Corrosion of metals in water systems: the basics

Corrosion is the deterioration of a metal as a result of a chemical reaction between it and the environment around it. Almost completely unavoidable in everyday life, the process is perhaps most commonly seen as rust on iron, on cars, boats and steel structures such as bridges.

This a process where there is a reaction between the metal and oxygen to form iron oxide compounds; the brown layer seen in the picture below:

corrosion-the-basics-image-2

What metals will corrode?

Corrosion is so common because most metals are more stable as compounds than as pure metal. This is because they have a desire to move from a high energy ‘unstable’ state to a lower energy ‘stable’ state.

All metals can corrode but the rate at which they corrode differs. For example, pure iron will corrode quickly while stainless steel, which does contain iron, is much slower to corrode.

A group of metals called the noble metals, which include silver, platinum and gold, will rarely corrode. But they also happen to be expensive so are used very sparingly in the types of water systems we would expect to come across. I have yet to see a gold cooling tower but I’m sure it would be quite a sight!

What types of corrosion are there?

There are many different reasons for metal corrosion. Some can be avoided by adding alloys to a pure metal. Others can be prevented by a careful combination of metals or management of the metal's environment, for example through the use of a corrosion inhibitor.

These are the most common types of corrosion:

General Attack Corrosion:

This very common form of corrosion attacks the entire surface of a metal structure. It is caused by chemical or electrochemical reactions. While general attack corrosion can cause a metal to fail, it is also a known and predictable issue. As a result, it is possible to plan for and manage general attack corrosion.

Localised Corrosion:

This corrosion attacks only portions of a metal structure. There are three types of localised corrosion:

  • Pitting - the creation of small holes in the surface of a metal.
  • Crevice corrosion - corrosion that occurs in stagnant locations such as those found under gaskets.
  • Filiform corrosion - corrosion that occurs when water gets under a coating such as paint.

Galvanic Corrosion:

This is probably the most common type of corrosion and it occurs when two different metals are located together in a liquid electrolyte such as salt water. In essence, one metal's molecules are drawn toward the other metal, leading to corrosion in one of the two metals.

Environmental Cracking:

When environmental conditions are stressful enough, some metal can begin to crack, fatigue, or become brittle and weakened.

What could this mean for me?

In terms of managing water systems, the main focus would be the prevention of internal corrosion. Cooling tower pipework and internals, closed system pipework and heat exchangers and steam boilers are all susceptible to corrosion. And if the corrosion process is not controlled, the results can be devastating to the system.

Look out for next week's blog - Corrosion prevention: keeping your water systems safe - to learn more about the potential problems and to discover what you can do to look after your systems.

Shane Lynch

Shane Lynch

Shane Lynch has been in the Water Treatment industry for 15 years, specialising previously in industrial water treatment applications. He is now working as a sales and support consultant for the B & V Chemicals team, offering customers technical and specialist product support.

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