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Author : Tim Stevenson, Ionix Advanced Technologies Heavy Lift Vessel
(Click here to view article in digital edition)
Sulfidic corrosion of steel components in downstream refining assets is a ubiquitous damage mechanism occurring in sulfur containing crudes when heated between 230°C and 425°,1 which is difficult to predict, costly to repair and has caused catastrophic ruptures in piping, tubing, pressure vessels and other refinery assets. Recent updates to recommended practices, require 100% inspection of components at risk, and those showing signs of accelerated corrosion necessitate monitoring. Recent advances in intrinsically safe ultrasonic testing (UT) technology based on new piezoelectric materials now allows for the inspection and continuous monitoring to be undertaken whilst components are on-stream, at temperature, and in potentially explosive atmospheres that refineries present, to obtain high-resolution wall thickness data and asset integrity intelligence.
Corrosion in refineries and petrochemical plants can be split into low- and high-temperature corrosion mechanisms around a critical temperature of ~260°C (500°F).2,3 Sulfur compounds in hydrocarbon products can react at high-temperatures above this critical point with low-alloy, carbon and 300- & 400-series stainless steels.
High temperature sulfidation (sometimes termed sulfidic) corrosion is the most prevalent corrosion mechanism in the oil refining industry as nearly all crude oil feeds contain naturally occurring sulfur compounds, such as H2S and other reactive species. This makes sulfidation corrosion ubiquitous in all crude processing refineries, affecting large sections of the plant, and is of increased concern when processing opportunity crudes with increased TAN and sulfur content.1
Sulfidation in iron-based alloys, the materials of construction for refinery components, often appears as uniform thinning (see Figure 1). This is particularly dangerous, as it reduces the pressure containment envelope, affecting components such as piping, pipe fittings and pressure vessels including, heaters, boilers, cokers, distillation units and fluid catalytic crackers (FCC)3,4 and can lead to rupture failure rather than leaks from defects like pin-holes. Most industry incidents have occurred in piping, due to their lower nominal wall thicknesses compared to other components.5 Where the damage has caused subsequent ruptures, it has led to the catastrophic release of large quantities of hydrocarbon streams5 making the detection of sulfidation a key to operating refinery assets safely.
Current best practices for inspection and detection
The rate and prevalence of sulfidation corrosion is dependent on factors such as materials of construction, sulfur concentration and species in the hydrocarbon. The presence of hydrogen, increasing temperature and increased flow rate further accelerates the sulfidation, making it hard to predict accurately where and the rate at which it will occur. However, one predictor of higher sulfidation rates is low silicon content in carbon steel components.
Figure 1 – High-temperature sulfidation corrosion of 12CrMo steel pipe (Photo by CEphoto, Uwe Aranas)
This has led to manufacturing standards for high-temperature piping to specify a minimum silicon content.6 As 95% of refineries in the US alone were built before these standards were introduced and as a result of refinery failure investigations,5 the recommended practices from bodies such as the American Petroleum Institute (API) now require either identification of carbon steel hydrocarbon circuits that may contain low-silicon content or the replacement of carbon steel for costly alloy steels with increased resilience to sulfidation.7
The determination of silicon content in existing carbon steel in the field is destructive, laboratory-based and difficult. In response to this, non-destructive testing (NDT) is recommended to be performed to determine the remaining wall thickness of these components in contact with high-temperature hydrocarbons and to locate areas of concern.
Updated (2020) API guidelines for avoiding sulfidic corrosion failures in refineries states that any circuits identified as susceptible to sulfidation corrosion now require 100% component inspection. Remaining wall thickness is to be determined by external ultrasonic thickness measurements and/or profile radiography, and where accelerated corrosion is found, should now be designated as a continuous monitoring location (CML) until the components can be replaced.3
Historically, NDT techniques Computed Radiography (CR) and Ultrasonic Testing (UT) have been implemented by asset integrity or inspection service companies to screen for accelerated sulfidation corrosion on carbon steel piping in line with the relevant codes and standards.
