ASTM D6386-22

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Designation: D6386 − 16a D6386 − 22
Standard Practice for
Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and
Steel Product and Hardware Surfaces for Painting
This standard is issued under the fixed designation D6386; 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.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope*Scope
1.1 This practice describes methods of preparing surfaces of new and weathered hot-dip galvanized steel for painting with liquid
paint and coating products. Preparing surfaces of new and partially weathered galvanized steel for powder coating is addressed in
Practice D7803. Hot-dip galvanized steel is produced by the immersion of fabricated or unfabricated products in a bath of molten
zinc, as specified in Specifications A123/A123M or A153/A153M. This practice covers surface preparation on iron and steel
products and hardware that have not been painted previously. Galvanized surfaces may have been treated with protective coatings
to prevent the occurrence of wet storage stain. This practice does not apply to sheet galvanized steel products nor to the coil coating
or continuous roller coating processes. Sheet and coil surface preparation can be done in accordance with Practice D7396.
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 to determine the
applicationapplicability 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
D6492 Practice for Detection of Hexavalent Chromium On Zinc and Zinc/Aluminum Alloy Coated Steel
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
D7396 Guide for Preparation of New, Continuous Zinc-Coated (Galvanized) Steel Surfaces for Painting
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 15, 2016Jan. 1, 2022. Published June 2016February 2022. Originally approved in 1999. Last previous edition approved in 2016 as
D6386 – 16.D6386 – 16a. DOI: 10.1520/D6386-16A.10.1520/D6386-22.
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’s Document Summary page on the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6386 − 22
D7803 Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Powder
Coating
E376 Practice for Measuring Coating Thickness by Magnetic-Field or Eddy Current (Electromagnetic) Testing Methods
2.2 Society for Protective Coatings SSPC Specifications:
Surface Preparation Specification No. 1 Solvent Cleaning
Surface Preparation Specification No. 2 Hand Tool Cleaning
Surface Preparation Specification No. 3 Power Tool Cleaning
Surface Preparation Specification No. 7 Brush-Off Blast Cleaning
Surface Preparation Specification No. 11 Power Tool Cleaning to Bare Metal
Surface Preparation Specification No. 16 Brush-Off Blast Cleaning of Non-Ferrous Metals
Paint Specification No. 27 Basic Zinc Chromate-Vinyl Butyral Wash Primer
SSPC Guide 15 Field Methods for Extraction and Analysis of Soluble Salts on Steel and Other Nonporous Substrates
3. Summary of Practice
3.1 This practice describes the preparation methods that provide clean and suitable galvanized surfaces for painting, specifically
so that an applied coating system can develop the adhesion necessary for a satisfactory service life.
3.2 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, next 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 climatological conditions. During the first stage, known as newly galvanized steel, the exposed surface
consists mainly of zinc metal with a small amount of zinc oxide. During the second stage, known as partially weathered galvanized
steel, the exposed surface consists mainly of zinc oxides and zinc hydroxides with some zinc carbonates. At the final stage, known
as weathered galvanized steel, when the part has been openly exposed to the elements for more than twelve months, the exposed
surface consists mainly of water-insoluble zinc carbonates, some zinc oxides, and rarely, zinc hydroxides. The surface preparation
for each of these stages must be treated separately.
3.3 Variations in surface preparation produce end conditions that differ, hence they do not necessarily yield identical results when
paints are subsequently applied. Service conditions will dictate the type of surface preparation to be selected, although the quality
produced by any individual process may vary with different compositions of the zinc surface.
4. Significance and Use
4.1 This practice describes the procedures that can be used to prepare new and weathered zinc-coated surfaces on after-fabrication
steel products for painting, and that can improve the bond of paint to the zinc surface.
5. Processes for Newly Galvanized Steel
5.1 Newly Galvanized Steel—The category of newly galvanized steel refers to zinc-coated steel 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 should be no visible signs of zinc oxide or zinc hydroxide, which first appear as a fine white powder.
5.2 Surface Smoothing—Hot-dip galvanized surfaces generally 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 galvanizing process, or high spots in the coating due
to included iron-zinc intermetallics (dross) or zinc oxide particles. These high spots and rough edges must be smoothed to avoid
paint film gaps in the areas of the high spots.
