Cooling water corrosion

General corrosion or localized corrosion of carbon steels and other metals is caused by dissolved salts, gases, organic compounds, or microbiological activity.

Materials susceptible to this damage

Carbon steel, all grades of stainless steel, copper, aluminum, titanium and nickel-base alloys.

Critical factors of this corrosion

1. Cooling water corrosion and scaling are closely related and must be considered together. Fluid temperature, water type (fresh, brackish water, salt), cooling system type (once-through, open circulating, closed circulating), oxygen content, and fluid velocities are critical factors.
2. Increasing the cooling water outlet temperature or the inlet temperature on the process side increases corrosion rates as well as scaling.
3. Increasing the amount of oxygen increases the corrosion rate of carbon steels.
4. If the process side temperature is above 140°F (60°C), there is a potential for scale formation in fresh water, and this probability increases as the process temperature and cooling water inlet temperature increase. In the case of salt water, the probability of scale formation increases significantly if the temperature is above 115°F (46°C).
5. Sedimentation may occur from mineral deposits (hardness), sludge, suspended organic matter, corrosion products, oxide layers, microbiological growth.
6. Fluid velocities should be high enough to minimize sedimentation, but it is worth noting that if the fluid velocity is too high, it will cause abrasion. In general, the range of velocity depends on the pipe material and water quality.
7. Low velocities can increase corrosion. In both fresh and salt water systems, velocities below 3 fps (m/s1) are prone to scale formation and increased corrosion. If cooling water is used in condensers/coolers on the shell side instead of the tube side, accelerated corrosion will occur in dead spots and stagnant areas.
8. 300 series stainless steels can suffer from pitting corrosion, crevice corrosion, and SCC in both fresh and salt water systems.
1. Copper/zinc alloys can undergo dezincification in both fresh and salt water systems. Copper/zinc alloys can undergo SCC in the presence of ammonia or ammonium compounds in the water or on the process side.
10. ERW carbon steel in fresh and salt water may suffer from severe heat-affected zone corrosion and welding.
11. Titanium may suffer from hydrating embrittlement when bonded to wet anodic materials. This problem generally occurs at temperatures above 180°F (82°C), but can occur at lower temperatures.

Affected units and equipment

Cooling water corrosion is a concern in water-cooled heat exchangers and cooling towers in all industrial applications.

Appearance and morphology of this injury

1. Cooling water corrosion can lead to various forms of damage including general corrosion, pitting corrosion (Figure 1), microbial corrosion (MIC), SCC, and scale formation.
2. In the presence of dissolved oxygen, general or uniform corrosion of carbon steel occurs.
3. Localized corrosion may be caused by sub deposit corrosion, crevice corrosion, or microbial corrosion.
4. Deposits and grooves can lead to crevice or pitting corrosion in any susceptible material.
5. Corrosion, either wavy or smooth, at the inlet and outlet of nozzles and pipe inlets may be due to flow induced corrosion, erosion or wear.
6. Corrosion in ERW weld areas will appear as grooves along the weld fusion lines.
7. Metallurgical analysis of tube samples may be required to confirm and identify the mode of failure.

Prevention and reduction

1. Cooling water corrosion (and scale formation) can be prevented by proper design, operation, and chemical treatment of cooling water systems.
2. The design should be such that the process side inlet temperature is below 140°F (60°C).
3. Maximum and minimum water velocities must be maintained, especially in saltwater systems
4. The metallurgy of heat exchanger components may require upgrading and increasing resistance, especially in waters with high chloride levels, low velocities, high process temperatures, and poor water chemistry retention.
5. Periodic mechanical cleaning of the inner and outer diameters of the tubes should be performed to maintain the cleanliness of the heat transfer surfaces.

Inspection and monitoring

1. Cooling water should be monitored for variables including pH, oxygen content, biocide residual, biological activity, cooling water outlet temperature, hydrocarbon contamination, and process leaks that affect corrosion and scale formation.
2. Periodic calculation of U-factors (a measure of heat exchanger efficiency) will provide information on fouling.
3. Ultrasonic flow meters can be used to check the velocity of water in pipes.
4. Pipe inspection using EC or IRIS method
5. Representative Pipe Inspection

Related mechanisms

MIC, CISCC and Galvanic Corrosion

Figure 1 - Cooling water corrosion on the inside diameter of a carbon steel heat exchanger tube with an operating temperature of 86°F (30°C).