|
|
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:
a. They provide a barrier to prevent or limit contact between a
structure's metal surface or components and its corrosive environment;
b. They release substances that inhibit the corrosion process and
protect the structure from deteriorating; and
c. 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.
corrosion tour - prevent & protect
|
