Magnesium corrosion products

In neutral and alkaline environments, the magnesium hydroxide product can form a surface film which offers considerable protection to the pure metal or its common alloys. Electron diffraction studies of the film formed ia humid air iadicate that it is amorphous, with the oxidation rate reported to be less than 0.01 /rni/yr. If the humidity level is sufficiently high, so that condensation occurs on the surface of the sample, the amorphous film is found to contain at least some crystalline magnesium hydroxide (bmcite). The crystalline magnesium hydroxide is also protective ia deionized water at room temperature. The aeration of the water has Httie or no measurable effect on the corrosion resistance. However, as the water temperature is iacreased to 100°C, the protective capacity of the film begias to erode, particularly ia the presence of certain cathodic contaminants ia either the metal or the water (121,122). 

Internal surfaces were covered with a tan deposit layer up to 0.033 in. (0.084 cm) thick. The deposits were analyzed by energy-dispersive spectroscopy and were found to contain 24% calcium, 17% silicon, 16% zinc, 11% phosphorus, 7% magnesium, 2% each sodium, iron, and sulfur, 1% manganese, and 18% carbonate by weight. The porous corrosion product shown in Fig. 13.11B contained 93% copper, 3% zinc, 3% tin, and 1% iron. Traces of sulfur and aluminum were also found. Near external surfaces, up to 27% of the corrosion product was sulfur. 

Filiform corrosion is characterised by the formation of a network of threadlike filaments of corrosion products on the surface of a metal coated with a transparent lacquer or a paint him, as a result of exposure to a humid atmosphere. This phenomenon first attracted attention because of its formation on lacquered steel, and for this reason it is sometimes referred to as underfilm corrosion, but although it is most readily observed under a transparent lacquer it can also occur under an opaque paint film or on a bare metal surface. Filiform corrosion has been observed on steel, zinc, magnesium and aluminium coated with lacquers and paints, and with aluminium foil coated with paper. Surface treatment of the metal by phosphating or chromating lessens the tendency for filiform corrosion to occur, but it is not completely... 

Rust of iron (the most abundant corrosion product), and white rust of zinc are examples of nonprotective oxides. Aluminum and magnesium oxides are more protective than iron and zinc oxides. Patina on copper is protective in certain atmospheres. Stainless steels are passivated and protected, especially in chloride-free aqueous environments due to a very thin passive film of Cr2C>3 on the surface of the steel. Most films having low porosities can control the corrosion rate by diffusion of reactants through the him. In certain cases of uniform general corrosion of metals in acids (e.g., aluminum in hydrochloric acid or iron in reducible acids or alkalis), a thin him of oxide is present on the metal surface. These reactions cannot be considered hlm-free although the him is not a rate-determining one.1... 

In pure H2Cr04, the magnesium is attacked at a very low rate. Boiling 20% H2Cr04 solution can be used to remove corrosion products from magnesium. 

Chloride ions attack oxide layers on iron, aluminum, and magnesium. Subsequently, the metal is electrochemically dissolved. The hydration of Fe " ", AP" ", or Mg " " releases protons and thereby leads to an acidification of the tip of the filament. At the cathodic site, the primary cathodic reaction, the reduction of oxygen to hydroxyl ions takes place. In between the anode and the cathode a potential gradient is estahhshed, which forces anions to migrate to the front and cations to the back. As the distance from the anode increases, the pH also increases on the basis of the dilution of hydronium ions and the migration of hydroxyl ions from the cathodic site. When favorable conditions are reached, the corresponding hydroxides of the cations are formed as gels. As the head advances, these hydrated corrosion products lose their water and convert to the dry corrosion products that fill the tail see Ref. [168] and references therein. 

M. Jonsson, D. Persson, D. Thierry, Corrosion product formation during NaCl induced atmospheric corrosion of magnesium alloy AZ91D, Corros. Sci. 49 (2007) 1540—1558. 

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. 

Aluminium and magnesium and their alloys are also used in galvanic anode cathodic protection systems. One advantage of these alloys is that they are lighter than zinc. However, their oxides and other corrosion products are voluminous and could attack the concrete. They are therefore less attractive for concrete applications. 

The reactions of magnesium sulphate occur analogously, but the reaction (6.19), that is the decalcification of C-S-H, occnrs more rapidly. This is due to a veiy poor solubility of magnesium hydroxide, known as bmcite. This hydroxide is precipitated as a white gel, forming a layer of corrosion products, mainly on the concrete surface. This layer, when dense and compact, plays a role of a barrier, impeding the diffusion of sulphate ions into the concrete. However, a ciystalline bracite can be occasionally formed [80]. 

Airborne contaminants, in the form of dust, corrosion products, and residues from paints, etc., can introduce many different elements into a sample, e.g., sodium, potassium, calcium, magnesium, and aluminum. This is a significant threat where open reaction vessels are used, and can be avoided by simple precautionary measures such as covering the dissolution vessel with a dish or beaker equipped with a side arm through which a stream of filtered air or gas is passed. More stringent measures include the use of a clean air room kept imder positive pressure, with the reaction itself carried out imder a laminar flow hood. Gaseous contaminants can be eliminated from the reaction vessel s air supply using appropriate adsorption filters. 

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