| Corrosion
Primer
What
is Rust?
Rust is probably the most familiar kind of corrosion. It is
generally classified as atmospheric corrosion, where the oxygen
of the atmosphere reacts with the material in question. Most
metals, with exception of the noble metals, like gold for example,
can be oxidized by such atmospheric oxygen. In the most common
case, water vapor must be present before any oxidation can take
place. With iron for example, about 40% of relative humidity
is needed at normal temperatures before rusting will occur.
In
a situation sometimes called concentration cell corrosion, two
solutions of different concentrations will set up an electrical
potential between them similar to a battery. If oxygen is present
in a liquid and if it is replenished continually by contact
with air, then the oxygen concentration in this liquid will
remain constant. Any liquid that is present in small holes or
cracks on a metal surface will not be able to get oxygen from
the main bulk of the solution, so when the supply in the holes
and cracks are exhausted, no more oxygen can enter and replace
it. Therefore, the oxygen concentration in the cracks is different
from the main bulk and a concentration cell is set up. This
minute electrical effect is sufficient enough to make corrosion
proceed quite rapidly.
A
similar type of corrosion is called two-metal corrosion. Two
different metals in contact will set up an electrical potential
between them. If the two metals are surrounded by electrolytes
so a closed circuit is made, corrosion takes place. The speed
and extent at which reaction takes place depends on the types
of metals. Generally, metal pairs farther apart in nobility
will corrode faster than those closer together.
It
is evident that oxygen and hydrogen play an important part in
metal corrosion. It can accelerate corrosion by participating
in cathodic reactions, or it can retard corrosion by forming
a protective film. The dual effect of oxygen is one of the factors
that complicates corrosion processes. Some forms of corrosion
are pitting resulting from local action currents, stress 'corrosion
cracking from environmental and internal stresses. around the
metal.
What
is corrosion?
Rust is corrosion, but not all corrosion is rust. Corrosion
is the deterioration of a material, usually a metal, that results
from a reaction with its environment. For corrosion to take
place, four components must be present:
• anode
• cathode
• electrical or mechanical connection between the anode
and cathode
• electrolyte with allows the electrons to move between
the anode and cathode.
The
four most common methods used to control corrosion are:
1.
Protective coatings and linings
Coatings and linings are principal tools for defending against
corrosion. They are often used in conjunction with cathodic
protection systems to provide the most cost effective protection
for the structure. Coatings and linings help protect against
corrosion in three ways:
I.
They provide a barrier to prevent or limit contact between
a structure's metal surface or components and its corrosive
environment;
2. They release substances that inhibit the corrosion process
and protect the structure from deteriorating; and
3. They serve as sacrificial materials, such as when galvanizing
is used.
To be effective, protective coatings and linings must be properly
selected and installed by personnel trained in surface preparation
and application of the material selected.
2.
Cathodic Protection
Cathodic Protection (CP) is a technology which uses direct electrical
current to counteract the normal external corrosion of a structure
that contains metal, such as a boat or a ship with steel reinforcing
components. The term "cathodic" refers to the area
of the metal where corrosion is controlled, as opposed to the
anodic areas where corrosion occurs. The principle behind Cathodic
Protection is to make the entire surface of a structure behave
like a cathode with respect to an external anode. This behavior
is induced by installing sacrificial materials to serve as anodes
or by applying an external direct current power source in conjunction
with anodes. On new structures, Cathodic Protection can help
prevent corrosion from starting; on existing structures, Cathodic
Protection can help stop existing corrosion from getting worse.
Effective
CP system design will take into account variables such as:
- variations in the environment surrounding a structure;
- he presence of protective coatings and linings;
- the metal to protected; the expected useful life of the structure;
- the ability to maintain the Cathodic Protection system;
- the total electrical current required for protection;
The
costs of installing and maintaining Cathodic Protection must
be considered in context of the direct expenses associated with
replacement of corroded structures and possible structural failure,
as well as indirect costs such as environmental damage. Installing
Cathodic Protection on any infrastructure can be very costly.
3.
Materials Selection
Materials selection refers to the selection and use of corrosion-resistant
materials such as stainless steels, plastics, and special alloys
to enhance the life span of a product. Materials selection personnel
consider the environment in which the product will exist and
the desired life span. If more than one
material is used, such as two metals joined together, controlling
corrosion requires that the materials have compatible electrochemical
properties.The two most common materials used in consumer products
is steel and aluminum, which can be severely affected by corrosion.
4.
Inhibitors, Vapor Corrosion
Vapor Phase Corrosion Inhibitors (VpCIs) are substances which,
when added to a particular environment, decrease the rate of
attack of that environment on material such as metal. VpCI's
are commonly added in small amounts to liquids such as acids,
cooling waters, and steam, either continuously or intermittently,
to prevent serious corrosion. Inhibitors can stop or retard
corrosion in many ways, such as "adsorption", forming
films to coat materials at a molecular level and protect them
from attack, and creating a "passive" layer on the
surface of a material which inhibits further deterioration.
VpCI's can: extend the life of equipment; prevent system shutdowns
and failures; avoid product contamination; prevent loss of heat
transfer; and preserve an attractive appearance of products.
Evaluating
the environment in which a structure is or will be located is
very important to corrosion prevention, no matter which control
method is used. Modifying the environment immediately surrounding
a structure, such as reducing moisture or improving drainage,
can be a simple and effective way to reduce the potential for
corrosion. |
