Electrochemical and microscopy studies of alloys exposed to pH

Studies equivalent to those described above were also carried out on alloy samples exposed to contrived corrosion in pH I solutions produced by a mixture of nitric and acetic acids. Potentiodynamic polarization studies revealed much 

In an effort to understand this lack of distinctive behavior of the alloys at acid pH, some additional sample treatment procedures were introduced. Samples of A and B alloys were heat-treated at 900°C, and quenched in cold water. One heat-treated sample for each alloy was then annealed at 500°C for 100 h. These treatments were intended to determine if the two alloys exhibited different corrosion behaviors as a result of a heat-treatment procedure that would alter the distribution of impurities at the grain boundaries. The original high-temperature heat treatment would be expected to cause. sensitizing impurities (such as B) to diffuse away from the boundaries an anneal at a lower temperature, on the other hand, would result in impurities drifting back to the boundaries if there were no structure in the bulk alloy capable of trapping the impurities. The latter condition is believed to be relevant to alloy B. where the phase composition (see Table 2) is close enough to the pha.se boundary to affect the solubilities of many impurities. 

The effect of the acid exposure on the surfaces of the two annealed alloys can be compared in the SEM micrographs in Figs. 21a and b. The extent of localized grain boundary attack on the annealed B surface is striking on A, however, grain boundary attack is considerably reduced. 

XPS analysis of these surfaces provided little additional information. All surfaces had a similar 1 1 Ni Cu compositional ratio. The real differences would be likely to be found in the grain compositions, such as were observed in Fig. 10. Unfortunately, no similar measurements were made of the heat-treated alloys. 

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. 

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