Galvanic anodes pure metals

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 impressed current method with metal oxide-coated niobium anodes is usually employed for internal protection (see Section 7.2.3). In smaller tanks, galvanic anodes of zinc can also be used. Potential control should be provided to avoid unacceptably negative potentials. Pure zinc electrodes serve as monitoring and control electrodes in exposed areas which have to be anodically cleaned in the course of operation. Ag-AgCl electrodes are used to check these reference electrodes. 

The galvanic series of metals and alloys in seawater is given in Table 7.20. From this series it is clear that steel and 2024 aluminum are in close proximity. From their positions it is inferred that steel is cathodic and aluminum is anodic in seawater. The corrosion potentials of iron and aluminum measured after immersion in various media for 24 h are given in Table 7.21. It is seen from these data that the corrosion potentials of iron and aluminum are very nearly the same in 0.1M sodium chloride. Some studies on the galvanic action of the steel-aluminum couple in fresh waters such as pure, river, lake and underground water and salt solutions are noted in Table 7.22. In one of the studies, the... 

An electrochemical cell in which electrolysis takes place is called an electrolytic cell. The arrangement of components in electrolytic cells is different from that in galvanic cells. Typically, the two electrodes share the same compartment, there is only one electrolyte, and concentrations and pressures are far front standard. As in all electrochemical cells, the current is carried through the electrolyte by the ions present. For example, when copper metal is refined electrolytically, the anode is impure copper, the cathode is pure copper, and the electrolyte is an aqueous solution of CuS04. As the Cu2f ions in solution are reduced and deposited as Cu atoms at the cathode, more Cu2+ ions migrate toward the cathode to take their place, and in turn their concentration is restored by Cu2+ produced by oxidation of copper metal at the anode. 

It is obvious that in case of a defect down to steel, which leads to the enhanced anodic dissolution of zinc, the delamination of the purely alkaline cleaned galvanized steel surface is not faster than that of a phosphated surface. Such a behavior can be explained by an anodic delamination process. If the corrosion conditions are such that no formation of a cathode is possible in front of the anode, then just the kinetics of zinc dissolution determine the degradation of the polymer-metal composite. 

The commercial purification of copper metal is carried out in electrolytic cells. The anode is composed of impure ( blister ) copper, and the electrolyte is a mixture of aqueous CUSO4 and H2SO4. During purification, copper is effectively transferred from the anode to the cathode, and pure copper is thereby produced, (a) How does an electrolytic cell differ from a galvanic cell (b) Write half equations for the cathode and anode reactimis. (c) Is the overall cell reaction spontaneous If not, how does it occur ... 

Traditionally, metallic coatings serve only one or two functions. For example, zinc has excellent corrosion resistance and functions as a sacrificial anode (Tsura, 2005). Zinc galvanizing provides sacrificial cathodic protection and acts as a barrier (Jones, 1996) but does not nsnally supply inhibitor ions. The release of zinc ions during the sacrificial protection of galvanized steels (Tsuru, 2005 Pourbaix, 1974) only provides a small additional benefit compared with galvanic protection provided by the potential driving force. Metallic coatings used to protect Al alloys (Reddy et al., 2000 Walton et al, 1953) consist of a thin layer of nearly pure Al mechanically bonded to standard precipitation age... 

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