COATING APPROACH OF CORROSION CONTROL AND PROTECTION
Corrosion
on it is a very basic entity that explains the reverse processes of extractive
metallurgy. It is the degradation of materials due to environmental interaction
on the material surface. Corrosion can as well be tagged as destructive attach
at material which poses an expensive cost of correction and maintenance challenges
on the Engineers.
Corrosion
control over time is achieved; by a critical investigation into its
mechanism, by using the corrosion-resistant material and designs, and by using the protective system, devices, and treatment which may come in any of the various
form of corrosion protection method, among which is coating to be discuss in this paper work.
|
Figure 1: illustration of mechanism of localize
corrosion in a metal surface |
COATING
A coating, from a surface engineering point of view, is a
layer of material deposited onto a substrate to enhance the surface properties
for corrosion and wear protection. Factors affecting the choice of a coating
include service environment, life expectancy, substrate material compatibility,
component shape and size, and cost.
There is a wide range of coating processes for depositing
many different types of material at thicknesses ranging from just a few
microns, up to several millimetres. The different types of coatings can be
categorized in many ways. One common approach is based on the manner in which
the coating material is deposited on the substrate surface. This includes
atomic deposition, particulate deposition and bulk coating or cladding.
There are two
types of surface coating:
1. Metallic
2. Non-Metallic
Metallic coating:
For centuries, metals have been the go-to choice for
multiples applications due to their durability, versatility and strength.
However, among the challenges that people face when using metals, corrosion is
arguably the most common and widely recognized.
Multiple solutions have been proposed to increase the
longevity of metallic structures and enhance their corrosion resistance.
Among them, metal coatings stand out as one of the most effective and
convenient protection methods.
Metallic
coatings contain a metallic
element or alloy. Metallic coatings can be applied by using a sprayer,
electrochemically, chemically or mechanically. These coatings are applied on
equipment requiring a shiny or glossy appearance and protection from sunlight,
corrosion and oxidation.
The structure is coated
with a layer of other metal which may be nobler than the structure or less
noble than it e.g. steel structures can be coated with copper which is nobler
than steel or zinc which is less noble. In case of coating the structure with a
more noble metal care should be taken that the coat is free from pores or
cracks to avoid the formation of dissimilar metal corrosion cells which would
lead to corrosion of the structure.
There are numerous methods for coating metallic surfaces,
each with its own set of limitations and benefits. In the following sections we
will take a detailed look at some of the most common types of metal coatings,
and discuss their suitability for various applications.
Factors that must be considered in
selection of a coating metal:
1.
The coating should be able to resist direct attack of the environment.
2.
The coating should be nonporous and continuous (no cracks) to a void
acceleration of corrosion especially in case of a more noble metal (e.g coating
of Fe by Cu).
3.
The coating should be hard.
In
case of coating the structure with a less noble metal the presence of pores and
cracks in the coat is not dangerous because in this case the less noble metal
will corrode by the formation of dissimilar metal cells while the structure
will remain protected.
How Metal Coatings Protect Surfaces and Structures
Metal corrosion is a deteriorative process that occurs under specific
conditions. The most common type of corrosion occurs when metals react with
moisture and oxygen to create various corrosion products. Iron, for example, reacts with water
and oxygen in the atmosphere to form iron (III) oxide, or rust.
The logic behind metal coatings, therefore, is to create an
inert (non-reactive) barrier around the metallic object being protected to
prevent it from reacting with air and moisture.
Types of Metal Coatings and Their Benefits
Below, is a list of the most common types of metal coatings
used across various industries, and the advantages and disadvantages of each.
Anodizing
Anodizing is
a process used to promote the formation of a protective oxide layer on the surface of a metal. The resulting
oxide layer forms more rapidly and is usually thicker than if it was produced
naturally. While several non-ferrous metals can be anodized, aluminum responds
most effectively to this process.
Anodizing is performed by immersing the aluminum component in
a tank filled with an electrolytic solution along
with a cathode (usually aluminum
or lead). An electrical current is passed through the aluminum, causing it to
oxidize and form a protective barrier.
Anodized finishes are perhaps the easiest to maintain of all
the coatings mentioned in this article. Anodized surfaces can be easily
periodically cleaned using mild detergents. Finished anodized surfaces are
also chemically stable and do
not decompose under normal conditions, allowing for a long-lasting coated
surface. Furthermore, because anodizing is a natural process, it is non-toxic
and does not produce any harmful or dangerous by-products.
The most significant drawback of this process is that it is
only useful on a handful of metals. This process is unsuitable for ferrous metals, which means
that common materials like steel and iron cannot be anodized. Additionally, due
to the processes used, the colors that can be achieved by anodizing is limited.
