engineering Design and Materials, Norwegian Uni- versity of Science and Technology, N-7491 Trondheim, Norway. Significance of Hydrogen Evolution During Cathodic Protection of Carbon Steel in Seawater T. Okstad,* Ø. Rannestad,** R. Johnsen,*** and K. Nisancioglu ‡, *** ABSTRACT Current requirement and calcareous scale properties on car- bon steel were investigated as a function of applied potential, flow rate, and time in natural seawater. The current require- ment was dominated by oxygen reduction during the initial stage of polarization after application of cathodic protection. However, as the calcareous scale deposited, oxygen reduction reaction rate decreased and hydrogen evolution became the dominating cathodic process. Furthermore, the rate of hydro- gen evolution appeared to become catalyzed by the presence of the calcareous scale deposit. At a given flow rate, a maxi- mum was obtained when the Ca/Mg ratio measured in the calcareous deposit was plotted as a function of applied poten- tial. The maximum became larger and shifted to more active potentials with increasing flow rate, as determined by the pH and potential-dependent competing kinetics of calcium hydrox- ide (Ca[OH] 2 ) and magnesium hydroxide (Mg[OH] 2 ) deposition. The applied potential giving the maximum Ca/Mg ratio in the calcareous deposit provided the best protection and minimum current requirement at a given flow rate. KEY WORDS: calcareous deposits, current requirement, polar- ization, seawater INTRODUCTION The significance of the oxygen reduction reaction on the formation of calcareous scale during cathodic pro- tection of steel in seawater is widely studied. 1-3 Fac- tors such as temperature, pH, flow conditions, and water composition strongly affect the precipitation of the calcareous deposit at the steel surface. During the early stages of polarization, oxygen reduction is the dominating reaction. It is well documented that oxygen reduction rate also affects the properties of the calcareous scale, which, in turn, determines the ex- tent to which the current requirement is reduced, an important aspect in the cathodic protection of steel in seawater. Little work is available on the significance of cal- careous scale on hydrogen reduction rate. Hydrogen evolution can cause hydrogen embrittlement if atomic hydrogen diffuses into the steel material. Although it is not straightforward to determine the fraction of hydrogen formed that diffuses into the metal, it is still of interest to ascertain the contribution of hydrogen evolution to the total current requirement during ca- thodic protection. Andresen, et al., 4 found that the hydrogen reduction rate was decreased as a result of cathodic protection in seawater because calcareous deposits at the surface reduced the active area. Simi- larly, Ou and Wu 5 reported that the calcareous scale acted as a barrier for diffusion of hydrogen ions to the steel surface, thereby reducing the rate of hydrogen evolution. The purpose of the present work was to in- vestigate the rate of hydrogen reduction relative to the rate of oxygen reduction during cathodic protection of carbon steel in natural seawater. The important ques- tion of hydrogen exchange at the solution-metal inter- face and hydrogen transport and entrainment in the metal bulk are outside the present scope. However, the present subject had to be treated as a part of work 0010-9312/07/000155/$5.00+$0.50/0 © 2007, NACE International