By Ted Huck, MATCOR, Inc.
A 1998 EPRI study, released in October of 2001, estimated the cost of corrosion in 1998 to steam generating facilities alone at $5.37 billion. The study, EPRI Report 1004662, also concluded that 22 percent of these costs are avoidable. Corrosion remains an ongoing problem for power generators and the industry has spent, and will continue to spend, millions of dollars on a wide range of corrosion protection measures.
One of the proven technologies for controlling corrosion is cathodic protection. Although used in numerous applications in power plants, there are still many untapped opportunities to apply this technology. Power plants using cathodic protection are able to reduce the economic impact of power plant corrosion considerably.
What is Corrosion?
Corrosion can be defined as the degradation and destruction of a metal by its chemical reaction with the environment. Corrosion reactions, with the exception of some forms of high temperature corrosion, are electrochemical oxidation/reduction reactions. Figure 1 shows a typical corrosion cell that is formed when small variations in potential occur over the metal’s surface. The corrosion is caused by differences in the metal or its surrounding electrolyte. The difference in potential results in a current flow from the anode to the cathode.
Cathodic protection is one option for controlling corrosion. In the typical corrosion cell, the metal structure has both anodic (area where metal is lost) and cathodic (area with no metal loss) regions resulting from electrical potential differences. Even small differences in potential can result in significant metal loss over time. Cathodic protection is accomplished by intentionally substituting the slightly anodic region of the structure to be protected with an even more anodic component called an anode.
The anode is intentionally coupled with the protected structure. In a galvanic system, current results from the inherent potential differences between the anode and the cathode. An impressed system, on the other hand, uses a DC power supply. As long as sufficient protective current is maintained, an impressed system can eliminate further corrosion.
The simplest system is the galvanic anode system, Figure 2. With a galvanic system, the potential difference results entirely from the electrical characteristics of the anode versus the structure to be protected. Although this type of system will protect the entire surface, it is important that the correct anode be selected. Some common galvanic anode materials are magnesium, zinc and aluminum.
Anodes are available in standard sizes weighing from one to 48 pounds. For example, the 17-pound magnesium pre-packaged anode, approximately 30 inches long and six inches in diameter, is the most commonly specified anode for many pipeline system applications. The package consists of a solid bar of magnesium alloy connected to an electrical lead that protrudes from a bag filled with a mixture of bentonite and gypsum.
The bentonite and gypsum provide an environment that absorbs and retains moisture while assuring a uniform low resistance around the anode. Prior to installation, the bag is thoroughly wetted and the lead is either welded, or mechanically connected, to the structure to be protected. Typically, multiple galvanic anodes are strategically located across the entire structure.
Although galvanic systems can be inexpensive, there is little margin for design error. The natural voltage differences are relatively small and fixed. Galvanic systems also do not allow for increased current output when operating conditions or the environment change.
For buried magnesium anodes, the electrical potential between the anode and carbon steel is a nominal -0.85V. The current output is a function of Ohms Law (E=IR) and is thus dependent on the resistance of the system. Spacing, quantity and location of the anodes must be determined during the design process. The rate of consumption of the anode can be calculated and its design life reasonably predicted.
Sacrificial anodes must be replaced on a regular basis to maintain system performance. Generally, buried or submerged anodes are not removed at the end of their useful life; new ones are simply added to the system to maintain the required protection.
Impressed Current Systems
The impressed current system, in contrast to the galvanic system, does not rely on the potential differences between the anode and the structure to be cathodically protected. Instead, an external power source, typically a rectifier converting electric power from AC to DC, is used. This creates a potential difference and helps to transmit the current from the anode through the electrolyte and onto the surface being protected, Figure 3.
Since the system uses a rectifier to provide the necessary current, the current can be regulated to provide optimum protection. It can also be adapted to changes in the structure, operating conditions, or the environment. When this type of system is used, the number of anodes required can be significantly reduced. A single anode is able to transfer more current over greater distances.
Anode materials are selected based on how fast they are consumed and not on their inherent potential difference with the structure to be protected. Many anode materials can be used in impressed current systems, including graphite, high silicon cast iron, platinum alloys and mixed metal oxides. The latter two have very low consumption rates and for this reason they are coated, in thin layers, onto a substrate of titanium or niobium giving the active element an inert but conducting base. With consumption rates of milligrams per amp year, impressed current anodes can be designed for long life.
Reference cells tied back to a control loop on the rectifier can be used to regulate the current output of the system. This is required when conditions change or where excessive current must be limited to prevent damage to the system. When designing impressed current systems the design engineer must take into consideration control points, test stations, instrumentation, and other operational and maintenance tools.
