Sacrificial anode-based cathodic protection

Sacrificial anode-based cathodic protection versus active corrosion inhibition... 

Tunable sacrificial anode-based cathodic protection in AI-TM-RE coatings... 

In addition, cathodic protection based on sacrificial anodes requires a cell potential as the driving force for a self-inposed spontaneous protective current imparted by sacrificial anodes liberating electrons at a specific rate [42]. The driving force for this current is the potential difference (overpotential) between the sacrificial anode and cathode that is, E = Ec + o.In fact, the resultant... 

Corrosion in these areas is sometimes effectively controlled by cathodic protection with zinc- or aluminium-alloy sacrificial anodes in the form of a ring fixed in good electrical contact with the steel adjacent to the non-ferrous component. This often proves only partially successful, however, and it also presents a possible danger since the corrosion of the anode may allow pieces to become detached which can damage the main circulating-pump impeller. Cladding by corrosion-resistant overlays such as cupronickel or nickel-base alloys may be an effective solution in difficult installational circumstances. 

Copper-base alloys will corrode in aerated conditions. It is, therefore, sometimes appropriate to consider cathodic protection. It becomes particularly relevant when the flow rates are high or when the design of an item causes the copper to be an anode in a galvanic cell (e.g. a copper alloy tube plate in a titanium-tubed heat exchanger). Corrosion can be controlled by polarisation to approximately — 0-6V (vs. CU/CUSO4) and may be achieved using soft iron sacrificial anodes. 

By contrast a cathodic protection system based on sacrificial anodes is designed from the outset to achieve the required protection potential. If this is not achieved in practice there is no control function that can be exercised to improve the situation. Some remodelling of the system will be required. Moreover, the currents from each current source (the sacrificial anodes) is modest so that field gradients in the environment are not significant. It is at once clear that potential measurements are less significant in this case and instant-off measurements are neither necessary nor possible. 

Anode efficiency is of little practical significance and can be misleading. For example, magnesium alloy anodes often have an efficiency ca. 50% whilst for zinc alloys the value exceeds 90% it does not follow that zinc alloy anodes are superior to those based on magnesium. Efficiency will be encountered in many texts on sacrificial anode cathodic protection. 

Cathodic protection equipment has been used very successfully in water tanks and HW and steam boilers as anticorrosion devices for 100 years or more. Such equipment comes in many shapes and sizes, and comprises a sacrificial anode of either zinc or magnesium alloy, either bolted directly to a suitable internal water-wetted (cathodic) metal surface, or self-contained by enclosing the anode with a suitable cathode (such as a silver plated base metal). Usually several devices are required for any boiler, more for larger units and less for smaller ones, and these require replacement every one to two years. 

Installing direct current (DC) electrical-based remediation systems in urban areas also requires containment of stray voltage and current. DC systems can cause corrosion of buried gas and water lines or wreak havoc on cathodic protection systems. A good design can minimize the impacts, but sometimes, extra sacrificial anodes need to be installed to contain the electric field, adding to the cost of installation. 

Cathodic protection from sacrificial anodes is based on the principle of galvanic corrosion. This means that a less noble material is connected to the structure (metal) to be protected. To select the right saeriflcial anode material, the galvanic series is important. Table 19.1 shows the galvanic series for seleeted materials in seawater. The table indicates that magnesium, zinc, and alnminum alloys are well suited as sacrificial anodes when protecting steel. 

Coatings such as a zinc silicate primer covered with a layer of an epoxy-based polymer are routinely applied to steel structures to protect them against corrosion. However, cracks or flaws in the coating expose Fe which then undergoes oxidation in an anodic process. To prevent this, a second protection system is put in place cathodic protection. By placing a block of a more electropositive metal on the surface, this second metal is preferentially oxidized. This is the same principle as the use of zinc in galvanized steel (see Section 6.7). From Table 8.1, you can see why Zn, A1 and Mg (or alloys of these metals) are typically chosen as sacrificial anodes. The most electropositive metals (Li, Na, K and Ca) are unsuitable because they react with cold water. The relevant half-equations (at pH 7) are now ... 

Considering the concept of sacrificial anode (cathodic protection), coatings that consist of high purity zinc dust dispersed in organic and inorganic vehicles have been designed in these materials, when applied in film form, there are dose contacts of the particles among themselves and with the base or metallic substrate to be protected. 

The cathodic protection technology of vessel hulls is based on principles developed over many years. Sacrificial anodes are usually applied for the protection of vessels up... 

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