Why paint?
A number of more or less obvious reasons can be readily given
below:
Decoration , Warning , Camouflage , Identification , Safety ,
Image and ease of cleaning
and decontamination etc. The primary function of the paint is protection for
the following substrates.
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*Steel
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Against
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Corrosion
|
|
*ships bottoms
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“
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fouling
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*wood
*commodities e.g. in tanks
*concrete
|
“
“
“
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rot and weathering
contamination, e .g from steel or corrosion products.
Erosion, deterioration and indeed, corrosion of
reinforcement bars (Carbonation)
|
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*ships deck
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“
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Corrosion & Abrasion
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In most cases the
paint is carried out to protect the steel from the corrosion however, the corrosion
is generally an electro-chemical process called electrolysis ,when iron and
steel are exposed to water and oxygen they quickly corrode . Reddish brown
deposits known as rust forms on the surface.
M------® M+++
ne
where M = element involved
n = a
number
e = electron (s)
· Anodic reaction:
Fe ------® Fe++ + z e-
i.e. iron gives iron ions and electrons.
· Cathodic reaction:
2H2O
+ O2 + 4e- ------® 4 OH
i.e. water +
oxygen + electrons give hydroxyl ions.
Fe ++
+ 2 OH ------® Fe (OH)2
i.e iron ions plus hydroxyl ions
gives iron hydroxide.
4 Fe (OH)2 + O2 ------® 2 Fe O3 H2O
+ 2H2O
i.e iron hydroxide plus oxygen gives rust.
Acceleration of corrosion reactions is caused by many factors
which include:
* Variations in oxygen content on the materials surface.
* Chlorides and sulphides
* Higher
nobility metals being in contact with the steel (e.g. mill scale)
* Acids or alkalis.
Mill scale is an
oxide of iron produced when the steel is manufactured; it is a result of the
white hot steel coming into contact with air and forming an oxide composed of
three layers: FeO nearest the steel, Fe3O4
then Fe2O3 on the outside. Mill scale
has a total thickness ranges from approximately 25 um to 100 um.
The following list shows some metals / metal compounds in
their order of nobility in sea water at ambient temperature; this list is known
as the galvanic series:
Gold ……………………
Noble
Platinum
Silver
Nickel
Copper
Mill scale
Steel (iron)
Aluminum
Zinc
Magnesium …………….
Ignoble
For example: if steel was in intimate contact with zinc or
attached to zinc via a wire in an electrolyte, e.g. soil or water, the zinc
would corrode first because steel is more noble than zinc. In this example the
zinc becomes the anode and the steel the cathode, i.e. the steel is being
cathodically protected.
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