Cathodic protection (CP) is a method wherein a sufficient amount of electric or impressed current (DC) is continuously supplied to a submerged or buried metallic structure to mitigate, slow down or stop altogether the natural corrosion processes from occurring.
There are two methods for supplying DC to cathodically protect a structure. They are the following:
- Galvanic or sacrificial anode cathodic protection system (SACP).
- Impressed current cathodic protection system (ICCP).
Electrical current leaves the positively charged anodes and travels through the masonry to the surface of the structural steel. Several important electro-chemical reactions occur because of this electron/ion transfer at the surface of electrodes and in the electrolyte:
• Steel surfaces receive enough protection current to mitigate or eliminate corrosion processes;
• Negatively charged corrosive ions are repelled away from the steel and pulled towards the positively charged anodes;
• Oxygen reacts with positively charged electrode (anode);
• Oxygen reduction occurs at the cathodic sites (structural steel).
We can design and install cathodic protection systems for :
- Water mains
- Gas and oil pipelines
- Storage tanks
- Steel pier piles
- Ship and boat hulls
- Offshore oil platforms
- Onshore oil well casting
- Wind farms
- Foundations and metal reinforcement bars in concrete buildings and structures
The galvanic anode cathodic protection system generates DC as a result of the natural electrical potential difference (electrochemical reaction) between the metal to be protected (cathode) and another metal to be sacrificed (anode). The sacrificing metals such as magnesium (Mg), zinc (Zn) or aluminum (Al) all have a lower more negative electrical potential with respect to carbon steel reinforcement. The current output of this system is affected by factors such as:
- Driving voltage difference between the anode and the cathode.
- Resistivity of the electrolyte (environment: soil or concrete).
- pH factor.
- Natural or man made environmental chemistry and/or contaminates.
Impressed Cathodic Protection System(ICCP)
The impressed current cathodic protection system comprises four main components which together constitute an electrical circuit. They are as follows:
- A controllable DC power source – usually a transformer rectifier.
- An applied anode – a material placed onto or into the concrete or surrounding electrolyte to enablecurrent flow.
- An electrolyte – normally the pore water present within the concrete, or in the case of remote anodes, also the water, soil or mud in which the anodes are placed.
- A return electrical path – normally the electrically continuous reinforcement steel to be protected.
- Impressed current cathodic protection systems employ the use of electrically forced galvanic reactions to protect steel in an electrolyte (such as masonry). Anodes are installed in the masonry in strategic locations near the steel to be protected. A cathodic protection rectifier applies a DC voltage to the system with the positive lead connected to the anodes and the negative lead connected to the structural steel (cathode).
- The CP transformer rectifier can be powered by external power sources, such as alternating current (AC). The CP rectifier converts the input power source into DC. DC is discharged from impressed current anodes made of metals such as steel, high silicon cast iron, graphite, platinum and mixed metal oxide coated titanium. The potential current output of an impressed current CP system is limited by factors such as available AC power, rectifier size, anode material, anode size and anode backfill material. However, higher maintenance during service is required and short circuiting of anode and masonry encased steel should be taken into consideration in design and implementation of this system.
Impressed current cathodic protection systems applied to under ground structures can be applied in a number of ways depending on several important factors. The major concerns when designing an ICCP system are as follows:
- Corrosivity of environment (Presence of moisture, chlorides, and oxygen);
- Current state of corrosion of masonry-encased steel members;
- Quality of encasing masonry;
- Electrical continuity between all encased steel members;
- Total surface area of steel to be cathodically protected;Total electrical current required to protect encased steel;
- Protection criteria.It is important to determine the condition of the steel beams and current requirements for the design of a reliable cathodic protection system.
Cathodic protection is a very effective corrosion mitigation procedure that has been widely used and refined for the past 100 years to protect fusion bonded epoxy coated pipelines, buried fuel lines,
utility structures and other buried and submerged assets from corrosion metal loss. Cathodic protection systems protect a wide range of metallic structures in various environments.The following variables should be looked at carefully:
- Foundation configuration and exposed steel surface areas
- Presence of coating and concrete
Given these parameters, anode configurations and bills-of-materials can be developed .
A cathodic protection system designed for structures should consider:
- Current Requirement
- Soil Resistivity
- Electrical Continuity
- Grounding at the Project Site
- Bare Surface Area for Piping, Galvanized Steel/Weathering Structures, underground tanks
- Coating Integrity and Degradation in time
- Stray Currents
- AC/DC interference
Stray current corrosion
Stray current corrosion is caused by the effects of a direct current that may be picked up on a pipe line of structure that is not part of circuit of interest. This current flows to other structure and at some point discharges(leaves) the other structure and travel back to the source. This cause stray current corrosion (sever) at the point of discharge. The form of attack is usally very localized..
Prior to cathodic protection design one needs to identify if there is a source for stray current. The following google map speaks to water lines with cathodic protection, AC transmission lines with cathpodic proteciton and deep anode wells.
The following images present our computerized approach for cathodic protection design for communication tower anchors,
We have NACE certified cathodic protection specialists with many years of experience in design , installation and corrosion monitoring of CP systems, both impressed and galvanic CP systems. Please contact us and we will be at your project site to assist you.