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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28 AUGUST 2018 W W W.MATERIALSPERFORMANCE.COM FEATURE ARTICLE The modified corrosion sensor was comprised of a thin metallic film track and a reference track for differential measurement of electrical resistance. It was connected to the logger with a 20-m long cable. The Corrosion Study To assess and compare corrosion rates through short-term testing, a commercial electrical resistance (ER) sensor, Aircorr O † , developed in collaboration with the Institut de la Corrosion (Brest, France), was adapted to facilitate field measurements while completely immersed. The ER sensor used a differential measurement between a sensitive track exposed to the environment (corrosion and temperature) and a refer- ence track protected from corrosion (tem- perature only). The sensor was attached to a logger using a 20-m long cable. The sens- ing element was a 25-μm thick AISI 1010 (UNS G10100) CS track with a surface area of 100 mm², which allowed sensitive and real-time corrosion measurements. The ER method measures the electrical resistance of a thin metallic film as it is affected by corrosion. Resistivity is depen- dent on the material and the temperature. When the thickness of the film decreases during the corrosion process, the resistance increases. The main advantage of this method is the real-time monitoring of cor- rosion, which obtains relevant data in a short time and in a changing environment. ER sensors were deployed along Berths A and B. To get comparative data and qual- ify the experimental procedure, similar sen- sors were also used in simulated labora- tory-scale environments. Because the short-term exposure of ER sensors only obtains initial corrosion data, typically the results are correlated with mass loss mea- surements on a CS coupon exposed over a longer time period. For this study, exposure of larger CS specimens (150 by 100 mm) was planned for 12 months at two depths along three selected berths. All coupons were weighed before exposure, and mass loss measurements will be performed after exposure to assess the actual corrosion rates without CP. At each site's two expo- sure depths, five steel coupons were deployed along a nylon rope. Samples will have to be recovered by local staff, cleaned, dried, and sent back to the Institut de la Corrosion for mass loss measurements and corrosion rate calculations. At time of pub- lication, the corrosion coupons were not yet recovered so the results cannot be presented. The Cathodic Protection Study Regarding the terminal's specific envi- ronment in terms of salinity and icing/deic- ing conditions, the literature and data from available standards could not be used to define an adapted CP design, so an in situ polarization study of CS coupons immersed in water at the Yamal LNG terminal was done. This technique, using sensor arrays to obtain in situ polarization data, has been used successfully several times by Institut de la Corrosion. 1-2 To facilitate potential and related cur- rent measurements, a set of coupons ( four coupons per set with different galvanic materials and resistors) was deployed at four defined locations at the two berth sites and two depths per site. By using several coupons exposed at several potentials, it was possible to acquire field and pseudo- polarization curves in the tested media. At the same time, the potentials of zinc anodes were measured on-site to check their ability to avoid passivation in this spe- cific resistive water. The anodes' ability to reactivate and deliver protection current after an icing period also was studied. The CP measurements were performed during one month of immersion before icing sea- son occurred, which was enough to reach stabilized and representative values. The Results During the field investigations, the water depth, temperature, DO, pH and con- ductivity were measured in different loca- tions along the two selected Yamal LNG ter- minal berths. For both sites, the distance from the pontoon to water level was ~3 m, and the water depth ranged between 8 to 12 m. The temperature, conductivity, pH, and DO measurements were performed in the two first meters of water, which is con- sidered the critical zone in terms of CP. For technical and security reasons, it was not possible to perform deeper measurements (where higher salinity was expected). Results for both sites indicate specific con- ductivity of ~7.5 mS/cm, a temperature of ~2 °C, ~12 ppm of DO, and pH of 7.6. Corrosivity without CP The initial CS corrosion was assessed with corrosion and CP sensors. During the field measurements, the temperature was ~2 °C, the specific conductivity was ~7.5 mS/cm (salinity of ~4 0 /00 [4,000 ppm]), and DO was ~12 ppm. Results were obtained after a series of six-day exposures with three replicates to get averaged values. The highest corrosion rate measured in the surface water after six days was ~6 μm, with similar corrosion rates at both Berths A and B. This would correspond to an ini- tial corrosion rate of ~370 μm/y. The rather high initial corrosion can be attributed to the agitated condition of the surface water, which induced high flow rates on the sur- face of the coupons and higher convection of DO. Closer to the sea floor, the initial corro- sion at Berths A and B after six days of exposure ranged from 3 to 4 μm (an initial corrosion rate of ~215 μm/y). The lower corrosion rate, as compared to the surface water, was attributed to less agitation closer to the sea floor. The corrosion in ice, determined by laboratory measurements in water extracted from Yamal LNG and simulated † Trade name.

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