Corrosion rate is an important key performance indicator in any industrial water system and must be followed closely. Once corrosion is a natural process for any metal, plant personnel is entitled to constantly measure and provide corrective actions if necessary once the detected rate is above recommended limits. The correct strategy for cooling water treatment plays a very important role on the corrosion rate results.
According to AWT – Association of Water Technologies, some guidelines are recommended as maximum limits for corrosion rates for the most common metallurgies (carbon steel and copper) used in industrial water systems.
Table 1 describes the limits for open recirculating (cooling towers) systems.
|
Description |
Carbon Steel |
Cooper Alloys |
|
Negligible or Excellent |
Less than or equal to 1 mpy |
Less than or equal to 0.1 mpy |
|
Mild or Very Good |
1 to 3 mpy |
0.1 to 0.25 mpy |
|
Good |
3 to 5 mpy |
0.25 to 0.35 mpy |
|
Moderate to Fair |
5 to 8 mpy |
0.35 to 0.5 mpy |
|
Poor |
8 to 10 mpy |
0.5 to 1 mpy |
|
Very Poor to Severe |
> 10 mpy |
> 1 mpy |
Table 1 – Corrosion rate allowances for open recirculating systems (source: Association of Water Technologies).
Table 2 describes the same limits recommended for closed loop cooling systems.
|
Description |
Carbon Steel |
Cooper Alloys |
|
Excellent |
Less than or equal to 0.2 mpy |
Less than or equal to 0.1 mpy |
|
Good |
0.2 to 0.5 mpy |
0.1 to 0.25 mpy |
|
Moderate |
0.5 to 0.8 mpy |
0.25 to 0.35 mpy |
|
Poor |
0.8 to 1 mpy |
0.35 to 0.5 mpy |
|
Very Poor to Severe |
> 1 mpy |
> 0.5 mpy |
Table 2 – Corrosion rate allowances for closed recirculating systems (source: Association of Water Technologies).
There are at least two different usual tools to measure corrosion rates: corrosion coupons and corrosion probes.
Details on corrosion coupons installation can be studied on the following link.
Corrosion coupons characteristics:
- Inexpensive
- Different metallurgies can be tested
- Visual inspection can offer extra evaluations (e.g. biofilm, suspended solids, etc)
- Demands long time exposure (minimum 30 days)
- Measures accumulated/average corrosion rate during the exposure period
Corrosion probes such as Linear Polarization Resistance (LPR) are specific probes/analyzers capable to measure and monitor instantaneous corrosion rate for one or more different metallurgies (one probe for metal). The concept is similar to other probes such as pH, conductivity, ORP, etc, where online 24/7 measurement (and perhaps control) is achieved. Corrosion probes characteristics:
- Instantaneous corrosion rate
- Early upset detection (contamination, lack of corrosion inhibitor product, etc)
- Different metallurgies can be tested
- Possibility of automatic control
- Possibility of localized corrosion detection (if probe can measure imbalance)
- Higher associated cost
Sure, the investment on corrosion rack and coupons is quite low compared to an automatic corrosion probe measurement system. However, one great advantage already described is the possibility of getting instantaneous measurement.
Figure 1 is a great example. On this process, one can observe that the corrosion rate (carbon steel) at large size (28000 m3/h recirculation rate) cooling tower was very well controlled – under 0.5 mpy. Very suddenly, a spike is observed, which generated and alarm for the management team. The problem was quickly identified and once corrected, the corrosion rate started to go back to its common result.
Figure 1 – Online corrosion rate measurement (carbon steel) of a cooling water system, performed by a LPR corrosion probe.
Such feature is not possible with a corrosion coupon, given the fact that one has wait at least 30 days to remove the coupon and measure the average rate. Also, if the upset occurs for a short period of time during, perhaps the average corrosion rate does not change dramatically. Then it becomes very hard or even impossible to detect the upset.
As a standard recommendation, it is wise to have both measurement techniques in your system. The corrosion probe will allow rapid detection and counter measures. The corrosion coupons will allow post exposure period visual identification of the metal surface.