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

corrosion products. Corrosion coupons can be mounted in different configurations to
study different types of corrosion mechanisms, such as crevice corrosion, galvanic
attack and stress corrosion. Coupons, which are the test specimens of the material of
interest, are carefully cleaned, weighed and measured before being assembled on the
corrosion test retractable probes. During assembly, two coupons are arranged so that
they are electrically insulated from the probe material so as to avoid galvanic attack.
After a prescribed period of exposure, the corrosion probe is disassembled and the
corroded coupons are chemically cleaned, weighed and corrosion rate is determined.
The corrosion rate is calculated from the weight loss, time of exposure and original
exposed surface area of the material by the following formula [Fontana, 1987].
where :
MPY =
Mils Per Year
W = Weight loss, mg
A = Area of specimen, in 2
MPY = 534 W
AtD
t = Exposure time, hr
D = Density of specimen, g/cm 3
In brine immersion zone, the coupon test method could be used successfully by using
large surface area coupons. The most serious problem encountered in immersion zone
is the high probability of scale formation. The scales can be adherent to metal which
give an extra protection to the metal or could be non-adherent which could lead to
initiate under-deposit corrosion. The difficult problem in calculation of corrosion rate
of immersed coupon is the determination of actual surface area due to the scale
formation and time parameters of this process.
During coupon testing, the coupon could be mounted inside the flash chamber at
different locations with different angles. The coupon should be insulated from flash
chamber materials, fastener and holder materials by using any non-conductive material.
3.3 Methodology
1626