Magnesium corrosion reaction

The importance of magnesium chloride has probably been exaggerated. There is little doubt that it can act as a catalyst in corrosion reactions by hydrolysing to form hydrochloric acid, being then regenerated by reaction between ferrous chloride and magnesium hydroxide. There is, however, little evidence that this reaction takes place in cold- or hot-water systems, and it is probably confined to steam boilers where it might be a cause of corrosive attack underneath scale deposits it does not constitute a problem in a properly conditioned boiler water. 

PROBLEM 18.16 Magnesium is often attached to the steel hulls of ships to protect the steel from rusting. Write balanced equations for the corrosion reactions that occur (a) in the presence of Mg and (b) in the absence of Mg. 

The overall corrosion reaction of magnesium in aqueous solution may be written as ... 

The energy of a corrosion reaction is used to prepare a meal that has a self-contained heat source. The heat comes from a packet containing a powder made of a magnesium-iron alloy and a separate packet of salt water. When the contents of the two packets mix, the reaction between the metal, salt water, and oxygen in the air releases enough energy to heat the food by 100°C in 15 minutes. The process is used to provide heated food or beverages to military personnel, truck drivers, and sports fans. 

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. 

This battery has seen applications in many military devices. Like the zinc air battery, the shelf life is very long in the unused state, in this case because magnesium is heavily passivated by the aqueous electrolyte. When the cell is used, however, the magnesium metal is exposed to fresh electrolyte and a strong corrosion reaction is initiated. This uses up much of the magnesium in wasteful reaction. For some military applications, the long shelf life is more important than the... 

The corrosivity of a magnesium alloy is governed by the corrosion reactions of the individual constituent phases of the alloy. If an alloy contains constituents that are very reactive with a particular environment, the alloy will often have low corrosion resistance in that environment. The reactions of pure magnesium are of particular interest. These reactions provide the basis for understanding the corrosion of magnesium alloys. 

The overall corrosion reaction of magnesium alloys has not yet received systematic study. It is, however, reasonable to expect the corrosion reactions of magnesium alloys... 

Magnesium exhibits a very strange electrochemical phenomenon known as the negative-difference effect (NDE). Electrochemistry classifies corrosion reactions as either anodic or cathodic processes. Normally, the anodic reaction rate increases and the cathodic reaction rate decreases with increasing applied potential or current density. Therefore, for most metals like iron, steels, and zinc etc, an anodic increase of the applied potential causes an increase of the anodic dissolution rate and a simultaneous decrease in the cathodic rate of hydrogen evolution. On magnesium, however, the hydrogen evolution behavior is quite different from that on iron and steels. On first examination such behavior seems contrary to the very basics of electrochemical theory. 

Marine environments typically have high RH, as well as salt rich aerosols. Studies have shown that the thickness of the adsorbed layer of water on a zinc surface increases with percent RH and that corrosion rates increase with the thickness of the adsorbed layer. There also seems to be a finite thickness to the water layer that, when exceeded, can limit the corrosion reaction due to limited oxygen diffusion [4]. However, when metallic surfaces become contaminated with hygroscopic salts their surface can be wetted at a lower RH. The presence of magnesium chloride (MgCy on a metallic surface can make a surface apparently wet at 34 percent RH while sodium chloride (NaCl) on the same surface requires 77 percent RH to create the same effect [5]. 

Metallic impurities such as copper, nickel, iron, and cobalt cause corrosive reactions with the zinc in battery electrolyte and must be avoided particularly in zero mercury constructions. In addition, iron in the alloy makes zinc harder and less workable. Tin, arsenic, antimony, magnesium, etc., make the zinc brittle. ... 

This chapter presents electrochemical reactions and corrosion processes of Mg and its alloys. First, an analysis of the thermodynamics of magnesium and possible electrochemical reactions associated with Mg are presented. After that an illustration of the nature of surface films formed on Mg and its alloys follows. To comprehensively understand the corrosion of Mg and its alloys, the anodic and cathodic processes are analyzed separately. Having understood the electrochemistry of Mg and its alloys, the corrosion characteristics and behavior of Mg and its alloys are discussed, including self-corrosion reaction, hydrogen evolution, the alkalization effect, corrosion potential, macro-galvanic corrosion, the micro-galvanic effect, impurity tolerance, influence of the chemical composition of the matrix phase, role of the secondary and other phases, localized corrosion and overall corrosivity of alloys. 

The probable primary overall corrosion reaction for magnesium in aqueous solutions is ... 

Natural waters. The corrosivity of natural waters depends on their constituents, such as dissolved solids, gases, and sometimes colloidal or suspended matter. The effects may either stimulate or suppress the corrosion reaction. Constituents or impurities in water include dissolved gases such as oxygen, COj, SOj, NHj, HjS, some of which are the result of bacterial activity. Dissolved mineral salts are mostly calcium, magnesium sodium, bicarbonate, sulfate, chloride, and nitrate. The effect of each of these ions on corrosion rate is different, but the chlorides have received the most study in this regard. Organic contaminants of water can directly affect the corrosion rate of metals and alloys. Bacteria, under optimum conditions can double their number in 10-60 minutes. This characteristic is typical of the widespread biodeterioration caused by microbes in aU indnstries, of which corrosion is a special case. With a few exceptions such as synthetic polymers, all materials can be attacked by bacteria. 

Because electrochemical corrosion reactions proceed only in a liquid aqueous phase, the chemical composition and properties determined by chemical composition of this phase are most important to consider. This phase is largely water and will be called water in the subsequent discussion. Water enters the various refinery process units in a number of ways. Of prime importance is water that is entrained and/or emulsified in the crude oil charge to the refinery, i.e. the feed to the crude still. This water is produced with crude oil and remains with the crude, despite oil-field separators, liquid traps in pipelines, etc. Although the amount of water is usually small in total volume, its effect on corrosion may be large, since it usually contains a high proportion of corrosive dissolved salts, mainly chlorides of sodium, calcium, and magnesium. 

Corrosion and Finishing. With few exceptions, magnesium exhibits good resistance to corrosion at normal ambient temperatures unless there is significant water content ia the environment ia combination with certain contaminants. The reaction which typically occurs is described by the equation... 

---