Corrosion coupons are one of the most frequently used methods to measure the severity of corrosion in oil and gas pipelines. Typically, operators use weight loss coupons to determine rates of general corrosion and occasionally pitting. Extended analysis (EA) coupons are less frequently used but can provide additional information such as rate of pitting and the corrosion mechanism which are critical when evaluating the need for additional information or considering the most effective course for mitigation. Improper mitigation can result in wasted time, resources, or worsened corrosion rates in some instances; therefore, proper understanding of corrosion rates and mechanisms is a critical step in the corrosion management process. Examples of analysis methods that are frequently applied to EA coupons include optical profilometry, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (XRD), and biological characterization methods such as quantitative polymerase chain reaction (qPCR) and adenosine triphosphate (ATP). This paper discusses these different analysis methods and the significance of the data obtained and presents a case study where a mitigation plan was developed based in part on the active corrosion mechanism identified using EA coupons.
Corrosion management is typically handled as a 4-step cyclical process that involves a threat assessment, identifying appropriate mitigation measures if required, monitoring the effectiveness of applied mitigation, and reviewing the various aspects of the corrosion management program to determine if threats are adequately identified and addressed. See Figure 1.
Corrosion coupons (also called weight loss coupons) have played an important role in the corrosion management programs of most operators in the oil and gas industry. As described in NACE Standard SP0775-20131, a corrosion coupon is a small, pre-weighed strip of metal that is exposed to the environment of interest for a period of time to determine the reaction of the metal to the environment. After removal from exposure, the coupons are cleaned and weighed to determine total mass loss, which is used to calculate the general (uniform) corrosion rate. In some cases, localized corrosion rates are determined by mechanical or optical measurement of the deepest corrosion pit on the coupon. Corrosion coupons are a relatively inexpensive monitoring method and therefore widely-accessible for assessment of the internal environment of a pipeline to determine its corrosivity as well as the severity of the corrosive environment.
