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For steel reinforcement in concrete the ionic flow running through the concrete and attachment between the steel cannot be disconnected since the corrosion circuit is buried inside the structure. It is possible by using an 'artificial' anode to add a new and higher current to the original corrosion circuit where it runs in the opposite of direction of the corrosion current. It converts all the previous anodes into current receivers. Thus all the steel reinforcement is made into a negative pole known as cathodic, hence it was named as cathodic protection.
For a careful designed and constructed concrete structures, the embedded steel are under protection from corrosion damage as the naturally highly alkaline cement provides a passive environment. A good compacted and adequate concrete cover will able to prevent corrosion of steel for years even it exposed to aggressive condition. However, those criteria are seldom met in construction practice. Besides, good current density and distribution path factors should be bear in mind as they are important to reduce, stop or prevent corrosion of the steel reinforcement.
Before proceed to next section, there are several issues that need to be avoided in cathodic protection. Excessive negative potentials can cause accelerated corrosion since environment contained alkaline are created in cathode. This condition may result of loss of adhesion between the coating and the structure. Besides, hydrogen evolution at cathode surface may result in hydrogen affect the brittleness of the steel and loss of strength. Another issue is when forming of spark hazards in a hazardous area. Therefore it is necessary to check the system after installation to avoid leakage of electric current.
Selection of a suitable current density output is crucial in this particular design since there are no references or clear guidelines provided. According to Concrete Society Technical Report No. 37(1991) the current densities within the range of 10 and 20 mA/m² of steel reinforcement are given as typical values. In practical the current density is mostly dependent on the steel corrosion state which related to the location and exposure of the steel structure such as concrete permeability and chloride level. For example, a concrete structure with a minimum concrete cover that is exposed to warm, cyclic wet environment will have a high current density requirement to reduce corrosion rate. In the other hand, a low current density will be sufficient if the concrete surrounding area is under alkaline activity where little chloride present and the steel is not strongly corroding.
Steel reinforcement arrangement, area of the corrosion spread and level of corrosion activity are important issues to achieve optimum current distribution. Normally the highest level of current should be installed in the area where most active corrosion are. There is no specified formula and terms to determine the current distribution in reinforced concrete. This is due to unstable resistivity of the concrete, resistivity from steel to concrete interface and density of the steel reinforcement.
There are various types of anode that are available in the market but it is important for designer to consider wisely according to the anode characteristics and effect to overall design. However, structures in the atmosphere have been successfully protected by using 200mm spacing between the anodes passing current. In general, anodes that are mounted on concrete would be expected to operate below a current density of 10mA/m² to reduce possibility of premature anode failure. For example, a conductive coating on the structure will require positive feeder wires and a discrete anode system require distribution boxes. Furthermore, well-applied and suitable coating will increase the effective spread of cathodic protection current. Usually good coating will have high electrical resistance, continuous to protect the steel surface.
A precast beam runs between the column and it supports a fan deck which loaded with dead and live loading. From observation, corrosion occurred on the top surface of the beam that being exposed to sun, with chloride contaminated water through the beam. The only access provided was a platform from basin floor therefore the cathodic protection has to be simple and easy installed for future maintenance. Structural engineers that involved in this project had issued about the severe corrosion damage, test loading and calculation of strength had been ignored and thus should only receive a low priority for cathodic protection.
A current density (15mA/m²) of steel was supplied to reflect to the surrounding environment of the beam. It is more convenient by using the same length of anode in the column and it was proposed to drill vertically upwards into the beam. However in the practice the drilling work cause difficulties with the dust and a horizontal positioning is used with closer spacing to compensate the reduced current distribution.
The sacrificial anode system consists of a galvanic cell system which the anode is made of active metal that are commonly used such as zinc, magnesium and aluminium alloy. They are useful when electrical power supply are not available, or in other situation where it is uneconomical to install power lines on the site. Moreover, they can serve as alternative sources of portable electrical energy. The anode is connected by welding to the structure and the anode output current can be measured. This causes a positive current to flow in the electrolyte from the anode to the steel. Thus, the surface of steel become negatively charged and become the cathode. In this circumstance, the structure is protected by the active metal until it is completely consumed and the base steel will be susceptible to corrosion.
However, the sacrificial anode system is unlikely to provide effective protection to structure other than those immersed in sea water owing to the low voltage supplied. Instead of that, there are some active corroding areas to cause the cracking of the cover. It is expected that the deterioration will continue for a time in the very early of the protection system.
Theoretically, aluminium operates at a voltage between magnesium and zinc. The main disadvantage of aluminium as a sacrificial anode is that it is tends to become passive in wet environment. Therefore, alloying additions such as mercury and antimony are galvanised to aluminium to avoid passivity. Compared to magnesium, aluminium are more reliable for long time performance. Magnesium anodes may be consumed and cannot last until the structure reached its end of lifetime. Below are the lists of metals that are normally used:
Current usually will be high due to the difference in potential between the anode and cathode are high. However the potential difference will decrease due to the effect of the current flow into the cathode, current gradually decrease due to the polarization of the cathode. The circuit resistance that are running between anode and cathode includes the water path, metal path and any cable related to the circuit. The primary value of this circuit is the resistance of the anode to the seawater since our main concern is the corrosion will occur in anode side. In practical the metal resistance can be ignored since the value is small compared to water resistance.
Different anode sizes contribute to different result gained. In most cases, long thin anodes have lower resistance than short fat anodes. As a result, they discharge more current but it will not last long. Therefore it is important for designer to decide and choose the right shape and surface area in order to discharge enough current to protect the structure for a long period. Another important point is the length of anode can determine how much the current can be produced by anode and thus the area of steel can be protected.
Compared to sacrificial anode protection system, impressed current method will be better option and more efficient to protect structures that are atmospherically exposed. For example, high currents involved in seawater and normally impressed current method is used. This technique works by delivering a small electrical current (DC) from a direct current source through an auxiliary anode to the surface of the reinforcement as shown in figure.
Impressed current method should be reasonably practical, safe and economical to install. Besides, the selection of anode type will be the main consideration to the effectiveness of the system. From the selection of the anode, there will be increased dead load in the system installed and this must be accounted into the system design stage. The load cases are extremely critical when the system is applied to existing structure. It is important to make sure that the system is able to provide a uniform and stable current to the reinforcement steel at an acceptable low DC output voltage.
The system components must be durable to resist the installation process and the design service life during the operation. They should not fail under reasonably vary environmental conditions else it would be costly to repair the system. Furthermore, the installed system should not adversely affect other components or the structures. Last but not least, the system should be designed in relatively simple for easier operation and maintenance in the future.