CLEANING AND PAINTING OF STRUCTURAL STEEL THE BASICS. FRANK D. REA PCS, NACE. CORROSION. WHAT - gradual destruction of a metal or alloy by chemical or electrochemical reaction. WHY - metals are in an unstable state.
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FRANK D. REA PCS, NACE
WHAT - gradual destruction of a metal or alloy by chemical or electrochemical reaction.
WHY - metals are in an unstable state.
HOW – electrons travel from a negative area (anode) of the metal to a positive area (cathode) through an external conductive intermediate (electrolyte) and then returns to the anode through the metal itself to complete the corrosion cell.
Protection by preventing water, oxygen and electrolyte from coming in contact with the metal substrate.
Contain chemicals which hinder the corrosion process. They react with the cathode, anode or other components (oxygen, electrolyte, acid).
Provide galvanic protection. They corrode (sacrifice) in preference to the substrate, providing electrons which reverses the corrosion cell and transforms anodes to cathodes.
THE RESIN ESTABLISHES THE CHEMICAL AND PHYSICAL PROPERTIES OF THE PAINT.
Solvent borne self-curing inorganic zinc rich ethyl silicate.
Used as a primer for steel, primarily in the fabrication shop.
ADVANTAGES: Excellent abrasion, heat and severe
Environment resistance. Fast-drying. Can be used without
a topcoat. Applied properly can last more than 20 years.
DISADVANTAGES: Requires a very clean, blasted surface;
user unfriendly; difficult to topcoat (porous); high initial
Modified drying oil paints which include silicone alkyds, epoxy esters, uralkyds and oleoresinous phenolics. Machinery, touch up, dowel rod assemblies.
ADVANTAGES: User friendly, surface tolerant, expand and contract with substrate.
DISADVANTAGES: Can contain high amounts solvent, slow drying, tack. Only oleoresinous phenolics can be used in severe environments.
Two-component thermosetting materials which cure chemically. Consist of a base and a curing agent (catalyst). FDOT uses polyamide epoxies as the intermediate coat in a three coat Inorganic Zinc system.
ADVANTAGES: Bond well to properly cleaned steel and concrete, very durable in most environments. Excellent barrier coating.
DISADVANTAGES: Pot life limitations, chalk (poor gloss and color retention), inflexible.
Higher solid version of epoxies with added pigment
and less solvent. Aluminum epoxy mastic used as touch-up
primer, overcoat primer and in some cases a finish coat.
ADVANTAGES: Very surface tolerant, can be applied to
marginally cleaned steel. Compatible with most coatings, low VOCs, durable, good adhesion.
DISADVANTAGES: Pot limitation, overspray, chalk, lack of flexibility, topcoating problems (short recoat window)
An epoxy which has been fortified with coal tar.
Immersion service, sheet piling.
ADVANTAGES: lower cost, improved water
resistance and greater film build.
DISADVANTAGES: Become brittle in sunlight,
pot life, toxic effects, high solvent content.
Also a two-component thermosetting coating. Consists of an
isocyanate and a polyol. Aliphatic polyurethane used as finish coat
of three coat inorganic zinc rich system.
ADVANTAGES: Best gloss and color retention, protects
epoxy intermediate coat from UV, hard but flexible.
DISADVANTAGES: Highly toxic (need personal protection),
lose gloss when applied in high humidity, user unfriendly,
pot life limitation, more expensive than an epoxy.
Emulsion of resin in water used to coat wood, masonry, and steel. Cure by coalescence, leaving a semi-permeable coating which “breathes”. Intermediate and topcoats.
ADVANATAGES: Excellent flexibility, ease of overcoat and repair; and gloss/color retention. User friendly.
DISADVANTAGES: Poor immersion resistance, difficulty bonding to smooth surfaces. Can only be used when ambient temperature is above 50ºF.
(removal of surface contaminants)
most expensive operation
of a painting project.
Pressure Water Jetting