Materials Performance Supplements

CORTEC 2018

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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Online Injection of Vapor Phase Corrosion Inhibitors to Extend Storage Tank Floor Life As A d Al Gh A fri, J A y A nt r n A ir, n A if Al Abri, A nd l A tif A Al s hibli, Oman Oil Refineries & Petroleum Industries Co. (ORPIC), Suhar, Oman The API 650 oil storage tanks in a refin- ery have experienced accelerated cor- rosion from the soil side at the rate of 1 to 2 mm/year, primarily due to ineffi- ciency of the impressed current ca- thodic protection system and underde- posit corrosion caused by the presence of corrosive bacteria in the soil. This caused failure of several tank bottom plates within eight years of commission- ing, resulting in loss of primary contain- ment. With no other means of corrosion control from the soil side, the priority was to apply a short-term stratagy by which the corrosion rate is substantially slowed and enable the refinery to dis- tribute these tank turnarounds across as many years as possible. If this is not achieved, a large number of tanks might be expected to perforate in any given year, overwhelming refinery mainte- nance and disrupting operations. The short-term strategy adopted with best likelihood of success and minimal dis- ruption to maintenance and operations was to drill under the tanks and inject vapor phase corrosion inhibitors. The primary objective was to increase the reliability by extending the floor life and meeting the agreed tank out-of-service schedule. The soil-side corrosion of aboveground storage tanks (ASTs) is a chronic prob- lem in the oil and gas industr y, not only in the Middle East but also in the world at large. 1 Corrosion of the underside of the tank bottom plates leads to metal loss and perforations, resulting in loss of primary containment. The results of such failures are dramatic on multiple fronts: loss of product, costly repairs, and environmental impact and safety concerns in the case of flammable fluids. Tr a d i t i o n a l l y, i m p r e s s e d c u r r e n t cathodic protection (ICCP) has been used for corrosion management of tank bottoms. However, experience has shown that CP fails to provide adequate protection in some types of AST construction. Suhar Refinery is part of Oman Oil Refin- eries and Petroleum Industries Co. (ORPIC). It was commissioned in 2006 and has 48 ASTs for various refiner y products. The refinery tanks have experienced severe cor- rosion at a very high rate from the underside (soil) side of the bottom plates and resulted in failures of more than six tanks within 10 years of commissioning (2006), with the first failure in 2013. Tanks floors are expected to provide service life of over 30 years. From a root cause analysis study, it was concluded that the tanks were corroding at an unacceptably high rate and the corrosion was specifically related to the inadequate strategy of CP design, which completely failed in its objective of corrosion control combined with a corrosive soil. The priority was to apply a short-term strategy by which the corrosion rate is sub- stantially slowed and enable the refinery to distribute these tank turnarounds across as many years as possible. Otherwise, a large number of tanks might be expected to perfo- rate in any given year, overwhelming mainte- nance and disrupting operations. The immediate strategy with the best likelihood of success and minimal disrup- tion to maintenance and operations was to drill under the tanks and inject vapor phase corrosion inhibitors (VCIs). These inhibitors have been increasingly used in this particular application for the last 15 years with proven results. The inhibitors can help control the corrosion and prevent any further corrosion by forming a layer on the metal surface and restricting the corrosion reactions. This article demonstrates the concept of using an amine carboxylate-based VCI slurry, as an online soil-side corrosion control method for in-service refinery storage tanks coupled with electrical resistance (ER) corro- sion probe monitoring, and exemplify the dif- ference in cost by using a VCI and other main- tenance options such as patch plate repair or jacking up and rebottoming the ASTs. Fundamentals of VCI Technology in ASTs Amin e carb oxy l at e - b a s ed V C Is are used beneath ASTs. They are effective for prevention of metal corrosion when a VCI product is released within the interstitial space between the bottom plate and sand pad. The mechanism for corrosion control is the formation of a monomolecular layer throughout the soil-side surface of the tank f loor. VCI molecules adsorb on the steel surface to suppress both metal dis- solution and the reduction reaction (both the anodic and cathodic processes). This adsorption is accomplished without the VAPOR PHASE CORROSION INHIBITORS 4 JUNE 2018 MATERIALS PERFORMANCE CORTEC SUPPLEMENT TO MP

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