Page 319 - Failure Analysis Case Studies II

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'l'he weight of sodium chloride in the 168 kg of the liquor used in the batch process is 0.084 kg.
With a partition coefficient of 40:1, this amount of sodium chloride would be partitioned thus: the
water-rich layer would contain 0.082 kg, and the butanone-rich layer would contain 0.002 kg. The
calculations given previously show that addition of the minimum volume of water required to cause
separation of the 8% water-in-butanone solution used in the batch process into two immiscible
liquids results in the formation of 0.23 1 of the water-rich phase. Using the data shown in Table 2
as a guide, the greatest amount of sodium chloride which could dissolve in 0.231 of water-rich
solution is 0.045 kg. Thus, if only 0.23 1 of the water-rich phase had been formed, it is probable that
sodium chloride would have precipitated from the water-rich phase.
Since no solids had been observed when the system was dismantled, it appears that at least 0.5 kg
of water-rich phase must have been formed. This led to a reappraisal of the amount of water which
had been introduced to the system. Calculations based on the Lever rule indicated that this would
have required the addition of just under 4 kg of water to the feed liquor.
Although the strict applicability of these calculations to the present case may be questioned, since
data for the solubility of sodium chloride in water-acetone mixtures have been used, they do suggest
that the hypothesis is tenable. The result would have been that the pump which had suffered the
crevice corrosion was not exposed to a very dilute solution of chloride but to a brine, and crevice
corrosion of 3 16L would inevitably have occurred.

5. A DEMONSTRATION

Since there were doubts about the applicability of these calculations to the present case, a
demonstration was carried out in which 5 mm nominal diameter 3 16L stainless steel rods were
exposed to: (i) an 8 wt%a solution of water in butanone which contained 0.05 wt% sodium chloride,
and (ii) the same solution after the addition of just sufficient water to cause the formation of two
liquids. Artificial crevices were formed by slipping Viton O-rings up the rod. In the second case, the
O-rings were positioned so that there was one in the water-rich phase and one in the butanone-rich
phase. The composition of the stainless steel used in this demonstration is given in Table 3.
After 12 days of exposure at 25 "C, a pale yellow discolouration of the originally clear water-rich
phase was observed, and a light ring of rust began to appear at the edge of the O-ring immersed in
this phase (Fig. 3). Neither discolouration of the solutions nor rings of rust at the O-rings were

Fig. 3. Light ring of rust which appeared at the O-ring immersed in the water-rich phase after 12 days of
exposure at 25 "C ( x 16)