FACTORS AFFECTING CORROSION RATES

1. TYPE OF METAL

2. CONDITION OF THE METAL SURFACE

3. TEMPERATURE

4. CONCENTRATION OF OXYGEN

PROBLEMS CAUSED BY MICROORGANISMS

Problems associated with the uncontrolled growth of microorganisms in cooling water systems are placed in two categories.

1. PLUGGING AND FOULING
2. MICROBIALLY INDUCED CORROSION

1. PLUGGING AND FOULING

MICROBIOLOGICAL growth can cause plugging and fouling in a system. Slime-forming bacteria can form masses which are large enough to reduce flow in a heat exchanger so that the capacity is lost or it is shut down. The types of bacteria that can do this are Aerobacter, Proteus, Bacillus, and Pseudomonas.

In other cases the growth of microbes can serve as a binding agent for suspended solids, corrosion products, etc., forming very bulky and adherent deposits. These deposits can foul or plug heat exchanger tubes causing efficiency losses, and can lead to corrosion.

2. MICROBIALLY INDUCED CORROSION

Microbially induced corrosion is caused by the growth of bacteria. This type of corrosion occurs by two basic mechanisms:

The first is the use of hydrogen by microorganisms in their metabolism. This consumes the gaseous hydrogen formed at the cathode of an electrochemical corrosion cell and leads to continue faster corrosion.

The second mechanism is the production of very corrosive materials by the bacteria during their metabolism, such as acids. For example, sulfate-reducing bacteria use sulfate and form hydrogen sulfide. This quickly attacks metals where the bacteria grow, leading to serious pitting corrosion. Other bacteria may form acids, which also destroy the system metals.

The most widespread contribution to corrosion by bacteria and other microorganisms is under-deposit corrosion.

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