The limitations in high equipment costs, portability, exclusion zones for radiation and isolation of process fluids for safety and contrast (or reduced pipe diameters) in CR has seen its use limited as a screening technique. In contrast, UT is a low-cost, high-resolution technique, and can be scanned using automated robotic platforms (AUT) but due to its requirement to be in close contact to the asset under test, means it has typically been restricted to shutdowns or planned maintenance schedules when the components are cool and the presence of hazardous atmospheres removed.
Figure 2 – (left) Automated high-temperature ultrasonic testing using an Ionix HS582i transducer in the EddyFi RMS scanner being used to determine remaining wall thickness in a carbon steel pipe susceptible to sulfidation corrosion at 270°C. (right) An ultrasonic A-scan (left inset) and C-scan (top right inset) generated by the automated scanner determining the remaining wall thickness of the internal diameter.
Until recently, UT has been limited to temperatures <200°C by the transducers, below the threshold for sulfidation corrosion susceptibility. However, advancements in new high-temperature piezoelectric ceramics, the electro-mechanical heart of an ultrasonic transducer,8,9 and acoustic materials technologies have seen a revolution in corrosion detection and monitoring.10
By example, new commercial extreme environment UT platforms can now offer high-temperature operation, allowing on-stream remaining wall thickness measurements up to 550°C without the need for cooling or duty cycling. These transducers are based on new piezoelectric technology9 and allow continuous measurements or scanning (with compatible magnetic crawlers or robotic scanners) for full coverage of refinery assets with in-depth inspections to determine the presence, location, and extent of any existing sulfidation corrosion damage (Figure 2). Enabling operators and service providers to undertake 100% inspection at on-stream temperatures, allows sulfidation detection at a fraction of the cost and time by negating the need for shutdown or isolation, and without impacting productivity. It can be started immediately by any asset integrity service provider competent in basic UT techniques, to identify areas of accelerated corrosion, or any other damage mechanism detectable by ultrasound, and can often be undertaken when the presence of explosive atmospheres can be temporarily confirmed not to be present by local gas monitoring.
The locating of high-risk corrosion areas is a key step in the determination of CMLs under new guidance, to subsequently monitor wall loss. With installed ultrasonic transducers, high-accuracy trending of corrosion rate can is invaluable to asset and process intelligence that can be used to inform risk-based inspection (RBI) strategies, operating efficiency and fitness for service assessments to defer or schedule appropriate maintenance.
To achieve on-stream monitoring of sulfidation, the same high-temperature piezo-technology that has been implemented in UT tools for inspection, has also been used to offer ultrasonic corrosion monitoring transducers.8,11 Critically however, these monitoring transducers (Figure 3) which can be installed under insulation to maintain weatherproofing integrity and other confined space locations, require hazardous location certification to allow them to be permanently installed in potentially explosive refinery atmospheres.
Conventional polycrystalline piezoelectric materials with high sensitivity, such as lead zirconate titanate (PZT), have a de-poling temperature around 200°C, with higher-temperature materials often being synthesised as single crystals (e.g. lithium niobate) or thin films (e.g. zinc oxide) which are relatively fragile, expensive to manufacture and/or orders of magnitude lower in activity, requiring high voltages to operate and susceptible to noise. New high-temperature, high-sensitivity polycrystalline materials9 however now offer operating voltages <5 V with excellent signal to noise characteristics, and reduced, or tailored capacitance characteristics to offer intrinsically safe transducers that can safely operate continuously in these hazardous refinery applications.
The thickness monitoring sensors are easily integrated into commercial intrinsically safe electronics, and can relay wall thickness data, in real-time, in-service, remotely (via wireless or cellular communication) into existing servers, distributed control system (DCS) or other data historians. With fixed installations, removing variables such as user, couplant, location and hardware differences has allowed very high precision measurements.12 More recently, Figure 3 shows HotSense continuous monitoring transducers which also feature integrated calibration blocks which offer better temperature compensation and increased accuracy to be able to plot absolute wall thickness to 20 µm, and plot wall loss trending to better than 2 mpy creating valuable insights in to process conditions, not just asset integrity.
