Corrosion Monitoring in Sour Systems Using Electrochemical Hydrogen Patch Probes
Authors: Martin Richard L. , French Eddie C. 
Published: December 2007

This paper discusses a probe that patches onto steel surfaces and monitors hydrogen that diffuses through to those surfaces. The probe was applied to two sour gas wells and a refinery absorber tower, and the results are presented. presented.

Introduction
A cell has been developed that patches onto the outside of steel pipes and vessels, and monitors the amount of hydrogen diffusing through the steel wall. In many cases, knowing the amount of hydrogen dissolved in steel is important because of blistering or cracking possibilities. Since corrosion in sour systems generates hydrogen as a by-product and much of that hydrogen dissolves into the steel, the electrochemical patch cell becomes a sensitive corrosion monitor. Particular advantages of this tool areno holes need to be cut into pressure systems;corrosion is measured on the natural inside diameter, so no foreign flow or metallurgical conditions are introduced;it is simple to apply and easy to relocate; andmeasurement is instantaneous minus the short lag for diffusion through the wall. The diffusion lag depends on steel thickness and temperature but is seldom more than 2 hours.
Other methods involving hydrogen are in use for corrosion monitoring in sour systems. However, because of the unique combination of sensitivity and flexibility, wider and more useful application can now be made. To date, the probe has been applied to gas wells, gas separators, and pumping wells on the production side, and to scrubbers and overheads on the process side.

Background
In any kind of corrosion control effort, corrosion monitoring is an important consideration. Granted, the best monitor is the performance of the metallic equipment itself, but in nearly all instances some quicker method is desirable to give early warning against corrosion damage and its consequences in the areas of safety, pollution, downtime, and economics. Many on-stream test methods have been developed to satisfy this need. The most desirable methods are those that measure corrosion under conditions that reproduce, as closely as possible, the actual corrosion conditions seen by the metallic surfaces in question. One example of efforts in this direction is the "flush mounted" probe, which gives corrosion rates at the plane of equipment surfaces using linear polarization principles. principles. A great deal of corrosion measuring effort has been made recently in the area of hydrogen detection since hydrogen is a by-product of many kinds of corrosion, certainly in sour (H2S) systems. All these efforts rely on the fact that a portion of freshly formed hydrogen (called nascent hydrogen or hydrogen atoms) from corrosion dissolves into the steel. This is an especially strong principle in the case of sour corrosion because the presence of principle in the case of sour corrosion because the presence of sulfide increases the fraction of hydrogen that enters the steel. One corrosion detection method in this category that has been used in both gas wells and refineries for some time is the hydrogen pressure probe. Many potential applications, however, call for greater sensitivity potential applications, however, call for greater sensitivity than pressure probes can provide. Therefore, work has been performed on methods that detect hydrogen diffusing through steel by using vacuum ion gauges of various configurations and by using electrochemical cells.
JPT
P. 1566