Grain boundaries Monel

Solders are cathodic to steel, zinc and cadmium, and anodic to Monel metal. Although tin or tin-coated metals may be used safely in contact with aluminium when they are not fused with it, a joint in aluminium made with a tin-lead solder is liable to destructive corrosion. The formation, on fusion, of the grain-boundary state, which, as already mentioned, makes aluminium so dangerous an impurity in tin, is responsible. Tin-zinc solders may be used the zinc gives a useful degree of protection. 

Figure 10. SIMS images of the distribution of B (bright) and Ni (dark) in polished cross sections of Monel 400 from A and B sources. The imaging field is 150 pin. The grain boundaries of B arc enriched in B. while in A, B is largely found in inclusions in the bulk of the alloy grains. Ni concentration is found to vary. somewhat within certain grains of the alloy. The faces of some grains arc darker than others because of orientation effects on contrast.

What have we learned about the differences in corrosion behavior of these two Monel specimens It appears from the contrived experiments that the structural differences in these two alloys actually provoke different corrosion behaviors in both acidic and basic solutions. The acidic case is clearer localized grain boundary attack is observed under conditions in which impurities in the defective B alloy are free to migrate to those boundaries. However, in a basic solution, there is also evidence that the oxide film structures on A and B" are somewhat different and that this difference influences corrosion properties. The difference is apparent in their appearance and in their response to the passage of an electrical current. Grain boundaiy chemistry is the major difference between the alloys, and it is indeed possible that the boundaries could provoke different galvanic responses on each of the two surfaces. 

When one is faced, however, with choosing the most appropriate test of the corrosion worthiness of a future Monel alloy specimen, there is no doubt that the contrived exposure in acid conditions would be the most reliable. All these studies have identified the grain boundary impurities as the root of the corrosion problem in the alloy. The experiments in acid are specific to the boundaries and illustrate the merits of an electrochemical approach. 

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