Quantification of hydrogen evolution in corrosion testing by buoyancy measurements

This report describes further development and improvements to an existing method for volumetric measurement of gas from a gas evolving electrode utilizing buoyancy measurements. The operating principle is that gas bubbles evolved at the electrode are captured in an inverted beaker suspended in the test solution, and the volume of the collected gas is determined by measuring the buoyancy that the gas exerts on the inverted beaker. The performance of the method was evaluated in a series of steady and dynamic polarisation tests of hydrogen evolution on inert cathodes and on magnesium electrodes subjected to both uniform and localised corrosion. The accuracy, precision, sensitivity, and sources of error have been analysed in detail. The method provides a useful tool for studying relatively high hydrogen evolution rates in corrosion testing. Knowledge of its limitations is however important. A general conclusion is that volumetric gas collection measurement at constant pressure has an inherent negative bias when it comes to quantifying the total production in a gas evolving electrochemical reaction due to the supersaturation needed to form gas bubbles and the displacement of supersaturated solution by the expanding gas pocket. It is also clear that such methods are amenable mostly for the higher end of corrosion rates, such as accelerated testing of less noble metals. The test results showed that, for a bare, inert electrode of 2.79 cm ² surface area, the method was accurate to -1% at cathodic current densities above 3.5 mA/cm ² , within -10% down to 1.5 mA/cm ² , and within -20% down to 0.7 mA/cm ² . Valuable semi-quantitative or qualitative information can be obtained for lower current densities if the performance of the method has been properly characterised. Long-term testing of Mg electrodes in alkaline solutions indicated that hydrogen evolution rates down to around 0.3 mA/cm ² could be measured with high accuracy in long term tests. This better performance compared to bare, inert electrodes may be attributed to the presence of thick, porous corrosion product layers on magnesium in alkaline solutions, in which the supersaturation of hydrogen may be higher.