Manufacturing of common metals used in cooling systems, such as mild steel, involves removing oxygen from the natural ore. Cooling water systems are an ideal environment for the reversion of the metal to the original oxide state. This reversion process is called corrosion.
What is Corrosion?
Corrosion is an electrochemical process by which a metal returns to its natural oxide state. For example, mild steel is a commonly used metal in cooling water systems that is very susceptible to corrosion. Corrosion causes loss of metal thickness or even penetration of tube walls which can cause leakage of process fluids into the cooling water or vice versa. Corrosion is generally a greater concern with the more common, lower cost materials like mild steel.
How does Corrosion take Place?
For corrosion to occur, a corrosion cell, consisting of an anode, a cathode, and an electrolyte must exist. Metal ions dissolve into the electrolyte (water) at the anode. Electrically charged particles (electrons) are left behind. These electrons flow through the metal to other points (cathodes) where electron-consuming reactions occur. The result of this activity is the loss of metal and often the formation of a deposit.
What Water Characteristics Affect Corrosion?
The most important factors are:
• Oxygen and other dissolved gases
• Dissolved and suspended solids
• Alkalinity or acidity (pH)
• Velocity • Temperature
• Microbial activity
What Methods are used to Prevent Corrosion?
Corrosion can be prevented or minimized by one or more of the following methods:
• When designing a new system, choose corrosion-resistant materials to minimize the effect of an aggressive environment.
• Apply protective coatings such as paints, metal plating, tar, or plastics.
• Protect cathodically, using sacrificial anodes or other methods such as impressed current. • Add protective film-forming chemical corrosion inhibitors that the water can distribute to all wetted parts of the system.
What is the Cost of Corrosion?
Corrosion can increase costs of any process. It causes fouling that reduces heat exchange efficiency and reduces the process efficiency. If severe enough, corrosion can cause failure of an exchanger, requiring either retuning or replacement. This has direct cost for the equipment being replaced and costs to shut down the process while repairs are completed (see Material Costs Chart above).