Materials Performance

AUG 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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29 MATERIALS PERFORMANCE: VOL. 57, NO. 8 AUGUST 2018 Cathodic Protection in Arctic Conditions The corrosion sensor system (left) featured two sensor arrays—one for surface water and one for deeper water— that consisted of corrosion sensors, CP sensors, and a reference electrode. The CP sensor (top) was comprised of one CS plate, one anode, and electrical connections for monitoring potential and electrical current. The four sensor arrays included a total of 16 CP sensors. water, was significantly lower and corre- sponded to corrosion rates of <10 μm/y. In the liquid phase, lower temperatures led to lower corrosion rates. However, the tem- perature appears to be a secondary factor when compared to agitation in aerated waters. In the literature, the DO content and flowing conditions are confirmed to be two main factors that affect the corrosion rates of CS. 3 Cathodic Protection Data The polarization curves obtained from exposures in water at Berths A and B were very similar, which provided confidence in the results since the environments at both berths are similar in terms of temperature, pH, conductivity, and tested depths. During the first day of exposure, the initial CD to achieve the protection criterion of −800 mV vs. a saturated calomel electrode (SCE) for both water depths was high, from 390 to 530 mA/m². The high CD required during the first day of exposure is in line with Insti- tut de la Corrosion's experience with sea - water. 4 The initial high CD for surface water was attributed to the water turbulence. Water near the sea floor was expected to be quiescent. The current demand decreased rapidly day after day for both surface and sea floor locations, and then stabilized after 25 days of exposure to ~110 to 130 mA/m² to achieve –800 to –900 mV vs. SCE. The results for stabilized protective current are in line with standard recommendations for Arctic seawaters (i.e., DNV RP401 5 and ISO 15589-2 6 both recommend 120 mA/m² for CP in Arctic seawater). The collected data for the Yamal LNG terminal facilitated an accurate CP design using actual polarization curves. Institut de la Corrosion's general experi- ence has determined a mean CD of 125 mA/m² measured with CP sensors for one year of exposure in Arctic seawater at tem- peratures ranging from –1 to 5 °C. 1 The lower conductivity of the Yamal LNG termi- nal water, compared to seawater, did not significantly affect the CD requirements for CP design. Because these CD measure- ments were performed with a surface salin- ity of ~4 0 /00 and not the lower salinity value for Yamal LNG terminal water, which can be as low as 0.5 0 /00 in summer, salinity should be considered if galvanic anode CP is selected, since it may significantly influ- ence the anode resistance. In addition to exposing CP sensors, the field investigation included the exposure of zinc anodes at open-circuit potential (OCP) for 30 days before the water froze to check their behavior in Yamal LNG terminal's resistive water. The OCP of zinc anodes remained very stable at –1,041 mV vs. SCE, ± 3 mV, for all tested locations. At a conduc- tivity of ~7.5 mS/cm, the zinc anodes showed no sign of passivation, suggesting that this material may be suitable for a gal- vanic protection CP system in this environ- ment. The results from CP sensors indi- cated that anodes are also able to be reactivated when deicing occurs after a complete icing season. Data from the field indicated the water near the sea floor did not show higher sea- sonal resistivity, but surface water can show higher resistivity during the summer. To prevent eventual passivation or under- polarization in resistive seawater, magne- sium anodes should be considered for ini- tial polarization of surfaces in the surface water. The galvanic current between a magnesium anode and a CS coupon was measured at laboratory scale as a function of temperature in simulated Yamal LNG terminal surface water with conductivity of ~7.5 mS/cm. The results indicated that the galvanic current was not significantly influ- enced by a temperature decrease from 20 to 0 °C while the water remains liquid. The galvanic current, however, decreased to very low values when solid ice formed. This means that anodes likely would not be consumed with solid ice

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