There are two ways in achieving cathodic protection. It is either by using galvanic anodes or by using impressed-current. Cathodic protection using galvanic anodes uses reactive metals as secondary anodes and they are directly connected electrically to the metal to be protected. On the other hand, cathodic protection using impressed-current uses inactive anodes and uses an external source of direct current power to impress a current from an external anode onto the cathode surface.
Before the design of cathodic protection systems, close interval surveys are done on test stations, test facilities and permanent half cells and data can be regularly collected. The survey aims to justify the financial viability of the proposed cathodic protection system while physical and chemical data are collected.
Measurement of potential between pipeline and a reference electrode such as copper sulfate is the most frequent test performed in the corrosion industry. A general idea of corrosion extent can obtained from the average pipeline potential. When potentials are measured with respect to a reference electrode every mile or so along a pipeline or at test stations and the readings are then plotted, the newer and less corroded pipelines will typically have more negative potential values.
CP test stations must have proper maintenance to avoid problems in protecting a structure from corrosion. Occasionally, a CP test station may be broken or missing as the result of accident or vandalism. Post mounted test stations, from time to time, require replacement of box covers or cover retention screws, cover gaskets or terminal nuts or screws within the test box. This is why corrosion personnel should carry a complete stock of spare parts and text boxes when making routine surveys so that minor maintenance can be implemented at that time. Also, CP test stations should be placed where they will be readily accessible during routine tests.
