Metallurgical structure

Differences in metallurgical structure. Grain boundaries, more reactive phases (solid solutions, intermetallic compounds, etc.). 

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

Since corrosion is essentially a reaction between a metal and its environment, the very significant effect of crystal defects and metallurgical structure on certain corrosion phenomena is to be expected. It is no more possible to... 

The accuracy of chemical analysis should be related to the degree of homogeneity of the object as reflected by its metallurgical structure. Thus, the objects sampled have been divided into three categories. Since the metallurgical structure is also related to the copper concentration, classes I through III also represent increasing copper concentration of the alloy and object. 

Class III the least homogeneous structure it is characterized by the presence of interdendritic, copper-rich eutectic. Figure 3 shows the metallurgical structure typical of this class. The Silver Nail and the Bull Bowl are representative of this class. 

This study is concerned with the comparison of data from x-ray fluorescence (2), electron microprobe (2), and thermal neutron activation (3) techniques with the metallurgical structure of the sample. The maximum precision which can be expected from all of these techniques, with normal precautions, is 5%. In all cases, the analytical parameters used in the measurements reported here were chosen to maximize the sensitivity and precision for silver-based alloys. The parameters used in each technique are listed in Table II. 

Sample Size. Since the aim of this study is to relate the data to the metallurgical structure, all analytical measurements were made on metallographical sections taken from the objects. This procedure limited the volume which could be sampled. 

---