MEASUREMENTS OF HYDROGEN ACTIVITY IN IRON DURING CATHODIC PROTECTION USING A POTENTIOMETRIC CONCENTRATION CELL

A solid-state potentiometric sensor, based on the well-tried principle of the thermodynamic concentration cell, has been constructed. The se nsor uses a conventional bi-electrode design with an entry side and an exit side at which the hydrogen is detected. The sensor has been used for the quantitative determination of the hydrogen activity (equivale nt H-2 pressure) generated during cathodic protection. Thus, the backg round hydrogen pressure in uncharged steel is 10(-17.7) (2 x 10(-18) a tm.) with a variation of about 20 times. During cathodic protection th is value rises by large amounts. Thus, using a zinc anode, the equival ent hydrogen pressure rose by 10(10.3) (to 4.4 x 10(-8) atm.) in 3.5% NaCl and by 10(16.9) (to 0.16 atm.) in artificial sea water, indicatin g the much larger amounts of hydrogen present in the latter case. This compares, for example, with literature data of 0.11 mu A cm(-1) and 0 .60 mu A cm(-2) for hydrogen permeation at -1000 mV (SCE) in 3.5% NaCl and artificial sea water respectively. Hydrogen entry and exit was al so considerably slowed in sea water. These differences are caused by l ocal surface pH buffering and deposit formation.