Power Plant Applications
There are several applications within a power plant where cathodic protection can be utilized:
- Underground piping
- Waterfront structures
- Cooling water process equipment
- Above ground tanks
Federal regulations have long mandated that cathodic protection be installed on underground hydrocarbon pipelines. In power plants there are several underground piping systems, including fuel oil, natural gas, fire water, cooling water, condensate and drains that need to be protected. Although underground steel piping systems typically are coated, damage to the coating can result in concentrated galvanic corrosion.
Depending upon the size and layout of underground piping systems, and the soil’s characteristics, these systems can be protected using either galvanic or impressed current designs. However, the design of the cathodic protection system should take into consideration other underground piping systems and structures: conduits, copper ground grids, electrical cables and foundation steel.
In a galvanic system it is essential that the lines be electrically isolated from the plant’s grounding grid system. Otherwise, the anodes will try to protect everything attached to the grid and would be unable to generate sufficient current.
In most power plant applications, the high concentration of underground structures and piping, along with an extensive grounding circuit, preclude the use of galvanic systems. It is simply too difficult to assure proper isolation of the piping. In these cases, an impressed current system must be used.
Many power plants have waterfront structures such as docks, steel piers and pilings, cooling water intake and outlet structures and inlet screens. Whether carbon steel or steel reinforced concrete, these structures are subject to extremely corrosive environments, particularly when they are located in saltwater or brackish water areas. Even river water can be corrosive. However, cathodic protection, typically impressed current, is extremely effective for protecting these types of structures.
Corrosion of steel in concrete (rebar) is often overlooked. Although steel should be protected by the alkaline nature of the cement used, this does not always happen. Corrosion of steel in concrete can occur from absorption of ions from seawater or ocean spray, industrial atmospheres (caustic, acids and other damaging chemicals), de-icing salts and chloride buildup in the re-circulating water.
When the steel in the concrete corrodes, ferric oxide particles are formed. Over a period of time the rust particles take up significantly more volume than the steel and eventually it expands within the concrete. The end result is cracking, spalling and ultimately separation of the corrosion from the reinforcing steel. However, cathodic protection can prevent corrosion of the steel and premature failure of the concrete.
Cooling Water Systems
Surface condensers, shell and tube heat exchangers, valves, strainers, screens and pumps can all be cathodically protected from corrosion. Frequently the main surface condensers in a power plant have carbon steel water boxes with an internal epoxy coating or rubber lining applied. However, without cathodic protection, any defect in the coating or liner can cause corrosion to develop more quickly than on surfaces that have no coating or liners. Still, the use of coatings and liners does significantly reduce the amperage required by cathodic protection systems.
Because galvanic systems require a system shutdown for inspection and replacement of the sacrificial anodes, impressed current systems are the best solution for large condenser water boxes. When an impressed current system is used frequent anode replacement is not required.
Special care must be taken when providing cathodic protection on condensers using titanium tubes and tube sheets. When these materials are subjected to excessive current they become susceptible to hydrogen embrittlement. A typical impressed current water box system is shown on Figure 4. To limit the current output below thresholds that might cause hydrogen embrittlement of the tubes and tube sheet, a reference cell must be connected to a rectifier. This allows automatic adjustment of the currant output if needed.
The bottoms of carbon steel tanks used for fuel storage are susceptible to corrosion. Today, cathodic protection for new fuel tanks is becoming increasingly common. However, older tanks can be retrofitted with cathodic protection. Tank size, geometry, and the physical characteristics of the underlying surface will determine the type of system that is most economical.
Operation and Maintenance
Cathodic protection systems prevent corrosion of the protected structure only when they function properly. As a result, their design should incorporate sufficient test stations and reference cells to allow plant staff to confirm that the system is indeed operating as intended.
Systems should be thoroughly tested on an annual basis by NACE certified specialists. For galvanic systems, testing is done to confirm that the protected structure is properly polarized. In addition, taking potential readings, and measuring instant on/off voltages in accordance to NACE standards, can confirm that proper protective current is being supplied. Impressed current systems also require similar performance testing. The rectifiers supplying the power should be checked on a regular basis and the output current and reference cell voltage readings should be taken and compared to the initial baseline readings.
Before installing a cathodic protection system, an economic evaluation should be conducted weighing the upfront and maintenance costs of cathodic protection versus the costs associated with the failure of power plant equipment due to corrosion. Even though the cost of providing cathodic protection is relatively low, and easily defined, the cost of power plant equipment failures resulting from corrosion are more difficult to quantify.
Besides the cost of repairing equipment damaged by corrosion, there are often consequential costs from equipment failing. These can include environmental costs in the event of an underground pipeline or above ground storage tank failure, lost energy production resulting from a condenser failure, or safety related issues stemming from firewater piping failures.
Corrosion will continue to be a problem in power plants. Still, electric utilities can save thousands of dollars from reduced downtime and repair by installing cathodic protection systems.