The main method of protecting metal components from
attack by the local environment is by use of surface
coatings. There are a number of surface
coating systems available;
- Vitreous Enamel
- Anodising (aluminium)
Various machines require grease and oil for
lubrication. These products also provide
a degree of corrosion protection of the lubricated
surfaces and adjacent surfaces. Grease
can be used to provide corrosion protection for tools
and equipment used in workshops which are stored for
short times when not in use.
There are a number of wax based products available
which can be sprayed on surfaces at risk and provide
a significant level of corrosion resistance at
minimum costs. Depending on the operating
conditions these products can last from one to ten
years or more..
Probably the most widely used system of corrosion
protection is by painting the metal surfaces at
risk. Notes on painting are provided
elsewhere on this site Painting
The advantages of painting is that the protection is
relatively convenient to apply and the paint can be
tailored to suit the duty. Paints can be
engineered for convenience of decontamination and
provide a degree of surface protection against the
environment in addition to the required corrosion
protection. Paint is also used for
decoration and equipment identification.
The main disadvantages of painting are that damaged
painted surfaces or low quality painted surfaces will
fail rapidly resulting in enhanced local corrosion of
the base metal.
Zinc Coating (Galvanising)
Galvanised steel which is steel electroplated with
zinc, uses the zinc layer as a protection to the
steel. Zinc is anodic and sacrificially removed. The
steel is cathodic and thus is protected.
The important advantage of this process
is that if the zinc coating is scratched the steel
surface beneath is still galvanically protected.
Notes on this process are provide elsewhere on this
Virtually all plastics can be applied as metal
surface coatings by spraying, fluidized-bed,
electrostatic, rotational moulding, flock or
'slush' coating or dipping The
coating system has the benefits of the strength of
the base metal with the relevant properties of the
plastic coating. This option has similar
disadvantages as the painting option.
The resulting surface must be sound and continuous
and the thermal and mechanical properties of the
coating are generally inferior to the base
Enamel (Vitreous Enamel) is a thin layer of glass
fused by heat on to the surface of a metal being
protected. The process involves dipping
or spraying the glass coatings onto metallic
substrates and subsequent fusion operations.
Vitreous enamelled components may have
single or multiple coatings and they may be fired
after each application or they may be fired as a
single operation. The firing process uses
a high temperature furnace to chemically bond the
enamel to the metal substrate.
Items that are typically vitreous enamelled include;
- white goods,
- industrial parts,
- architectural steel,
- jewellery andcraft items.
Enamelled steel surfaces have excellent corrosion
protection. Enamelled surfaces are
protected, smooth and very decorative.
Ceramic coatings are used to provide corrosion
resistance against numerous chemicals as the ceramic
materials are inert. They also provide
erosion resistance as the ceramics are very hard.
Butterfly disc valves, plug valves and
valve bodies can be coated by this process to
overcome corrosion and erosion. Ceramic coatings can
also withstand high temperature
conditions. A typical coating thickness
is 50 to 100 microns and can be done
selectively. These coatings provide
excellent finish and are deposited as slurry on
metallic substrates, like cast iron, steel, stainless
steel and aluminium. They are
subsequently chemically treated to attain hard,
impervious and corrosion resistant layers of
excellent bond strength.
Aluminium has a high natural resistance to corrosion
because an oxide film
Al2O3forms on the surface
providing a protective layer. Anodising
is a technical electrolytic process to form a thicker
Al2O3 film than forms
naturally. Better protection can be
obtained in more aggressive environments such as sea