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any storage terminals and petrochemical facilities are located near a navigable waterway and have shore marine structures nearby to allow for tanker traffic to receive or send product. These could include docks, jetties, piers, seawalls, mooring, and turning/mooring dolphins and other similar structures. These metallic structures are subject to corrosion and are generally protected by cathodic protection systems. This article provides a general overview of cathodic protection for these structures and discusses some of the common strategies that are frequently employed to protect them.
Two basic types of cathodic protection There are two basic types of cathodic protection and both are used to protect near shore marine structures. It is not uncommon to employ both strategies to protect different structures in the same facility. Each of these systems have their advantages and disadvantages, and it is important that the design engineer consider the application specifics to choose the appropriate system(s) for the application. The first type of system is a galvanic anode system (also commonly referred to as a sacrificial anode system). Galvanic anodes are cast hunks of a specific metal alloy that are typically attached directly to the structure. For seawater applications, these are commonly aluminium or zinc anodes, while brackish or freshwater applications might utilise zinc or magnesium anodes. These metals are inherently more electro-negative than steel so that when they are coupled to a steel structure, current will flow from the more electro-negative anode to the metallic structure. When properly applied, this electrical current, generated from the anode, flowing through the electrolyte (water) to the structure (or cathode), reduces the corrosion rate to virtually nothing – this is, by definition, cathodic protection. The other type of system is an impressed current system. Impressed current systems work in the same basic manner as Summer 2021 42
galvanic systems; however, for these systems, an external power supply is used to drive current off the anode, through the electrolyte, to the structure being protected. The use of an external power supply allows the cathodic protection system to utilise anodes that are not required to be more electro-negative than the structure being protected. These anodes can be chosen for other properties, namely their ability to discharge current much more cost-effectively than massive hunks of metal alloy. They can do this because of the use of an external power supply that will drive the flow of current.
Key differences between galvanic and impressed current The key differences between an impressed current anode system and a galvanic anode system for near shore marine structures are discussed here:
Anode consumption rates Aluminium alloy anodes, such as those typically used in seawater applications, have a nominal capacity of 2000 amp-hr/kg, while titanium anodes with a mixed metal oxide coating (MMO), commonly used in near shore applications, have a capacity of approximately 1000 times the equivalent capacity of aluminium anodes. This means that far fewer anodes are required and the expected life of the system can be much longer.
Current density limits Aluminium alloy anodes can operate at a maximum current density (current being discharged for a given surface area) of 15 amps/m2 but are typically designed to operate at much lower current densities. Impressed current MMO type anodes can operate at current densities up to 500 amps/m2; however, in most cases the operating levels are much lower. The higher current density limit means that the designer can use fewer impressed current anodes at a much higher output.
