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 |
Control
Techniques
The principle
methods to prevent or minimizing corrosion include:
1. Selecting suitable materials of construction to resist
corrosion
2. Adding protective film- forming chemical inhibitors that the
water can distribute to all wetted parts of the system.
3. Controlling scaling and micro-biological growth
4. Protect cathodically, using sacrificial metals
5. Apply protective coatings such as paints, metal plating, tar or plastics on external surfaces
