Corrosion causes a lot of problems in maintenance. Machines get worn out, structures become weakened, appearance gets tarnished and metals like iron do not last as long as we wanted them to.
Yet not every wearing of machines can be attributed to corrosion. Erosion, abrasion and scuffing can also wear down machines. However the major reason for machine wear down is still corrosion. Why is that so?
We live in a world of chemicals and corrosion is a chemical reaction. In the process, metal is dissolved. Since iron and steel is used in almost every machine and structure, we particularly want to avoid corrosion in them.
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.
Potential Difference
Acids are the chief culprit in any corrosion. Sulphuric acid from acid rain causes a lot of corrosion damage. However, many people do not realize that it is the movement of electrons in the chemicals that causes the iron to get dissolved. The potential difference is the one that causes the electrons to flow in a closed electrical circuit, similarly to a battery.
Galvanic Corrosion
In the galvanic series of metals, noble metals like gold, silver, and platinum are almost immune to the attack of acids. They are considered inactive. The most active are metals like potassium and sodium. Somewhere in between these extremes are metals like iron, copper, zinc, tin, etc.
In a normal galvanic corrosion, electrical current flows from a less active metal to a more active metal, i.e. from the cathode to the anode. The anode gets wasted away. When a steel structure is immersed in salt water together with a less active metal like copper, the steel gets corroded very quickly.
When dissimilar metals are placed together in an electrolyte, a potential difference is created between them. The more active metal is corroded when a current flows through the acidic water or electrolyte. The faster the flow, the faster is the rate of corrosion. In normal atmospheric corrosion, the concentration of the acid in the water electrolyte is very low, so the rate of corrosion is slow. With a higher concentration, the corrosion also quickens.
Presence of Oxygen
Rust is one of the most common effects of corrosion. It is the oxidation of iron to form ferrous oxide. Corrosion is therefore highest in water tanks at the surface of the water where it is wet and contains a lot of dissolved air from the atmosphere. In a closed vessel, corrosion stops when all the oxygen has been used up for rusting.
Corrosion Prevention Measures
Methods that are used to prevent corrosion are therefore aimed at preventing the presence of an electrical potential difference, using less active metals and removing the presence of moisture and oxygen.
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.
Thomas Yoon specializes in cartoon illustrations that will make an impact on people's opinions.
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