By Russell Kane and Phillip Ng Chemical processing plants are changing the way they approach corrosion. It’s no longer an unavoidable evil. Online, real-time monitoring systems are being used to find and remove the root causes of corrosion.
| | Implementation of an online, real-time corrosion monitoring system has allowed the BASF chemical plant in Freeport, TX, to reduce costs due to corrosion and improve plant reliability.
‘It’s a change that potentially can save up to $10 billion annually in corrosion costs on a global basis.’ During the past 10 years, the process control industry has been blazing new trails to increase productivity and reduce costs. There remains, however, one untapped area with great potential for productivity improvement and cost containment. It is corrosion control.There are several reasons this area has been ignored. One involves the notion that replacing plant assets damaged by corrosion is a cost of doing business. Another assumption is that living with corrosion problems is the only option. New technology for monitoring corrosion is changing these beliefs and giving plant operators the capability to proactively identify and solve corrosion problems in real-time. It’s a change that potentially can save up to $10 billion annually in corrosion costs on a global basis. Understanding Cost of Corrosion Corrosion has been considered a phenomenon that occurs over long periods of time, usually seen after damage is done. As a result, corrosion detection and measurement have not been online functions in most chemical plants. Corrosion measurement is usually performed offline over a period of months or even years. The most common techniques are visual inspections during plant turnarounds and periodic inspections using radiography and ultrasonic testing of plant piping and components. In some locations, corrosion measurements are made with corrosion coupons (metal specimens) that are exposed to the process environment. Coupons are periodically removed and manually cleaned, weighed, visually inspected, and analyzed for corrosion rate and the presence of localized corrosion, often referred to as pitting. The above methods are examples of work processes and analytical efforts that identify the rate and modality of accumulated corrosion over long periods, but they do not get to the root cause of corrosion and solve the problem before damage accumulates. No offline data can determine the real process conditions that cause corrosion. In some cases, electric resistance (ER) and linear polarization resistance (LPR) techniques are employed to measure corrosion in hours or days, using probes in place of corrosion coupons. ER and LPR procedures can provide quicker semi-quantitative measurement of corrosion so that trends can be observed in a shorter time frame. However, the instrumentation for these devices is limited in onboard analysis capabilities, which restrict their usefulness. In other words, they cannot differentiate between general (uniform) corrosion and localized (pitting) corrosion. But, differentiation is needed because the latter form of corrosion is associated with unexpected and catastrophic failures 70 to 90 percent of the time. Therefore, controlling pitting corrosion will make a substantial difference and bring more value and potential cost reduction to plant operations. An added problem with conventional corrosion monitoring is that corrosion data are usually logged remotely and manually gathered offline by corrosion or maintenance personnel. This makes corrosion monitoring slow, labor intensive, and costly. It is also isolated from other process plant data. It’s the job of the corrosion engineer to obtain the process information, if available, and manually make the correlations. Additionally, once the corrosion data are evaluated, the findings are usually presented in a report to the process engineer or manager weeks or months after the damage has occurred. Thus, conventional corrosion monitoring data only bring limited value to the process control environment. One realization that businesses have come to understand is that receiving timely data has value so that action can be taken. Offline corrosion data, as described in the above scenario, cannot be used in a real-time plant production run, which demonstrates the limited value of offline data. So what is the solution? New Online Variable Over the past several years, substantial development has taken place in the area of corrosion measurement systems. Smarter data storage and processing capabilities have made it possible to have sophisticated techniques in remote plant instruments that previously only were available in the corrosion laboratory. This technological development has brought greater accuracy to corrosion rate measurements and the ability to differentiate different types of corrosion. It also has resulted in a more rapid measurement cycle, enabling integration of corrosion as a process variable within the plant distributed control system (DCS). With these changes, the corrosion signal can be taken directly to the plant DCS and viewed along with process information. The simplicity of this concept might cause the significance of this change to be overlooked by DCS users since they routinely bring new data into the DCS. But, it is a major accomplishment to bring corrosion into the online, real-time process control environment because process or corrosion engineers can examine the root causes of conditions that cause corrosion. They also can take ownership for both maximizing productivity while preserving valuable plant equipment. Offline techniques do not offer these capabilities. Online, real-time corrosion monitoring provides the actual variability of corrosion due to process conditions. Its measurement capabilities approach corrosion as a real-time, online variable with a cycle time of minutes instead of weeks, months, or years. Process engineers can now see corrosion rates as much more dynamic, sometimes varying over one or two orders of magnitude in a matter of minutes, hours, or days. In fact, with this new technology, it is possible to observe corrosion events and take proactive intervention before corrosion damage has a chance to accumulate and cause a problem. This is particularly important when dealing with pitting corrosion because the rate of pitting corrosion is typically 10 or more times that of uniform corrosion. Plant equipment is primarily made from stainless steels and other stainless and corrosion-resistant alloys. Stainless alloys depend on a protective oxide layer on the metal surface to maintain passivity and immunity to corrosion. The surface layer can be affected by changes in process pH, aeration, halide concentrations, feedstock impurities, flow, entrained particles, and other operating or