Galvanic corrosion microstructures

Locations. Galvanic corrosion of any type is most severe in immediate proximity to the junction of the coupled metals. Galvanic corrosion of weld metals is frequently microstructurally localized. The less-noble weld material will corrode away, leaving behind the skeletal remnants of the more-noble metal (Figs. 15.1 and 15.2). 

Galvanic corrosion may occur at stainless steel welds if sensitization has taken place or if welding has produced unfavorable dissimilar phases (see Chap. 15, Weld Defects, particularly Case History 15.1). These forms of microstructural galvanic corrosion do not involve the joining of two different metals in the usual sense. 

Another form of microstructural galvanic corrosion, graphitic corrosion, is unique to gray and nodular cast irons. It may be encountered in cast iron pumps and other cast iron components. It is a homogeneous form of galvanic corrosion, not requiring connection to a different metal. 

Unalloyed zinc corrodes uniformly unless in contact with more cathodic materials, in which case a more localized galvanic corrosion can occur at the interface. In other cases, nonunifoim corrosion is generally not encountered. Zinc alloys, particularly those with aluminum, can suffer from pitting and intergranular attack. This tends to be influenced by the fineness of the alloy microstructure. 

Galvanic Corrosion between dissimilar metal/alloys or microstructural phases (pearlitic steels, o-/3 copper alloys, 0-/8 lead alloys). 

It is important to realize that galvanic corrosion effects can be manifested not only on the macroscopic level but also within the microstructure of a material. Certain phases or precipitates will undergo anodic dissolution under microgalvanic effects. Because the principle of galvanic corrosion is widely known, it is remarkable that it still features prominently in numerous corrosion failures. Figure 7.17 illustrates the main factors affecting the formation of a galvanic cell [14]. 

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. 

Galvanic corrosion or bimetallic corrosion is important to consider since most of the structural industrial metals and even the metallic phases in the microstructure alloys create galvanic cells between them and/or the a Mg anodic phase. However, these secondary particles which are noble to the Mg matrix, can in certain circumstances enrich the corrosion product or the passive layer, leading to a decrease or a control of the corrosion rate. Severe corrosion may occur in neutral solutions of salts of heavy metals, such as copper, iron and nickel. The heavy metal, the heavy metal basic salts or both plate out to form active cathodes on the anodic magnesium surface. Small amounts of dissolved salts of alkali or alkaline-earth metal (chlorides, bromides, iodides and sulfates) in water will break the protective film locally and usually lead to pitting (Froats et al., 1987 Shaw and Wolfe, 2005). 

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 ... 

The only significant corrosion observed in the entire system was confined to the weld beads along the internal surface. Corrosion occurred due to a microstructural galvanic couple formed between two distinct phases in the weld-bead microstructure (Figs. 15.22 and 15.23). The less-noble phase corroded away, leaving behind the skeletal remnants of the more-noble phase. 

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