Galvanizing
Galvanizing involves
immersing the metal (mostly steel or iron) in a molten zinc bath. Once removed,
the coated metal reacts with oxygen and carbon dioxide in the atmosphere to
form a protective zinc carbonate layer.
The galvanizing process has multiple advantages that make it
a popular choice for numerous applications. For example, the zinc
oxide coating is highly stable and adheres tightly to the
metal substrate; it is very durable
and does not flake off easily.
Galvanizing is also renowned for its galvanic protection. In
other words, if the metal's surface becomes exposed due to scratches, cuts or
dents, the zinc coating will
sacrifice itself by corroding preferentially. This process helps protect the
steel substrate between maintenance operations.
The biggest disadvantage of the galvanizing process is its
cost. While hot-dip galvanizing
(HDG) may be cheaper for coating large steel structures, it
can be less cost-effective for smaller pieces such as nuts and fasteners.
Additionally, galvanized surfaces have a dullish grey appearance that may not
be aesthetically pleasing for some applications.
Electroplating
Electroplating, also known
as electrodeposition, involves
depositing a thin layer of one metal on the surface of another metal. During
electroplating, both metals are placed in an electrolytic solution. The metal
to be coated acts as the anode,
while the coating metal acts as the cathode.
An electric current is applied to the electrolytic cell, causing
metal ions to move from the cathode to the anode, thus forming the coating.
Electroplating offers excellent corrosion resistance and can
enhance some of the metal's mechanical properties.
Electroplating also produces an aesthetically pleasing surface finish, making
it ideal for coating jewelry and ornaments.
However, electroplating can produce non-uniform coating
thicknesses, making it unsuitable for high-precision applications. Also, the
process itself has numerous requirements and is too costly to be used on an
industrial scale.
Potentially toxic and harmful compounds are used as
electrolytes in the electroplating process. Therefore, care must be taken when
discarding electrolyte chemicals to avoid environmental contamination.
Powder Coating
Powder coating, as its name
implies, involves coating an object with a powder-based substance. It is an
electrostatic process, whereby the coating particles are electrically charged
with a polarity that is opposite to the part to be coated. The difference in
charge causes the powdered particles to adhere to the metal's surface. The
coated object is then heat-treated in an oven to harden the coating.
Powder coatings are renowned for their durability and
aesthetically pleasing appearance. Additionally, because powder coatings do not
contain solvents, there are little to no volatile organic compound (VOC) emissions.
While powder coatings may be cost-efficient in the long-term,
the initial start-up costs can be significant. The coating process requires
special spray booths, ovens and
spraying equipment. This can also limit the size of objects that can be coated.
It is also difficult, or even impossible, to achieve thin
coating layers. Furthermore, the finished surface is not the smoothest when
compared to other coating methods. Projects that require a coating thickness of
less than six mils should rely on another coating process.
Paint Coating
A painted coating is
essentially the application of liquid paint. It is the most accessible and
cost-effective type of coating. Different paint formulations can be used
depending on the type of metal, the operating environment and the performance
requirements.
For industrial applications, paint coatings are slowly being
replaced by other coating methods. Some paints may contain toxic elements and
other volatile compounds (VOCs), making them harmful to the environment.
Their durability is also lower
than other coating methods, as they are likely to fade, peel or flake off due
to prolonged environmental exposure.
In practice metallic coating is
carried out by different methods such:
(1)
Electroplating,
(2)
Hot dipping of the work piece in molten metal covered with a flux,
(3)
Spraying of the molten metal on the work piece.
(1) Electroplating
Electroplating is the method of coating one
metal with another. It is most commonly used for decorative purposes,
appearance and protection.
Electroplated items include chrome bumpers, jewelry, electronics,
circuit boards and airplane parts.
Electroplating procedure
1.
Preparation of the work piece. Solutions such as alkaline cleaners, solvent
degreasers or acidic pickling mixtures are used to remove dirt, greases,
oxidation and contaminants from the piece.
2.
The piece to be plated is connected to the negative pole (cathode) of the d. c.
power supply while the plating (coating) metal anode is connected to the
positive pole (anode). Multi-range ammeter (in series) and voltmeter (in
parallel) are connected to the cell to measure the cell current and voltage.
3.
The piece is then immersed in the plating solution until coated and rinsed and
then buffed or polished, if necessary.
(2) Hot dipping of the work piece
in molten metal covered with a flux
There
are two common processes of hot dipping:
1. Hot dipping galvanizing
It
is a hot coating process whereby the cleaned steel is immersed in molten zinc
usually at a temperature of between 445 °C and 450 °C. When the cleaned steel
is immersed into the molten zinc, a chemical reaction results, which is refer
to as metallurgical laws.
