ASTM A385-2008 Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip).pdf

1、Designation: A 385 08Standard Practice forProviding High-Quality Zinc Coatings (Hot-Dip)1This standard is issued under the fixed designation A 385; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number

2、 in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope*1.1 This practice covers the precautions that should be takento

3、 obtain high-quality hot-dip galvanized coatings.1.2 Where experience on a specific product indicates arelaxing of any provision, the mutually acceptable change shallbe a matter for agreement between the manufacturer andpurchaser.1.3 The values stated in inch-pound units are to be regardedas standar

4、d. The values given in parentheses are mathematicalconversions to SI units that are provided for information onlyand are not considered standard.2. Referenced Documents2.1 ASTM Standards:2A 143/A 143M Practice for Safeguarding Against Em-brittlement of Hot-Dip Galvanized Structural Steel Prod-ucts a

5、nd Procedure for Detecting EmbrittlementA 384/A 384M Practice for Safeguarding Against Warpageand Distortion During Hot-Dip Galvanizing of Steel As-sembliesA 563 Specification for Carbons and Alloy Steel Nuts3. Steel Selection3.1 The production of a galvanized coating has as its basisthe metallurgic

6、al reaction between the steel and the moltenzinc, resulting in the formation of several iron-zinc compoundlayers, for example, gamma (not always visible microscopi-cally), delta, and zeta in Fig. 1. In addition, a layer of themolten zinc adheres to the surface of the compound layers asthe steel is w

7、ithdrawn from the galvanizing bath. Uponsolidification, this adherent zinc forms the eta layer.3.2 It is known that the exact structural nature of thegalvanized coating, as typified by Fig. 1, may be modified inaccordance with the exact chemical nature of the steel beinggalvanized. Certain elements

8、found in steels are known to havean influence on the coating structure. The elements carbon inexcess of about 0.25 %, phosphorus in excess of 0.04 %, ormanganese in excess of about 1.3 % will cause the productionof coatings different from the coating typified by Fig. 1. Steelswith silicon in the ran

9、ge 0.04 % to 0.15 % or above 0.22 % canproduce galvanized coating growth rates much higher thanthose for steels with silicon levels below 0.04 % and between0.15 % and 0.22 %. Recent studies have shown that even incases where the silicon and phosphorous are individually heldto desirable limits, a com

10、bined effect between them canproduce a coating as shown in Fig. 2, which typically wouldhave a mottled or dull gray appearance.3.3 These elements manifest their structural effect as anaccelerated growth of the compound layers, particularly thezeta layer, and the virtual elimination of the eta layer.

11、 Cosmeti-cally this accelerated growth is seen as a gray matte finishedcoating as opposed to the usual bright and smooth appearanceof galvanized coatings. Sometimes, a large surface may haveadjacent areas of matte finish and bright finish leading to amottled appearance.3.4 There is some evidence tha

12、t the coatings resulting fromthis accelerated growth are more brittle and less adherent thannormal coatings. There is also evidence that these coatings aresubject to a premature red staining in atmospheric exposure;however, this staining has been found not to be associated withcorrosion of the subst

13、rate steel.3.5 A problem with steel chemistry is not usually apparentuntil after an item has been galvanized. Not all combinations ofsilicon, phosphorus, carbon, and manganese can be galvanizedsuccessfully. When the steel chemistry is known beforehand,experienced galvanizers can in some, but not all

14、, instancesexercise limited control over the coatings as shown in Fig. 2.Also, the combination of two different steel types or thick-nesses in one item may result in a nonuniform galvanizingfinish. The experience of the steel supplier, designer, manufac-turer, and galvanizer should determine the ste

15、el selection.3.6 In general, galvanized coatings are specified because oftheir corrosion resistance, not because of their appearance. Therelative corrosion resistance of the normal and abnormalcoatings is, for all practical purposes, equal.1This practice is under the jurisdiction of ASTM Committee A

16、05 on Metallic-Coated Iron and Steel Products and is the direct responsibility of SubcommitteeA05.13 on Structural Shapes and Hardware Specifications.Current edition approved May 1, 2008. Published June 2008. Originallyapproved in 1955. Last previous edition approved in 2005 as A 385 05.2For referen

17、ced 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 Document Summary page onthe ASTM website.1*A Summary of Changes section appears at the end of this standard.Copyri

18、ght ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.4. Assemblies of Different Materials or Different Surfacesor Both4.1 Whenever possible, assemblies should consist of ele-ments of similar steel chemistry and surface condition.4.2 Whenever dif

19、ferent analyses of steel or different sur-faces of steel are united in an assembly the galvanized finish isnot generally uniform in appearance. These differences in-clude:4.2.1 Excessively rusted surfaces.4.2.2 Pitted surfaces.4.2.3 Machined surfaces.4.2.4 Cast iron (especially with sand inclusion).

