Materials Performance Supplements


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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need for direct contact of the VCI chemical on the metal surface. Th e ef fe ctiven e ss of any c orro sion inhibitor can be evaluated using ER probes and the same is valid for VCI used for soil- side corrosion in ASTs. Since the sensing element of the ER probe is usually made of an alloy with the same grade as the tank bottom plate, it is believed that the data obtained from these probes are more rep- resentative of AST bottom plate corrosion. 2 The probe sensing tip is mild steel and will corrode at a rate similar to the tank floor in that area. Readings of metal loss in mils are obtained from the ER probes with a special meter, which is incorporated into a formula that produces a rate of corrosion in mils per year (mpy). VCI Application Project The refinery experienced its first tank fail- ure in July 2013 when a sour water tank expe- rienced bottom plate perforations due to soil- side corrosion after seven years in service. A magnetic f lux leakage (MFL) survey was carried out for the floor and it revealed more than 40% reduction in a majority of the plates. The most severe corrosion (80 to 100%) with perforations was observed only in the periph- eral sketch plates adjacent to the annular plates (Figure 1). This gives a corrosion rate of almost 1 mm/year in the corner plates and 0.4 mm/year at the other bottom plates. The tank was patch-plate repaired and VCI was injected for soil-side protection along with ER probes that were installed to monitor the efficiency of the chemical and corrosion rates. The injection of VCI was from inside the tank during an off-stream period. There were seven injection ports in different locations of the tank f loor to fog VCI powder, and four points in the ring wall were cored to introduce sleeves for installing ER probes to monitor the effec- tiveness of the VCI slurry (Figure 2). Subsequently, multiple tank failures were obser ved with anoth er five tank s experiencing severe bottom plate under- side corrosion. Similar MFL results as the sour water tanks were revealed. The patch plate repair method was adopted for three tanks, whereas for two tanks rebottoming FIGURE 2 Plan view with ER probes and VCI injection port locations. FIGURE 1 MFL floor scan results for sour water tank. ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? VCI powder inject port, seven places Electrical resistance probes, four places 5 CORTEC SUPPLEMENT TO MP MATERIALS PERFORMANCE JUNE 2018

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