Negative-difference effect

When the potential of the magnesium electrode is made more positive, the rate of Mg+ ion formation increases, and with it that of reaction (16.5). Therefore, the rate of hydrogen evolution increases instead of falling off, with increasing anodic polarization of the magnesium (see Section 13.7). This phenomenon has become known as the negative difference effect. 

OCP of magnesium is 0.6 to 1.1 V more positive than the thermodynamic value. The negative difference effect that is seen in anodic magnesium dissolution (see Section 16.2) can, in part, be attributed to partial mechanical disintegration of the passivating layer during magnesium dissolution. 

The behavior of aluminum in neutral and weakly alkaline solutions resembles the behavior of magnesium, but the negative difference effect is much less pronounced at aluminum. The steady-state potential of aluminum is approximately 1V more positive than the thermodynamic value. Yet unlike magnesium, aluminum will not passivate in strongly alkaline solutions, but undergoes fast dissolution to soluble aluminates. 

Considerations of the negative difference effect [see Section III(5(iv))] indicate the second case to be the likely one, but no direct experimental evidence has been obtained so far. 

When aluminum is anodically dissolved in halide solutions, the rate of hydrogen evolution linearly increases with increasing current density as shown in Fig. 25. This phenomenon is historically, and somewhat misleadingly, termed the negative difference effect 124 (NDE). It is contrary to what one would normally expect, for hydrogen evolution should subside with the potential going positive (as indeed is observed in alkaline solutions) or at least stay constant at a constant-potential plateau. 

Reaction (10.92) is a chemical reaction that results in hydrogen evolution at potentials associated with Mg dissolution. Due to the anomalous production of hydrogen at anodic rather than cathodic potentials, this reaction was termed the Negative difference effect (NDE). The result presented in Example 10.3 can be obtained from that presented by Baril et al. by setting their from reaction (10.92) equal to zero. 

T.R. Thomaz, C.R. Weber, T. Pelegrini, L.F.P. Dick, G. Knomschild, The negative difference effect of magnesium and of the AZ91 alloy in chloride and stannate-containing solutions, Corros. Sci. 52 (2010) 2235-2243. 

Neither theories, however, explain the negative difference effect and this is considered independently as being due to three possible causes (a) the... 

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. 

Figure 4-7. The negative-difference effect (Song et al., 1997b).

The phenomenon that the measured deviates from the normally expected Iq is designated as the negative-difference effect (NDE). The NDE is usually defined for a galvanostatic applied current ppi, in terms of the difference A given by ... 

Mg" might be involved in the negative-difference effect as illustrated in Fig. 4-10, because the calculated valence of dissolved magnesium has been reported to be in the range 1.33 to 1.66 (Petty etal., 1954 Rausch et al., 1954, 1955 Przyluski and Palka, 1970). It is assumed that the monovalent ion Mg" is produced electrochemi-cally according to ... 

Figure 4-14. Electrochemical corrosion and the negative-difference effect on the magnesium surface (Song et al 1997 a).

Song, G., Atrens, A., Li, Y., Zhang, B. (1997a), Negative difference effect of magnesium, in Proc. Corrosion and Prevention-97, Australasian Corrosion Association Inc., 1997, p. 038. 

When A <0, the phenomenon is termed the negative difference effect... 

The strange anodic polarization behaviors (negative difference effect, lower apparent valence, low anodic dissolution efficiency, low anodic polarization resistance and poor passivity ) are closely associated with the AHE process which is further related to the onset of localized corrosion or pitting . A comprehensive anodic dissolution model can be employed to understand these. 

Mitrovic-Scepanovic and Brigham (1990) considered a schematic and theoretical approach for the dissolution of magnesium in de-aerated alkaline solution at pH 13. In this approach the divalent state dissolution has been considered the dissolution of magnesium in the monovalent state accompanied by the oxidation-reduction reaction of hydrogen ions leading to the negative difference effect on the anodic curve and the formation and stability of magnesium hydride at this pH are not considered. 

Influence of negative difference effect (NDE) and Mg on corrosion rate... 

Atrens A and Dietzel W (2007), The negative difference effect and unipositive Mg , Advanced Engineering Materials, 9, 292-297. 

NaCI saturated with Mg(OH)2 (151. Simultaneous measurements of hydrogen evolution gave independent measurements of the corrosion rate, designated as Ph- There was excellent agreement between the two independent measurements of the corrosion rate, Pi was less than Ph as expected from the negative difference effect (NDE), and the apparent valence of Mg was equal to 1.4 from the average value of 2PJP. ... 

In addition to the 3-phase, the most potent cathodes in Mg-Al alloys were thought to be the iron-rich phases the iron-aluminium intermetallic phase FeAl is one of the most detrimental cathodic phases in Mg-Al alloys on the basis of its potential and low hydrogen over-voltage [22]. AlMn [17,18] is also detrimental, and Mg2Si [18] seems to have no influence, while Mg2Pb facilitates localised corrosion and leads to a negative difference effect. 

Keywords P-phase, negative difference effect, microcrystallization, rare earth (RE) element, thin electrolyte layer. 

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