METALIC/INORGANIC COATINGS

andrewfei

METALIC/INORGANIC COATINGS
Thin metallic/inorganic coatings
- act as a barrier to the environment
many different methods of application:
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ELECTRODEPOSITION (ELECTROPLATING)
Immersion of material in solution of coating metal
– then hooked up electrically to another electrode
Coating – dependent on T, current density, time and composition of
the bath
Coatings - <0.01mm up to 20mm
soft, hard, brittle, ductile
pure, alloy, dull, bright
etc….
A hard plating will combat erosion corrosion
Automotive bumpers:
1) thin copper layer (good adhesion)
2) intermediate nickel layer (corrosion protection)
3) thin chromium layer (appearance)
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FLAME SPRAYING (METALLIZING)
Feed a metal wire or powder through a hot flame
- liquid metal is then blown onto surface
coatings = porous and not very protective
surface must be sand blasted to improve adhesion
coating == good base for paints
CLADDING
Thin surface layer of sheet metal rolled on to original metal
e.g. nickel on steel
e.g.
high strength aluminium alloy
- strong but not corrosion resistant
∴ clad with thin sheet of pure Al
good process where protection is required on large, bulky areas
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HOT DIPPING
Applied to metals by immersing them in a molten metal bath of low
Tm metal
e.g. Zn, Sn, Pb, Al(?)
Best example – galvanized steel
- gives much thicker coating than electroplating
+ much older (cheaper) technology
Can be heat treated to form an alloy bond with the substrate
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VAPOR DEPOSITION
Uses a high vacuum chamber
coating metal – vaporized by heat – then vapour ‘sticks’ to substrate
(adsorbs)
very expensive ∴ only used on specialist items

andrewfei

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DIFFUSION (SURFACE ALLOYING)
Parts to be coated
– packed in solid materials containing diffusing species
- or sat in a gas
sherardizing – zinc coating
chromizing – chromium coating
calorizing (alonizing) – aluminium coating
Alonized steel – very common in petroleum and chemical industries
as heat exchangers
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CHEMICAL CONVERSION
i.e. deliberately ‘corroding’ the metal surface
Best example – anodizing
i.e. place metal in acid bath to form oxide layer
common with aluminium
the layer is very porous ∴ used as an adherent for paints
- layer can be sealed by placing in boiling water
another technique – bonderizing i.e. P in phosphoric acid
common process for car bodies – protects and creates bond for paint
ORGANIC COATINGS
i.e. paint, varnish, lacquer - very common!
BUT/ very complex technology
Generally – they are not used where the environment would rapidly
attack the substrate if damaged
3 main factors:
1 – surface preparation – dirty surfaces don’t bond well with paint
2 – selection of primer – must have good adherence and compatibility
3 – selection of top coat

Surface preparation
e.g. roughening and cleaning
roughen – to obtain a mechanical bond
clean – remove grease, wax etc.
best way = sand blast
can also pickle, scrape, brush etc.. or use chemical treatments
sharp edges must be ground out – or no contact is made
edges will be unprotected
Primers
Usually – a rust inhibitive pigment e.g. zinc chromate
good wettability must be present – or gaps result
+ rapid drying – especially in the field
- minimizes crud etc. getting into the layer
Top coat
Must be durable and have the desired appearance
i.e. aesthetic factors
Usually – multiple layers are required somewhere
- single coat will not completely cover the substrate
- must also be thick enough – most coatings thin with age
through simple erosion
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Preventative maintenance in the form of a ‘touch up’ regime is
essential to minimize the appearance of bald spots
- must initiate repair before the base metal is exposed
Best application method – hot spraying
- good sprayability gives thick, uniform coating

again – edges are hardest to protect
There are many types of paints…..
Asphalt/bituminous paints (with cloth reinforcement)
good for pipelines
Silicone/acrylic paints
high temperature uses
Thermosetting paints
epoxy-based, polyester-based, phenolic-based
quite durable and hard-wearing
Thermoplastic paints
acrylic, nylon, PVC etc..
easiest to repair
Elastomeric paints
neoprene, butadiene etc.
overriding concern – must be resistant to the corrosive environment!