Aluminum alloys sacrificial anodes

Ship hulls Painting cannot always protect hostile marine conditions, in ships and, areas above keel blocks. Stem and mdder areas suffer erosion and corrosion due to the high turbulence caused by the propeller coupled with the galvanic effects of the noble bronze propeller. Effective cathodic protection of ship hulls and similar marine structures in seawater against corrosion can be apphed using either aluminum or zinc alloy sacrificial anodes. Twenty percent of the anodes required for full hull protection are required for stern protection only. 

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. 

Sacrificial anode systems operate without external power source. The anodes are reactive metals such as magnesium and zinc or aluminum alloys. The energy for the process is derived from the anode material. Careful design is required to match the output and lifetime of the anodes with the polarization and life-expectancy requirements of the plant. Sacrificial anode CP is used for offshore platforms, sub-sea pipelines and the inside of ballast tanks on tanker ships. 

This method uses a more active metal than that in the structure to be protected, to supply the current needed to stop corrosion. Metals commonly used to protect iron as sacrificial anodes are magnesium, zinc, aluminum, and their alloys. No current has to be impressed to the system, since this acts as a galvanic pair that generates a current. The protected metal becomes the cathode, and hence it is free of corrosion. Two dissimilar metals in the same environment can lead to accelerated corrosion of the more active metal and protection of the less active one. Galvanic protection is often used in preference to impressed-current technique when the current requirements are low and the electrolyte has relatively low resistivity. It offers an advantage when there is no source of electrical power and when a completely underground system is desired. Probably, it is the most economical method for short life protection. 

Nanostructured aluminum [74—78], iron [74] and aluminum-manganese alloys [74] have been prepared from a Lewis acid A1Q3/[BMIM]Q mixture (65 mol% AICI3, 35 mol% [BMIMJC1) whereas palladium alloys have been deposited from a Lewis basic system (45 mol% Aid , 55 mol% [BMIMJC1). The electrochemical cell and all parts which are in contact with the electrolyte have to be built from inert materials. As cathode material glassy carbon can be used. A constant ion concentration in the electrolyte can be realized by the use of a sacrificial anode consisting of the... 

The sacrificial anode system consists of a galvanic cell system in which the anode is made of a more active metal than the structure. The anode is attached to the structure and the anode output current may be measured. Magnesium and zinc anodes are commonly use in underground operations, zinc and aluminum alloy anodes in salt water. 

Some measures involving cathodic protection of aluminum using zinc as the sacrificial anode,47 and protection of aluminum ship hulls are found in the literature.37 Overprotection may lead to alkali (cathodic corrosion) attack. Alclad alloy systems are also some form of self-contained cathodic protection systems. 

The sacrificial anode system typically uses magnesium, zinc or aluminum and their alloys Figure 7.25. These metals or alloys act as anodes when coupled with steel and its... 

Sacrificial anode — is a piece of metal used as an anode in electrochemical processes where it is intended to be dissolved during the process. In -+ corrosion protection it is a piece of a non-noble metal or metal alloy (e.g., magnesium, aluminum, zinc) attached to the metal to be protected. Because of their relative -+ electrode potentials the latter is established as the -+ cathode und thus immune to corrosion. In -+ electroplating the metal used as anode may serve as a source for replenishing the electrolyte which is consumed by cathodic deposition. The sodium-lead alloy anode used in the electrochemical production of tetraethyl lead may also be considered as a sacrificial anode. 

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 ... 

Sacrificial Anodes Incontrastto the impressed current technique, the use of sacrificial anodes does not depend on the creation of driven electrochemical cell. Rather, a galvanic cell is formed between the structure and the sacrificial anode in which electrons pass spontaneously from the latter to the former (Fig. 9). Thus, the source of the electrons (the sacrificial anode) must have a more negative electrode potential than the structure. It was for this reason that Humphrey Davy chose zinc or iron to protect copper, and it also explains why magnesium, aluminum and zinc alloys are used to protect steel today. 