upset conditions. Such situations can initiate localized damage of the protective surface layer and lead to the onset of pitting corrosion. Consequently, the predominant failure mode of stainless alloys is pitting corrosion, which is not readily monitored using conventional corrosion devices. Coupling corrosion data with process data creates a better working relationship between corrosion and process engineers. Pitting corrosion may be understood by corrosion engineers but not necessarily by process operators and engineers. Including corrosion as an online process variable makes plant personnel aware of the process conditions that can initiate pitting corrosion. Examples of such conditions include unintentional aeration by the venting of equipment to the atmosphere, the addition of oxidizing agents and aggressive catalysts, the lack of dew point control in normally dehydrated systems, and excessively high velocities to increase unit productivity. Online corrosion detection in a process control environment allows plant operators to correlate highly corrosive conditions with process conditions so that they can reduce corrosion costs. Direct-to-Plant DCS A limited number of new plant corrosion measurement devices have both high resolution and shorter data cycle times. They generally employ electrochemical techniques that are very sensitive to changes at the metal/environment interface, affecting the charge transfer across the surface. Measurements may take only a few minutes. New corrosion field transmitters can offer a combination of corrosion monitoring capabilities using a suite of electrochemical techniques (e.g. linear polarization, harmonic distortion, and electrochemical noise) running simultaneously and remotely in the transmitter that are applied continuously in an automated data acquisition and analysis cycle. They provide the foundation for a total plant corrosion solution, supplying both general and pitting corrosion data, online, real-time, direct to the DCS, and enabling transfer of corrosion information into the process knowledge system. Integration of corrosion data into a process knowledge system synchronizes people with process, business, and asset management. It allows knowledge to flow from the field to the control room and the boardroom or wherever and whenever it’s needed. Online, real-time corrosion data give plant operators the ability to improve equipment reliability, availability, and integrity. Also, with access to current, actionable process variable information, they can have a better, quicker, more accurate measurement of corrosion rate and, more important, an assessment of pitting corrosion. The Numbers Behind Corrosion A federally funded study analyzing all major sectors of the economy puts the direct cost of corrosion at about $300 billion annually in the U.S., which is about 4 percent of the GDP. The study also found that $1.807 billion is spent annually on corrosion control and asset damage in just the chemical industry. This figure doesn’t include the costs associated with catastrophic outages and accumulated lost production. When the petrochemical and pharmaceutical sectors are added into the equation, the annual cost of corrosion increases to about $2.5 billion. In other words, the annual cost of corrosion in the process industries is more than 10 percent of annual plant capital expenditures. Looking globally, the numbers are even more astounding. The cost of corrosion is estimated at about $50 billion per year in the global process industries. Comparing Corrosion Failure Modes According to a study by the Materials Technology Institute of the Chemical Process Industry, localized (pitting) corrosion and general (uniform) corrosion account for an equal number of chemical plant failures. However, the impact of pitting corrosion jumps to nearly 60 percent of corrosion failures if stress cracking, which usually initiates with pitting, is included. The study, which examined various modes of corrosion failure at five companies in the chemical process industry, compared data involving more than 1,200 failures. Common Causes of Pitting Changes in the following conditions can initiate pitting in the protective surface layer of stainless alloys found in chemical plants. Process pH Aeration Halide concentrations Feedstock impurities Flow Entrained particles Case in Point: Corrosion Monitoring at BASF An example of a chemical facility that has adopted online, real-time corrosion monitoring is the BASF plant in Freeport, TX. It runs a predominantly organic stream and has carbon steel, 304L, and 316L in its construction. When BASF made the change, it began using the SmartCET corrosion field transmitter to examine real-time corrosion behavior. As a result, its process engineers, plant operators, and materials engineers were able to see changes caused by specific process parameters. Within a matter of weeks, process adjustments were identified that reduced corrosion rates while maintaining acceptable product yields and quality. The online, real-time corrosion monitoring system used flange-style probes to allow monitoring around the complete circumference of pipes. These probes consisted of three-ring sensor elements separated by electrical insulators sandwiched between a standard ANSI flange. The sensor elements matched the inside bore. An advantage of this probe design is it can be inserted into actual process piping, thus making any flange location a monitoring point with little or no in-plant fabrication or modification. About the Authors: Dr. Russell Kane, an internationally recognized expert in corrosion evaluation and modeling, is the director of corrosion services at Honeywell Process Solutions, which offers a corrosion field transmitter called SmartCET that provides the foundation for a total plant corrosion solution, supplying corrosion data online, real-time to the distributed control system, and enabling transfer of corrosion information into the process knowledge system. Kane received NACE’s A.B. Campbell and Technical Achievement Awards and ASTM’s Sam Tour Award for distinguished contributions to corrosion research, development, and evaluation. His doctorate is in metallurgy and materials science at Case Western Reserve University. Phillip Ng is the senior product manager for corrosion at Honeywell Process Solutions. He has been involved in the process control industry for 18 years and has held a variety of positions including advanced control engineering. His bachelor’s and master’s degrees in chemical engineering are from the University of Utah. Questions about this article can be addressed to Kane at 281-444-2282, ext. 32. Honeywell Process Solutions is headquartered at 2500 W. Union Hills Dr., Phoenix, AZ 85027. Additional information is available at www.honeywell.com.