Chemical engineering related topic

2. Erosion is the general destruction of material by
mechanical wear or abrasion e.g. wearing of railway
tracks and train wheels etc.
Corrosion is the disintegration of an engineered
material into its constituent atoms due to chemical
reactions with its surroundings.

3. Corrosion is electrochemical oxidation of metals in reaction
with an oxidant such as oxygen.
A significant amount of energy is put into a metal when it is
extracted from its ores, placing it in a high-energy state.
These ores are typically oxides of the metal such as hematite
(Fe2O3) for steel or bauxite (Al2O3¢H2O) for aluminum.
One principle of thermodynamics is that a material always
seeks the lowest energy state. In other words, most metals
are thermodynamically unstable and will tend to seek a lower
energy state, which is an oxide or some other compound. The
process by which metals convert to the lower-energy oxides is
called corrosion.

4. The corrosion process involves the removal of electrons (oxidation) of the
metal and the consumption of those electrons by some other reduction
reaction, such as oxygen or water reduction.
The oxidation reaction is commonly called the anodic reaction and the
reduction reaction is called the cathodic reaction. The oxidation reaction
causes the actual metal loss but the reduction reaction must be present to
consume the electrons liberated by the oxidation reaction, maintaining
charge neutrality.

5. The oxidation and reduction reactions are sometimes referred to as half-cell
reactions and can occur locally (at the same site on the metal) or can be
physically separated.
When the electrochemical reactions are physically separated, the process is
referred to as a differential corrosion cell.
There are four necessary components of a differential corrosion cell.
1.There must be an anode
2.There must be a cathode
3.There must be a metallic path electrically connecting the anode and cathode.
(Normally, this will be the pipeline itself.)
4.The anode and cathode must be immersed in an electrically conductive
electrolyte (normally, moist soil).

7. Passivation
A thin film of corrosion products can form on a metal's
surface spontaneously, acting as a barrier to further
oxidation. Passivation in air and water at moderate pH is
seen in such materials as aluminium, stainless steel,
titanium, and silicon.
Pitting
Localized corrosion of a metal surface that is confined to
a small area and takes the form of cavities called pits.

8. Cathodic protection (CP) is a technique used to
control the corrosion of a metal surface by making it
the cathode of an electrochemical cell.

9. Galvanic anodes are designed and selected to have a more "active"
voltage (more negative electrochemical potential) than the metal of the
structure (typically steel). For effective CP, the potential of the steel surface
is polarized (pushed) more negative until the surface has a uniform
potential. At that stage, the driving force for the corrosion reaction is
removed. The galvanic anode continues to corrode, consuming the anode
material until eventually it must be replaced. The polarization is caused by
the electron flow from the anode to the cathode. The driving force for the
CP current is the difference in electrochemical potential between the anode
and the cathode.
Galvanic or sacrificial anodes are made in various shapes and sizes using
alloys of zinc, magnesium and aluminium. ASTM International publishes
standards on the composition and manufacturing of galvanic anodes.
In order for galvanic cathodic protection to work, the anode must possess a
lower (that is, more negative) potential than that of the cathode (the
structure to be protected).

10. For larger structures, galvanic anodes cannot deliver
economically enough current to provide complete protection.
Impressed current cathodic protection (ICCP) systems use
anodes connected to a DC power source. Usually this will be
a cathodic protection rectifier, which converts an AC power
supply to a DC output. In the absence of an AC supply,
alternative power sources may be used, such as solar panels,
wind power or gas powered thermoelectric generators.
Anodes for ICCP systems are available in a variety of shapes
and sizes. Common anodes are tubular and solid rod shapes
or continuous ribbons of various materials. These include high
silicon cast iron, graphite, mixed metal oxide, platinum and
niobium coated wire and others.

11.  Pipelines are routinely protected by a coating supplemented with cathodic
protection. An ICCP system for a pipeline would consist of a DC power source,
which is often an AC powered rectifier and an anode, or array of anodes buried in
the ground (the anode groundbed).
 The DC power source would typically have a DC output of between 10 and 50
amperes and 50 volts, but this depends on several factors, such as the size of the
pipeline. The positive DC output terminal would be connected via cables to the
anode array, while another cable would connect the negative terminal of the
rectifier to the pipeline, preferably through junction boxes to allow measurements to
be taken.
 Anodes can be installed in a vertical hole and backfilled with conductive coke (a
material that improves the performance and life of the anodes) or laid in a prepared
trench, surrounded by conductive coke and backfilled. The choice of grounded type
and size depends on the application, location and soil resistivity.
 The output of the DC source would then be adjusted to the optimum level after
conducting various tests including measurements of electrochemical potential.
 It is sometimes more economically viable to protect a pipeline using galvanic
anodes. This is often the case on smaller diameter pipelines of limited length.