Corrosion control impressed current system

Cathodic protection is used widely for the protection of submerged steel in waterfront structures. It also can provide considerable benefit in the intertidal zone and can even reduce the usually high corrosion rate experienced at the boundary between the intertidal zone and the splash and spray zone. Cathodic protection also is used to prevent corrosion of the soil side of steel in marine structures such as sheet steel bulkheads. Cathodic protection also is effective in the control of the corrosion of reinforcing steel in concrete in all exposure zones in waterfiont structures. Particularly for impressed current systems, it is important to select materials for the cathodic protection system components such as rectifiers and junction boxes with consideration of the environment to which they will be exposed. When considering cathodic protection, periodic inspection and maintenance is required for proper system operation. The costs for inspection and maintenance must be considered in the overall cost of cathodic protection. While there are no specific standards for cathodic protection of piers and docks, information in NACE RP0176 (Corrosion Control of Fixed Offshore Platforms Associated with Petroleum Production) and NACE RP-0187 (Design Considerations for Corrosion Control of Reinforcing Steel in Concrete) contain information that is applicable to marine piers and docks. 

Figure 20-9 shows the negative effect of uninsulated heating elements on corrosion protection. In a 250-liter tank, an electric tube heating element with a 0.05-m surface area was screwed into the upper third without electrical separation, and in the lower third a tinned copper tube heat exchanger with a 0.61 -m surface area was built in. The Cu heat exchanger was short-circuited for measurements, as required. For cathodic protection, a potential-controlled protection system with impressed current anodes was installed between the two heating elements. The measurements were carried out with two different samples of water with different conductivities. 

Corrosion damage is a major factor in ship maintenance and availability. Coatings combined with the impressed current cathodic protection (ICCP) system are the most common means for shipboard corrosion control. They interact with each other to protect the shipboard. Coatings provide primary corrosion protection by isolating the hull metal from seawater, while ICCP systems protect the hull by applying an external source of current to the ship where the paint is damaged or degraded. 

Sacrificial anode CP systems have been used for the corrosion control of bridge decks as long as impressed-current anode systems for corrosion control of bridge decks. Two of the earliest field trials (1977) for sacrificial anode systems were the following ... 

Because of the relatively high resistivity of atmospherically exposed concrete substructures, most anodes utilize impressed current to achieve the necessary driving voltages to supply the current required for corrosion control. However, an exception to this is the use of sacrificial zinc anodes for CP of coastal bridges in Florida, which have a relatively low concrete resistance. However, studies continue to examine the use of sacrificial anodes because of the benefit of its low maintenance compared to impressed-current CP systems. Two of these studies are the following ... 

CP is the required method of corrosion control of buried pipelines. The two forms of CP are impressed-current and sacrificial anode systems. Both forms of protection have been in use in industry for quite some time and the industrial personnel are familiar with their installation and operation (NACE Standard RP0169-96). 

Cathodic protection (CP) is an electrical method of mitigating corrosion on metallic structures that are exposed to electrolytes such as soils and waters. Corrosion control is achieved by forcing a defined quantity of direct current to flow from auxiliary anodes through the electrolyte and onto the metal structure to be protected. Theoretically, corrosion of the structure is completely eliminated when the open-circuit potentials of the cathodic sites are polarized to the open-circuit potentials of the anodic sites. The entire protected structure becomes cathodic relative to the auxiliary anodes. Therefore, corrosion of the metal structure will cease when the applied cathodic current equals the corrosion current. There are two basic methods of corrosion control by cathodic protection. One involves the use of current that is produced when two electrochemically dissimilar metals or alloys (Table 19.1) are metallically connected and exposed to the electrolyte. This is commonly referred to as a sacrificial or galvanic cathodic protection system. The other method of cathodic protection involves the use of a direct current power source and auxiliary anodes, which is commonly referred to as an impressed-current cathodic protection system. Then cathodic protection is a technique to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell [3]. 

Corrosion probes of the electrical resistance type are rarely used in concrete. They can suffer from localized pitting around the ends of the corrodible steel leading to rapid failure once corrosion initiates in this environment. They have been used to show that corrosion is under control, for example, in impressed current cathodic protection systems, but in other cases have been found to be poor indicators of rates of corrosion. 

Figure 9.14 Cathodic protection with an impressed current, (a) Leadjsilver anode for the protection of a ship s hull. Note that the anode is insulated from the hull and surrounded by a non-conducting shield to improve the potential distribution over the hull The anodes will be placed at intervals around the hull (b) Anode cans for the protection of an underground pipeline (some distance away). The anode cans contain a Fe/Si rod surrounded by coke breeze (to increase anode area), (c) Monitor and controlled power supply for a ship s corrosion protection system. Photographs supplied by Corrintec UK Ltd.

Where stray currents from a man-made source of direct current are a potential problem, metallically bonding the structure of concern to the source of the dc is often used to mitigate the corrosion that would otherwise occur. This is a common method of control where pipelines are subject to the adverse effects from stray currents generated by light rail transit systems, d-c welding machines, and impressed current cathodic protection systems installed on other nearby structures. 

Control of External Corrosion on Underground or Submerged Metallic Piping Systems Design, Installation, Operation, and Maintenance of Impressed Current Deep Groundbeds Corrosion Control of Electric Underground Residential Distribution Systems Design, Installation, Operation, and Maintenance of Internal Cathodic Protection Systems in Oil... 

The first reinforced concrete-impressed current CP system was an experimental system installed on a bridge support beam in 1959 [5]. A more advanced system was subsequently installed on a bridge deck in 1972 [6]. The anode system used in both applications was based on a conventional-impressed current CP system for pipelines, but "spread out" over a bridge deck. CP has since then become one of the few techniques that can be applied to control corrosion on existing structures. 

Figure 4.18 illustrates the principles based on an impressed anodic protection system. An active-passive metal possesses three regions in the polarization curve the active, the passive, and the transpassive regions. In the active region, the corrosion potential and corrosion current are controlled by the Tafel kinetics of the individual redox reactions. 

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