(Metal O2(g) à 2Cu2O­(s­) (red) The oxidation-reduction reaction

(Metal supermarkets, 2016).

6.      Design
Modifications can improve the durability of the structure and any protective anti-corrosion
coatings. Designs that encourage the movement of air and avoid the trappings of
dust and water as well as allowing for easy access for regular maintenance can
be great ways to increase longevity of a structure.

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5.      Corrosion
Inhibitors are chemical inhibitors that react with the surface of the metal or
its surrounding gases to create a protective film. An example of this can be
seen through passivation.

4.      Cathodic
Protection is a type of sacrificial coating where one metals is coated with
another type of metal which is more reactive. This reactive metal then oxidizes
more freely with the oxygen in the environment, saving the metal below from
corrosion. The top layer of metal can be reapplied as needed. An example can be
seen from galvanizing zinc on top of steel alloys, which is used on steel
pipelines.

3.      Environmental
Measures can be obtained by controlling the environment around the structure to
limit unwanted reactions. This can be done simply by reducing exposure to rain
and water or by controlling the amounts of oxygen, sulfur and chlorine in the
surrounding environment.

2.      Protective
Coatings via plating or paint coatings are major tools in protecting against
corrosion, since they provide a protective layer over the metal, inhibiting
water and oxygen to react with it, and are very cost effective.

1.      Materials
Selection is the use of materials that are resistant to corrosion, such as
stainless steel, plastics, and special alloys that increase the lifespan of the
structure.

There are several different ways to protect and
control corrosion:

Corrosion
Protection

 

(OpenStax, 2016)

4CuO(s)
+ SO3(g) + 3H2O(l) à
Cu4SO4(OH)6(s)   (green in colour)

3CuO(s)
+2CO2(g) + H2O(l) à
Cu2(CO3)2(OH)2(s)   (blue in colour)

2CuO­(s)
+ CO2(g) + H2O(l) à
Cu2CO3(OH)2(s)   (green in colour)

Since coal, which is high in sulfur, was burned to a
large extent in the beginning of the last century, the copper(II) oxide then
reacted with sulfur trioxide, carbon dioxide and water. The combined
precipitate of all 3 reactions is what constitutes patina:

2Cu2O­(s­)
+ O2(g) à 4CuO(s) (black)

4Cu(s)
+ O2(g) à 2Cu2O­(s­)  (red)

The oxidation-reduction reaction of copper with its
environment occurs in several steps. To start, the copper metal reacts with
oxygen to form copper(I) oxide (red in colour). The copper(I) oxide then reacts
with oxygen again to form copper(II) oxide (black in colour). In both reactions
the copper is oxidized and the oxygen is reduced:

An example of Generalized Corrosion can be seen
through the formation of blue-green patina produced from the oxidation of
copper on the Statue of Liberty (Figure 3) or the roofs of Parliament
buildings. The patina creates a protective layer over the surface of the
copper, stopping further corrosion of the copper below.

 

(Hinds, 2003).

4Fe(OH)2(s)
+ O2(g) à 2H2O(l) + 2Fe2O3.H2O(s)­

iron(II)
hydroxide + oxygen à water + hydrated iron(III) oxide (red
rust)

Lastly, since oxygen is in excess supply and readily
dissolves in water, it once again reacts with the iron hydroxide to form red
rust:

2Fe(s)
+ O2(g) + 2H2O à 2Fe(OH)2(s)

iron
+ water with oxygen dissolved in it à iron(II)
hydroxide

This then leaves the iron ions to combine with the
hydroxide ions to form iron(II) hydroxide. The overall oxidation-reduction
reaction can be rewritten as:

O2(g)
+ 2H2O(l) + 4 e- à
4OH-(aq)

The electrons then travel through the metal to the
cathode site where they are used to reduce oxygen molecules and produce
hydroxide ions:

2Fe(s)
à
2Fe2+(aq) + 4 e-

Firstly, in the presence of oxygen and moisture
(water), iron oxidizes at the anode site, forming iron ions and 4 electrons:

An example of Localized Corrosion can be seen
through the rusting of iron (Figure 2). Rust is the flakey reddish-brown
material that is produced when iron containing metals like steel, corrode. It
does not stick well to the metal underneath and flakes off easily to expose the
fresh metal to further corrosion. As stated earlier, corrosion is the creation
of a galvanic cell on the surface of the iron.

