For liquid transportation pipelines, internal corrosion is rarely the primary failure mode. Corrosion coupons are one of the primary ways to get an indication of how corrosive the internal pipeline environment is and are an essential part of the toolbox that operators use to confirm if internal corrosion is occurring or can be controlled by chemical treatment. The previous version of the SP 0775 document had inadequate guidance on how to prevent oxidation between removal and analysis. Additionally, an informal survey showed that many other practices are currently used by practitioners.
Our aim for this project was to run a suite of exposure tests using different coupon handling techniques and examine the exposed surfaces at various time intervals, which could potentially help identify best practices on sample removal and preservation.
API (American Petroleum Institute) tracks and analyzes the cause of leaks / releases of hydrocarbons based on data reported to PHMSA (Pipeline and Hazardous Materials Safety Administration). One study took a statistical look at the causes of internal corrosion for onshore liquid pipelines.1 In a 10-year period, a total of 503 incidents were responsible for 53,000 barrels spilled and property damage of $188,000,000. Corrosion was reported to be one of the top three causes of hazardous liquid pipeline failures.
Corrosion coupons are the main methodology by which many pipeline companies monitor the corrosivity of the products carried within the pipelines. Sampling is another common methodology for monitoring.
Upcoming changes in US regulations, the "Mega-rule" is expected to increase the usage of coupons as gas pipeline operators are now required to prove either a lack of corrosivity of products, or prove that the corrosion is controlled.
In many companies, the practice is to not dry or remove deposits from coupons in order to be able to analyze any scales or corrosion products present. While investigating some corrosion on coupons, one of the authors noticed that significant changes occurred between coupon removal and analysis. Figure 1 shows a corrosion coupon upon removal from a pipeline (on a Thursday) and then the same coupon the following Monday, four days later. The coupon had been placed directly into a VCI (volatile corrosion inhibitor) impregnated paper bag after removal and left in the bag over the weekend.
