Galvanic anodes magnesium

G lv nic Corrosion. Galvanic corrosion is an electrochemical process with four fundamental requirements (/) an anode (magnesium), 2) a cathode (steel, brass, or graphite component), (J) direct anode to cathode electrical contact, and (4) an electrolyte bridge at the anode and cathode interface, eg, salt water bridging the adjacent surfaces of steel and magnesium components. If any one of these is lacking, the process does not occur (133,134). 

The cathodic protection of plain carbon and low-alloy steels can be achieved with galvanic anodes of zinc, aluminum or magnesium. For materials with relatively more positive protection potentials (e.g., stainless steels, copper, nickel or tin alloys), galvanic anodes of iron or of activated lead can be used. 

Magnesium anodes usually consist of alloys with additions of Al, Zn and Mn. The content of Ni, Fe and Cu must be kept very low because they favor selfcorrosion. Ni contents of >0.001% impair properties and should not be exceeded. The influence of Cu is not clear. Cu certainly increases self-corrosion but amounts up to 0.05% are not detrimental if the Mn content is over 0.3%. Amounts of Fe up to about 0.01% do not influence self-corrosion if the Mn content is above 0.3%. With additions of Mn, Fe is precipitated from the melt which on solidification is rendered harmless by the formation of Fe crystals with a coating of manganese. The addition of zinc renders the corrosive attack uniform. In addition, the sensitivity to other impurities is depressed. The most important magnesium alloy for galvanic anodes is AZ63, which corresponds to the claims in Ref. 22. Alloys AZ31 and M2 are still used. The most important properties of these alloys are... 

The quality control of galvanic anodes is reduced mainly to the analytical control of the chemical composition of the alloy, to the quality and coating of the support, to an adequate joint between support and anode material, as well as to restricting the weight and size of the anode. The standards in Refs. 6, 7, 22, 27, 31 refer to magnesium and zinc anodes. Corresponding specifications for aluminum anodes do not exist. In addition, the lowest values of the rest potentials are also given [16]. The analytical data represent the minimum requirements, which are usually exceeded. 

Osborn, O. and Robinson, H. A., Performance of Magnesium Galvanic Anodes in Underground Service , Corrosion, April (1952)... 

Severe corrosion may occur in active A1 or Mg alloys in neutral solutions of heavy metal salts (salts of Cu, Fe, or Ni). This type of corrosion occurs when the heavy metal salts plate out to form active cathodes on the anodic magnesium surface. This type of galvanic corrosion can lead to localized pitting corrosion. 

Magnesium and zinc are the predominantly used galvanic anodes for the cathodic protection of pipelines [13—16]. The corrosion potential difference of magnesium with respect to steel is 1 V, which Umits the length of the pipeline that can be protected by one anode. Economic considerations have led to the use of aluminum and its alloys as anodes. However, aluminum passivates easily, decreasing current output. To avoid passivation, aluminum is alloyed with tin, indium, mercury, or gallium. The electrochemical properties of these alloys, such as theoretical and actual output, consumption rate, efficiency, and open circuit (corrosion) potential, are given in Table 15.1. 

Aluminium and magnesium and their alloys are also used in galvanic anode cathodic protection systems. One advantage of these alloys is that they are lighter than zinc. However, their oxides and other corrosion products are voluminous and could attack the concrete. They are therefore less attractive for concrete applications. 

Galvanic anode gal- va-nik a- nod. Source of current for cathodic protection provided by a metal less noble than the one to be protected, e.g., magnesium, zinc or aluminum as used for cathodic protection of steel. Baboian R (2002) Corrosion engineer s handbook, 3rd edn. NACE International -The Corrosion Society, Houston, TX. 

Anticorrosion protection of metal structures using sacrificial magnesium, zinc, and aluminum anodes is the oldest and, at the same time, the simplest method of electrochemical protection. Galvanic anodes are mainly used in cases where the structure is covered with a good insulation coating and low currents are required for protection, and also when lack of a power supply makes realization of cathodic protection impossible. 

In the galvanic or impressed current system, the metallic structure is made the cathode (negative) by connecting it to galvanic anodes, which are more negative than the metallic structure to be protected. In this system, the current is generated by the corrosion of active metals, such as magnesium, zinc and also aluminum, which are... 

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