Figure 3 (left) – A photograph of four high-temperature capable, intrinsically safe, ultrasonic wall-thickness monitoring transducers with integrated calibration block, installed on a refinery crude distillation unit (CDU), 8” diameter carbon steel outlet pipe operating at 350°C continuously, under insulation (removed for photo) (Photo © Ionix Advanced Technologies 2020).
The introduction of new materials technology is addressing the challenges of sulfidation corrosion, an insidious damage mechanism that has increased in prevalence with increased processing of opportunity crude slates, and implementing associated updated recommended practices and standards for its detection and mitigation. In the current market climate where downstream operations are under ever increasing pressure to maintain productivity, reduce shutdowns and defer maintenance whilst operating with fewer personnel on-site, the growing use of high-temperature capable ultrasonic testing tools to offer increased asset intelligence is fast becoming an important strategy for refining operators and their management of corrosion.
When it can be applied effectively, using on-stream UT inspection tools to determine installation locations, it can provide major benefits in terms of increased process intelligence, risk minimisation, productivity, and profitability enhancement, and of course safety improvements.
The adoption of ultrasonics for this role carries the advantages of being built on mature technology, available at low cost, and with new piezo-materials has been successfully certified intrinsically safe removing the barriers for full scale adoption.
[1] R. B. Rebak, “Sulfidic corrosion in refineries - A review,” Corros. Rev., vol. 29, no. 3–4, pp. 123–133, 2011.
[2] Y. T. Al-janabi, “An Overview of Corrosion in Oil and Gas Industry: Upstream, Midstream and Downstream Sectors,” Corrossion Inhib. Oil Gas Ind., pp. 1–39, 2020.
[3] American Petroleum Institute (API), RP 571 Damage mechanisms affecting fixed equipment in the refining industry, no. 3. 2020.
[4] Inspectioneering, “The Fundamentals of Sulfidation Corrosion,” Inspectioneering. [Online]. Available: https://inspectioneering.com/airs/2017-02-13/6123/a-primer-on-sulfidation-corrosion. [Accessed: 02-Aug-2020].
[5] 2012-03-I-CA, “FINAL INVESTIGATION REPORT CHEVRON RICHMOND REFINERY #4 CRUDE UNIT,” 2015.
[6] ASTM, ASTM Standard A106/A106M Standard Specification for Seamless Carbon Steel Pipe for High-Temperature Service. 2011.
[7] American Petroleum Institute (API), RP 939-C Guidelines for Avoiding Sulfidation (Sulfidic) Corrosion Failures in Oil Refineries. 2019.
[8] T. Stevenson, “High-Temperature Piezoelectric Sensors for the Energy Industry,” Eng. Technol. Ref., vol. 1, no. 1, Jan. 2017.
[9] T. Stevenson et al., “Piezoelectric materials for high temperature transducers and actuators,” J. Mater. Sci. Mater. Electron., vol. 26, no. 12, pp. 9256–67, Aug. 2015.
[10] T. Stevenson, “FROM INSPECTION TO MONITORING,” Hydrocarbon Engineering, no. August, p. 4, Aug-2018.
[11] W. Vickers, “Automated corrosion monitoring,” Hydrocarbon Engineering, 2020.
[12] S. Rex, “Continuous Thickness Monitoring: Precise Corrosion Rates in Less Time,” Inspectioneering, vol. 26, no. 3, pp. 52–54, 2020.
Tim Stevenson is Development Director of Ionix Advanced Technologies. Tim holds a Ph.D in Materials Science, and is a Fellow of the Institute of Materials Minerals and Mining. He has been an expert in high-temperature piezoelectrics for in-service ultrasonic asset-integrity and process control sensors and systems for over a decade.
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