Aluminum and aluminum-zinc alloy anodes have become the preferred sacrificial anodes for the cathodic protection of offshore platforms. This preference is because aluminum anodes demonstrate reliable long-term performance when compared with magnesium, which might be consumed before the platform has served its useful hfe. Aluminum also has better current/weight characteristics than zinc. Weight can be a major consideration for large offshore platforms. The major disadvantage of aluminum for some applications, for example, the protection of painted ship hulls, is that aluminum is too corrosion resistant in many environments. Aluminum alloys will not corrode reliably onshore or in freshwater [37]. In marine... 

The two types of cathodic protection are (i) sacrificial and (ii) impressed current systems. The sacrificial anode system typically uses magnesium, zinc, or aluminum and their alloys (Fig. 5.25). These metals or alloys act as anodes when coupled with steel and its alloys. These metals or alloys act as anodes when coupled with steel and preferentially corrode. Magnesium is often used in fresh water media while zinc and aluminum are used in seawater and brackish water media. 

Bond failure can also occur if the surface is anodic relative to another joint component. An example would be clad aluminum adherends where a thin layer of pure aluminum overlays the base alloy. Such a surface layer is designed to be more corrosion resistant than the alloy, but to act as a sacrificial anode should corrosion occur. Although this approach works well for corrosion protection of the substrate material, it can be a disaster for bonded material if the adherend surface/interface corrodes. As a result, American companies tend to use unclad aluminum for bonding and provide other means of corrosion protection, such as painting [1,70]. On the other hand, European companies commonly use clad adherends, but with a thicker oxide (CAA) [6,18,71-73] that provides bondline corrosion protection. 

Galvanized steel is a common example of galvanic coupling where steel (Fe), with a standard electrode potential of —0.440 V vs. SHE, is cathodicaUy protected by zinc, which has a more active standard electrode potential of —0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects steel from corrosion through its sacrificial properties. Because zinc is less noble than iron in terms of the standard electrode potentials, it acts as an anode. The sacrificial anode (zinc) is continuously consumed by anodic dissolution reaction and protects the underlying metal (iron in steel) from corrosion. In practice, sacrificial anodes are comprised of zinc, magnesium alloys, or aluminum. 

The two principal alloys of aluminum usually employed as sacrificial anodes are Al-Zn-Hg and Al-Zn-I. These are exclusively used in seawater, which is a major disadvantage. As well, they spark when struck with rusting iron and therefore may not be very useful in the petroleum industries. In addition, these alloys passivate when operated in the cold. This problem is worse in muddy environments. Another limitation to their usage is that they produce poisonous dissolution products. 

Cathodic control protection protects the substrate by coating with a less noble metal, for which the slopes of the cathodic polarization curves are steep. The cathodic overpotential of the surface is increased by the coating therefore, the corrosion potential becomes more negative than that of the substrate. Coating materials used for this purpose are zinc, aluminum, manganese, cadmium, and their alloys. The electrode potential of these metals are more negative than those of iron and steel. When exposed to the environment, these coatings act as sacrificial anodes for the iron and steel substrates. 

Aluminum and 55% Al-Zn alloy provide galvanic protection for the steel substrate. In rural and industrial atmospheres an aluminum coating does not act as a sacrificial anode. However, in a chloride atmosphere, such as a marine area, it does act as a sacrificial anode. 

Zinc used as a sacrificial material should be characterized by high purity (99.99% Zn, less than 0.003% Fe). The presence of impurities such as iron, copper, and lead very negatively affects the work of a sacrificial anode. They cause passivation of the surface of zinc as a result of which the polarization current decreases in the protection system and the current output is decreased. In order to improve the sacrificial properties of zinc, small amounts of alloy additives are introduced. The following have an advantageous effect aluminum (0.1-0.5% Al) and cadmium (0.02-0.15% Cd), and aluminum (0.5% Al) and silicon (0.1% Si). 

In practice. Mg alloys are much more popular than pure Mg in industrial applications. One of the well-known uses of Mg alloys is for cathodic protection, and some Mg alloys are used as sacrificial anodes due to their negative potential. Although their rapid rate of corrosion is a disadvantage. Mg alloys are superior to aluminum and zinc anodes in some environments such as in soil and water. The most important application of Mg alloys is for aerospace and military purposes, and some high-strength and creep-resistant... 

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