Materials Performance

MAY 2015

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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contact with water; and once the amines are loaded with the absorbed CO 2 gas, they can be very corrosive to the metallurgy of the CO 2 capture plant due to their high oxygen and CO 2 levels as well as impurities such as fly ash, SO x , and NO x . Corrosion mechanisms seen in CO 2 capture plants include general corrosion, stress corrosion cracking (SCC), crevice corrosion, pitting corrosion, and erosion-corrosion. Addi- tionally, corrosion can be exacerbated by oxidative and thermal degradation of the amine solvent, which can vary with a CO 2 capture plant's run time and gas throughput. "Corrosion is a profound safety issue in industry," says Srinivasan. "Most plant accidents happen because there is either inadequate or ineffective corrosion man- agement." He notes that corrosion is a dynamic phenomenon—it changes in real time as the process changes. "When you look at corrosion, it is similar to looking at the changes in blood pressure of a person who is undergoing different activities. The changes in corrosion are a function of a number of process variables [feed rates, temperature excursions, variations in product purity, etc.]." Because of that, he adds, process plants have to be extremely careful when managing and mitigating cor- rosion, especially with acid gases like CO 2 . The way to accurately and meaning- fully track corrosion is with online corro- sion monitoring in real time, Srinivasan says. For decades, however, process corro- sion has been monitored with manual, offline techniques that indicate the pres- ence of corrosion after it has occurred in the process equipment. This, he notes, leads to the accumulation of corrosion that can cause failures and outages, and corrosion management that is reactive rather than proactive. While common cor- rosion monitoring techniques, such as lin- ear polarization resistance (LPR) probes, can be used in online monitoring systems, typically the protocols to link corrosion data to a plant's process control systems have not been sufficient. Now, due to advanced plant process control system technology that is being used to regularly monitor plant process conditions, it is possible to incorporate a relatively new corrosion monitoring approach. Industrial plants have a distrib- uted control system (DCS) where process data are sent and evaluated for production optimization. Online corrosion monitor- ing technology can also send real-time corrosion data to the plant's DCS where they can be regularly monitored along with process control data, making it possi- ble to correlate changes in corrosion rates with process events. This proactive, online approach allows plant operators to deter- mine the presence and cause of corrosion and make process changes before substan- tial corrosion damage occurs. Real-Time, Online Monitoring Technology The online corrosion monitoring tech- nology measures the minuscule electrical current that results from corrosion occur- ring in the system. The technology utilizes a sensor device that comprises a three- electrode corrosion probe and a transmit- ter. The three electrodes are identical and function as the working electrode, refer- ence electrode, and auxiliary/counter elec- trode (which supplies current to the work- ing electrode). Since they are constructed of the same metallurgy as the piping mate- rial being monitored, the electrodes are essentially replicating the corrosion behav- ior of the pipe, says Srinivasan. The trans- mitter measures and analyzes the current flow and sends it to the DCS. By integrating measurements from multiple electrochemical monitoring tech- niques—LPR, harmonic distortion analy- sis (HDA), and electrochemical noise (ECN)—the technology is able to charac- terize the corrosion rate and pitting factor every 30 seconds, and provide four output variables to accurately quantify corrosion that stems from oxidation of the metal. The LPR technique, Srinivasan explains, defines the polarization resistance by applying a potential to the working elec- trode and measuring the current flow between the working electrode and the counter electrode. By finding the polariza- 29 MATERIALS PERFORMANCE MAY 2015 NACE INTERNATIONAL: VOL. 54, NO. 5

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