Photo: Sacrificial anodes installed to protect chiller condenser end plate. No red iron rust or tuberculation present.
Corrosion is best defined as a reaction between a metal and its environment. Several forms of corrosion have been identified such as general, pitting and dissimilar metal. In these cases, the corrosion mechanism is described as an electrochemical reaction whereby current flows from anodic to cathodic sites on the metal surface. If this process is allowed to continue, costly damage will result on the metal component or structure.
Protection of metal structures from electrochemical corrosion by means of cathodic protection is achieved by making the metal object the cathode of an electric circuit. The anode of this circuit may consist of an element, such as zinc, magnesium or aluminum, which is more “active” in the electrochemical series than is the structure to be protected.
Cathodic protection is only one of a number of corrosion control methods that are in common use and may not be applicable in every situation. It is only effective in controlling corrosion on metallic structures that are in contact with an electrolyte (such as water). If the structure is buried in the soil, the damp earth serves as the electrolyte.
Savings in maintenance, repair and replacement costs resulting from the use of cathodic protection systems are substantial. It is successfully used to protect above ground metal structures, buried pipes and tanks, heat exchangers and ship hulls amongst others.
CATHODIC PROTECTION – IN THEORY
Whenever corrosion is occurring in an electrolytic environment, current flows. The direction of current flow is from the area of the metal that corrodes through the electrolyte to the non-corroding area of the metal. The area where corrosion occurs is called the anode and the areas where corrosion does not occur is called the cathode.
The convention for assigning the direction of the current flow is taken as the flow of positive current, which is in the opposite direction to the flow of the electrons. This is an important distinction in that in some areas of science, current is defined as the flow of electrons through a conductor (metal). In corrosion science, however, the current flow is visualized as the flow of positively charged metal ions that move through the electrolyte from the anode to the cathode of the corrosion cell.
At the anode, the positively charged metal ions dissolve into the electrolyte. As this dissolution process continues, the metal steadily deteriorates. Using iron as an example, the chemical reaction at the anode is:
Fe —-> Fe+2 + 2e-
Iron —> Ferrous ion + 2 electrons
At the cathode, the hydrogen ions are chemically reduced by the flow of electrons to form hydrogen gas. This reaction occurs readily in acidic environments.
2e- + 2H+ —> H2(gas)
2 electrons + hydrogen ion —> hydrogen gas
If dissolved oxygen is present, hydroxide ions are produced at the cathode.
H2O + 2e- + 1/2O2 —> 2(OH)-
Water + 2 electrons + oxygen —> hydroxyl ions
The hydroxyl ions (OH) formed at the cathode are then available to react with ferrous iron that has dissolved into solution at the anode. These combine to form ferrous hydroxide, which is the very common rust-red deposit found on corroded surfaces.
The driving force for the current flow from the anode to the cathode is determined by the potential difference between them. There is an electrical potential at the anode that tends to drive the metal into solution and an associated anodic resistance. At the cathode, there exists a similar, but smaller electrode potential and a cathodic resistance. Assuming the anode and cathode resistances are about the same, the current flow will be determined by the relative magnitude of the electrode potentials. If the potential at the anode is greater than the potential at the cathode, current will flow through the electrolyte from the anode to the cathode. This corrosion current results in the deterioration of the anode by virtue of dissolution of the metal into the electrolyte.
The flow of current in an electrochemical circuit produces chemical effects that cause polarization of the electrodes. The accumulation of metal ions at the anode reduces the electrode potential and thereby decreases the driving force of the corrosion cell. The accumulation of hydrogen gas at the cathode increases the resistance at the cathode and thereby limits the flow of current. These reactions modify the flow of current. This shift in potential is called polarization. Polarization effects act to limit the flow of corrosion current and has a controlling effect on the corrosion rate.
Application of cathodic protection serves to stop the flow of corrosion current. When the potential of the cathode is equal to the potential of the anode, no current will flow and corrosion ceases.
