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vv, v VJanuary 1, 2007
<br />new coating system. The hierarchy of blasting standards is
<br />as follows: white metal blast cleaning, near-white metal blast
<br />cleaning, commercial blast cleaning, industrial blast cleaning,
<br />and brush-off blast cleaning.
<br />A2 MAINTENANCE COATING WORK: When this stan-
<br />dard is used in maintenance coating work, specific instructions
<br />should be provided on the extent of surface to be blast cleaned
<br />or spot blast cieaned to this degree of cleanliness. in these
<br />cases, this degree of cleaning applies to the entire specified
<br />area. For example, if all weld seams are to be cleaned in a
<br />maintenance operation, this degree of cleaning applies to
<br />100 percent of all weld seams. If the entire structure is to be
<br />prepared, this degree of cleaning applies to 100 percent of the
<br />entire structure. SSPC-PA Guide 49 provides a description of
<br />accepted practices for retaining old sound coating, removing
<br />unsound coating, feathering, and spot cleaning.
<br />A3 REFERENCE PHOTOGRAPHS AND COMPARA-
<br />TORS: SSPC-VIS 1 provides color photographs for the various
<br />grades of surface cleaning as a function of the initial condition
<br />of the steel. The photographs B SP 6, C SP 6, D SP 6, G, SP 6,
<br />G2 SP 6, and G3 SP 6 depict surfaces cleaned to commercial
<br />grade. Other available reference photographs and comparators
<br />are described in Section 11 of SSPC-SP COM.
<br />A4 SURFACE IMPERFECTIONS: Surface imperfections
<br />can cause premature coating failure when the service is severe.
<br />Coatings tend to pull away from sharp edges and projections,
<br />leaving little or no coating to protect the underlying steel. Other
<br />features that are difficult to cover and protect properly include
<br />crevices, weld porosities, laminations, etc. The high cost of
<br />the methods to remedy surtace imperfections (such as edge
<br />rounding and weld spatter removal) should be weighed against
<br />the costs of a potential coating failure.
<br />Poorly adhering contaminants, such as weld slag residues,
<br />loose weld spatter, and some minor surface laminations, may
<br />be removed during the blast cleaning operation. Other surface
<br />defects (steel laminations, weld porosities, or deep corrosion
<br />pits) may not be evident until the surface cleaning has been
<br />completed. Repair of such surface defects should be planned
<br />properly because the timing of the repairs may occur before,
<br />during, or after the blast cleaning operation. Section 4.4 of
<br />SSPC-SP COM and NACE Standard SP017870 contain addi-
<br />tional information on surface imperfections.
<br />A5 WET ABRASIVE BLAST CLEANING: Steel that is
<br />wet abrasive blast cleaned may rust rapidly. Clean water should
<br />be used for rinsing. It may be necessary to add inhibitors to
<br />the water or apply them to the surface immediately after blast
<br />cleaning to temporarily prevent rust formation. The use of
<br />inhibitors or the application of coating over slight discoloration
<br />should be in accordance with the requirements of the coating
<br />manufacturer. CAUTION: Some inhibitive treatments may
<br />interfere with the performance of certain coating systems.
<br />A6 ABRASIVE SELECTION: Types of inetallic and
<br />nonmetallic abrasives are discussed in SSPC-SP COM. Blasting
<br />abrasives may become embedded in, or leave residues on, the
<br />surface of the steel during cleaning. While such embedment or
<br />residues are normally not detrimental, care should be taken to
<br />ensure that the abrasive is free from detrimental amounts of
<br />water-soluble, solvent-soluble, acid-soluble, or other soluble
<br />contaminants (particularly if the cleaned steel is to be used in
<br />an immersion environment). Criteriafor selecting and evaluating
<br />abrasives are in SSPC-AB 1, SSPC-AB 2, and SSPC-AB 3.
<br />A7 SURFACE PROFILE: Surtace profile is the rough-
<br />ness of the surface that results from abrasive blast cleaning.
<br />The profile height is dependent on the size, shape, type, and
<br />hardness of the abrasive, particle velocity and angle of impact,
<br />hardness of the surface, amount of abrasive recycling, and the
<br />proper maintenance of working mixtures of grit and/or shot.
<br />The allowabie minimum/maximum height of profile is
<br />usually dependent on the thickness of the coating to be applied.
<br />Large particle-sized abrasives (particularly metallic) can
<br />produce a surface profile that may be too high to be adequately
<br />covered by a single thin-film coat. Accordingly, the use of
<br />larger abrasives should be avoided in these cases. However,
<br />larger abrasives may be needed for thick-film coatings or to
<br />facilitate removal of thick coatings, heavy mill scale, or rust. If
<br />control of surface profile (minimum/maximum) is deemed to be
<br />significantto coating performance, it should be addressed inthe
<br />procurement documents (project specification). Typical surface
<br />profile heights achieved with commercial abrasive media are
<br />shown in Table 6 of SSPC-SP COM. Surface profile should
<br />be measured in accordance with NACE Standard RP0287"
<br />or ASTM D 4417.12
<br />A8 FILM THICKNESS: It is essential that ample coating
<br />be applied after blast cleaning to adequately cover the peaks
<br />of the surtace profile. The dry-film thickness of the coating
<br />above the peaks of the profile should equal the thickness
<br />known to be needed for the desired protection. If the dry-film
<br />thickness over the peaks is inadequate, prematu re rust-through
<br />or coating failure will occur. To ensure that coating thicknesses
<br />are properly measured, the procedures in SSPC-PA213should
<br />be used.
<br />A9 CHEMICAL CONTAMINATION: Steel contaminated
<br />with soluble salts (e.g., chlorides and sulfates) develops rust-
<br />back rapidly at intermediate and high levels of humidity. These
<br />soluble salts can be present on the steel surface prior to blast
<br />cleaning as a result of atmospheric contamination. In addition,
<br />contaminants can be deposited on the steel surface during
<br />blast cleaning if the abrasive is contaminated. Therefore, rust-
<br />back can be minimized by removing these salts from the steel
<br />surface and eliminating sources of recontamination during and
<br />after blast cleaning. Wet methods of removal are described in
<br />SSPC-SP 12/NACE No. 5.14 Identification of the contaminants
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