1、Designation: D7803 12Standard Practice forPreparation of Zinc (Hot-Dip Galvanized) Coated Iron andSteel Product and Hardware Surfaces for Powder Coating1This standard is issued under the fixed designation D7803; the number immediately following the designation indicates the year oforiginal adoption
2、or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice describes methods of preparing surfaces ofhot-dip galvanized ir
3、on and steel for powder coating and theapplication of powder coating materials.1.1.1 Powder coating is a dry finishing process which usesfinely ground particles of pigment and resin, electrostaticallycharged, and sprayed onto a part to be coated. The parts areelectrically grounded so that the charge
4、d particles projected atthem adhere to the surface and are held there until melted andfused into a smooth coating in the curing oven.1.1.2 Hot-dip galvanized iron or steel is produced by theimmersion of fabricated or un-fabricated products in a bath ofmolten zinc, as specified in Specification A123/
5、A123M orA153/A153M. This practice covers surface preparation andthermal pretreatment of iron and steel products and hardwarewhich have not been painted or powder coated previously(Practice D6386). Galvanized surfaces may have been treatedwith protective coatings to prevent the occurrence of wetstora
6、ge stain. This practice neither applies to sheet galvanizedsteel products nor to the coil coating or continuous rollercoating processes.1.2 The values stated in SI units are to be regarded as thestandard. The values given in parentheses are for informationonly.1.3 This standard does not purport to a
7、ddress all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2A123/A123M
8、Specification for Zinc (Hot-Dip Galvanized)Coatings on Iron and Steel ProductsA153/A153M Specification for Zinc Coating (Hot-Dip) onIron and Steel HardwareA780 Practice for Repair of Damaged and Uncoated Areasof Hot-Dip Galvanized CoatingsB201 Practice for Testing Chromate Coatings on Zinc andCadmiu
9、m SurfacesD4285 Test Method for Indicating Oil or Water in Com-pressed AirD6386 Practice for Preparation of Zinc (Hot-Dip Galva-nized) Coated Iron and Steel Product and HardwareSurfaces for PaintingD7091 Practice for Nondestructive Measurement of DryFilm Thickness of Nonmagnetic Coatings Applied toF
10、errous Metals and Nonmagnetic, Nonconductive Coat-ings Applied to Non-Ferrous MetalsE376 Practice for Measuring Coating Thickness byMagnetic-Field or Eddy-Current (Electromagnetic) Test-ing MethodsF21 Test Method for Hydrophobic Surface Films by theAtomizer Test2.2 Society for Protective Coatings Sp
11、ecifications:3Surface Preparation Specification No. 1 Solvent CleaningSurface Preparation Specification No. 2 Hand Tool CleaningSurface Preparation Specification No. 3 Power Tool Clean-ingSurface Preparation Specification No. 12 Surface Prepara-tion and Cleaning of Metals by Water-Jetting Prior toRe
12、coatingSurface Preparation Specification No. 16 Brush-Off BlastCleaning of Coated and Uncoated Galvanized Steel,Stainless Steels, and Non-Ferrous Metals3. Summary of Practice3.1 This practice describes the procedures that can be usedto prepare surfaces for powder coating application on new,partially
13、 weathered, and fully weathered zinc-coated surfaceson after-fabrication iron and steel products. These procedures1This practice is under the jurisdiction of ASTM Committee on Paint andRelated Coatings, Materials, and Applications and is the direct responsibility ofSubcommittee D01.46 on Industrial
14、Protective Coatings.Current edition approved June 1, 2012. Published June 2012. DOI: 10.1520/D7803-12.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards
15、Document Summary page onthe ASTM website.3Available from Society for Protective Coatings (SSPC), 40 24th St., 6th Floor,Pittsburgh, PA 15222-4656, http:/www.sspc.org.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1improve the bond of
16、the powder coating to the zinc surfaceproviding for long life.3.2 The proper preparation of galvanized surfaces prior toapplication of powder coating is dependent on cleaning,profiling, and thermal pretreatment.4. Significance and Use4.1 This practice describes the methods of preparation ofhot-dip g
17、alvanized surfaces prior to the application of powdercoating. The key to achieving proper adhesion between powdercoatings and galvanized steel is surface preparation. Thesurface must be entirely free from metal oxides prior to powdercoating. Any metal oxides that remain on the surface of thegalvaniz
18、ed steel can potentially retain air or moisture. Uponheating during the curing stages of the powder application, theoxides may release water vapor or air, which can expand andpenetrate the powder coating, causing blisters or voids.4.2 The zinc coating is constantly in a state of change. Fromthe time
