Cathodic protection is mostly applied as a secondary corrosion protection method while coatings being the primary protection method. Nevertheless in case of coating failure, cathodic protection system ensures that adequate current is impressed on to the metallic structure so that it does not lose its positive ions and so corrosion reaction (anodic or oxidation reaction) is suppressed which otherwise would be corroding the metal at that localized area (coating breakdown areas) at much faster rate in absence of CP system. Metallic materials in corrosive environments corrode at anodic sites i.e. local anodes and cathodes on the surface of a single metal/alloy or a galvanic couple of different metals/alloys. Therefore in cathodic protection system, an anode is forcefully converted into cathode by impressing a negative current to protect it from corrosion at the expense of another metal/alloy which acts as anode. Anodic protection is exactly opposite in methodology to cathodic protection where an anode is impressed with a more positive current so that it eventually turns into a passive state and corrosion reaction is suppressed. While anodic protection finds limited applications in industry, cathodic protection is most widely used and technologically more advanced.
Selection and design of CP system require specialized knowledge, skill and experience. Choice of CP system type, whether sacrificial or impressed current system, depends on various factors such as electrolyte resistivity, surface area, pH, current requirement, maintainability and design life. For well casings, E Log I test is carried out to determine cathodic protection current requirement whereas for piping, pipelines and tanks, current density and/or current drain test criteria are used. Pipelines also require a temporary CP system if there is a considerable time duration between pipeline laying and permanent CP system energization.
CP system design engineer must have thorough knowledge and experience of soil corrosion and liquid submerged corrosion damage mechanisms, different protection criterias, polarization phenomena, current attenuation, parallelism, anode types, ground bed types, effect of temperature and bacteria in soil, anode efficiency and consumption rate, interference effects and their mitigation, power sources such as transformer rectifiers, thermoelectric generators, solar panels, minimum distance between different structures to avoid interference, key design equations, effects of coating and coating breakdown factors, safety factors and margins and other design parameters. All these factors need to be integrated into design calculations. Highest standards of safety protocols must be followed in design, installation and commissioning as per guidelines established by NACE, ISO and API standards.
Cormat has extensive experience in providing FEED and EPC solutions to its Clients. Our experts have required qualifications, trainings, certifications, skills and experiences to support our Clients for all stages of a project i.e. Concept select, FEED and EPC.
