CORROSION MONITORING OF SWCC PLANTS - I:1 Vapor-Side ...

This provides a plausible explanation for the significantly higher corrosion rates of
cupronickel alloy in vent line. The corrosion products from carbon steel specimens
exposed to non-condensable gases in flash chambers invariably contain sulfur though in
small concentrations. The vent line corrosion products contain no or insignificantly low
concentration of sulfur. Copper is present in small but in perceptible concentrations in
corrosion products of steel exposed in vent lines. There is no plausible explanation for
the existence of copper in the scales or corrosion products of the carbon steel coupons.
A detailed systematic study in collaboration with chemistry and Biology Departments
shall be useful in investigating the source of sulfur in the non-condensable gases. The
EDAX data also indicate the presence of significant concentration of bromide in the
corrosion products. The source of bromide could be the seawater.
Considering the corrosion behavior of materials in vent line and evaporator at different
stages, the shape of the weight loss vs. no stages of curves can be explained broadly as
follows:
(i)
(ii)
Steep increase in weight loss (or corrosion rates) is due to breaking up or
spalling of scales when fresh alloy surface comes into contact with gases.
Decrease in weight loss could be due to progressive formation of adherent
protective scales.
(iii) Presence of plateau (no change in weight) could be due to the formation of
protective scales.
An analysis of the corrosion rates data (by weight loss technique) from corrosion
monitoring studies in flashing brine in the evaporator and water boxes provide some
useful information regarding the corrosion of carbon steel and cupronickel alloys.
Whilst the corrosion rate of carbon steel varies from 0.6 MPY (stage # 6) to 1.8 (stage #
19) after 25000 hours, the corrosion rates of cupronickel (70-30) in water boxes are 0.18
(# 7-8, unit 100) and 0.13 (# 5-6, unit 200) after the same exposure time. The low
corrosion rate values for both carbon steel and cupronickel (70-30) alloys are well
within the allowable range. Comparing these values with those for vapor phase
corrosion in evaporator (9000 hrs exposures) and vent line (2000 and 4000 hrs
exposures), corrosion rates are of the same magnitude.
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