Presentation on theme: "CLEANING AND PAINTING OF STRUCTURAL STEEL THE BASICS FRANK D. REA PCS, NACE."— Presentation transcript:
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. 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.
BARRIER COATINGS Protection by preventing water, oxygen and electrolyte from coming in contact with the metal substrate.
INHIBITIVE COATINGS Contain chemicals which hinder the corrosion process. They react with the cathode, anode or other components (oxygen, electrolyte, acid). STOP
SACRIFICIAL COATINGS SACRIFICIAL COATINGS Provide galvanic protection. They corrode (sacrifice) in preference to the substrate, providing electrons which reverses the corrosion cell and transforms anodes to cathodes.
THE INGREDIENTS OF PAINT 1.VEHICLE A.RESIN (non-volatile) B.SOLVENT (volatile) 2.PIGMENT A.INHIBITIVE B.SACRIFICIAL C.OPACITY (HIDING) 3.ADDITIVES A. VISCOSITY B.ANTI-FOAMING
PAINTS ARE USUALLY DESCRIBED BY THE RESIN THE RESIN ESTABLISHES THE CHEMICAL AND PHYSICAL PROPERTIES OF THE PAINT.
TYPES OF PAINTS BY RESIN 1.ACRYLICS (Latex)B, I 2.ALKYDSB, I 3.EPOXIESB, I A.COAL TAR EPOXIESB B.EPOXY MASTICSB 4.ZINC RICH A. ORGANICB, S B. INORGANICB, S 5.POLYURETHANESB, I, S
INORGANIC ZINC RICH (IOZ) 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 cost.
ALKYDS 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.
EPOXIES 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.
EPOXY MASTICS 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)
COAL TAR EPOXIES 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.
POLYURETHANES 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.
LATEX ACRYLICS 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.
SURFACE PREPARATION Pre-Cleaning (removal of surface contaminants) Cleaning Profiling
SURFACE PREPARATION Why is it so important? Poor surface preparation is the most frequent cause of premature coatings failure. Surface preparation is the most expensive operation of a painting project.
PRE – CLEANING Surface Contaminants Types Loose Rust Loose Paint Salts Dirt and Dust Oil and grease Chalk Efflorescence (cement only) Latence (cement only) Removal Methods Solvents Steam Potable Pressurized Water
CLEANING HAND TOOLS (Stainless Steel or Aluminum) POWER TOOLS VACUUM POWER TOOLS ABRASIVE BLASTING SAND RECYCLABLE STEEL SHOT AND GRIT SLAG (Black Beauty) SPONGE WATER BLASTING OR JETTING CHEMICAL STRIPPING
DEGREE OF CLEANILINESS SSPC SURFACE PREPARATION STANDARDS SP1Solvent Cleaning SP2Hand Tool Cleaning SP3Power Tool Cleaning SP5White Metal Blast(NACE 1) SP6Commercial Blast(NACE 3) SP7Brush-Off Blast(NACE 4) SP10Near White Blast(NACE 2) SP12High and Ultrahigh(NACE 5) Pressure Water Jetting
PROFILE (ANCHOR PATTERN) CORRECT TOO DEEP TOO ROUND
PROFILE (ANCHOR PATTERN) FOUR FACTORS DETERMINE THE SHAPE AND DEPTH OF THE PROFILE OF BLASTED SURFACES 1.Size of the Particle 2.Shape of the Particle 3.Hardness of the Particle 4.Velocity of the Particle
SUMMARY CORROSION CORROSION TYPE OF COATINGS TYPE OF COATINGS DIFFERENT PAINTS DIFFERENT PAINTS SURFACE PREPARATION SURFACE PREPARATION