Corrosion congruent

For both coordinated and congruent control, the pH depends upon the phosphate concentration and the sodium to phosphate ratio. Generally, however, phosphates are unsuitable for use at boiler pressures above 100 bar as their low solubility and high concentration factors developed lead to corrosive conditions. 

After one day, a direct correlation between leachant pH and release of Si in the solution can be seen (Fig. 9a). Later on (3- and 10-day corrosion), the increase in pH becomes much slower, suggesting that the leaching of alkaline elements is not a congruent process, and pH stabilizes rapidly during matrix dissolution. In fact, the proportion of CaO in the matrix of HT materials (11-38 wt%) directly controls the amount of Ca that will be readily leached out (see Fig. 9b). It can thus be expected that Ca-poor/Si-rich HT materials possess a higher durability than Ca-rich/Si-poor ones. The relationship between CaO in the matrix of HT materials, Ca2+ extracted into solution by the leachant, and pH is given by ... 

There are five mechanisms for the corrosion of ceramics. The first one is the congruent dissolution by simple dissociation. If the ceramic corrodes by this mechanism, then the ratio of the elements dissolved will be the same as that of them in the ceramic. For example, let us consider sodium chloride. If it dissolves by this mechanism, then the ratio of the number of Na+ ions to the number of CT ions will be 1. This is the same ratio of these ions in the sodium chloride crystal. This kind of dissolution will lead to uniform removal of the surface— that is, the reduction in thickness will be the same at any point on the surface. Hence, the concentration profile on the surface that is exposed will be same before and after the dissolution. This and the solid undergoing dissolution are represented in Figure 5.1. Here, represents the original surface, and X the surface after equilibrium concentration is achieved. 

The second mechanism for the corrosion of ceramics is congruent dissolution by chemical reaction with the solvent. Here, the dissolution takes place by the attack of the solvent on the surface of the ceramic. The attack is uniform. Hence, the aftereffect of this kind of dissolution is the same as that of the first—that is, there will be uniform retreat of the surface. 

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