In
most surroundings - inside or in the open air - Aluminium has a very
high resistance to corrosion . One reason for this is that Aluminium
is covered spontaneously by a thin but effective coating of oxide
which protects the Aluminium from further oxidation. The Aluminium
oxide is impermeable and is integral with the base metal. If the coating
of oxide is damaged mechanically this coating will be renewed immediately.
The coating of oxide is a main reason for the good corrosion characteristics
of Aluminium. The coating stays stable for pH-values between 4 and
9. Generally alloys with more than 0.5% copper have a worse resistance
to corrosion and they should not be used in unprotected, strong chlorine
surroundings.
The most frequent types of corrosion are Galvanic Corrosion, Pitting
and Crevice Corrosion.
Galvanic Corrosion:
Galvanic corrosion takes place when two different metals have contact
with each other in the prescence of an electrolyte. The less noble
metal (anode) of the combination corrodes while the noble (cathode)
will be protected. A small surface of the cathode and a large surface
of the anode lead to a low corrosion. In reverse case the Aluminium
will be attacked quickly. In most combinations with other metals,
Aluminium is the less noble. Aluminium is therefore of greater risk
of galvanic corrosion than that of the other constructive materials.
These dangers of galvanic corrosion only exist in metallic contact
with more noble metals (or other conductors of electrons, for example
graphite) and then when an electrolyte with good conductivity exists
between the metals. The occurrence of galvanic corrosion will be encouraged
because of a disadvantageous construction of the extrusion.
Galvanic corrosion does not happen in dry surroundings, however contrary
to this, in surroundings with chloride the risk of galvanic corrosion
always exists, for example near the seaside. In these surroundings
it could be that copper, carbon steel and stainless steel suffer galvanic
corrosion.
It should be noted that problems can occur in the combination of galvanised
steel and Aluminium. This is due to the fact that the Aluminium will
be protected by the coating of zinc but when the zinc is used up the
bare surface of steel can attack the Aluminium. For this reason the
Aluminium should be combined with warm zinced material in an aggressive
surrounding because warm galvanising, gives a harder coating of zinc,
than hot-dipped
However, galvanic corrosion can be prevented if certain steps are
taken. One being by electric insulation between the metals, where
the insulation has to interrupt the metallic contact completely.
In large constructions where electric insulation is difficult it is
possible to use an electrolyte insulator between the two metals for
example by painting to interrupt the connection. Often it is beneficial
to conceal the surface of the cathodes (that of the nobler metal),
however another possibility is the installation of an intermediate
layer
Another method of protection is Cathodic protection. Cathodic protection
can be achieved in two ways. Often anodes, consisting of less noble
metal can be in contact with the surface of Aluminium that has to
be protected. Within this process the less noble metal was sacrificed
(it is corroding ) and is therefore called a sacrificial anode. The
existence of a contact of liquid between the coating which has to
be protected and the anode is another condition for prevention of
corrosion. Often Z- or Mg-anodes are used for Aluminium. Cathodic
protection can be reached by an exterior constant-potential supply
and by connecting an Aluminium object to the negative pole.
Pitting :
Pitting is a frequent type of corrosion of Aluminium and occurs due
to the presence of an eletrolyte. The corrosion is seen as very small
pits and only penetrates a small way into the surface when induced
in air however, in water and earth larger pits can occur. As residues
of corrosion often cover the small pits the attacked places in the
surface of Aluminium are rarely visible.
Small pits are mainly a problem of appearance and have no negative
influence on the temper, however treatment of the surface, anodising
and painting can prevent this corrosion. The treated surface is cleaned
so that the attractive appearance is kept and corrosion is prevented.
Occasional washing with water is sufficient to prevent pitting on
untreated metal. Alkaline cleaning materials must be avoided.
Pitting can be prevented by cathodic protection. It is important that
the extrusions are shaped in a way that can dry easily and edges and
hollows in which dirt and moisture can accumulate should be avoided.
Stagnant water can be prevented by inclination of the extrusions or
drain holes (minimum 8mm respectively 6 x 20mm because otherwise the
water cannot run off due to the capillary force). Also airing of "closed"
constructions reduces the risk of condensation.
Crevice corrosion:
Crevice corrosion can arise in small crevices filled with fluid however
this rarely happens in extruded constructions and this risk is increased
in marine atmospheres. It can happen during transport and storage
that water gathers in the crevices between Aluminium surfaces opposite
to each other and this can lead to corrosion on the surface (water
spots). The rain water is sucked up between the surfaces of the metal
or condensation water occurs when cold material is brought into warm
surroundings. Condensation water can also arise when Aluminium is
stored carefully covered in the open air because of the varying temperatures.
The use of sealing compound or tape that is adhesive on both sides,
before joining the pieces prevents water from entering into crevices.
Sometimes rivets or screws can be substituted by glue or they can
be combined. By this formation crevices corrosion can be prevented.
Aluminium in the Atmosphere
The corrosion of metal in the atmosphere depends on the time of exposure
and the composition of the electrolyte on the surface. The time of
moisture is the time in which the surface of metal is so wet that
corrosion can happen and the time of moisture is when the relative
moisture exceeds 80% and the temperature is over 0oC at the same time
(for example the formation of condensation).
