How often corrosion coupons should be taken out for analysis?
This is one of the most frequently asked questions. There are no particular guidelines about how frequently the corrosion analysis needs to be done.
The frequency of how often you should do this analysis depends on some factors; the most important factor being the observed data.
If you observe low rate of corrosion, the frequency of analysis should be increased. The corrosion analysis should also be more at sensitive areas, where the probability
of risk is more or where there is a history of corrosion.
Now there is a negative consequence if the corrosion coupons are pulled out for analysis too often, especially if you are using the weight loss process.
This may provide you with misleading results as the nature of corrosion growth and the calculation involved in measuring the General and Pitting corrosion rates.
The expert engineers consider corrosion growth as a non-linear activity and the calculation of prorating the measured corrosion to mpy (mils per year) is linear.
Again if you pull the corrosion coupons after long intervals in the weight loss process, this may also lead to misleading corrosion data.
This is due to the fact that, if the process is delayed, the corrosion rates are prorated on an annual basis and hence severe corrosion evens may get
lost in the average calculation. For example, if the corrosion data is obtained annually and if there was severe corrosion at a single month, that month's
corrosion attributes only 1/12th of the total calculated corrosion data. Moreover, if the corrosion data is not quite large, it may
totally get lost in the annual corrosion data.