Q&A: Corrosion Monitoring & Control

Michael I. McElroy, Ph.D., is business development manager for Pepperl+Fuchs’s Corrosion Monitoring product line. He earned his Ph.D. in Physical Chemistry from Case Western Reserve University. Pepperl+Fuchs’s Process Automation Division supplies products and services to the automation industry, including intrinsic safety interfaces, HART and fieldbus products, corrosion measurement products, and purge-pressurization systems. Dr. McElroy can be reached at mmcelroy@us.pepperl-fuchs.com.

Q: Generally speaking, what are the primary corrosion vulnerability points in common industrial applications? Why is the occurrence of corrosion problematic in these application scenarios?

A: There is an assortment of vulnerability points in any industrial system that carries fluids, gasses or slurries. It depends a lot on the corrosion phenomena that are going on. Traditionally I think people believe that, simply put, things “rust.”

In reality, pipelines may rust, may pit, they may even abrade or erode. These are the easy ones to imagine happening. Other phenomena are crevice corrosion, cavitation corrosion, and fretting corrosion, to name a few. It is a complicated process; there are lots of different corrosion actions that can occur in an industrial application that may have to be considered by the user.

The problem is two-fold (at least). First, there is no universal method to identify corrosion from all sources. Each corrosion phenomenon may require a different measurement technique. Second, one does not monitor the whole area under concern, or in some cases does not directly monitor the affected area.  Material properties change from location to location within the enclosed area, and you cannot monitor all of them. Erosion may be greater in certain pipe layouts, while pitting may be very selective in the areas in which it occurs.

Q: Considering the spectrum of corrosion monitoring and control techniques employed by end-users from the low end to the high end, what would be considered base-level corrosion monitoring and control as compared to high-end corrosion monitoring and control in modern-day application environments?  For what sort of applications is the base-level solution suitable, and for what sort of applications is a high-end solution more appropriate?

A: Probably the simplest corrosion monitoring system is weight-loss coupons.  This is also the traditional and only direct method of monitoring corrosion.  Essentially, coupons of the same material are weighed before and after prolonged exposure to the corrosive medium in question. At the other end of the spectrum are fully integrated electronic transmitters, which use different methods to emulate what is occurring in the corrosion process.

Suitability for a given type of corrosion monitoring is driven by the application. For weight-loss coupons, there are two data points, the weight when the coupon went in and the weight when the coupon came out. If the coupon is left in for, say, six months, the only inference one can make is that the weight loss occurred linearly over that time. In reality, however, most of the corrosion may have occurred in the first month due to some process difference, but it would be impossible to tell because you didn’t measure at more intervals than the beginning and end of the test.

Meanwhile, some electronic transducers can produce data in real-time or in near real-time. In that case, process upsets can be easily identified and correlated to other events such as temperature changes, pH, ORP, flow, pressure, etc. When this happens, a better understanding of the root cause of the corrosion phenomena is possible and corrective action can be taken.

Q: What are – and what is the difference between – active corrosion and passive corrosion? How do these different types of corrosion affect different metal types? How can end-users mitigate the negative effects of active and/or passive corrosion?

A: Active corrosion control changes the reaction the corrosive agent has on the surfaces one is looking to protect. Active corrosion control can be implemented by, for instance, using inhibitors or corrosion-resistant alloys.

In passive corrosion control, a mechanical separation is made between the stream and the material to be protected – for instance, a liner or some other type of coating process. These mechanisms don’t change the material property (as in the case of alloying) or the corrosive nature of the stream (inhibitors). In the case of passive corrosion control, the ability to affect corrosion is 100-percent dependent on the ability to isolate the surface from the corrosive medium. Without that, there is no protection afforded.

Q: How has corrosion monitoring and control technology evolved over the past 10-20 years?  How are the corrosion monitors of today more effective than the corrosion monitors of yesterday?

Modern corrosion-detection systems are capable of providing multivariable outputs, including general and local corrosion rates and conductivity, from a single transmitter.

Some 40 years ago, online corrosion-monitoring instrumentation became available, using either electrical resistance (ER) or electrochemical (LPR) measurements. Initially, the devices were fairly simple, limited by the electronic circuitry then available. Over the last 10-20 years, advances in digital electronics, such as embedded microprocessors, have made it possible to refine the measurement circuitry and to employ much more sophisticated measurement techniques for online corrosion monitoring – for example, harmonic distortion analysis (HDA) and electrochemical noise analysis (ECN).

