Corrosion Control MS: Cathodic Protection and Corrosion/Process Monitoring

By John Reynolds, Principal Consultant at Intertek. July 15, 2013

This week’s post takes up right where last week’s post left off in our discussion on Corrosion Management and Control (CM&C) Management Systems. Here are the last two Corrosion Management and Control Management Systems.

#9 Cathodic Protection

Now we come to Cathodic Protection (CP). For those sites with underground piping and vessels, as well as above ground atmospheric storage tanks and wharf/dock facilities, there will almost always be a need for some sort of CP. And just as with materials selection and coating/linings issues, this is typically the purview of a C/M engineer/specialist with special training and experience in CP facilities to design and overview them.

Equipment corrodes in contact with the ground because of corrosion currents. If you design, install, monitor and maintain CP systems, you can achieve long-term, cost- effective preservation of buried equipment and tank bottoms. But it takes a concerted management system to make sure that effective CP is maintained throughout the life of the equipment. Sometimes there are mistakes made in CP design, stray currents creep in, insufficient current reaches the protected item, anode beds are not maintained, the rectifiers are not monitored and maintained, structure to soil potential measurements are not taken in the right place, or no one group has specific responsibility for maintaining effectiveness of the entire system. I have even seen the rectifiers turned off for maintenance activities and no one notices for many months that they were turned off. Sometimes short-term cost cutting needs, can result in loss of system maintenance, thereby trading the large long-term value of the asset for small short-term budget needs. Few things in our plants are more expensive than the inspection and maintenance of tank bottoms. Yet, effective CP systems can virtually halt bottom-side corrosion of tanks, and allow us to extend tank inspection intervals out to the maximum allowable by API 653; thereby allowing us to achieve the lowest total life cycle costs for storage tanks. API RP 651 (3rd ed.) section 11 has excellent guidance on operating and maintaining an effective CP system.

#10 Corrosion and Process Monitoring

And finally, we come to Corrosion and Process Monitoring. This is another of the important sections that needs to be documented in the CCDs referred to previously, and is very closely integrated with Integrity Operating Windows (IOW) covered in my previous article.  In order to establish a comprehensive set of IOW’s, it takes solid team work between C/M engineers/ specialists (leading the process), process engineers and operations, among others, to fully understand all the probable corrosion and damage mechanisms that could impact pressure equipment integrity.

Once all the IOW’s are established and implemented, there is usually a need to do a significant amount of process monitoring to make sure that the IOW’s are not exceeded, and when/if they are exceeded that the appropriate corrective actions are implemented. Process condition monitoring is especially important in those processes that are prone or susceptible to changes in process conditions. If we are to rely on the selected materials of construction that will resist certain process conditions, but will deteriorate more rapidly outside of a given set of conditions, then we should be monitoring those conditions that could give rise to significant changes in deterioration rates. Temperature monitoring is fairly common, but often there are other variables that really need to be monitored. Examples include pH limits, hydrogen partial pressures, chloride (inorganic and organic) contents, water contents, percentages of salts, sulfide contents, organic acid contents, amine/caustic carryover, iron in solution, oxygen contents and a host of other process specific variables. And, of course, equally important as monitoring for these variables, is monitoring in the right places in the operating process. A C/M engineer/specialist should know where the right places are.

Where corrosion rates are not well established, the C/M engineer/specialist typically would need to implement one or more of the variety of corrosion monitoring techniques to establish corrosion rates, other than relying just on ultrasonic thickness measurements, which is an inspection technique, not a corrosion monitoring technique. Often we assume things about how our process conditions affect corrosion rates that may not always be true. Hence there is often a real need for corrosion rate as well as process variable monitoring to validate our assumptions and validate that our materials of construction are the right choices. These corrosion monitoring methods are embodied in the electronic and potentiostatic systems, by which we can quickly ascertain changes in corrosion rate, when process conditions change, like e.g. resistivity and linear polarization measurements. Two classic cases of need come to mind for effective use of corrosion rate monitoring: 1) when we change process fluids that may impact our historic corrosion rates in ways that we are not sure about, and 2) when we have a new process or process change where we don’t have historic data on which to effectively estimate corrosion rates and future service lives of equipment.

That covers the list of the Top 10 Corrosion Management & Control Management Systems. Next week’s post will outline how CM&C MS integrate with other Pressure Equipment Integrity Management Systems.

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