THE GALVANIC ANODE METHOD
The galvanic anode method of cathodic protection depends upon the establishment of an intentional corrosion cell in the environment such that the structure to be protected becomes the cathode of the cell. This is accomplished by selecting a sacrificial anode that is less noble (more electrochemically reactive) than the metal to be protected. The protective anode will tend to corrode and thereby produce a protective current flow onto the structure.
The most commonly used materials for sacrificial anodes are magnesium, zinc and aluminum.
Magnesium has an open circuit potential against steel of about one (1) volt; that is, one volt is the driving force for the protective current. The use of a magnesium anode for one year to produce a current of one ampere results in an anode weight loss of about 17 lbs.
Zinc has an open circuit potential to steel of about one-half (1/2) volt. Zinc anodes operate at about 90% efficiency, which is equivalent to 25 lbs of zinc consumed per ampere per year. Zinc is commonly used to protect ship hulls.
Aluminum anodes produce an open circuit potential against steel of 0.175 volt and operates at about 70% efficiency. This results in the consumption of about 9 lbs of aluminum per ampere per year. Aluminum is commonly used in water systems since it does not introduce any soluble impurities or color.
The galvanic anode system is generally designed with many small anodes placed symmetrically in relation to the object to be protected. The galvanic anode current output depends essentially on the size and shape of the anode. Anode length is the major factor in determining its resistance to ground as a rod. For this reason, the most efficient shape is long and narrow.
IMPRESSED CURRENT METHOD
The impressed current method of cathodic protection employs a direct electric current from an external source (such as a rectifier) that is forced from a ground bed of anodes, through the electrolyte (water or soil), onto the structure that is to be protected. The ground bed anodes may or may not be completely consumed in this reaction. Since the current is supplied by an external source, the ground bed acts merely as the anode through which the current may be applied.
The materials used for impressed current anodes are carbon, iron or steel, high silicon iron, aluminum, stainless steel and platinum. Some of these anodes are permanent, such as platinum, stainless steel and high silicon iron. Others are semi-permanent as in the case of carbon. Scrap iron or aluminum, however, are completely consumed.
The type of current source to be used in an impressed current system depends in large part on the location and availability of electric power. At locations where alternating current is available, rectifiers are typically used. Rectifiers convert AC current into DC current. In other locations, solar power, wind generators and diesel electric generators may be used. The direct current wiring to the ground bed and to the protected structure must be carried in insulated wire of the proper size. The insulation should be of a type to have a long life in the electrolyte in which it is going to be used.
Impressed current systems offer several advantages. They can be designed to cover a wide range of applied voltage and current output, whereas the voltage of galvanic anodes is limited. They can be designed to protect a very extensive area of steel with one installation. A high level of amperes per year can be obtained from a single ground bed. And the applied voltage and current output can be varied over a wide range to meet changing conditions.
For projects that include numerous and different underground structures, they have to be designed with care to avoid cathodic interferences. Cathodic protection, in order to be fully efficient, must be continuous. Any system that depends on an alternating current power source is subject to occasional outages when the current is interrupted. Therefore, impressed current systems require some maintenance.
GALVANIC VERSUS IMPRESSED CURRENT
The galvanic sacrificial anode system is used mostly for the protection of small isolated tanks on both the interior and exterior; the protection of moderate sized tank farms; “hot spots’ on pipelines; and other small or moderate area installations. Galvanic anodes are also favored for use in restricted area such as condenser water boxes and in many types of marine installations.
Impressed current cathodic protection systems are used for large general installations of all types. This includes the protection of the interior of hot and cold water storage tanks, process water tanks and equipment such as beer pasteurizers, and large extensive marine installations such as piers, pilings and dry docks.
Notwithstanding the method chosen to employ cathodic protection, this method of corrosion control has been successfully and economically employed throughout the world to protect and prolong the useful life of metal structures, equipment and infrastructure.