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Cleaning and Painting of Structural Steel

Steel which is to be painted must be thoroughly cleaned of all loose mill scale, loose rust, dirt, and other foreign matter. Cleaning can be done by hand tool, power tool and a variety of levels of abrasive blasting. Abrasive blasting in most fabrication shops is done with centrifugal wheel blast units. The various surface preparations are described in specifications by the Society for Protective Coatings.

Unless the fabricator is otherwise directed, cleaning of structural steel is ordinarily done with a wire brush. Sophisticated paint systems require superior cleaning, usually abrasive blast cleaning and appropriate quality systems. Knowledge of the coating systems, equipment maintenance, surface preparation and quality control are all essential.

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Gradual destruction of a metal or alloy by Chemical or electrochemical reaction.

Metals are in an unstable state.

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.

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PRE-CLEANING (Abnormalities)

Steel

  • Remove sharp edges, weld splatter
SURFACE PREPARATION
  • Pre-cleaning
  • (removal of surface contaminants)
  • Cleaning
  • Profiling
  • Poor surface preparation is the most frequent cause of premature coatings failure.
  • Surface preparation is the most expensive opearation of a painting project.
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Pre - Cleaning Surface Contaminates

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Types

  • Loose Rust
  • Loose Paint
  • Salts
  • Dirt
  • Oil and Grease

Removal Methods

  • Solvents
  • Degreaser
  • Portable Pressurised Water
METHODS OF CLEANING

Hand Tools

  • Sand Paper
  • Chipping
  • Scrapping

Power Tools

  • Wire Brush
  • Disc Grinder
  • Wire Wheel Brush

Abrasive Blasting

  • Sand
  • Recyclable Grit and Shot
  • Copper Slag

Water Blasting / Jetting

  • Use Portable Water
PROTECTIVE COATING

1. 

BARRIER COATING

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

2. 

INHIBITIVE COATING

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

3. 

SACRIFICIAL COATING

Provide galvanic protection. they corrode (sacrifice) in preference to the substrate, providing electrons which reverses the corrosion cell and transforms anodes to cathodes.

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Types of paints by resin
1. 
Acrylic(latex)
2. 
Alkyds
3. 
Epoxies
  • Coal tar epoxies
  • Epoxy mastics
4. 
Zinc rich
  • Organic
  • Inorganic
THE INGREDIENTS OF PAINT
1. 
VEHICLE
  • Resin
  • Solvent
2. 
ADDITIVES
  • Viscocity
  • Anti-Foaming
3. 
PIGMENT
  • Inhibitive
  • Sacrificial
  • Opacity
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