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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tion resistance, which is inversely propor- tional to the measured corrosion current, he adds, the technique can determine the overall rate of metal loss (corrosion rate). Because each corroding system has its own unique behaviors (signatures), the HDA technique is used to determine varia- tions in the current, known as the harmon- ics, to find the localized Stern Geary constant (B value). The B value represents a system's "constant," which is determined by the mechanism and kinetics of the sys- tem's corrosion process. The B value can be different for each corroding system depending on the characteristics of the system and the type of corrosion activity (i.e., the anodic and cathodic currents and potentials, which can vary quite a bit from system to system). However, the LPR tech- nique typically uses a default B value to calculate the corrosion rate, which can lead to significant errors when computing the corrosion rate. By using a measured B value, a more accurate corrosion rate can be determined. The corrosion process also generates ECN, which is the low-amplitude (<1 mV), low-frequency fluctuation of corrosion current and potential. The ECN technique measures these low-level fluctuations between working and counter electrodes, which can determine the type and speed of corrosion. Based on the magnitude of the variations in the corrosion current (intrin- sic current noise), Srinivasan comments, the ECN measurement can establish whether the corrosion is likely to be uni- form or localized and be used to determine the pitting factor, which indicates the probability of the corrosion mechanism generating localized corrosion over time. Typically, general corrosion processes have low levels of ECN, but the onset of localized corrosion (e.g., pitting) leads to increasingly higher levels of ECN. The last output delivered by the elec- trochemical monitoring techniques is the corrosion mechanism indicator (CMI), which shows the presence and likely effects of surface films. This is determined by the characteristics of the interface between the metal and the environment (electrolyte) and its influence on the behavior of the corrosion current. The four output variables from the monitoring device are calculated continu- ously at time intervals determined by the plant operator and immediately commu- nicated to the plant DCS via hardwire or wireless channels, or stored remotely for later download. Since the reported corro- sion data represent real-time measure- ments, Srinivasan notes, the plant operators are able to correlate corrosion events with specific process changes (tem- perature, flow rate, injection of neutraliz- ers or catalysts, etc.) and determine potential cause-effect relationships. Corrosion Monitoring in a CCS Pilot Plant To investigate the corrosion process at the post-combustion CCS pilot plant at the Maasvlakte coal-fired power plant in Rot- terdam, The Netherlands, the advanced, real-time online corrosion monitoring technology was implemented in the unit's CO 2 capture process. The objective of the study was to verify the interrelationship between amine solvent degradation, ammonia (NH 3 ) emissions from oxidative solvent degradation, and corrosion, as well as validate the accuracy of the online cor- rosion monitoring system in terms of reporting the correct corrosion rate. The study was conducted over two monitoring campaigns during a two-year period, with each campaign running about five to six months. Corrosion coupons were also installed parallel to the online corrosion monitoring device to provide baseline cor- rosion data for validating results from the online corrosion monitoring system. A review of the monitoring approach and results of the case study were presented during the CORROSION 2015 symposium, "Corrosion Monitoring Technologies: Past Present and the Future," sponsored by TEG 100X. The CCS pilot plant components are mainly comprised of 304L (UNS S30403) and 316L (UNS S31603) austenitic stain- less steel (SS) to minimize general corro- sion. For both monitoring campaigns, 30 wt% MEA was used as the CO 2 capture solvent. Based on an evaluation of the plant's CO 2 capture process, the online corrosion monitoring device was placed in an area suspected to be particularly sus- ceptible to corrosion—between the lean solvent pump and the amine stripper. In this portion of the process, changes in amine composition, as well as degradation products and impurities, operating tem- perature, local flow rates, and localized turbulence corrosion, often affect the cor- rosion rate. "Corrosion behavior is a func- The new Maasvlakte coal-fired power station, with an installed capacity of 1,100 MW, is owned by E.ON (Düsseldorf, Germany). Wikipedia photo. The Maasvlakte CO 2 capture plant with major components identified. FEATURE ARTICLE 30 MAY 2015 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 54, NO. 5

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