Corrosion


Corrosion is by far the greatest course of failure of cooling water equipment. It takes place to some degree whenever water, metal and a depolarizing agent such as oxygen are brought together. Also, water inherently contains aggressive species such as chloride and sulfate. Corrosion is a chemical process, therefore, its rate of reaction will increase with temperature. In general, a low pH will increase the rate of corrosion, while inhibiting scale. A high pH will decrease corrosion and promote scale formation. Low pH can result from gases such as sulfur dioxide being scrubbed from the air by the cooling tower; acidic process fluids being leaked in at the heat exchangers; or an overfeed of acid, if being used to control the pH in the cooling tower.



Non-chemical factors such as water velocity, or deposit formation, also play a considerable role in localized corrosion, which can accelerate equipment failures. Because corrosion is an electrochemical process, any condition that causes one point on a metal surface to differ from another will allow a localized corrosion cell to establish.

Localized corrosion is produced by crevices, deposits of any type and local stresses. Low flow areas can allow suspended solids to settle out, and localized corrosion can then take place under the resulting deposit. This is one of the most common causes of localized corrosion in cooling water systems.

Another form of localized corrosion is galvanic coupling corrosion caused by the joining of two different metals in the system. Galvanic corrosion is well known and understood, particularly in the case of copper and steel. The areas of these different metals which form the galvanic cell will also play a part in the damage caused by corrosion- A large anodic area (more corrodible) connected to a small area of cathodic (more corrosion resistant) metal is commonly acceptable because the total corrosion is spread over a large area. When the reverse situation is present, a small anodic area connected to a large cathodic area, corrosion will be accelerated because the corrosion is concentrated in a small area which can quickly lead to the failure of that area.

When two metals from this galvanic series are in contact in solution, the corrosion rate of the more anodic (corrodible) metal increases and the corrosion rate of the more cathodic (corrosion resistant) metal decreases.



Initial Conditioning | Scale | Fouling | Corrosion | Microbiological Control | Cooling Water Treatment
 
 
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