Magnesium corrosion potentials

This definition covers all the behavior of magnesium - start of the NDE behavior at a cathodic external current density, or at a potential negative to the magnesium corrosion potential. This new definition describes the main features of NDE. 

Both silicon and aluminium are added to zinc to control the adverse effects of iron. The former forms a ferro-silicon dross (which may be removed during casting). Aluminium forms an intermetallic compound which is less active as a cathode than FeZn,] . Similarly in aluminium and magnesium alloys, manganese is added to control the iron . Thus in aluminium alloys for example, the cathodic activity of, FeAl, is avoided by transformation of FeAlj to (Fe, Mn)Al/. This material is believed to have a corrosion potential close to that of the matrix and is, therefore, unable to produce significant cathodic activity . 

Almost any epoxy adhesive can be used on magnesium provided that proper surface protection is maintained. In view of the corrosion potential, water-based adhesives or adhesives that allow water permeation may be expected to cause problems with magnesium substrates. 

Example 17.1 Plots for Mg Alloys Consider the global impedance response for an as-cast magnesium alloy AZ91 presented in Figure 17.11 for the AZ91 alloy at the corrosion potential after different immersion times in 0.1 M NaCl. What quantitative information can be obtained without considering a detailed process model such as discussed in Example 10.3 ... 

The polarisation resistance can be related to the rate of general corrosion for metals at or near their corrosion potential, corr. Polarisation resistance measurements are an accurate and rapid way to measure the general corrosion rate (CR) of bioinert Ti alloys, biotolerant austenitic surgical steels or biodegradable magnesium alloys. 

Magnesium and zinc are the predominantly used galvanic anodes for the cathodic protection of pipelines [13—16]. The corrosion potential difference of magnesium with respect to steel is 1 V, which Umits the length of the pipeline that can be protected by one anode. Economic considerations have led to the use of aluminum and its alloys as anodes. However, aluminum passivates easily, decreasing current output. To avoid passivation, aluminum is alloyed with tin, indium, mercury, or gallium. The electrochemical properties of these alloys, such as theoretical and actual output, consumption rate, efficiency, and open circuit (corrosion) potential, are given in Table 15.1. 

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. 

Thus, lead immersed in sulfuric acid, or magnesium in water, or iron in inhibited pickling acid, would be called passive by Def. 2 based on low corrosion rates, despite pronounced corrosion tendencies but these metals are not passive by Def. 1. Their corrosion potentials are relatively active, and polarization is not pronounced when they are made the anode of a cell. 

Figure 4.41 presents a series of cathodic polarization curves measured on iron [17] in de-aerated 4% NaCl solutions of different pH. The limiting current is highest at the lowest pH values, because the cathodic current is limited by proton transport. If, in such a system, the corrosion potential lies in the plateau region, the corrosion rate is entirely limited by the rate of mass transport of protons. Figure 4.42 shows the effect of HCl concentration on the corrosion rate of a magnesium cylinder rotating at different speeds. At concentrations under O.IM, the corrosion rate varies with the... 

Galvanic corrosion and the factors affecting it have been discussed in Chapter 20. However, a few precautionary comments are in order for aluminum, since it is anodic to most common materials of construction, with the exception of magnesium and zinc. In the presence of a good electrolyte, as little as 15 mV difference in corrosion potential of the two metals can have an effect, and if the difference is 30 mV or greater the anodic material will definitely corrode sacrificially to protect the contacting cathodic metal. A recently revised report on galvanic corrosion, with emphasis on automotive applications is available [73]. 

Many users do not recognize that aluminum alloys themselves span a range of about 400 mV in their respective corrosion potentials. In aerated sodium chloride solutions, pure aluminum, 3xxx alloys, and many other alloys have a potential of about —740 mV when measured with a saturated calomel reference electrode (SCE). Aluminum alloys with high magnesium or zinc contents will be more anodic by as much as 260 mV, while high copper content alloys will be more cathodic up to about 140 mV. Care must be taken therefore, that all alloys and tempers are compatible, even in an all aluminum structure. 

Table 4-3. Rates of corrosion of magnesium and various magnesium alloys at the free corrosion potential in 1 M NaCl solution at pH 11.

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. 

Table 3.2 Typical corrosion potential values [17] for common magnesium second phases (after 2h in de-aerated 5% NaCI solution saturated with Mg(OH)2 (pH 10.5))...

It is well known that the overall corrosion reaction of magnesium in aqueous solution at its corrosion potential can be expressed as follows ... 

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