Galvanic anodes aluminum

Pure aluminum cannot be used as an anode material on account of its easy passivatability. For galvanic anodes, aluminum alloys are employed that contain activating alloying elements that hinder or prevent the formation of surface films. These are usually up to 8% Zn and/or 5% Mg. In addition, metals such as Cd, Ga, In, Hg and T1 are added as so-called lattice expanders, these maintain the longterm activity of the anode. Activation naturally also encourages self-corrosion of the anode. In order to optimize the current yield, so-called lattice contractors are added that include Mn, Si and Ti. 

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. 

Oxide, hydroxide and basic salts of aluminum are less soluble at pH values of about 7 than those of zinc [17], which explains the easy passivatability. Galvanic anodes of aluminum alloys are primarily employed in the area of offshore technology. The anodes work in relatively pure seawater flowing with a high velocity so that by using suitable alloys, passivation phenomena are rare. Their low weight is particularly favorable in view of a service time of 20 to 30 years. 

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. 

Due to its galvanic activity (aluminum is anodic to most common metals), aluminum is often used as a sacrificial anode and hence it corrodes preferentially to protect less active metals such as carbon steels. For evaluating aluminum alloys, contact ... 

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. 

However, the current outputs from galvanic anodes for all practical purposes are relatively small and depend much on the electrical resistivity of the local environment of the structure to be protected. Thus, anodes made of aluminum, zinc, and their alloys, with potential difference relative to steel of about 0.5 V, are limited to applications in high-conductivity electrolytes like seawater. 

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. 

FIGURE 5.175 In a galvanic couple of aluminum and steel, anodic aluminum corrodes while cathodic steel is relatively unaffected. It is important that fasteners be compatible with building materials. 

What happens is this the aluminum and the dental filling act as electrodes in a galvanic cell. Aluminum acts as the anode, and the amalgam acts as the cathode. In effect, when the aluminum comes into contact with the amalgam, an electrochemical cell is established in the mouth, causing a current to flow. This current stimulates the nerve of the tooth, causing a very unpleasant sensation. 

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... 

The high consumption rates, as well as low-driving voltage, are the primary disadvantages of the galvanic anode method of cathodic protection. The open circuit potential between steel and magnesium is on the order of 1 V, while zinc and aluminum are somewhat less. Thus, with this method, it is imperative that a low-resistance circuit be established by installation of many anodes in a low-resistance medium. The anodes installed should also be sized in accordance with their respective consumption rates to provide the necessary design life. 

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