Corrosion control magnesium anodes

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

Deaeration has occasionally been used as a means of controlling bimetallic corrosion under conditions of total immersion, and this method of control can be used successfully, if physical conditions permit, provided that the less noble metal is not sufficiently electrochemically active to permit rapid evolution of hydrogen at the more noble metal, as is observed, for instance, in many bimetallic couples involving magnesium anodes. 

Corrosion control technology is a mainstay of automobile coatings as well as household appliances for example, the lifetime of water heaters is extended and often governed by the presence of a magnesium sacrificial anode that represents a small fraction of the appliance price. 

Anodized films are most often applied to protect aluminum alloys. However there are processes for other metals such as titanium, zinc, and magnesium. Anodized titanium is used in dental implants and sometimes in art and costume jewelry because it generates various colors without dyes. Each color depends on a specific thickness of the oxide [9]. To ensure the preparation of a consistent oxide layer, one must control conditions such as electrolytic concentrations, acidity, and current. Also a sealing process is often needed to achieve corrosion resistance because thick coatings (oxide layers) are generally porous. 

Naturally occurring oxide films on most metals do not usually provide optimum corrosion protection, and this may be modified or replaced to provide a further means of corrosion control. Common examples are the anodizing of aluminium alloys or the chromating of aluminium, zinc, cadmium or magnesium. With anodizing, the natural oxide film on the aluminium is thickened electrolytieally by up to 5 p.m. Chromating, described in detail later in the chapter, replaces the existing metal oxide film with a mixed chromium/metal oxide film of better corrosion resistance. 

The magnesium anode can be obtained as a bare or packaged anode. If packaged, the anode is delivered in a prepared backfill consisting currently of 75% gypsum, 20% bentonite and 5% sodium sulfate contained in a cloth bag (Farwest Corrosion Control Company, 2009). Where the soil conditions are poor it is customary to surrouud magnesium anodes with a mixture of... 

Song, G, Shi, Z, Hinton, B, McAdam, G, Talevski, J Gerrard, D. (2006), Electrochemical evaluation of the corrosion performance of anodized magnesium alloys. In 14th Asian-Pacific Corrosion Control Conference, Shanghai, China. Keynote-11. 

The reserve battery is activated by electrolyte addition, either manually or automatically, by electrical or mechanical means. The heat of corrosion of the magnesium anode, at a controlled rate, enhances operation over a wide range of temperature and discharge rates. The batteries are non-hazardous, being vented to atmosphere, and non-explosive the electrolyte is far less corrosive than that used in conventional alkaline batteries. 

A serious limitation of the use of anodic inhibitors is that they must be used in sufficiently high concentration to eliminate all the anodic sites, otherwise the anodic area that remains will carry the whole corrosion current, which is usually cathodically controlled. Intense local corrosion may then result, possibly leading to failure of the specimen. Cathodic inhibitors, on the contrary, are helpful in any concentrations for example, the blanketing of only half the cathodic surface will still roughly halve the corrosion rate. The presence of temporary hardness or magnesium ions can help reduce corrosion through deposition of CaCOs or Mg(OH)2, specifically on the cathodic surfaces where OH is produced in the oxygen absorption reaction ... 

Phenomena of this type are well known and exploited typically when we protect by means of passivating and anodic oxidation treatments, that is, when we precorrode" in controlled environments materials such as magnesium, aluminum, zinc, copper, and stainless steel in order to create a layer of protective corrosion products able to reduce, if not to cancel, their corrosion rate in the different environments. Obviously, any heterogeneity in the topography of the layer of corrosion products, even if in the presence of homogeneous environments and homogeneous materials, may involve the localization of the cathodic and anodic processes and the consequent localized corrosion. 

Both resistance of the electrolyte and polarization of the electrodes limit the magnitude of current produced by a galvanic cell. For local-action cells on the surface of a metal, electrodes are in close proximity to each other consequently, resistance of the electrolyte is usually a secondary factor compared to the more important factor of polarization. When polarization occurs mostly at the anodes, the corrosion reaction is said to be anodically controlled (see Fig. 5.7). Under anodic control, the corrosion potential is close to the thermodynamic potential of the cathode. A practical example is impure lead immersed in sulfuric add, where a lead sulfate film covers the anodic areas and exposes cathodic impurities, such as copper. Other examples are magnesium exposed to natural waters and iron immersed in a chromate solution. 

Galvanic corrosion or bimetallic corrosion is important to consider since most of the structural industrial metals and even the metallic phases in the microstructure alloys create galvanic cells between them and/or the a Mg anodic phase. However, these secondary particles which are noble to the Mg matrix, can in certain circumstances enrich the corrosion product or the passive layer, leading to a decrease or a control of the corrosion rate. Severe corrosion may occur in neutral solutions of salts of heavy metals, such as copper, iron and nickel. The heavy metal, the heavy metal basic salts or both plate out to form active cathodes on the anodic magnesium surface. Small amounts of dissolved salts of alkali or alkaline-earth metal (chlorides, bromides, iodides and sulfates) in water will break the protective film locally and usually lead to pitting (Froats et al., 1987 Shaw and Wolfe, 2005). 

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