Micro-galvanic corrosion

K.B. Deshpande, Numerical modeling of micro-galvanic corrosion, Electrochim. Acta 56 (2011) 1737-1745. 

Figure 3.16 presents the corrosion rates for GW103K in the different heat treatment conditions, measured by immersion in 5% NaCl solution for 3 days. The as-cast (F) condition had the highest corrosion rate due to micro-galvanic corrosion of the a-Mg matrix by the eutectic. Solution treatment led to the lowest corrosion rate, attributed to the absence of any second phase and a relatively compact protective surface film. Ageing at 250 °C increased the... 

For Mg-Y alloys, Y in the surface layer can increase the protectiveness of the surface film. The Mg-Y intermetallic Mg24Ys can cause micro-galvanic corrosion, so Y can have a dual effect on the corrosion of a Mg alloy. Which effect is more important depends on the electrolyte. In 0.1 M NaCl, the chloride ions can penetrate the surface film, and localised corrosion initiated as filiform corrosion. The important effect is the micro-galvanic corrosion. The corrosion rate increases with increasing Y content once the Y content exceeds the Y solid solubility and the microstructure contains a second phase ... 

This multi-piece system can also be extended to a micro-galvanic corrosion system. For example, an alloy sometimes consists of different... 

Galvanic corrosion can occur in a polycrystaUine alloys, such as pearMc steels, due to differences in microstmctural phases. This leads to galvanic-phase coupling or galvanic microceUs between ferrite (a-Fe) and cementite FesC) since each phase has different electrode potentials and atomic stmcture. Therefore, distinct localized anodic and cathodic microstmctural areas develop due to microstmctural inhomogeneities, which act as micro-electrochemical cells in the presence of a corrosive medium (electrolyte). This is an electrochemical action known as galvanic corrosion, which is mainly a metallic surface deterioration. 

Because Ni-Mo alloys do not develop a passivating oxide film on the surface, they do not suffer localized attack in the classical sense, such as the halide-induced pitting and crevice corrosion of stainless steels. In some service applications these alloys would not corrode uniformly but might develop some shallow cavities on their surface. These cavities could be the result of confined enhanced corrosion as a consequence of micro galvanic couples with oxidizing agents or other impurities. 

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 galvanic corrosion can also occur within an alloy if micro-anodes and cathodes are present in the alloy. Mg alloys are not uniform in terms of their composition, microstructure and even crystalline orientation. These differences can result in various electrochemical activities within a Mg alloy and thereby generate galvanic couples on a micro-scale. 

Generally speaking, the following factors should be considered in analyzing the micro-galvanic cell related corrosion of Mg alloys. 

Corrosion model of Mg alloy with various micro-galvanic cells. 

A very small addition of impurities of Fe, Ni, Co or Cu can dramatically increase the corrosion rate of Mg or a Mg alloy through the micro-galvanic effect as discussed earlier (Hanawalt et al., 1942 Hillis and Murray, 1987 Lunder et al., 1995 Nisancioglu et al., 1990a). It is well known that higher purity can lead to higher corrosion resistance for Mg and Mg alloys (Aune, 1983 Avedesian and Baker, 1999 Busk, 1987 Emley, 1966 Frey and Albright, 1984 Fronts et ah, 1987 Hillis, 1983). 

The second phase (the 3-phase in AZ91) can cause micro-galvanic acceleration of corrosion. 

The corrosion rate of the alloys was measured [37,39] by immersion in 1% NaCl (Fig. 3.14). The corrosion rate decreased with increasing Gd content up to 10%. In particular the corrosion rate for Mg-lOGd in the T4 condition was 0.7 mm/yr, which appears to be somewhat smaller than that of pure Mg. The corrosion rate in the T6 condition was even smaller at 0.4mm/yr despite the presence of nano-sized precipitates these appear not to have had an adverse influence on the corrosion rate. The corrosion rate for Mg-15Gd was much higher, consistent with micro-galvanic acceleration by the second phase. 

Particular micro-climates can favour the development of galvanic corrosion of aluminium in contact with steel. This can be observed in humid zones, close to factories that emit a great deal of dust fertiliser plants, cement works, coal-fired power stations, etc. Experience shows that this situation, which is highly unfavourable for the resistance of materials, can be controlled to a large extent by suitable design and especially by frequent cleaning of accumulated dust. 

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