19、 the steel part is removed from the galvanizing kettle,the exposed zinc coating interacts with the environment toform, first zinc oxides and zinc hydroxides, and then zinccarbonates.4The process of complete conversion of the outerlayer of zinc carbonates can take up to two years of exposureto the en
20、vironment, depending on the local weather andmoisture conditions.4.3 The zinc surface after full weathering is very resistant toatmospheric corrosion because the tight patina that is formed(zinc oxide, zinc hydroxide and zinc carbonate) is dense andtenacious. However, during the formative stages of
21、patinadevelopment, the oxide/hydroxide layer is poorly adhered andmust be removed in order for the powder coating to adhereproperly to the galvanized coating. The second is pinholing/blistering of the coating which can severely limit its potentialperformance, especially in aggressive chloride enviro
22、nments.Entrapped gasses developed during the galvanizing processescape the surface through the coating as it cures at hightemperatures. If these volatile materials are not removedthrough an outgassing process prior to the baking of thepowder, then pinholing or blistering can occur. The presence ofpi
23、nholes gives chlorides and other corrosive agents access tothe zinc substrate consequently producing zinc corrosionproducts which may leach out through the coatings. While thepresence of these corrosion products may not result in associ-ated delamination of the coating, unsightly white staining ofth
24、e coating can occur. Blisters are defects that are not adheredto the surface and may easily be broken into or off duringhandling, which creates performance and aesthetic issues. Theproper preparation of the galvanized coating surface canincrease the adhesion and coverage necessary to overcomethese p
25、roblems and results in a satisfactory service life of thepowder coating and the galvanized coating together.4.4 Variations in surface preparation produce end conditionsthat differ as far as surface roughness and zinc composition,hence they do not necessarily yield identical results whenpowder coatin
26、gs are subsequently applied. The age of the zinccorrosion products on the galvanized coating will dictate thetype of surface preparation to be selected.5. Processes for Cleaning and Preparing Hot DippedGalvanized Iron and Steel Surfaces for Powder Coating5.1 Newly Galvanized MetalThe category of new
27、ly gal-vanized metal refers to zinc-coated metal that has no surfacetreatment after galvanizing, such as water quenching or chro-mate conversion coating, and has been galvanized within theprevious 48 h. There also shall be no visible signs of zinc oxideor zinc hydroxide, which first appear as a fine
28、 white powder.5.1.1 Surface SmoothingHot-dip galvanized surfaces, ingeneral, are relatively smooth after galvanizing. There may besome thick/rough edges at the drip line due to excess liquidzinc run-off during the galvanization process, or high spots inthe coating from included iron-zinc intermetall
29、ics (dross) orzinc oxide particles. These high spots and rough edges must besmoothed to avoid powder coat film gaps in the areas of thehigh spots. Zinc high spots shall be removed by cleaning withhand or power tools as described in SSPC Surface PreparationSpecification 2 or 3 until they are level wi
30、th the surroundingzinc area, taking care that the base coating is not removed bythe cleaning methods. After smoothing, the surface shall beinspected for conformance to the required zinc thickness inaccordance with Specification A123/A123M or A153/A153Mutilizing a magnetic thickness instrument in acc
31、ordance withPractice E376 and/or D7091. Any area falling below therequired zinc thickness, before or after removal of any highspots, shall be repaired in accordance with Practice A780 usingan appropriate method that is compatible with the curingtemperature and time of the powder coating.5.1.2 Surfac
32、e CleaningHot-dip galvanized surfaces mustbe clean and free of oil and grease before they are powdercoated. Adhesion problems have been experienced with newlygalvanized articles that have been water quenched or treatedwith chromate conversion coatings. These two post-galvanizing treatments are not r
33、ecommended for galvanizedarticles that are to be coated. Test Method F21 can determineif contamination is on the galvanized surface prior to powdercoating.5.1.2.1 Checking for Chromate Conversion CoatingBefore preparing the surface of any state of the galvanized steelfor powder coating, if the galva
34、nizer has not indicated if heavoided the chromate bath, the surface of a sample group fromeach shipment must be checked for the presence of a chromateconversion coating. The presence of a chromate conversioncoating can severely impair the adhesion of powder coating.The presence of chromate conversio
