Hazardous product pipelines are routinely protected by a coating supplemented with cathodic protection. An impressed current cathodic protection system (ICCP) for a pipeline consists of a DC power source, often an AC powered transformer rectifier and an anode, or array of anodes buried in the ground (the anode groundbed).
It is sometimes more economically viable to protect a pipeline using galvanic (sacrificial) anodes. This is often the case on smaller diameter pipelines of limited length. Galvanic anodes rely on the galvanic series potentials of the metals to drive cathodic protection current from the anode to the structure being protected.
Water pipelines of various pipe materials are also provided with cathodic protection where owners determine the cost is reasonable for the expected pipeline service life extension attributed to the application of cathodic protection.
Plant facility piping system mostly protected by remote groundbed within the plant battery limit. Cross country pipeline for transporting oil, gas and other products are mostly protected by ICCP system.
External Tank Bottom of Storage Tanks
Carbon-steel water storage tanks are a common storage for all oil producing companies as well as other utility and power companies for their water storage and other liquid product storage facility. They come in two general configurations – the underground storage tank and the aboveground storage tanks. As with any steel structure, corrosion is an ever present concern. The effects of corrosion on storage tanks include premature failures and disruptions in service during repairs. Cathodic protection (CP), however, stops the corrosion reaction when properly applied underneath the tank bottom surface.
Internal cathodic protection
Vessels, pipelines and tanks which are used to store or transport liquids can also be protected from corrosion on their internal surfaces by the use of cathodic protection. ICCP and galvanic systems can be used. A common application of internal cathodic protection is water storage tanks, crude storage tanks etc. Cooling Water piping system mostly carry seawater and face severe corrosion problem, therefore modern day power plant with large diameter CW piping system require internal ICCP system for corrosion protection.
Ships and boats
The white patches visible on the ship’s hull are zinc block sacrificial anodes.
Cathodic protection on ships is often implemented by galvanic anodes attached to the hull and ICCP for larger vessels. Since ships are regularly removed from the water for inspections and maintenance, it is a simple task to replace the galvanic anodes.
Galvanic anodes are generally shaped to reduced drag in the water and fitted flush to the hull to also try to minimize drag.
Smaller vessels, with non-metallic hulls, such as yachts, are equipped with galvanic anodes to protect areas such as outboard motors. As with all galvanic cathodic protection, this application relies on a solid electrical connection between the anode and the item to be protected.
For ICCP on ships, the anodes are usually constructed of a relatively inert material such as platinised titanium. A DC power supply is provided within the ship and the anodes mounted on the outside of the hull. The anode cables are introduced into the ship via a compression seal fitting and routed to the DC power source. The negative cable from the power supply is simply attached to the hull to complete the circuit. Ship ICCP anodes are flush-mounted, minimizing the effects of drag on the ship, and located a minimum 5 ft below the light load line in an area to avoid mechanical damage. The current density required for protection is a function of velocity and considered when selecting the current capacity and location of anode placement on the hull.
Marine cathodic protection covers many areas, jetties, harbors, offshore structures. The variety of different types of structure leads to a variety of systems to provide protection. Galvanic anodes are favored, but ICCP can also often be used. Because of the wide variety of structure geometry, composition, and architecture, NACE certified CP specialist and engineers are often required to engineer structure-specific cathodic protection systems. Sometimes marine structures require retroactive modification to be effectively protected.