CLEANING AND PAINTING OF CLEANING AND PAINTING OF STRUCTURAL STEELSTRUCTURAL STEEL

THE BASICSTHE BASICS

FRANK D. REA PCS, NACE

CORROSIONCORROSIONWHAT - 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.

PROTECTIVE COATINGSPROTECTIVE COATINGS

1. BARRIER

2. INHIBITIVE

3. SACRIFICIAL

4. COMBINATION

BARRIER COATINGSBARRIER COATINGS

Protection by preventing water, oxygen and electrolyte from coming in contact with the metal substrate.

INHIBITIVE COATINGSINHIBITIVE COATINGS

Contain chemicals which hinder the corrosion process. They react with the cathode, anode or other components (oxygen, electrolyte, acid).

STOP

SACRIFICIAL COATINGSSACRIFICIAL 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 INGREDIENTSTHE INGREDIENTSOF PAINTOF PAINT

1.1. VEHICLEVEHICLEA. RESIN (non-volatile)B. SOLVENT (volatile)

2.2. PIGMENTPIGMENTA. INHIBITIVEB. SACRIFICIALC. OPACITY (HIDING)

3.3. ADDITIVESADDITIVESA. VISCOSITYB. ANTI-FOAMING

PAINTS ARE USUALLY PAINTS ARE USUALLY DESCRIBED BY THE RESINDESCRIBED BY THE RESIN

THE RESIN ESTABLISHES THE CHEMICAL AND PHYSICAL PROPERTIES OF THE PAINT.

TYPES OF PAINTS BY RESINTYPES OF PAINTS BY RESIN

1. ACRYLICS (Latex) B, I2. ALKYDS B, I3. EPOXIES B, I

A. COAL TAR EPOXIES BB. EPOXY MASTICS B

4. ZINC RICHA. ORGANIC B, SB. INORGANIC B, S

5. POLYURETHANES B, I, S

INORGANIC ZINC RICH (IOZ)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 severeEnvironment resistance. Fast-drying. Can be used withouta topcoat. Applied properly can last more than 20 years.

DISADVANTAGES: Requires a very clean, blasted surface;user unfriendly; difficult to topcoat (porous); high initialcost.

ALKYDSALKYDS

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.

EPOXIESEPOXIES

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 MASTICSEPOXY MASTICS

Higher solid version of epoxies with added pigmentand less solvent. Aluminum epoxy mastic used as touch-upprimer, overcoat primer and in some cases a finish coat.

ADVANTAGES: Very surface tolerant, can be applied tomarginally 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 EPOXIESCOAL 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.

POLYURETHANESPOLYURETHANES

Also a two-component thermosetting coating. Consists of an isocyanate and a polyol. Aliphatic polyurethane used as finish coatof three coat inorganic zinc rich system.

ADVANTAGES: Best gloss and color retention, protectsepoxy 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 ACRYLICSLATEX 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 PREPARATIONSURFACE PREPARATION

Pre-Cleaning (removal of surface contaminants)

Cleaning

Profiling

SURFACE PREPARATIONSURFACE PREPARATIONWhy is it so important?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 – CLEANINGPRE – CLEANINGAbnormalitiesAbnormalities

STEEL Remove sharp edges, weld splatter

CONCRETE Patch, Caulk

PRE – CLEANINGPRE – CLEANINGSurface ContaminantsSurface 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

CLEANINGCLEANING

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 CLEANILINESSDEGREE OF CLEANILINESS SSPC SURFACE PREPARATION STANDARDS

SP1 Solvent Cleaning SP2 Hand Tool Cleaning SP3 Power Tool Cleaning SP5 “White Metal” Blast (NACE 1) SP6 “Commercial” Blast (NACE 3) SP7 “Brush-Off” Blast (NACE 4) SP10 “Near White” Blast (NACE 2) SP12 High and Ultrahigh (NACE 5) Pressure Water Jetting

PROFILEPROFILE(ANCHOR PATTERN)(ANCHOR PATTERN)

CORRECT

TOO DEEP

TOO ROUND

PROFILEPROFILE(ANCHOR PATTERN)(ANCHOR PATTERN)

FOUR FACTORS DETERMINE THE SHAPE ANDDEPTH OF THE PROFILE OF BLASTED SURFACES

1. Size of the Particle2. Shape of the Particle3. Hardness of the Particle4. Velocity of the Particle

SUMMARYSUMMARY

CORROSIONCORROSION

TYPE OF COATINGSTYPE OF COATINGS

DIFFERENT PAINTSDIFFERENT PAINTS

SURFACE PREPARATIONSURFACE PREPARATION