Sacrificial anode output

Sacrificial anode systems operate without external power source. The anodes are reactive metals such as magnesium and zinc or aluminum alloys. The energy for the process is derived from the anode material. Careful design is required to match the output and lifetime of the anodes with the polarization and life-expectancy requirements of the plant. Sacrificial anode CP is used for offshore platforms, sub-sea pipelines and the inside of ballast tanks on tanker ships. 

The question of attenuation is not significant in the case of sacrificial anode protection systems where the individual source outputs are small. 

The proximity of the anodes to structures is also important. For example, if the sacrificial anodes are placed on, or very close to, steel pipework in soil then the output from the face of the anodes next to the steelwork can be severely limited. Alternatively, in high conductivity environments, corrosion products may build up and wedge between the anode and the structure. The resulting stresses can lead to mechanical failure of the anode. On the other hand, when anodes are located at an appreciable distance from the steelwork, part of the potential difference will be consumed in overcoming the environmental resistance between the anode and cathode. 

The reason for the use of zinc as a power-impressed rather than a sacrificial anode is that the high concrete resistivity limits the current output, and a higher driving voltage than that provided by the e.m.f. between zinc and steel in concrete is used to provide the necessary current output. No cementitious overlay is required, although it may be advisable to paint the top surface of the sprayed zinc to prevent atmospheric corrosion of the zinc anode. 

The sacrificial anode system consists of a galvanic cell system in which the anode is made of a more active metal than the structure. The anode is attached to the structure and the anode output current may be measured. Magnesium and zinc anodes are commonly use in underground operations, zinc and aluminum alloy anodes in salt water. 

The basic design of sacrificial CP system includes calculation of cathodic protection circuit resistance, potential difference between the anode and structure, anode output, number of anodes, and the anode life expectancy. A schematic of the cathodic protection test is given in Fig. 15.11. To estimate current requirements, a test is needed to determine the current i ) necessary to provide adequate protection for the pipeline. This can be done by applying current using a temporary test setup and adjusting the current from the rectifier until the cathodic protection criteria is reached. 

For bare steel on offshore structures in seawater, the sacrificial anodes are commonly made of special aluminium alloys because these give the highest eurrent output for a certain anode weight (Tables 10.15 and 10.16) as well as flie lowest eost. Zn anodes are usually applied on coated and buried pipelines offshore, where flie risk for passivation of A1 anodes is higher due to a lower CD. 

For pipe systems of stainless steel carrying chlorinated seawater, internal localized corrosion can be very efficiently prevented by the application of Resistance-controlled Cathodic Protection (RCP). A resistance is simply inserted between the sacrificial anode and the pipeline, and this makes a system that is particularly suitable when there is a low diffusion-limiting cathodic current in the critical potential range [10.29]. Typical of the method is that the current output from the anode is kept low, which has the consequence that the voltage drops are low and the protected pipe length from each anode is long. 

The anodes should be self-regulating. However, because their current output is usually less than their output capability controlled by the difference in potential between the two metals and calculated resistance as described in Section 16.7, and which is a function of the anode geometry, the current from the anodes is not normally controllable. Thus, local changes in the structure as a result of probable deterioration of applied coatings in service, which will cause an increase in protection current demand, may necessitate the installation of additional sacrificial anodes to maintain protection. However, if adequate calculations using the anode utility factor and anode resistance (Section 16.7) are performed and the benefits of experience and hindsight are employed, the disruptions to the system for maintenance purposes will be minimal. 

Sacrificial anodes are normally electrically connected to the structure to be protected through welding or bolting. One way to monitor the current output from selected anodes is to connect the anode through a resistor with known value to the structure. By measuring the potential drop across the known resistor, the current from the anode can be calculated. This principle is shown schematically in Figure 19.5. The use of a Swain meter is an alternative if all the anodes are electrically connected without any resistor. 

FIGURE 19.5 Sacrificial anode instrumented for monitoring current output [5]. 

In this equation U is the output voltage of the power supply. The fact that its value can be adjusted according to needs gives more flexibility for controlling the current than the use of a sacrificial anode, whose corrosion potential in a given environment can not be varied. 

As a result of self-corrosion of sacrificial anodes, leading to loss of their material, the real output differs from the theoretical cur-... 

Zinc used as a sacrificial material should be characterized by high purity (99.99% Zn, less than 0.003% Fe). The presence of impurities such as iron, copper, and lead very negatively affects the work of a sacrificial anode. They cause passivation of the surface of zinc as a result of which the polarization current decreases in the protection system and the current output is decreased. In order to improve the sacrificial properties of zinc, small amounts of alloy additives are introduced. The following have an advantageous effect aluminum (0.1-0.5% Al) and cadmium (0.02-0.15% Cd), and aluminum (0.5% Al) and silicon (0.1% Si). 

Magnesium is characterized by a high theoretical current output, but simultaneously undergoes strong self-corrosion, especially in the presence of such impurities as iron, nickel, copper, silicon, lead, and tin. In such conditions, the current efficiency can be equal to only 25%. Magnesium sacrificial anodes are used for the protection of metal structures in sea water, fresh water, and soils (resistivity of soil < 100 m). The... 

In the discussion of cathodic protection monitoring, two important distinct areas can be identified. The first domain lies in monitoring the condition and performance of the CP system hardware. Monitoring of rectifier output, pipe-to-soil potential and current measurements at buried sacrificial anodes, inspection of bonds, fuses, insulators, test posts, and permanent reference electrodes are relevant to this area. The second domain concerns the condition of the pipeline (or buried structure) itself and largely deals with surveys along the length of the pipeline to assess its condition and to identify high corrosion-risk areas. 

Avoid using too few high output anodes relatively poor distribution efficiency increases the requirement of current (applies to sacrificial anode and impressed current systems). 

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