35、n coatings can be detectedby the procedure outlined in Practice B201. Chromate conver-sion coating can be removed completely by weathering galva-nized metal for six months outdoors, sanding in accordancewith 5.1.3.3, or sweep blasting in accordance with 5.1.3.1.5.1.2.2 Aqueous Alkaline CleaningAn al
36、kaline solution,pH in the range of 11 to 12, definitely not greater than 13, canbe used to remove traces of oil, grease, or dirt. The alkalinesolution nominally is 2 to 5 % sodium compounds, with small4This interaction is described in “Duplex Systems,” van Eijnsbergen, J. F. H.,Elsevier Science, New
37、 York, NY 1994, and in Zinc Handbook, Porter, F., MarcelDekker, Inc., New York, NY 1991.D7803 122additions of emulsifying, chelating, or sequestering agents, ora combination thereof. This solution can be applied throughimmersion in a tank filled with the solution, sprayed, orbrushed with a soft bris
38、tle brush, usually nylon and not steel orcopper. When dipping or spraying, the solution works best inthe temperature range from 60 to 85C (140 to 185F). Aftercleaning, rinse thoroughly in hot water or water under pressure.Allow to dry completely before proceeding. Whenever galva-nized steel is rinse
39、d, it is desirable to use heated drying toaccelerate the complete removal of water from the surface.NOTE 1An alkaline cleaner is unsuitable for removal of heavybuild-up of zinc oxide or wet storage stain (see American GalvanizersPublication, Wet Storage Stain,5for description of these conditions). S
40、ee5.1.3 for removal of zinc oxide layer.5.1.2.3 Solvent CleaningTypical cleaning solvents, suchas mineral spirits or high-flash naphtha, can be used to removeoil and grease. The procedure to be used is as specified in SSPCSP 1. Proper rags or brushes shall be used to wipe thegalvanized parts. After
41、cleaning, rinse thoroughly in hot wateror water under pressure. Allow to dry completely beforeproceeding. Whenever galvanized steel is rinsed, it is desirableto use heated drying to accelerate the complete removal ofwater from the surface. (WarningThese rags or brushesshould be cleaned or recycled o
42、ften since oil can accumulate ontheir surfaces and be transferred back to the galvanized part.Small parts may be dipped or cleaned in ultrasonic baths ofsolvents.)5.1.3 Surface PreparationHot-dip galvanized surfaceshave a layer of zinc oxide and zinc hydroxide that must beremoved before powder coati
43、ng will adhere to the zinc coating.Zinc coatings are generally smooth and must be slightlyroughened prior to powder coating. One of the following threemethods shall be used to prepare the galvanized surface forpowder coating.5.1.3.1 Sweep BlastingAbrasive sweep or brush blastingin accordance with SS
44、PC SP 16, which uses a rapid nozzlemovement, will roughen the galvanized surface profile. Theabrasive material must be chosen carefully to provide astripping action that removes the oxide reaction productswithout excessive removal of the zinc. One of the materialsthat has been used successfully is a
45、luminum/magnesium sili-cate with a particle size in the range of 200 to 500 m (8 to 20mils.). Other materials that can be used are soft mineral sandswith a Mohs hardness of five or less, organic media, such ascorn cobs or walnut shells, corundum, or limestone. Dependingon the value of hardness for t
46、he abrasive medium, blastingpressure may need to be adjusted for the appropriate nozzle towork-piece distance, geometry of the component, and blastingmedium. For reactive steel with all-alloy coatings which mayhave compromised adhesion, even the relatively low-pressureblast of 0.15 to 0.25 MPa (20 t
47、o 40 psi) can be too great,causing adhesion problems. Care must be taken to leave zinclayers intact. Oil contamination of the compressed air willdegrade adhesion to sweep-blasted hot-dip galvanized surfaces(Test Method D4285). Care is needed in averting this type ofcontamination. The purpose of swee
48、p blasting is to deform, notremove the galvanized metal. Any area falling below therequired zinc thickness, before or after sweep blasting, shall berepaired in accordance with Practice A780. Sweep blasting ofzinc shall be not less than 110 m2/h (1200 ft2/h) using theseabrasive materials. The substra
49、te shall be maintained at atemperature of at least 3C (5F) above the dew point tem-perature. Following abrasive blast cleaning, surfaces shall beblown down with clean, compressed air. In some atmosphericconditions, such as high humidity, high temperature, or both,the formation of zinc oxide on the blasted surface will beginvery quickly. Because newly formed zinc oxide is not visible tothe naked eye, powder coating shall be applied as soon aspossible after surface preparation.5.1.3.2 Phosphate TreatmentThis conversion-coating pro-cess consists of reacting the newly gal