As
a result of this process the coating consists of a series of zinc iron alloy
layers (intermetallic layer) and usually a top pure zinc layer, the adhesion of
the coating to the steel is therefore determined by means of a chemical bond,
or a “metallurgical bond”. Such bonding is considered to be far superior to
that of a mechanical bond. A hot dipped galvanized coating will provide greater
corrosion protection to steel when compared to that of an electroplated
product.
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Figure 2: Some examples for electroplating process |
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Figure 3: Hot dipping galvanizing of steel |
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Figure 4: Galvanization of steel by hot dipping and
electroplating |
2. Hot-dipped tin plating
Tinning
is the process of thinly coating sheets of iron or steel with tin, and the
resulting product is known as tinplate. It is most often used to prevent rust.
Tinplate
made via hot-dipped tin plating is made by cold rolling steel or iron, pickling
or remove any scale, annealing to remove any strain hardening, and then coating
it with a thin layer of tin.
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Figure 5: Basic concept of hot dipped tin plating process |
Advantage of hot dipping process
i.
No waste from production process.
ii. No
hazardous substance (such as cyanogens, lead, etc.) is used at all in
production process.
iii. The
coating metal and the base metal are strongly bonded as inter-metallic layer
formed.
iv. Provide
greater corrosion protection to steel when compared to that of an electroplated
product
(3) Spraying of the molten metal on
the work piece.
Thermal spraying refers to a process by which
a metal wire or powder is melted and sprayed onto a surface to form a
coating. A thermal spray gun is used to
apply the coatings. The thermal spray gun heats the metallic wire or powder to
a molten state and compressed air or other gas propels it onto the surface to
form a coating.
The
compressed gas also aids in division and atomization of the molten coating. The
two metals most commonly applied by thermal spray are zinc and aluminum. These
metals and their alloys provide excellent protection in a variety of marine and
industrial corrosive environments.
Factors
affecting coating quality:
The quality of coating
depends on:
1. Average thickness of
coating
2. Porosity and
continuity
3. Uniformity of
thickness
4. Adherence
Non-metallic coating
There
are two types of nonmetallic coating:
A. Inorganic coating
Inorganic protective
layers - also called conversion layers - are non-metallic, very thin
coatings on a metal surface and serve to protect against corrosive
attacks. Inorganic protective coatings are usually produced by the targeted
chemical reaction of an aqueous electrolyte solution with the metallic
substrate.
Inorganic Protective Coatings
Inorganic
protective layers - also called conversion layers - are non-metallic, very thin
coatings on a metal surface and serve to protect against corrosive attacks.
Inorganic
protective coatings are usually produced by the targeted chemical reaction of
an aqueous electrolyte solution with the metallic substrate. This is also known
as passivation. In addition, spontaneous passivation is also possible - for
example in the formation of metal oxides. In the elemental state, aluminium
immediately forms aluminium oxide on the surface and thus prevents further
oxidation as a dense layer. Anodising processes can intensify this phenomenon.
Further
examples of inorganic protective coatings are phosphating,
alkali passivation and chromating.
Technical
terms can not always be avoided. As corrosion experts, we not only want to give
you comprehensive advice, we are also interested in making you a corrosion
expert yourself.
The
variety around the topic of corrosion and corrosion protection is also in our
glossary at home: explanations from A as in Adhesion to T as
in Thread tolerance. Have fun clicking through!
Methods of inorganic coating:
i.
Oxidation (passivation)
Steel
can be coated with an oxide film by;
Heating at high temperature, chemical oxidation by treating steel with
hot alkaline nitrate, or persulphate or perchlorate and
anodic oxidation by making the steel structure an anode in electrolytic
cell
ii. Phosphating
Steel is coated with a layer of iron phosphate
by dipping in a solution containing phosphoric acid and zinc phosphate. The
iron phosphate film is not highly protective because it is porous so it usually
covered with paint. The phosphate film improves the bond between the metal and
the paint.
iii. Enamels
Enamels
are glassy layer applied to the metal by dipping it in a suspension of powdered
glass, and then the metal is heated in a stove (furnace) at high temperature
where the glass powder melts and coat the metal.
iv. Cement coating
It
is used to coat the inner side of steel pipelines carrying water or wastewater.
|
Figure 6: Cement coating of the inner wall of water
steel pipeline |
B. Organic coating
An organic coating is
a type of coating whose primary ingredients are derived from either
vegetable or animal matter or from compounds rich in carbon. These
coatings are primarily used to provide additive type finishes on the materials
on which they are applied.
What Does Organic Coating Mean?