In the mainland atmosphere and in a moderate sulphurous atmosphere
the durability of Aluminium is excellent. In strong sulphurous atmospheres
it is possible that small corrosion may appear on the surface, but
generally the durability is better than the durability of carbon steel
and galvanised steel. The occurrence of salts, especially chlorides,
in the atmosphere reduce the durability of Aluminium slightly compared
with other materials. Mostly the maximum depth of small pits is only
a fraction of the thickness of the material, so the characteristics
of durability are nearly constant. However those of carbon steel are
totally different.
Experiments in the open air with different untreated metals were carried
out by the institute of Research into Corrosion. The losses in weight
of sheet metals with untreated surfaces after 8 years of research.
The average for the deepest corrosion on Aluminium sheets was 0.07mm
after the first 8 years. The table shows how huge the loss of weight
is near the seaside at approximately 1/100 of carbon steel and approximately
1/10 of galvanised steel. The rate of corrosion decreases rapidly
with greater distance to the sea. 1km away from the sea Aluminium
has the same characteristics as inland. The rate of increase of the
small pits decreases over time.
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Losses
in weight after 8 years
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Atmosphere
at the sea
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Aluminium
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7
g/mē
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Copper
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57
g/mē
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Zinc
|
133
g/mē
|
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Carbon
steel
|
933
g/mē
|
|
|
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Atmosphere
in Stockholm
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|
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Aluminium
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2
g/mē
|
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Copper
|
31
g/mē
|
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Zinc
|
61
g/mē
|
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Carbon
steel
|
676
g/mē
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Aluminium in the Ground
The ground is not a uniform material. It varies in the composition
of minerals, the moisture content, degree of acidity, existence
of organic materials and electric conductivity. Because of these
differences it is problematic to predict the durability of metals
in the ground. In addition to these, other factors like the leakage
current of constant -potential supplies can influence their durability.
The characteristics of corrosion of Aluminium in the ground depends
on the moisture, the specific resistance and the pH-value of the
earth. The present knowledge about the characteristics of corrosion
in different types of ground is not sufficient. When using Aluminium
in the ground a protective treatment e.g. A coating of tar, is recommended.
The corrosion can be interrupted by a cathodic protection.
Aluminium in Water
The corrosion of metal in water mainly depends on the composition
of water. Chlorides and heavy metals influence the durability of
Aluminium. In fresh water and drinking water small pits in the Aluminium
can occur. By regular cleaning the risk of corrosion is very low.
Pots and other household objects can be used for decades without
damage from corrosion. Stagnant water and long exposure increases
the risk.
The Aluminium can be protected from small pits by designs which
prevent the risk of banked-up water level, by using cathodic protection
and chemical additives to delay the corrosion, for example, car
radiators. The rate of pitting in fresh water decreases strongly
over time. The corrosion present after three years will only have
doubled over 24 years. In sea water A1Mg-alloys with more than 2.5%
but also A1MgSi-alloys show a good stability. Cupriferous alloys
should be avoided, however when still using them they have to be
effectively protected from corrosion. When using the correct construction
in combination with other metals (please note the risk of galvanic
corrosion) Aluminium is an excellent material in the surroundings
of the sea. An example is useful, frequent use of Aluminium in boats.
Very often the protection from corrosion of this material is a cathodic
protection.
Corrosion at the Water Level
Aluminium, which is only partly in the water, can corrode directly
under the water surface (sea level of corrosion). This type of corrosion,
which only appears in stagnant water, can be protected by varnishing.
Aluminium and Alkaline Building Materials
Splashes of alkaline building materials like mortar and concrete
cause visible spots on the surface of the Aluminium. These are difficult
to remove. For that reason Aluminium should be protected on building
sites. Aluminium cast in concrete will be attacked similarly and
the adherence between the materials increases. After the cementation
of the concrete corrosion cannot happen. Longer moisture can continue
the process of corrosion so that the corrosion produces splits in
the concrete. This type of corrosion can be stopped by coating the
surface of Aluminium with tar or by coating with alkaline varnish.
Anodising does not improve the durability because the coating of
oxide is not stable in an alkaline surrounding. Indoors in a dry
environment, the surface of Aluminium does not have to be protected.
Aluminium and Chemicals
Thanks to the protective characteristics of the coating of oxide,
Aluminium has a good durability against many chemicals. Low or high
pH-values (less than 4 and more than 9) lead to a dissolution of
the coating of the oxide and to a rapid corrosion of the Aluminium.
Inorganic acids and strong alkaline solutions can then easily attack
Aluminium. In moderate alkaline solutions of water the corrosion
can be restricted by insertion of silicates as an inhibitor of corrosion.
Normally such kinds of inhibitors are included in dishwasher detergents.
Most inorganic salts have no marked effect of corrosion of Aluminium.
Exceptions are heavy metal salts, which can start a strong galvanic
corrosion on the surface of Aluminium because of the reduction of
the heavy metals ( for example copper and mercury).
Aluminium has a strong durability against many organic combinations.
Often objects of equipment for the production and the storage of
chemicals are made of Aluminium.
Aluminium and Dirt
Coatings and accumulation of dirt on the intermediate coating can
cause a reduced durability. Very often this is a consequence of
long-term moisture. Dirty surfaces should be cleaned once or twice
a year depending on the grade of contamination. See Cleaning
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