5.2.1 Zinc high spots, those that would cause paint film gaps such as the metal drip line, should be removed by smoothing with
hand or power tools as described in SSPC Surface Preparation Specification No. 2 or No. 3. The zinc should be removed until it
is level with the surrounding area, taking care that the base coating is not removed by the smoothing methods. After smoothing,
the surface shall be inspected for conformance to the required zinc thickness in accordance with Specifications A123/A123M or
Available from Society for Protective Coatings (SSPC), 40 24th St., 6th Floor, Pittsburgh, PA 15222-4656, http://www.sspc.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.
D6386 − 22
A153/A153M utilizing a magnetic thickness instrument in accordance with Practice E376 and/oror Practice D7091. , or both. Any
item falling below the required zinc thickness, before or after removal of any high spots, shall be repaired in accordance with
Practice A780.
5.3 Surface Cleaning—Hot-dip galvanized surfaces must be clean and free of oil and grease before they are painted. Soluble salts
shall be removed to the degree specified in the painting/coating specification. Removal of Soluble salts can be extracted using
SSPC Guide 15. 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 painted. Practice B201 or D6492 can determine if contamination is on the galvanized surface prior to painting.
5.3.1 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°C85 °C (140 to 185°F).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 Publications, Wet
Storage Stain, for description of these conditions). See 5.3.3 or 5.4 for removal of zinc oxide layer.
NOTE 1— An alkaline cleaner is unsuitable for removal of heavy build-up of zinc oxide or wet storage stain (see American Galvanizers Publications, Wet
Storage Stain, for description of these conditions). See 5.3.3 or 5.4 for removal of zinc oxide layer.
5.3.2 Solvent Cleaning—Typical cleaning solvents, such as mineral spirits or high-flash naphtha, can be used to remove oil and
grease. The procedure to be used is as specified in SSPC Surface Preparation Specification No. 1. Proper rags or brushes should
be used to wipe the galvanized parts. (Warning—WarningThese—These rags or brushes should be cleaned or recycled often since
oil can accumulate on their surfaces and be transferred back to the galvanized part.) Small parts may be dipped or cleaned in
ultrasonic baths of solvents. 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.
5.3.3 Hand or Power Tool Cleaning—Hand or power tool cleaning using equipment as specified in SSPC Surface Preparation
Specification No. 2 or No. 3 is one method to clean light deposits of zinc reaction by products, such as light wet-storage stain.
5.4 Surface Preparation—Hot-dip galvanized surfaces have a layer of zinc oxide and zinc hydroxide that must be removed before
paint will adhere to the zinc coating. Zinc coatings generally are relatively smooth and may be slightly roughened prior to painting.
The following five methods may be used to prepare the galvanized surface for painting.
5.4.1 Sweep Blasting—Abrasive sweep or brush blasting in accordance with procedures described in SSPC Surface Preparation
No. 16, which uses a rapid nozzle movement will roughen the galvanized surface profile. The abrasive material must be chosen
with care to provide a stripping action without removing excess zinc layers, removal of up to 25 micronsμm (1 mil) is acceptable.
One of the materials that has been used successfully is aluminum/magnesium silicate. Particle size should be in the range of 200
to 500 μm (8 to 20 mils). Other materials that can be used are soft mineral sands with a MOHMohs hardness of five or less, organic
media, such as corn cobs or walnut shells, corundum, and limestone. limestone or cast zinc shot. The use of abrasive media with
Mohs hardness of five or greater, or a bulk density over 200 lb/ft is capable of producing roughened galvanized surfaces with
increased peak heights, but additional mitigations may necessary by the blaster to significantly reduce the risk of damage.
Depending on the value of hardness for the abrasive medium, blasting pressure may need to be determined for the appropriate
nozzle to work-piece distance, geometry of the component, and blasting medium. For some all-alloy coatings, even the relatively
low-pressure blast of 0.15 to 0.25 MPa 0.25 MPa (20 to 40 psi) can be too great, causing cohesion problems. Oil contamination
of the compressed air will degrade paint adhesion to sweep-blasted hot-dip galvanized surfaces, Test Methodtest method for
determining this contamination is Test Method D4285. Care is needed in averting this type of contamination. Care must be taken
to leave zinc layers intact. The purpose of sweep blasting is to deform, not remove the galvanized metal. Any area falling below
the required zinc thickness, before or after sweep blasting, should be rep
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