Introduction to Corrosion Control and Coatings and Coating

Introduction to Corrosion Control and Coatings and Coating Types

Topics • Coating system selection • Coating components • Curing mechanisms of coatings • Elements of a coating system • Coating types

Selecting a Coating System • Mission of system • Service environment • Substrate • Environmental and occupational health considerations

Service Environments to Consider • Temperature • Likelihood and type of physical damage • Surrounding operations • Prevailing environment (e. g. saltwater mist) • Biological life

Basic Components of a Coating • Solvent • Resin (Binder) • Pigments • Additives • Low % of overall makeup

Components of a Coating Non-Volatile Components • Remain on surface • Include: • Resin (Binder) • Pigments • Additives Volatile Components • Evaporate from coating into atmosphere • Include: • Solvent system included in paint wet film

Resin (Binder) Definition Properties • Adhesion • Film-forming; nonvolatile component of a coating • Compatibility with different coatings • Ease of application, topcoating, and repair • Performance in different environments

Pigment Definition • Heavier, solid; nonvolatile component of a coating Properties • Opacity (Hiding) • Color • Inhibitive corrosion resistance • Level of gloss • Reinforcement • Wet paint properties • Weather and moisture resistance

Solvent Primary Solvents Secondary Solvents • Formulated in coating to reduce viscosity of resin, pigment, and additives for ease of application • Formulated to stay in wet film, because a slower evaporation helps the coating to flow -out and knit together • This relationship is necessary for a uniform, continuous film

Curing Mechanism Definition Types of Mechanisms • Solvent evaporation • Method by which a coating converts from a liquid to a solid state • Chemical reaction of components (Polymerization) • Moisture cure • Heat and UV cure

Solvent Evaporation Definition • Coatings that form protective films by simple evaporation of organic solvent Coating Examples • Vinyls • Acrylics • Chlorinated rubbers

Chemical Reaction of Components (Polymerization) Definition Coating Examples • Coatings that cure by chemical reaction of multiple components • Epoxies • A ratio that is specified by the manufacturer is provided to obtain a film with optimum properties • Polyureas • Polyurethanes • Polyesters/vinyl esters • Phenolics • Silicones (Polysiloxanes)

Moisture Cure Definition • Reaction of a coating with moisture in a process known as hydrolysis Coating Examples • Single-component polyurethanes • Inorganic zinc coatings

Heat and UV Cure Definition • Heat or ultraviolet light reacts with resin, causing a chemical reaction and subsequent cure of the coating film • Heat may also be used to accelerate coating cure Coating Examples • Powder coatings • Specialty coatings

Coating System Stackup • Consists of multiple coatings that are compatible with one another and used to create a system • Example: • Pretreatment • Epoxy (Primer) • Polyurethane (Topcoat)

Functions of Coating Components • Pretreatment • Improves coating adhesion and promotes corrosion protection • Primer • Bond coating system to the substrate • Provide corrosion protection to the substrate using barrier membrane, inhibitive, galvanic, or combination of the three • Topcoat • First line of defense in terms of corrosion • Aesthetics • Color • Gloss • Compatible with primer

Coating Types • Conversion coatings • Pretreatment wash primer • Epoxies • Polyurethanes (single- and two-component) • Waterborne acrylic • Polyurea • Polysiloxane

Conversion Coating • Chrome and Non-chrome options • Often incorrectly referred to as Alodine • Increases corrosion resistance of aluminum • Promote strong adhesion properties • Applied with either a “wipe-on” and “blot-off” method or a disposable application pen • Apply according to manufacturer’s product data sheet

Conversion Coatings Advantages • Promotes adhesion • Provides some corrosion protection Limitations • Can contain hazardous materials • Hexavalent chromium • Fast-drying Hexavalent chromium is an excellent corrosion inhibitor but is a known human carcinogen Must apply appropriate engineering controls during application, removal, and disposal

Pretreatment Wash Primers • Alternative to conversion coatings • Two-component product • Similar to a vinyl • Components react with each other and with metal to form a tightly adhering film • Commonly used to promote adhesion and corrosion protection

Pretreatment Wash Primers Advantages • Promotes adhesion of some primers • Provides temporary corrosion protection • Fast-drying Limitations • High VOCs • Limited uses • Film may undergo cohesive failure when applied too thickly

Epoxies Advantages • Good solvent, water, and chemical resistance • Good abrasion resistance • Good adhesion • Rapid cure time Limitations • Limited pot life • Chalk in sunlight • Limited flexibility • Amine blush • Strict surface preparation

Two-Component Polyurethanes Aliphatic • Better weathering characteristics • Color • Gloss • Used as Topcoat on Exteriors Aromatic • Chemical Resistance • Chalk and yellow in sunlight

Two-Component Polyurethanes Advantages Limitations • Water resistance • Highly toxic (isocyanates) • Hardness or flexibility • Moisture-sensitive • Durability/abrasion resistance • Limited pot life • Lower temp curing achievable

Waterborne Acrylic Coatings Advantages Limitations • Lower VOC • Limited durability • Easy application • Fast drying • Poor chemical and solvent resistance • Low odor • Poor wetting of surface • Low cost • Poor immersion service • Safer • Best cured above 50°F • Flexibility

Two-Component Polyureas Advantages • No VOC • Low odor • Low permeability • Resistant to various solvents, caustics and mild acids • Excellent bond strength • Weather-tolerant Limitations • Difficult application due to short pot life • Highly toxic (isocyanates) • Weight

Polysiloxanes Advantages Limitations • Low VOCs • Shorter pot life • Cure without the use of isocyanates • Brittleness • Color and gloss retention • Good abrasion resistance • Good weatherability • Film-thickness sensitive

Powder Coatings Advantages • Excellent abrasion resistance and hardness • Very low VOC Disadvantages • Difficult to repair/touch-up • Thin coatings are hard to achieve • Part has to be grounded • Oven size

Coatings and Coating Types Summary • Factors that contribute to the satisfactory performance of a protective coating include surface preparation or application characteristics, resistance to exposure conditions, safety and environmental considerations, and maintainability. • While some coating types are clearly best for certain uses, each coating type has its advantages and disadvantages.

Agenda • Corrosion Basics • Fundamentals of Corrosion Control • Surface Preparation for Coatings • Coatings and Coating Types • Coating Application and Safety • Coating Defects and Inspection