The
Chemistry Behind Corrosion

High Temperature Corrosion occurs when fuels containing
vanadium or sulfates, form compounds during combustion. These compounds, with
low melting points, are very corrosive towards metal alloys which are normally
resistant to high temperatures and corrosion. High temperature corrosion can
also be caused by high temperature oxidization, sulfidation, and carbonization.
(Bell, 2017).

Fretting Corrosion is a result of constant wearing, weight
and or vibration on an uneven, rough surface, resulting in pits and grooves, which
occur on the surface of the metal. (Bell, 2017).

De-Alloying is when only one specific element of an
alloy is corroded.

Intergranular Corrosion is a chemical attack that occurs
on the grain boundaries of the metal. Impurities in a metal are present in
higher concentration near the grain boundaries, thus making them more susceptible
to corrosion than the bulk of the metal. (Bell, 2017).

·       
Cavitation is caused by the implosion of
gas bubbles on a metal surface. It is associated with the sudden change of
pressures related to hydrodynamic parameters of a fluid.

·       
Impingement is the combined effect of corrosion
and erosion caused by rapidly flowing water.

·       
Erosion-assisted Corrosion is the wearing
down of a material surface due to mechanical action.

Flow-Assisted Corrosion
happens when the protective layer of oxide coating above a metal is removed or
dissolved by wind or water. This exposes the underlying metal to further
corrosion and deterioration. (Bell, 2017).

·       
Liquid Metal Embrittlement occurs when a
liquid metal atom is absorbed onto a susceptible metal which causes stress,
creating a crack that will grow quickly against the grain boundaries of the
metal.

·       
Hydrogen-Induced Cracking occurs when
lone hydrogen ions within the metal recombine to form hydrogen molecules,
creating pressure within the metal. This pressure will increase until it cracks
open the metal.

·       
Corrosion Fatigue is the mechanical
degradation or fatigue of a material in a corrosive environment, under the
joint action of corrosion and cyclic stressors.

·       
Stress Corrosion Cracking is when a
crack forms and grows due to a corrosive environment with tensile stress.

Environmental Cracking is a process that results
from conditions like stress, chemical and temperature related environmental
conditions, which affect the metal. (Bell, 2017).

Galvanic Corrosion occurs when two different types
of metals are placed together with an electrolyte. An oxidation-reduction
reaction occurs similar to that of a galvanic battery, whereby one metal
becomes an anode and the other a cathode. The anode metal acts as a sacrifice
and corrodes faster than it would if the second metal wasn’t present, while the
cathode metal corrodes slower than it would otherwise. (Bell, 2017).

·       
Passivation is the creation of an outer
layer of protective material that is applied as a micro film over the metal
below. This layer is created from the metal oxide produced from the oxidation
of the metal, with air. It stops any future corrosion of the metal below, and the
protection will continue unless the protective layer is removed by external
forces.

Generalized Corrosion is when corrosion appears over
the entire surface of a metal, which leads to the overall thinning of the
metal. This type of corrosion is a rare occurrence. (Bell, 2017).

·       
Filiform Corrosion is when water seeps
under painted or plated coatings. Corrosion will begin from small defects in
the coating and spread which will cause weakness in the overall structure of
the metal.

·       
Crevice Corrosion, similar to pitting,
occurs at a specific area. Acidic conditions or the loss of oxygen in a crevice
leads to crevice corrosion.

·       
Pitting is when a small cavity or hole
forms in a metal. This area becomes an anode while the rest of the metal
becomes a cathode to produce a localized galvanic reaction.

Localized Corrosion is the most common and dangerous
type of corrosion, where the attack happens in a single location on the surface
of the metal and creates a small pit or cavity. It is hard to prevent and often
too difficult to detect before the metal cracks and causes structural failure. (Bell,
2017).

How
Corrosion Occurs

 

3.      An
electrolyte (usually water)

2.      Oxygen
(usually from the atmosphere)

1.      Metal
(eg: iron)

Corrosion is defined as the deterioration of metals
as a result of oxidation – the process in which chemical entities lose
electrons. (Davies, 2004). This process is an oxidation-reduction reaction
where the metal is oxidized by the oxygen in its environment. These reaction
occurs spontaneously. Corrosion is basically the creation of a galvanic cell on
the surface of the metal, where the metal acts as the anode and over time
deteriorates or loses functional stability. The reactivity series (Figure 1)
shows metals in their order of their reactivity. For corrosion to occur, three
components are needed:

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