These are some of the common questions end users and owners often ask. However, often they are asked after equipment replacement has occurred or about to be due to corrosion. In this series of columns, I hope to provide you some guidelines and helpful advice for corrosion testing of the components in your cooling water system so that they will last longer.
Step 1: Metals IdentificationKnow what metals are contacted by the cooling water in your system. Fill in this list or develop your own. If you do not know the exact metal, go to your supplier or contractor to get the specifications. An example is provided in table 1.
Now you have identified all of the metals within your cooling system. Be sure to indicate if any components are coated, such as the water box or cooling tower. This prevents the metal from contacting the cooling water. Also, determine if the condenser tubing may be "enhanced" or "super enhanced" (see previous columns) because they are more sensitive to corrosion and thus require special attention. Special considerations may also apply to the process equipment metallurgy, so identify these concerns.
Step 2: Corrosion Metal to be TestedDetermine the metals to be tested for corrosion rates within the cooling water system so as to estimate the life expectancy of the metal components. Common corrosion testing is with the metals that are either the most sensitive to water corrosion or the "thinnest." These metals will fail most quickly. Thus, what are the common metals tested? Among the most often tested metals are:
- Condenser tube.
- Metal cooling tower.
- Process equipment.
- Condenser water box.
- Condenser tubesheet.
Specialized corrosion testing is available if certain cooling water-contacted equipment has had premature failure or excessive corrosion. These may be pump impellers, valves or some process equipment.
Thus, in cooling water systems, the following metals are (or should be) tested: mild steel, copper/copper alloys, galvanized steel and stainless steel.
Step 3: Corrosion Testing MethodsThere are several different corrosion testing methods that may be utilized. They involve both online (while cooling system is operational) and offline (while the system is shut down). They involve monitoring, equipment testing and equipment inspections. Both are useful in predicting equipment life expectancy as well as equipment integrity. Typical methods for corrosion testing while the system is online include:
- Corrosion coupons (metal strips).
- Instantaneous corrosion by linear polarization.
- Heat transfer test units.
Typical methods for corrosion testing while the system is offline include:
- Eddy current testing.
- Robotics inspection.
- Equipment inspection.
Corrosion Coupons. Corrosion coupons, or small inexpensive strips of the metal to be tested, are put in contact with the cooling water. These remain for a period of time such as 30, 60 or 90 days and then are removed and evaluated as to their metal loss. Coupons often are 0.5 x 3" and 0.0625" thick. They can be larger, or even as 0.5" rods, and there are various configurations (figure 1).
A calculation is made to determine the annual average corrosion rate occurring. This measurement is most often reported as the average mils per year metal loss over the entire coupon metal surface. A mil is one thousandth of an inch. Thus, 10 mils per year (mpy) is a hundredth of an inch. It should be noted that copper condenser tubes often are about 20 mils in thickness and thus would be totally corroded at 10 mpy within two years. Mild steel, schedule 40 pipe, on the other hand, is about 120 mils thick and would be totally corroded in 12 years at the 10 mpy rate.
Corrosion coupons are not only valuable in determining the average corrosion rate but, upon their close inspection after cleaning, can reveal localized and/or pitting corrosion. This selective corrosion can be 10 to 50 times greater than the calculated average corrosion rate. Thus, penetration can occur much more rapidly and leaks may occur in a year or less. Examples of mild steel corrosion coupons both before cleaning and after can reveal the presence of localized or pitting corrosion (figure 2).
The characteristic localized and/or pitting corrosion on mild steel coupons might also reveal the cause of the corrosion. The presence of "concentric rings" often indicates the corrosion is due to sulfate-reducing bacteria. This is shown in the figure 3.
Step 4: Desired Corrosion RatesGuidelines and industry standards are published for the desirable corrosion rates of different metals. When maintained, equipment life is very acceptable. To determine how corrosion rates are assessed vs. the desirable values, the following tables provide these guidelines. Table 2 shows cooling tower systems while Table 3 shows closed cooling systems.
How well are you doing in controlling corrosion in your cooling water systems? More on corrosion testing in future articles. I'd be pleased to answer any questions the reader may have on corrosion testing.