ASTM D7803-19

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it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
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Designation: D7803 − 12 D7803 − 19
Standard Practice for
Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and
Steel Product and Hardware Surfaces for Powder Coating
This standard is issued under the fixed designation D7803; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This practice describes methods of preparing surfaces of hot-dip galvanized iron and steel for powder coating and the
application of powder coating materials.
1.1.1 Powder coating is a dry finishing process which uses finely ground particles of pigment and resin, electrostatically
charged, and sprayed onto a part to be coated. The parts are electrically grounded so that the charged particles projected at them
adhere to the surface and are held there until melted and fused into a smooth coating in the curing oven.
1.1.2 Hot-dip galvanized iron or steel is produced by the immersion of fabricated or un-fabricated products in a bath of molten
zinc, as specified in Specification A123/A123M or A153/A153M. This practice covers surface preparation and thermal
pretreatment of iron and steel products and hardware which have not been painted or powder coated previously (Practice D6386).
Galvanized surfaces may have been treated with protective coatings to prevent the occurrence of wet storage stain. This practice
neither applies to sheet galvanized steel products nor to the coil coating or continuous roller coating processes.
1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
A123/A123M Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products
A153/A153M Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware
A780 Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings
B201 Practice for Testing Chromate Coatings on Zinc and Cadmium Surfaces
D4285 Test Method for Indicating Oil or Water in Compressed Air
D6386 Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting
D7091 Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals
and Nonmagnetic, Nonconductive Coatings Applied to Non-Ferrous Metals
E376 Practice for Measuring Coating Thickness by Magnetic-Field or Eddy Current (Electromagnetic) Testing Methods
F21 Test Method for Hydrophobic Surface Films by the Atomizer Test
2.2 Society for Protective Coatings Specifications:
Surface Preparation Specification No. 1 Solvent Cleaning
Surface Preparation Specification No. 2 Hand Tool Cleaning
Surface Preparation Specification No. 3 Power Tool Cleaning
This practice is under the jurisdiction of ASTM Committee D01 on Paint and Related Coatings, Materials, and Applications and is the direct responsibility of
Subcommittee D01.46 on Industrial Protective Coatings.
Current edition approved June 1, 2012Feb. 1, 2019. Published June 2012February 2019. Originally approved in 2012. Last previous edition approved in 2018 as D7803
– 18. DOI: 10.1520/D7803-12.10.1520/D7803-19.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’sstandard’s Document Summary page on the ASTM website.
Available from Society for Protective Coatings (SSPC), 40 24th St., 6th Floor, 800 Trumbull Dr., Pittsburgh, PA 15222-4656,15205, http://www.sspc.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7803 − 19
Surface Preparation Specification No. 12WJ-1 Surface Preparation and Cleaning of Metals by Water-Jetting Prior to Recoating
Surface Preparation Specification No. WJ-2 Surface Preparation and Cleaning of Metals by Water-Jetting Prior to Recoating
Surface Preparation Specification No. WJ-3 Surface Preparation and Cleaning of Metals by Water-Jetting Prior to Recoating
Surface Preparation Specification No. WJ-4 Surface Preparation and Cleaning of Metals by Water-Jetting Prior to Recoating
Surface Preparation Specification No. 15 Commercial Power Tool Cleaning
Surface Preparation Specification No. 16 Brush-Off Blast Cleaning of Coated and Uncoated Galvanized Steel, Stainless Steels,
and Non-Ferrous Metals
2.3 NACE:
NACE 6G186 Surface Preparation of Soluble Salt Contaminated Steel Substrates Prior to Coating
3. Summary of Practice
3.1 This practice describes the procedures that can be used to prepare surfaces for powder coating application on new, partially
weathered, and fully weathered zinc-coated surfaces on after-fabrication iron and steel products. These procedures improve the
bond of the powder coating to the zinc surface providing for long life.
3.2 The proper preparation of galvanized surfaces prior to application of powder coating is dependent on cleaning, profiling,
and thermal pretreatment.
4. Significance and Use
4.1 This practice describes the methods of preparation of hot-dip galvanized surfaces prior to the application of powder coating.
The key to achieving proper adhesion between powder coatings and galvanized steel is surface preparation. The surface must be
entirely free from visible metal oxides prior to powder coating. Any metal oxides that remain on the surface of the galvanized steel
can potentially retain air or moisture. Upon heating during the curing stages of the powder application, the oxides may release
water vapor or air, which can expand and penetrate the powder coating, causing blisters or voids.
4.2 The zinc coating is constantly in a state of change. From the time the steel part is removed from the galvanizing kettle, the
exposed zinc coating interacts with the environment to form, first zinc oxides and zinc hydroxides, and then zinc carbonates. The
process of complete conversion of the outer layer of zinc carbonates can take up to two years of exposure to the environment,
depending on the local weather and moisture conditions.
