MSEN 446 Test 2

Purple: Coating Fundamentals----Blue: Coatings case study

What is a coating?

A material (usually a liquid) that is applied to a substrate, resulting in a “dry” film

Different coating properties?

• chemical resistance

• water resistance

• ease of application

• adhesion to substrate

• cohesive strength

• flexibility and elongation

• impact resistance

• abrasion resistance

• temperature resistance

Chemical Resistance

• Must resist breakdown due to chemicals

• Primary function

Water resistance

• water affects virtually all coatings

• greater water resistance equates to more effective corrosion control

Ease of application

• very important characteristic, especially for intricate structures

• difficult application leads to opportunity for defects, can lead to failure

Adhesion to substrate

• based on chemical interactions between coating and substrate

Cohesive strength

• withstanding the stresses of the curing process and changes in temperature or moisture content

Flexibility and elongation

• ability to expand and contract with the substrate

Factors that affect the rate of corrosion?

• type of metal

• type of corrosion

• service environment

Factors that affect

Galvanic corrosion

• corrosion between two dissimilar materials in electrolyte

• prevented by appropriate metal selection, insulating contact, coatings, large anodes and small cathodes, sacrificial anode

Crevice corrosion

• localized attack at the gap between two joining surfaces

• prevented by inhibitors, cathodic protection, minimizing crevices, material selection

Pitting corrosion

• localized attack where holes are produced

• prevented by maintaining electrode potential below pitting potential, inhibitors, material selection

Intergranular corrosion

• localized attack at grain boundaries of an alloy

Selective leaching or dealloying

selective removal of an element from an alloy by a corrosion process

Erosion corrosion

acceleration in rate of corrosion due to motion of corrosive fluid

Environmentally induced cracking

• corrosion fatigue is cracking due to cyclic stress and corrosive environment

• stress-corrosion cracking is cracking due to combination of stress and environment

  • prevented by eliminating stress, inhibitors, cathodic protection

Service environment

• characteristics of environment where the metal functions

• immersion in liquid or soil or atmospheric

• desert/rural areas give lower corrosion rates, chemical/marine settings give higher corrosion rates

  • changing environment give higher corrosion rates compared to stable environments

Earth materials

• soil, clay, earth materials are good conductors

• some soils cause more severe corrosion

Barrier Coatings

• impedes the entering of oxygen, water, soluble salts through the film

• retards moisture but not moisture permeation, resistant to corrosive chemicals

Sacrificial coatings

• usually contains zinc dust

• zinc acts as anode when damage exposes substrate

coating classification

• organic coatings

  • come from refined petroleum products

  • contains carbon

• inorganic coatings

  • uses inorganic binders, mostly silicone or silicates

• performance differences in heat resistance, organic coatings have weak carbon-carbon bond

coating composition

• pigment

• vehicle

  • binder/resin

  • solvent

• additives

pigment

• discrete solid particles used to give specific properties

• are suspended in coating, not dissolved

• can give color, weathering protection, inhibit, modify mechanical/electrical properties

PVC (pigment characteristics)

• pigment volume concentration

  • pigment to resin ratio

CPVC (pigment characteristics)

• critical pigment volume concentration

Exceeded CPVC (Pigment characteristics)

• Insufficient binder to wet the pigment particles fully and bind them to the substrate

slightly below CPVC (Pigment characteristics)

• lower impermeability but better blistering resistance

lower PVC (Pigment characteristics)

• higher binder content gives higher gloss

• resin rich coatings have “hot spots” (glossier area)

• finer dispersion gives a greater gloss

Resin/Binder

• where the coatings gets its name from

• must convert from a liquid state to a solid state to form protective film on substrate

Desired properties of resin/binder

• good wetting/adhesion

• transmission resistance of water, oxygen, chemicals

• tolerates variability in application process

• resist chemical and physical change in service environment

• dry within acceptable period

• form a stable film that maintains strength, hardness, flexibility

Solvents

added to dissolve binder and reduce coating viscosity for easier application

Two major properties (solvents)

• solvency powder: ability to dissolve resin

• volatility: controls evaporation rate

Evaporation rate (solvents)

speed at which the solvent leaves the coating film during and after application

Role of solvent in protective coating (solvents)

• fleeting role

• serves no purpose once coating is cured