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Galvanic corrosion is one of the most common forms of corrosion found in oil industry facilities and in the transportation of liquids generally. This is due to the fact that the transportation equipment comprises a wide variety of fittings, which are made from a range of materials that create galvanic corrosion cells when placed in contact with each other. Galvanic series have been considered a source of information for predicting the behavior of galvanic pairs between different kinds of noble or active metals.
However, there are metals or metal alloys that, despite having a noble electrochemical potential, present very low levels of corrosion currents when paired with another metal, which makes for a galvanic pair that is non-aggressive to the most active metal.
The objective of the study was to analyze the galvanic pairs formed with the Nickel-Cobalt-Boron alloy when used as a coating for steel (also known by the brand DOX steel™) in contact with conventional steels, such as stainless and B7 steel, and steels with protective coatings such as cadmium-plated and galvanized steel.
The evaluation was carried out using three well-established techniques: potential sweeping, polarization curves and monitoring of galvanic potential and galvanic current using a zero-resistance ammeter.
The results obtained show that DOX steel does not create an aggressive galvanic pair since the current output that develops is very low when compared with other metals, allowing it to be successfully used as a long-term anti-corrosion coating. This behavior is attributed to the Nickel-Cobalt-Boron alloy that is deposited on the DOX steel forming a stable passivation film which is polarized by cathodic reaction at very negative values. The alloy presents a very low current output which reduces the speed of galvanic corrosion when compared with the speed of corrosion observed in the metals in the pair independently.