4.3 The zinc surface after full weathering is very resistant to atmospheric corrosion because the tight patina that is formed (zinc
oxide, zinc hydroxide and zinc carbonate) is dense and tenacious. However, during the formative stages of patina development,
the oxide/hydroxide layer is poorly adhered and must be removed in order for the powder coating to adhere properly to the
galvanized coating. The second is pinholing/blistering of the coating which can severely limit its potential performance, especially
in aggressive chloride environments. Entrapped gasses developed during the galvanizing process escape the surface through the
coating as it cures at high temperatures. If these volatile materials are not removed through an outgassing process prior to the
baking of the powder, then pinholing or blistering can occur. The presence of pinholes gives chlorides and other corrosive agents
access to the zinc substrate consequently producing zinc corrosion products which may leach out through the coatings. While the
presence of these corrosion products may not result in associated delamination of the coating, unsightly white staining of the
coating can occur. Blisters are defects that are not adhered to the surface and may easily be broken into or off during handling,
which creates performance and aesthetic issues. The proper preparation of the galvanized coating surface can increase the adhesion
and coverage necessary to overcome these problems and results in a satisfactory service life of the powder coating and the
galvanized coating together.
4.4 Variations in surface preparation produce end conditions that differ as far as surface roughness and zinc composition, hence
they do not necessarily yield identical results when powder coatings are subsequently applied. The age of the zinc corrosion
products on the galvanized coating will dictate the type of surface preparation to be selected.
5. Processes for Cleaning and Preparing Hot Dipped Galvanized Iron and Steel Surfaces for Powder Coating
5.1 Newly Galvanized Metal—The category of newly galvanized metal refers to zinc-coated metal that has no surface treatment
after galvanizing, such as water quenching or chromate conversion coating, and has been galvanized within the previous 48 h.
There also shall be no visible signs of zinc oxide or zinc hydroxide, which first appear as a fine white powder.
5.1.1 Surface Smoothing—Hot-dip galvanized surfaces, in general, are relatively smooth after galvanizing. There may be some
thick/rough edges at the drip line due to excess liquid zinc run-off during the galvanization process, or high spots in the coating
from included iron-zinc intermetallics (dross) or zinc oxide particles. These high spots and rough edges must be smoothed to avoid
powder coat film gaps in the areas of the high spots. Zinc high spots shall be removed by cleaning with hand or power tools as
described in SSPC Surface Preparation Specification 2 or 3 until they are level with the surrounding zinc area, taking care that the
Available from NACE International (NACE), 15835 Park Ten Pl., Houston, TX 77084, http://www.nace.org.
This interaction is described in “Duplex Systems,” van Eijnsbergen, J. F. H., Elsevier Science, New York, NY 1994, and in Zinc Handbook, Porter, F., Marcel Dekker,
Inc., New York, NY 1991.
D7803 − 19
base coating is not removed by the cleaning methods. After smoothing, the surface shall be inspected for conformance to the
required zinc thickness in accordance with Specification A123/A123M or A153/A153M utilizing a magnetic thickness instrument
in accordance with Practice E376 and/or D7091. Any area falling below the required zinc thickness, before or after removal of any
high spots, shall be repaired in accordance with Practice A780 using an appropriate method that is compatible with the curing
temperature and time of the powder coating.
5.1.2 Surface Cleaning—Hot-dip galvanized surfaces must be clean and free of oil and grease before they are powder coated.
Soluble salts shall be removed to the degree specified in the powder coating specification. Detection of soluble salts can be
achieved following SSPC Guide 15. Removal of soluble salts can be achieved following NACE 6G186. Adhesion problems have
been experienced with newly galvanized articles that have been water quenched or treated with chromate conversion coatings.
These two post-galvanizing treatments are not recommended for galvanized articles that are to be coated. Test Method F21 can
determine if contamination is on the galvanized surface prior to powder coating.
5.1.2.1 Checking for Chromate Conversion Coating—Before preparing the surface of any state of the galvanized steel for
powder coating, if the galvanizer has not indicated if he avoided the chromate bath, the surface of a sample group from each
shipment must be checked for the presence of a chromate conversion coating. The presence of a chromate conversion coating can
severely impair the adhesion of powder coating. The presence of chromate conversion coatings can be detected by the procedure
outlined in Practice B201. Chromate conversion coating can be removed completely by weathering galvanized metal for six
months outdoors, sanding in accordance with 5.1.3.3, or sweep blasting in accordance with 5.1.3.1.
5.1.2.2 Aqueous Alkaline Cleaning—An alkaline solution, pH in the range of 11 to 12, definitely not greater than 13, can be used
to remove traces of oil, grease, or dirt. The alkaline solution nominally is 2 to 5 % sodium compounds, with small additions of
emulsifying, chelating, or sequestering agents, or a combination thereof. This solution can be applied through immersion in a tank
filled with the solution, sprayed, or brushed with a soft bristle brush, usually nylon and not steel or copper. When dipping or
spraying, the solution works best in the temperature range from 60 to 85°C (140 to 185°F). After cleaning, rinse thoroughly in hot
water or water under pressure. Allow to dry completely before proceeding. Whenever galvanized steel is rinsed, it is desirable to
use heated drying to accelerate the complete removal of water from the surface.
NOTE 1—An alkaline cleaner is unsuitable for removal of heavy build-up of zinc oxide or wet storage stain (see American Galvanizers Publication,
Wet Storage Stain, for description of these conditions). See 5.1.3 for removal of zinc oxide layer.
5.1.2.3 Solvent Cleaning—Typical cleaning solvents, such as mineral spirits or high-flash naphtha, can be used to
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