Sacrificial anode materials

Examples of the sacrificial-anode method include the use of zinc, magnesium, or aluminum as anodes in electrical contact with the metal to be protected. These may be anodes buried in the ground for protection of underground pipe lines or attachments to the surfaces of equipment such as condenser water boxes or on ship hulls. The current required is generated in this method by corrosion of the sacrificial-anode material. In the case of the impressed emf, the direct current is provided by external sources and is passed through the system by use of essentially nonsacrificial anodes such as carbon, noncor-rodible alloys, or platinum buried in the ground or suspended in the electrolyte in the case of aqueous systems. 

In practice, with one minor exception (pure zinc), the commercially pure metals are unsuitable as sacrificial anode materials. This is because they fail to meet one or more of the pre-requisites outlined above. In each generic type of material alloying elements are added to ensure more acceptable properties. 

Magnesium is the predominant sacrificial anode material for onshore use. 

Tests of sacrificial anode materials are generally conducted for three reasons for screening (or ranking), performance information and quality control. 

The total net weight of sacrificial anode material is determined from the following formula... 

The sacrificial anode method is the practical in-service application of galvanic cell. Hence, the corrosion potential for a sacrificial anode material must be lower than that of the structure to be protected. 

A sacrificial anode material must also polarize the structure sufficiently (i.e., bring the material of the structure to its equilibrium potential). This implies that the potentials of the structure and the anode will polarize toward each other when coupled, with the anode affording the protection. 

Sacrificial anode material Composition Typical anodic current density (A.m") Consumption rate (g-A-. yr ) Cost per unit surface area (US /m ) 1-mm-thick anode Notes... 

The advantage of this technique over the impressed current is that it does not require a power supply since the structure and the anode are coupled through a wiring system or by mounting the anode on the structure forming a galvanic cell. Table 8.2 lists relevant data for common sacrificial anode materials used... 

Table 8.2 Sacrificial anode materials and operating parameters for protecting steel structures [2,6].

Table 8-10. Topical information on sacrificial anode materials.

Modern specifications for the cathodic protection of active oceangoing ships were first described in 1950 [4]. Since that time progress has been rapid. Considerable advances in cathodic protection technology have been made, better sacrificial anode materials have... 

The corrosion process of a sacrificial Mg alloy is evaluated in the laboratory in either 3% NaCl solution or a solution containing 5g CaS04. 2H2O and 0.1 g Mg (0H)2/L), as recommended after the ASTM G97 standard. Beside the ASTM procedure, there is also the Mexican test method (NMX-K-109-1977, Magnesium anodes used in cathodic protection ) that considers a test environment made of artificial seawater. Both standards consider galvanostatic tests, in which a known direct current is passed through test cells connected in series in order to determine efficiency of sacrificial anode materials (Guadarrama-Mu-oz et al., 2006). 

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