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Corrosion Prevention Measures

By: Thomas Yoon

In the previous article, we found out the corrosion of iron and steel occurs only when 3 conditions occur:

1. A potential difference exists.
2. The surfaces are covered by an electrolyte.
3. Oxygen is present.

Painting

 The most common method is to cover up the surfaces of bare metal with a coating of paint. This prevents the flow of current whenever the metal is in contact with water. To be effective, the coating on the bare metal must have a very strong bond. This first paint coat is called the primer coat.

The purpose of the primer paint coat is to protect the base metal area and also have adhesion of the topcoat. Some examples of primers are zinc chromate primer, red oxide and red lead primer. For galvanized steel and light alloys you will need to use a polyvinyl butyral-based, acid-catalyzed etching primer.

Some alkyd-based paint coatings pigmented with micaceous iron oxide provides good protection for iron and steel surfaces and are suitable for storage tanks, pipes, steel structures and grills.

For heavy-duty paint coatings, the surfaces need to be cleaned by sand-blasting. The idea of sand-blasting is to bombard the surface with abrasives under high pressure to scrape it clean to the bare metal.

Epoxy paints provide very good protection from corrosion. This is because of the impervious nature of the epoxy coating layer. These are excellent for protection against corrosive chemicals, salt and fresh water abrasion and are ideal for steel piles, marine structures, ballast tanks and sewage treatment plants.

Polyurethane paints are used in body paint and auto paint of cars. These produce high gloss and are excellent body repair paints with good durability.

Rubber and Plastic linings

 By using rubber and plastic linings and laminates to coat the bare metal, a chemical resistant elastomer layer can be strengthened by the strong steel reinforcement underneath. Their use is not so much to protect the steel but to use the steel for support.

Cathodic Protection

 Another way to protect steel is to coat the metal with a more reactive metal. This can be found in galvanizing. This is the coating of iron with zinc. The iron can be dipped in molten zinc to coat it. Because the zinc is reactive, it forms a tough oxide layer that prevents further oxidation and so protects the iron underneath. Many outdoor structural parts are often protected in this way. Some examples are cooling towers, air conditioning ducting, sheet metal, pipes, street lamps, etc.

In some submerged steel installations, zinc is used as sacrificial anodes. When zinc is placed together with iron, the former is sacrificed in favor of the latter. An example of this usage is in tube and shell heat exchangers where the presence of copper tubes will cause an electrical potential to exist between the steel shell and the copper tube nest. Sacrificial anodes of zinc are often then placed at the shell end covers to protect the iron from wasting away.

Impressed Current System

 By putting the flow of current to the opposite direction, a way has been found to counter the inherent electrical potential caused by dissimilar metals placed together. By using noble metals like titanium, platinized niobium or lead/silver alloy and injecting electrical current in the opposite direction, the parent steel metal can be protected from the effects of corrosion.

A typical impressed current system controls current so that the inert anodes are made positive relative to the steel. When the steel is immersed in seawater, the normal galvanic current is counteracted by the supplied current. Different potentials are also applied for newly painted steel and for clean bare steel. The controller unit with its step-down transformer, rectifier, and the reference electrode are essential components of this impressed current system. The amount of current, and its evenly distribution is very important in this system. Over potential can produce OH- ions that can attack the surrounding paint.

Alkaline Chemical Treatment

 Cement wash or alkaline chemical treatment for the water in contact with the metal is used in tanks where no painting or coating is allowed. Chemical treatment of water is also used in steam boilers where any coating on the heat transfer surfaces will be disastrous.

Use of less active metals

 It is difficult to replace steel in many applications where strength is important. In applications where strength and low cost is not so critical, alloys of cupro nickel, bronze and brass can be used to substitute steel parts. Stainless steels containing iron and chromium can also be used where higher cost is not a concern.

Painting to prevent corrosion does make our world a more colorful place to live. We can prevent corrosion in so many different ways. Corrosion need not be a dirty word. In most cases, just a coat of paint will do.

Until next time...

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