Metalizing System, Specification for Fuel Storage Tanks

Shell Oil Company

1.0.0 Blast Specification:

1.1.0 Material:

1.1.1 Washed, salt-free, slag abrasive, low free silica, mesh size 20 - 40 with a minimum of 40% retained on a 30 mesh screen. California certified for open air blasting.

1.1.2 Washed, salt-free crushed garnet, free of feldspar or other mineral constituents which tend to break down and remain on the surface in visible quantity. Mesh size 20 to 40 (850 to 425 microns) with a minimum of 40% retained on a 30 (600 microns) mesh screen. (US Standard Sieve series.)

1.1.3 Angular chilled iron grit, clean and reasonable sharp. Old grit which is rusty, noticeably worn or dull when compared with new grit shall not be used. Grit having a mesh size of SAE G-25 to G-50 shall be used; a 50/50 mixture of G-40 and G-50 is ideal.

1.1.4 Aluminum Oxide, clean and free of excessive fines. Mesh size shall be 20 to 50 (850 to 300 microns), with a minimum of 40% retained on a 40 mesh (425 microns) screen. (US Standard Sieve series.)

1.2.0 Equipment: This shall be of the conventional force-feed pressure type. Nozzle size shall be such that a pressure of not less than 75 psi (5.27 kgf/sq. cm.) is maintained at the blast generator (50 psi (3.52 kgf/sq. cm. if aluminum oxide is used)). Centrifugal blasting equipment may be used with angular chilled iron grit only. Abrasive velocity must be equivalent to that produced with force-feed, pressure type machines at 75 psi (5.27 kgf/sq. cm.).

1.3.0 Blast Operation:

1.3.1 All surfaces to be electric arc sprayed shall be thoroughly cleaned and roughened by blasting with an abrasive described under "Material." If paint, oil or bituminous materials are present, they must be removed by flame cleaning or by blast cleaning with fine sand prior to the final blast preparation. If the surface is chemically contaminated, it may be necessary to wash it or neutralize it before final blasting. The abrasive used for cleaning heavily contaminated surfaces should not be re-used for the final blast, even though it is re-screened.

1.3.2 The air supply must be sufficiently free of oil and moisture so that no visible oil or moisture appears on the blasted surface. The air supply must be adequate to maintain 75 psi (5.27 kgf/sq. cm.) minimum at the blast generator for the abrasives described in paragraphs 1.1.1 and 1.1.2. Where aluminum oxide is used, a minimum pressure of 50 psi (3.52 kgf/sq. cm.) is sufficient.

1.4.0 Inspection:

1.4.1 The abrasive shall be checked periodically to see that it conforms to the requirements of Section 1.1.0.

1.4.2 The blasted surfaces shall be inspected and approved as suitable for electric arc spraying before moving or dismantling the blasting equipment. A sample steel plate shall be blasted until the surface cannot be further cleaned or roughened. This should be used for visual comparison, and any areas which do not compare favorably with the plate as to roughness or cleanliness should be re-blasted. In general, a SSPC-SP10, near-white metal blast, with minimum 2 mil profile is sufficient.

2.0.0 Electric Arc Spraying Specification

2.1.0 Material: The wire shall be 3/32 (2.4 mm) diameter Aluminum 99.0% purity.

2.2.0 Equipment: Any electric spray gun shall be used. Use parameters shown in Gun Instruction.

2.3.0 Electric Arc Spraying Operation:

2.3.1 Clean dry air shall be used with not less than 60 psi (4.22 kgf/sq. cm.) air pressure at the Air Control Unit. There shall not be more than 20 feet (6 m) of 5/8" (15.8 mm) I.D. hose between the Air Control Unit and the gun.

2.3.2 The metal coating shall be applied to a minimum thickness of .005" (.125 mm). (This minimum thickness is based on a nominal average thickness of .006"/.125 mm.

2.3.3 Any surface which shows visible moisture, rust, scale, or other contamination shall be re-blasted before spraying. At least one layer of the coating must be applied within four hours of blasting. The surface must be completely coated to the specified thickness within eight hours of blasting.

2.3.4 The specified thickness of coating shall be applied in multiple layers, and in no case shall less than two passes of the electric arc spray gun be made over every part of the surface. The sprayed metal shall overlap on each pass of the gun to assure uniform coverage.

2.3.5 The coating shall be firmly adherent. The surface after spraying shall be uniform and free of lumps, loosely adherent spattered metal and uncoated spots.

2.4.0 Inspection:

The metal coating shall be inspected for thickness by means of an approved Magnetic Thickness Gage (Elcometer or equal). Inspection shall follow as closely as possible after the completion of spraying.

2.5.0 Rejection and resubmission for inspection:

Articles which have been rejected shall have the defective sections blasted clean of all sprayed metal prior to respraying; except that where the rejection has been solely due to thin coatings, additional sprayed metal may be added if the surface is completely dry and free of visible contamination.

3.0.0 Finish Coating Specification

3.1.0 Material: The finish coatings shall be a modified epoxy.

3.2.0 Application

3.2.1 Finish coatings must be applied to clean, dry electric arc sprayed surfaces. Any oil, grease, or other contamination should be removed by thorough washing with a thinner until no visible traces exist. The surfaces should be allowed to dry for at least 15 minutes before applying sealant. Coatings must be applied heavily enough to produce a thoroughly wet appearance. These coatings may be applied by brush or spray. If sprayed, pressure type spray guns must be used.

3.2.2 If required, the first coat shall be a primer. The minimum drying time shall be as specified by the coating manufacturer.

3.2.3 The second coat shall be applied by spray or brush. Thinners may be used in the quantities shown on the containers.

3.3.0 Inspection:

The inspector shall require that coatings are applied in accordance with Section 3.2.0, and that each coating completely and uniformly covers the metal or previous coating over the entire surface.

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