The new hydrogen permeation sensor based on solid polymer fuel cell technology was evaluated on laboratory and a field prototype was installed in an oil production facility. This sensor shows significant advantages in comparison with the other available hydrogen permeation detectors.
It has been shown that hydrogen permeation technique can be successfully used in the field, as a non- intrusive on line method for assessing the effect of actual service conditions on materials susceptibility to ? ? 1 corrosion damage, with remarkable advantages in sour service applications .
The methods used in the field to monitor internal corrosion rates of pipelines or reactors, non-intrusively, are based on the detection of the atomic hydrogen permeating out of the external wall by either pressure increase in an evacuated chamber or electrochemical oxidation. The advantages and limitations of the non intrusive hydrogen probes, as well as a mathematical model developed for correlating electrochemical with vacuum loss probes have been discussed in previous communications 2'3.
The electrochemical technique, developed by Devanathan and Stachurski 4, is based on the oxidation of the atomic hydrogen exiting the metallic surface, by maintaining its electrode potential at a sufficient anodic value in a suitable electrolyte. In the field, the electrochemical method was adapted as a non- intrusive hydrogen permeation monitor 5, with some limitations mainly due to the need for liquid electrolytes that could leak and were generally aggressive.
Fig. 1 shows schematically the process of detecting hydrogen permeation through a metallic membrane or pipe wall using the electrochemical technique. It can be observed that some of the atomic hydrogen produced in the cathodic reaction, due to the corrosion process or cathodic polarization, is absorbed into the metal, diffuses through the wall and is finally oxidized at the outer surface. The anodic current needed to oxidize these hydrogen atoms is a direct measurement of the hydrogen flux J through the metallic wall.
I
Equation (1)
where F is the Faraday's constant and I is the hydrogen permeation current. The external surface is exposed to a liquid electrolyte that is in general aggressive to the carbon steel. The exposed surface is usually covered by a palladium sheet or palladium plating in order to protect the surface from corrosion and to improve detection.
Recently, a new electrochemical sensor based on fuel cell technology was developed, which requires no liquid electrolytes, has longer life time, needs no external power and can be adapted for high temperature applications ' . Fuel cells are electrochemical devices that convert chemical energy from a reaction between a fuel and oxidant directly into electrical energy (DC current). Contrary to batteries, fuel cells do not need recharging and will provide energy as electricity and heat as long as there is fuel available.
There are at least five types of fuel cells, differing in the type of electrolyte used and temperature of operation 8. Fig. 2 shows schematically a typical fuel cell with solid electrolyte (protonic exchange membrane, PEM) which uses hydrogen as a fuel and operates at low temperatures (below 80 °C). This consists of two electrodes separated by a protonic membrane? In the anode, the hydrogen is oxidized to protons:
Equation (2)
Protons migrate through the solid electrolyte (membrane) towards the other side of the cell (cathode) where the oxygen is reduced in presence of protons to produce water:
Equation (3)
The electrons flow from the anode to the cathode through the external conducto