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Repeated cyclic potentiodynamic polarization (CPP) measurements were performed in different environments for both carbon steel and Type 304L stainless steel. The effect of different environmental species, nitrite and hydroxide on repassivation/reactivation mechanism of carbon steel is examined.
Metallic coupons are generally used close to buried pipes under cathodic protection in order to evaluate potential and current demand at coating defects. This technique assumes that a metallic coupon is representative of a coating defect. However considering the inhomogeneity of soils and their potential high resistivity the coupon design and the measurement techniques must be carefully studied and understood to get reliable results. For this purpose specific on-shore coupons have been designed allowing adapted measurement at direct proximity of the virtual coating defect. These coupons are used in field soil for cathodic protection survey of buried pipes. However the response depends on many parameters which are difficult to measure and control in the field (actual soil composition humidity soil compaction at coupon location ageing of the coupon etc.). In a former program laboratory soil exposure cells have been developed and used at the Institut de la Corrosion to study soil corrosion of different materials. These cells allow controlling physicochemical parameters of the tested soils which are considered to potentially affect corrosion of materials (texture humidity salt composition pH etc.) and results were in very good line with field results. In the present study these cells were adapted to study and characterize cathodic protection on the specific on-shore coupons. The main objective of the study was to better understand the polarization (polarization curves) of the used coupons as a function of soil nature and humidity. This paper presents the experimental technique that was developed to get exploitable polarization curves of carbon steel coupons in soils. It provides the polarization curves at different levels of humidity for two different types of soil. A rather good reproducibility was found which allowed studying the influence of significant parameters affecting cathodic protection in soils.
Key words: downloadable, pit repassivation, pit reactivation, carbon steel, nuclear waste, repeated cyclic potentiodynamic polarization
This paper will describe a multi-channel corrosion and stray-current monitoring system and the design consideration/philosophy used when determining the measurement locations throughout the system and the challenges presented and overcome. Follows NACE Corrosion 2012 (paper C2012-01727).
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