Steels have been extensively used for construction of oilfield equipment, particularly tanks and vessels. However, steel materials fail to resist corrosion when exposed to aggressive media. Two methods are mainly used to protect metal structures against corrosion in specific environments. These are application of insulating coatings and electrochemical protection. Electrochemical (sacrificial or cathodic) protection is provided by application of direct current having negative polarity onto metal structure. Cathodic protection systems require an outside DC power source, namely special-purpose AC to DC converters or cathodic protection stations. During corrosion protection, the negative pole of cathodic protection station is connected to metal structure to be protected, while the positive pole is connected to electrode (anode). Reliability of cathodic protection systems largely depends on anode material and structure. Nowadays, low-solubility electrodes have gained wide acceptance in anodic grounding of cathodic protection units, particularly electrodes made of high silicon cast iron - ferrosilide. Ferrosilide, as anode-resistant material, has limited applications confined to moderate operating conditions. Severe conditions associated with protection of offshore structures, saline soils-buried pipelines, and the interior of oilfield tanks and vessels require anode materials exhibiting better resistance in chloride-containing environments. Manufacturers of equipment for electrochemical corrosion protection systems contribute to the development of technologies for production of grounding anode materials with platinum group-metal mixed oxide coatings to ensure an excellent corrosion resistance without sacrificing the electrical conductivity.
The paper discusses the advantages and disadvantages of various grounding anode materials, presents available types of grounding anodes used for cathodic protection against corrosion. The paper also reveals the findings of laboratory and bench tests on samples of titanium electrodes with manganese-dioxide corrosion-resistant coating, as well as the results of field trials of titanium electrode application in anode grounding for internal cathodic protection of vertical steel tanks.
References
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