Cooling Water Problems and Solutions /مشاكل المياه المبردة وحلولها4(ازالة التاكل)

CORROSION INHIBITION

Next to scale, the next most important concern is the prevention of “corrosion”.
Corrosion is defined as the destruction or loss of metal through chemical or electrochemical reaction with its surrounding environment.

Mild steel is a commonly used metal in the cooling water system that is most susceptible to corrosion.

Other metals in general, such as copper, stainless steel, aluminum alloys also do corrode but the process is slow.

However in some
waters and in presence of dissolved gases, such as H2S or NH3, the corrosion to these metals is more severe & destructive than to mild steel.

What causes corrosion?

Corrosion is a three step electrochemical reaction in which free oxygen in the water passes into a metal surface a one point (referred to as the cathode) and reacts with water and electrons, which have been liberated by the oxidation of metal at the anode portion of the reaction at another spot on the metal surface.

The combination of free electrons, oxygen and water forms hydroxide ions.

The hydroxide ions then combine with the metal ions, which were liberated at the anode as part of the oxidation reaction, to form an insoluble metal hydroxide.

The result of this activity is the loss of metal and often the formation of a deposit.

Corrosion Problems

Common problems arising from corrosion are reduction in heat transfer and water flow resulting from a partial or complete blockage of pipes, valves, strainers, etc.

Also, excessive wear of moving parts, such as pump, shaft, impeller and mechanical seal, etc. may resist the movement of the equipment.

Hence, thermal and energy performance of heat exchange may degrade.

Factors

Many factors affect the corrosion rates in a given cooling water system. Few important factors are:

1. Dissolved Oxygen - Oxygen dissolved in water is essential for the cathodic reaction to take place.

2. Alkalinity & Acidity - Low alkalinity waters have little pH buffering capability. Consequently, this type of water can pick up acidic gases from the air and can dissolve metal and the protective oxide film on metal surfaces. More alkaline water favors the formation of the protective oxide layer.

3. Total Dissolved Solids - Water containing a high concentration of total dissolved solids has a high conductivity, which provides a considerable potential for galvanic attack. Dissolved chlorides and sulphates are particularly corrosive.

4. Microbial Growth - Deposition of matter, either organic or inorganic, can cause differential aeration pitting (particularly of austenitic stainless steel) and erosion/corrosion of some alloys because of increased local turbulence.

Microbial growths promote the formation of corrosion cells in addition; the byproducts of some organisms, such as hydrogen sulphide from anaerobic corrosive bacteria are corrosive.

5. Water Velocity - High velocity water increases corrosion by transporting oxygen to the metal and carrying away the products of corrosion at a faster rate.

When water velocity is low, deposition of suspended solids can establish localized corrosion cells, thereby increasing corrosion rates.

6. Temperature - Every 25-30°F increase in temperature causes corrosion rates to double. Above 160°F, additional temperature increases have relatively little effect on corrosion rates in cooling water system.

Some contaminants, such as hydrogen sulfide and ammonia, can produce corrosive waters even when total hardness and alkalinity are relatively high.

Corrosion Types

Many different type of corrosion exist, but the most common is often characterized as general, pitting and galvanic corrosion.

1. General attack: exists when the corrosion is uniformly distributed over the metal surface. The considerable amount of iron oxide produced contributes to fouling problems.

2. Pitting attack: exists when only small area of the metal corrodes. Pitting may perforate the metal in short time. The main source for pitting attack is dissolved oxygen.

3. Galvanic attack: can occur when two different metals are in contact. The more active metal corrodes rapidly.

Common examples in water systems are steel & brass, aluminum & steel, Zinc & steel and zinc & brass. If galvanic attack occurs, the metal named first will corrode.

How to monitor Corrosion Rates?

Corrosion rates are most commonly tested using metal coupons.

Mild steel and copper coupons are frequently used since these represent the metals that are most vulnerable to corrosion in the
system.

An acceptable cooling water treatment program should be able to reduce corrosion rates to the following average levels reported as mils/yr:

Corrosion Rate Standards Rating Rate (Mils/yr)

Poor >5
Fair 3.5 – 5.0
Good 2.0 – 3.5
Excellent 0.0 – 2.0

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