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 |  موضوع: Guidelines for Steel Surface Preparation الإثنين 10 فبراير 2014, 11:16 am | |
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Guidelines for Steel Surface Preparation
General Polymers Guideline Instructions for Surface Preparation of Structural Steel (Form G-2 Revised 07/2001)
Introduction
Surface preparation is the most important step in the process of applying coatings to any surface. Metal surfaces exposed to atmospheric environments, splash and spill of corrosive chemicals or immersion conditions are subject to corrosive degradation and oxidation. The contaminants, corrosion by-products, and oxidation of metallic surfaces must be removed, while creating a surface profile in order to insure a sound bond of the coating or lining system.
Carbon Steel
Atmospheric Conditions
For surfaces that will not experience immersion conditions, but may experience splash and spill conditions the following procedures must be followed:
Clean and degrease the surface in accordance with SSPC SP-1 prior to grit blasting. Use organic solvents, caustic solutions, steam or hot water with detergents to remove dirt and grease. For applications requiring heat force cure, pre-baking of the old tank is recommended.
The surface must be cleaned to an SSPC SP-2 Hand Tool Cleaning as a minimum. The preferred profile should be SSPC-SP10 or NACE No. 2 Near White Metal surface. The finished surface should be free of loose mill scale and old paint.
The blasting media should be a natural abrasive, steel grit or slag grit. The ideal grit is angular with hard cutting surface, properly graded, dry, free of contaminants and produces little dust.
After blasting, traces of grit should be vacuumed from the surface. Care must be taken not to contaminate the surface with moisture or fingerprints.
The surface must be maintained at 5 above the dew point to prevent oxidation of the surface. The coating should be applied immediately after the surface has been prepared. If delays are anticipated, consider the use of a hold primer such as General Polymers COR-BOND MP. Consult General Polymers Technical Service Department.
Carbon Steel
Immersion Conditions
Tanks should be designed and fabricated in accordance with NACE Standard RP-0178-89 Design Fabrication, and Surface Finish of Metal Tanks and Vessels to be Lined for Chemical Immersion Service.
Butt welds are recommended instead of continuous lap welded joints. Rivets and bolted joints are not recommended. Welds should be continuous and not spot welded. All rough welds should be ground to remove sharp edges, undercuts and pinholes. All weld splatter must be removed.
Flanged connections are preferred over threaded screw connections. Connections greater than two inches in diameter shall be lined. Those less than two inches in diameter should use chemical resistant fiberglass inserts.
Appurtenances, including agitators, baffles and internal piping, should be lined or constructed of corrosion resistant materials. Dissimilar metals should be electrically insulated from the steel tank surface. Heating elements should be at least six inches from the tank surface. Structural reinforcements should be fabricated of smooth round bars or pipe for ease of lining.
Surface finish should be free of gouges, deep scratches, slivers and other flaws. Sharp edges and fillets should be ground to a smooth radius of 1/8"-1/4".
Lining for Immersion Conditions
Clean and degrease the surface prior to grit blasting. Use organic solvents, caustic solutions, steam or hot water with detergents to remove dirt and grease. For applications requiring heat force cure, pre-baking of the old tank is recommended.
The surface should be grit blasted to an SSPC-SP5 or NACE No. 1 White Metal surface. This is accomplished using a Venturi blast nozzle supplied with 80-120 psi. The anchor pattern in metal shall correspond to about 20% of the film thickness of the coating. The anchor pattern should be sharp with no evidence of a polished surface.
The blasting media should be a natural abrasive, steel grit or slag grit. The ideal grit is angular with hard cutting surface, properly graded, dry, free of contaminants and produces little dust.
After blasting, traces of grit should be vacuumed from the surface. Care must be taken not to contaminate the surface with moisture or fingerprints.
The surface must be maintained at 5� above the dew point to prevent oxidation of the surface. The lining should be applied immediately after the surface has been prepared. If delays are anticipated, consider the use of a hold primer such as General Polymers COR-BOND MP or consult General Polymers Technical Service Department.
Other Metals:
Stainless Steel
Stainless Steel is produced in various grades, including 304, 316, 316L, and 904. The steps outlined above for Carbon Steel preparation apply to Stainless Steel. Due to the increased hardness of Stainless Steel, an anchor profile is achieved using 120-psi blast nozzle pressure. Avoid the use of iron or steel grit abrasives.
Galvanized Metal
The surface must be clean and free of grease and properly etched with a phosphating solution. The surface then is washed and dried prior to coating.
Aluminum
The surface must be clean and free of grease with a blast profile as outlined under Carbon Steel. Normally the blast media should be 80 mesh or finer grit. The surface should also be etched with Alodine 1200S (Parker & Amchem 800/521-1355) or Iridite 14-2 (MacDermid Inc. 800/325-4158).
Copper or Brass
These surfaces should be clean and free of grease. It should then be lightly blasted to remove oxides or treated with a solution consisting of 5% ferric chloride and 5% muratic acid. Allow the material to react for a few minutes then wash and dry prior to coating.
Cast Iron
Cast iron is porous and will absorb moisture or other liquids. Heat the surface for 8-12 hours at 300�F or with torches to remove any absorbed liquids.
For additional information refer to NACE and SSPC publications on surface preparation of metallic substrates. 
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