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paint, body repair paint, corrosion, chemicals, galvanic corrosion, steel, iron,
rust, paint, painting, primer, galvanizing, cathodic protection, impressed
What You Should Know About Corrosion
By: Thomas Yoon
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.
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.
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.
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.
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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