Although largely transparent to the end-user, these techniques result in greatly improved measurement accuracy and reliability, especially in situations where the old simple devices would not work – such as low-conductivity environments or localized corrosion situations.Having a method such as LPR changes the means by which one can collect data to monitor and improve electrochemical corrosion performance in an industrial setting. The near real-time nature of the measurement technique allows corrosion engineers to correlate changes in general or pitting corrosion and immediately understand the cause-and-effect relationship of this very complex process.

Q: In your experience, what are some common causes of corrosion problems in fluid handling systems? How does w corrosion typically become an issue in a fluid handling environment?

A: There are a variety of different causes of corrosion in fluid handling systems.  They are all related to the presence of an oxidizer, such as oxygen, in contact with a metal. Each metal reacts differently to oxidization; there are some that are more susceptible to corrosion in certain streams than others.

Further, one can affect the corrosion properties of a material by changing its physical properties. Bending, rolling and welding, for instance, are all processes that can fundamentally change the properties of a material. As an example, it is not uncommon to find excessive pitting along weld seams in tanks where no pitting, or very little, occurs just a short distance away out of the original heat zone of the weld.Flow can also affect the deposition (and removal) of a natural protective oxide film, on which many materials rely for their corrosion-resistance.

Q: What are some common pitfalls you see end-users encountering when it comes to corrosion monitoring and control strategy? What are some of the mistakes end-users typically make when addressing corrosion in their fluid handling applications?

A: Users tend to become wrapped up in the quantitative information. Corrosion is a very complicated process – all the causes are not necessarily evident as you look at the process occurring.

It is important to understand that outside of metal loss coupons, you don’t necessarily have an absolute value of the corrosion rate. What you do have with the advanced sensor-type corrosion monitoring devices (for instance, LPR) is an instantaneous or almost instantaneous advisory that “something happened” that is different than what has been going on for the last two months, or two days, or two hours. That allows you to look at what happened that might be related. Did the pressure change? pH? Temperature? Did a pump turn on? Did one turn off?

It is often tempting to ignore instances when online corrosion monitoring shows unexpectedly high corrosion rates. However, the corrosion damage is cumulative and can be very costly. A plant operator would not ignore sudden pressure or temperature increases that take the plant beyond its design limits, but might be tempted to overlook short-term corrosion rate spikes.  Unfortunately, the consequences may be just as catastrophic.

Q: On the materials front, how are metals and alloys evolving to offer better resistance to corrosion in fluid handling applications? What sort of advice can you offer end-users in terms of evaluating the cost-benefit of high-end materials for corrosion resistance?  For what sort of applications might a high-end metal/alloy be worth the investment; for what sort of applications might such an investment be ill-advised and/or unnecessary?

A: There is always a trade-off between the cost of the plant material and its longevity. If the expected corrosion rate of the material is known, and if online corrosion monitoring can be used to verify that this is not being exceeded in operation, then a cheaper and less corrosion-resistant material can often be used.

High-end metal/alloys are important when used with certain corrosive media. There are custom-engineered stainless alloys for use with a variety of acids; there are super-alloys for specified situations that are valuable in protecting the integrity of a system. The nature of the importance is driven off the combination of the containment and the corrosive medium, which makes the material decision for you.

With companies paying attention to anything related to reliability and sustainability, especially when it concerns safety and environmental discharges, all of these hot-button topics are affected by corrosion.  To ensure both proper management of assets and sustainability of equipment, processors must know the condition of their piping, tanks, valves, pumps and other assets.

Q: Going forward, how do you see corrosion monitoring and control techniques evolving over the next 10-20 years? How will the corrosion monitoring and control techniques of tomorrow be more effective than those of today?

A: The scientific basis of the currently available corrosion monitoring techniques is well understood, and the prospect of some “magical” new technique being developed is probably unlikely. After all, electrochemistry is quite an old science.  What is likely to happen is that as the computing power available in the corrosion monitoring hardware continues to increase, the reliability of the measurements will continue to improve. And there is still a lot of scope for improvement of the design of the actual corrosion probes themselves.

Hopefully, corrosion monitoring will lose some of its mystery and become just as routine and accepted as, for example, pressure or temperature measurement. 

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