Lecture 14 : Corrosion Control by Barrier Coatings

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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

fig 14.3

(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 coatins

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

fig 14.8

Aluminium

in Cl environment, anodic to steel

in others, may be cathodic to steel

Nickel and chromium

all cathodic to steel; barrier coats

fig 14.9


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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