Function and Classification
(1) to separate the metal/environment
(2) to control the micro-environment
Classification of Coatings:
(1) metallic
(2) non-metallic (paints)
Composition of paint
(1) a vehicle: controls the paint fluidity, drying
(2) a pigment: controls corr rxn or diffusion
(3) additives: accelerate the drying process
Paint Failure
(1) poor or inadequate surface preparation: surface rust, surface too rough
(2) applied under unsuitable condition or by inappropriate methods
relative humidity too high
temperature variations
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Metallic Coatings
ideal properties:
(1) more resist to attack
(2) do not accelerate local corr at defects
(3) good physical properties elasticity, hardness
(4) compatible with fabrication of the complete components
(5) uniform thickness, pore free
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Metallic Coating Methods
(1) electroplating
to be plated => cathode
plating metal => anode
(2) hot dipping
object dipped into a bath of molten coating metal => good bonding due to interfacial alloying
hot dipping in zinc/aluminium => galvanizing fig 12.6,
12.7, 14.7
(3) spray coats
melted metal is sprayed on substrate surface; more porous than (1) & (2)
(4) clad coatingss
corr resistant skins applied by rolling (high pressure), explosive welding for building up
a welded coat
(5) diffusion coatings
limited to small objects
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The Corrosion Behavior of Metallic Coatings
factors:
(1) nature of electrolyte (ph, toc)
(2) O2 concentration
(3) cathode/anode area ratio
(4) nature of coat/substrate
Zinc coating
anodic to steel substrate
service life depends on thickness
Aluminium
in Cl environment, anodic to steel
in others, may be cathodic to steel
Nickel and chromium
all cathodic to steel; barrier coat
Tin coating
used for cans in food industry
inside anodic to steel (organic acids)
outside cathodic to steel
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Summary
Coatings either metallic or non-metallic in nature serves as a physical barrier
between the metal substrate and its environment. Anodic (metallic) coatings such as zinc on steel can also provide
sacrificial protection. Cathodic (metallic) coatings may induce or accelerate localised attack on the substrate
.
Reading Assignments
To reinforce learnings in this lecture read pages 302-335
(textbook)
To prepare yourself for the next lecture
read pages 375-393 (textbook)
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