Just a few more comments about corrosion coupons: Some suppliers will supply pretreated or prefilmed coupons. They use an effective corrosion inhibitor such as a chromate compound to establish a protective film. They claim that this will determine if the water treatment corrosion inhibitor is capable of maintaining that protective film.
I'm firmly opposed to this process. I believe the corrosion coupon should not be pretreated. This will determine if the water treatment can refilm or repair a "break" in the protective film on the system metals. Pretreated coupons always will have lower corrosion rates than raw metal coupons and thus can provide a false sense of metal protection.
Corrosion coupons commonly are utilized due to the low cost for supply and evaluation. However, it is important to identify the advantages and disadvantages of this corrosion monitoring method vs. other techniques (table 1).
Two additional corrosion-monitoring techniques are available: electric resistance (ER) and linear polarization resistance (LPR).
Electric Resistance Corrosion MonitoringThis technique utilizes a low level of direct current through a special probe containing the metal or alloy being monitored. The current measures the variation in resistance through the metal probe. As the metal is corroded, the resistance changes, and this change is used to determine the corrosion rate. There are several different styles of the metal probe for use in measuring low, medium or highly corrosive cooling water. A solid wire loop is for highly corrosive waters, a tube loop is for medium, and a thin ribbon loop is for low corrosive cooling waters (figure 1).
The electric resistance and corrosion coupon results are similar methods in that the corrosion rates are an average rate since the last reading. However, unlike coupons, the resistance probe does not have to be removed. The frequency for the corrosion-rate measuring can be hours or days. Corrosion coupons generally are left in place for 30 to 90 days before being removed, cleaned and corrosion rate calculated.
Linear Polarization Resistance MonitoringThe linear polarization resistance corrosion monitoring method is used extensively in cooling water systems. It functions by passing low direct current through a two- or three-element probe. Technically, it is based on the relationship of potential vs. current on a corroding electrode element (figure 2).
The elements are short (1 to 1.5") and about 0.125" dia. They do not touch but are screwed into the probe. Two element probes are used for medium to high conductivity cooling water while the three- element probe is used for all cooling waters and is sensitive to low conductivity waters. They remain in the cooling water for extended periods -- at times for one year -- or until any bridging occurs between two elements. This method requires waters to have a conductivity sufficient to pass a current between the elements (more than 10 micro?; preferably several microhms). Once the probe has stabilized in the cooling water, usually within 24 hr, corrosion rate measurements can be taken.
Unlike corrosion coupons and the ER methods, the corrosion rate for LPR is essentially instantaneous -- simply push the activation button and immediately the occurring corrosion rate, at the moment, is identified. With an automatic activator and recorder, the instantaneous corrosion rates can be taken every minute of the day. This enables detection of variations in corrosion rate over days, weeks and even months.
The overall average corrosion rate by the LPR method will be essentially the same as with corrosion coupons and the ER corrosion rates. However, the LPR corrosion monitoring method is ideal for identifying variations in corrosion, making it suitable for troubleshooting as well as optimizing the corrosion-inhibitor dosages.
Figure 3 shows the comparison of the corrosion coupon corrosion rates vs. linear polarization resistance rates. The LPR corrosion method also can provide instantaneously a pitting tendency of the cooling water.
Advantages and DisadvantagesThe three corrosion rate-monitoring methods have advantages and disadvantages. They all should provide similar corrosion rates, with both the linear polarization resistance and electric resistance methods usually somewhat lower than coupons. This is due to higher initial corrosion rates of coupons. One important consideration for all three of these methods is that they measure corrosion but not under heat transfer conditions. Heat transfer, such as cooling water passing through condensers, chillers or all types of heat exchangers picks up heat. This increase in cooling water temperature can increase corrosion rates substantially.
So, keep in mind that use of any of these methods discussed in Part 1 and this article may duplicate nonheat transfer surfaces such as piping. Coupons start with a bare metal surface while both the electric resistance and linear polarization resistance, after a short period following installation of the probe, will more closely duplicate piping with corrosion products and/or an established protection.
Corrosion monitoring is essential to ensure your cooling water-contacted equipment is being protected -- much quicker and sooner than by inspections. My next column will discuss corrosion measuring under heat transfer conditions. PCE