EXPERT TIP #12: METALS USED IN CATHODIC PROTECTION

EXPERT TIP #12: METALS USED IN CATHODIC PROTECTION

Metals

Many materials have been developed to mitigate corrosion in buried and submerged metals (pipelines, storage tanks, etc.) corrosion.  The development of corrosion resistant materials i.e., plastic composites, epoxies, urethanes and the like have significantly contributed to the industry. For many applications, the “non-metallic” products are used to simply replace the metal or are applied to the metal as protective coatings. Despite this, metals and their unique properties are still major players in the corrosion control industry. 

In certain applications, specific metals are selected for their physical resistance to corrosion. Popular choices include copper and copper-based materials (brass and bronzes), stainless steels and nickel alloys.  Less cost effective, but very corrosion resistant, are titanium, platinum and the “king” of corrosion resistant metals, gold. 

Looking Back

One metal that changed the world and was a major contributor to many industries is steel.  Developed in the 1740s, steel is an alloy of iron. Originally, very expensive with small production quantities, steel became a major player in the mid-1800s when a process was developed to produce steel economically and on a large scale. Steel production continued to evolve into the early 20^th century with the Siemens-Martin process. While refined, this leading steel-making process is still used today.

The availability of inexpensive steel allowed larger bridges, railroads, skyscrapers and ships, as well as improvements to automobiles.  In addition, many other products improved greatly thanks to steel, which includes better and stronger pipelines. Despite all its great physical properties, steel is quite vulnerable to corrosion.

The Cathodic Protection System

Cathodic Protection (CP) is a proven method to mitigate corrosion on buried and submerged steel structures.  CP is an “electro-chemical” process.  Therefore, any CP design will include elements that require some form of electrical energy along with consideration for the chemical and physical properties of the metals within the CP system.  To be effective, a typical CP system design will take advantage of these properties.

There are two basic types of CP systems:

1. Galvanic System – This is where two dissimilar metals are connected together in an electrolyte. The “anode” (or corroding electrode) is the more electro-negative metal in an electrical circuit and the “cathode” (or corroding electrode) will receive cathodic protection current from the anode.  A typical galvanic anode used in CP is manufactured of magnesium, zinc and aluminum alloys.
2. Impressed Current System – This is where a DC power supply provides the necessary voltage to power a buried anode, which can be made of graphite (technically not a metal), high silicon cast iron (HSCI), platinum or mixed metal oxide (MMO).

In all CP systems, the anode discharges an amount of DC current that is collected on the structure to be protected.  When adjusted for maximum effect, all sections of the structure receive a portion of the DC current and the entire structure becomes cathodic.  In designing a CP system, it is important to understand the physical and electrical properties of the metals that make up the CP system.   In either type of CP system, the choice of anode material will be a major factor. 

In a Galvanic CP System, the technology can be as simple as connecting a single galvanic anode directly to the structure to be protected.  Often this interconnect is through a copper wire between the anode and structure or the anode can be connected directly to the structure by bolting or welding. 

The Impressed Current CP System will require an anode(s) and interconnect wiring between the power supply and to the anode(s).  The power supply provides DC power to the system and will require an external AC power source.  The conductivity or electrical resistance of metals included in a CP system is an important consideration.  

A typical impressed current system will include the following metals:

Metal Conductivity Trivia

All metals can conduct electricity but certain metals are more conductive, the most common being copper. Copper is highly conductive, which is why it is has been used in electrical wiring since the days of the telegraph. Interestingly, brass, which contains typically contains approximately 60% copper, is far less conductive because it is made up of additional materials that lower its conductivity.

Copper is NOT the most conductive metal, despite being used in many common electrical applications.  Pure silver is the most conductive metal, but also considerably more expensive than copper.  Therefore, the increase in conductivity is usually not worth the added cost.

A common misconception is that pure gold is the best conductor of electricity. While gold does have a relatively high conductive rating, it is less conductive than copper.  Gold is used extensively in computers and other high-end electronics. The gold is normally a thin coating (plating) over the copper connectors. The purpose of the gold plating is to provide a stable (corrosion resistant) and reliable electrical contact. 

Understanding the relative difference in the electrical conductivity of metals is important when designing electrical systems, i.e., CP systems.  These systems carry relatively large electrical currents and are sensitive to series voltage drops. Often not considered is the linear voltage drop in steel pipelines. This must be taken into account to ensure an effective CP system. 

The following chart shows the relative conductivity of various metals. 

Note that copper is the baseline reference and that a 100% rating does not indicate zero resistance.

The differences in electrical conductivity varies considerably depending on the metal.  Therefore, it is very important not make assumptions on the electrical conductivity of a metal.