For these reasons your water must be treated and monitored for corrosion and one of the most popular methods of doing this is by use of Corrosion Coupons.
Corrosion coupons are used to monitor the corrosivity of a system. Corrosion coupons are installed to duplicate the corrosion rate experienced in piping
Corrosion coupons are typically constructed from carbon steel, usually these are generic carbon steel or can be selected to match the material used for a specific purpose.
The coupons are installed in a parallel stream with the water for a given time specified, usually between 3 – 6 months. After the exposure time, the coupon is removed and its surfaces are cleaned. After cleaning, coupon weight is measured and compared to the initial weight, over the exposure time. The result is the corrosion rate for that period.
It is a simple and effective tool that gives a quantitative estimate of corrosion rates that take place within a specific operating system.
This method also comes with some distinct disadvantages which include
In a typical monitoring program, coupons are exposed for a 3 month – 12 months duration before being removed for laboratory analysis. As such, an indication of a corrosion event may only be obtained significantly after the event may have occurred. When the problems become noticeable from the outside it is often far too late and major, very costly repairs may already be required
- Lack of Time Determination
If a specific corrosion event occurs during the period of exposure, the coupon will not be able to identify the time of occurrence
Depending upon the peak corrosion rate of a corrosion event and its duration, the coupon will not indicate the peak corrosion rate of the event, and short-term events may not even register a statistically significant increased weight loss
Corrosion coupons are typically isolated from any metal-metal contact. Thus, they are unaffected by the galvanic interactions present in the system