An organic coating is a type of coating whose primary
ingredients are derived from either vegetable or animal matter or from
compounds rich in carbon. These coatings are primarily used to provide additive
type finishes on the materials on which they are applied. Organic coatings can
be monolithic (consisting of only one layer) or two or more layers.
Organic coatings act as a protective barrier against
corrosion and oxidation. These are durable coatings applied to a substrate for
their decorative or specific technical properties. Organic coatings depend
primarily on their chemical inertness and impermeability. Various types of
organic coatings are available for industrial purposes including primers,
adhesive cements and topcoats (varnish and paints).
Organic coatings are easy to apply with the help of brushes,
sprays, rollers, dips, or by electrostatic means. Brush application is a slow
and lengthy procedure. The coating cures or dries by evaporation or loss of
solvent, polymerization and oxidation.
Some examples of common organic
coating
i. Paints:
Paint
consists of; a film forming substance such as linseed oil or a polymer
(resin), an organic solvent and a
pigment (usually an inorganic oxide or metal powder).
Before
applying paint to a steel surface, the metal surface should be cleaned of
oxides by sand blasting or acid pickling. After cleaning, the metal surface is
coated with a thin layer of primer.
A
primer is a paint containing a pigment such as lead oxide (Pb3O4
red lead) or zinc chromate which oxidizes the steel surface and inhibits its
corrosion. Besides, the primer film increases the strength of the bond between
steel and final paint film. When the primer film dries a thick film of the
required paint is applied over the primer. Polymers such as alkyd resins, PVC,
polyethylene, polyesters, acrylics, polyurethanes, chlorinated rubber, epoxy
resins, etc. are used in paint manufacture. For severe conditions such marine
and industrial atmosphere, a paint containing epoxy resin (water resistant) is
suitable.
ii. Lacquers:
A
lacquer consists of a thermoplastic polymer dissolved in an organic solvent.
Lacquers can be used to line steel tanks holding corrosive chemicals such as
acids.
iii.Coal tar
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Figure 7: Coating of outside surface of pipeline with
coal tar + epoxy |
iv.Temporary coating:
It is used to protect metallic structures
during shipping and storage by coating the structure with layer of lubricating
oil which can be removed by an or ganic solvent when the structure is put to
service.
Chemical Conversion Coating
A chemical conversion coating is a coating that is produced
by electrochemical or chemical reaction of metals, giving a superficial layer
which contains the metal compound.
It offers economical protection against corrosion and a
surface suitable for powder coating and painting. It also preserves electric
conductivity, in contrast to anodizing which produces coatings that are
non-conductive.
Unlike anodizing, chemical conversion coatings do not need
electricity, making production more cost effective. It can be colored or clear,
depending on preference.
A chemical conversion coating is also known as chromating,
chromate conversion and alodining.
Chemical
Conversion Coating
Conversion coatings are applied on metal parts for corrosion
protection. These are acidic in order to transform a metal substrate to a zinc
phosphate or iron surface. It is the chemical reaction that makes the metal surface
improve field performance and paint adhesion.
Chemical conversion coatings can go through either
electro-chemical or chemical processes, which may include any of the following:
- Chromate conversion - Mainly utilized on aluminum
surfaces
- Zinc and iron phosphate conversion - Mainly applied on
steel substrates
- Anodizing - Used primarily on aluminum
This type of coating is utilized to provide a surface for
paint to adhere to throughout the process of curing. Without it, metal and
other surfaces would have paint that only sits on top of the surface rather
than being bonded to it mechanically. Due to this, paint may undergo
delamination or flaking from the steel surface, especially in cases of
inadequate or no conversion.
Chemical conversion coatings that are distributed evenly are
a thin film of crystals that overlaps throughout the pre-treatment period. When
properly prepared, such coating offers a very solid base for adhesives and
paints.
Miscellaneous surface coating
Miscellaneous surface
coating operations, where the coating contains compounds of chromium (Cr), lead
(Pb), manganese (Mn), nickel (Ni), or cadmium (Cd) and is spray-applied
to any part or product made of plastic and/or metal substrate that are not
motor vehicles or mobile equipment.
Cladding
It is defined as a
physical process of bonding two or more metals by applying high pressure under
specific condition. It can take the following form:
(a) Cold and hot roll
bonding.
(b) Extrusion bonding.
(c) Explosive bonding.
(d) In cladding of
steel sheets with aluminum, aluminum sheet is initially cold rolled and
roughened by wire brushing, then is rolled with steel sheet at 100-200 ºC to
effect the lamination. Finally the sheet is reduced by cold rolling. However,
the wire is claded by the application of compacted aluminum powder to a high
strength steel rod
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