20、4.2.5 Cast steel.4.2.6 Malleable iron.4.2.7 Hot-rolled steel.4.2.8 Cold-rolled steel.4.2.9 Steel containing chemical elements in excess of thoserecommended in 3.2.4.3 Where combinations are unavoidable, thorough abrasiveblasting of the entire assembly will normally improve galva-nizing quality.5. Ov

21、erlapping or Contacting Surfaces5.1 Overlapping or contacting surfaces that have not had alledges seal welded are undesirable.5.2 When the distance between the overlapping surfaces isless than332 in. (2.38 mm), these surfaces will not normally bewet by molten zinc. Furthermore, cleaning solution com

22、poundsthat remain on these surfaces volatilize during the galvanizingprocess and may interfere with zinc wetting in adjacent areas.Such uncoated surfaces cause a rust staining after exposure tothe environment. Traditionally however, steel grating has beenmanufactured without seal welding and when pr

23、operly ex-ecuted, this manufacturing means has permitted the galvanizedcoating to satisfy the quality requirements of the applicableASTM specifications.5.3 When the overlap surface area is large and the edgeshave been seal welded, air or moisture or both entrappedtherein can develop destructive pres

24、sures when the assembly isheated to the galvanizing temperature, which is nominally850F (454C). Vent holes or unwelded area around theadjoining surfaces should be provided through one or bothsides into the lapped area per the following tables.FIG. 1 Photomicrograph of Normal Galvanized Coating (X 40

25、0)FIG. 2 Photomicrograph of Dull Gray, Thick-Galvanized Coating (X 200)A3850826. Sheet Steel Rolled Over a Wire or Rod Stiffener6.1 All oil or grease should be removed from both the sheetsteel and wire or rod before rolling (Fig. 3). Grease or oilbecomes volatile at the galvanizing temperature and w

26、illgenerate gas which will prevent zinc from sealing the contactedges. All steel should be degreased before pickling and in thecase of folded assemblies, before folding and assembling (seeFig. 4).7. Weld Flux Removal and Welding Rods7.1 Welding flux residues are chemically inert in normalpickling so

27、lutions. Thus, they will not be removed by standardgalvanizing cleaning techniques and are best removed at thetime of fabrication by grit or sand-blasting or by a wire needlegun.7.2 It is desirable to choose a welding rod with a chemicalcomposition as close as possible to the parent metal.7.3 Weldin

28、g rods high in silicon may cause excessivelythick or darkened coatings or both to form in the welded area.8. Flame Cut Cope Edges Preparation8.1 Flame cut copes on beams can be extremely sensitive toresidual stresses in the steel beam and, with the rough surfacefrom the flame cutting operation, can

29、be sources of crackingduring the thermal cycling of the hot-dip galvanizing process.The steel beams start near ambient temperature then areimmersed in the molten zinc and heated to above 800F forusually 5 to 10 minutes. The steel beams are then cooled backto ambient temperature so the thermal cyclin

30、g can createthermal stresses in the area of the cope.8.2 One method that has had fairly good success at mini-mizing the cracking at the edges of the flame cut cope is toweld a bead along the sides of the cope in the area where theflame cutting was done before hot-dip galvanizing. Thiswelding operati

31、on will reheat the area and may relieve some ofthe residual stress near the cope edges. The weld bead will noteliminate all incidents of cracking but will greatly reduce thelikelihood of cracking.8.3 The weld rod material should be chosen as described inSection 7.9. Cold Forming Before Galvanizing9.

32、1 Refer to the latest revision of Practice A 143/A 143M.10. Shearing, Cutting and Punching Before Galvanizing10.1 Refer to the latest revision of Practice A 143/A 143M.11. Warpage and Distortion11.1 Refer to the latest revision of Practice A 384.12. Design Recommendations for Providing for the FreeF

33、low of Cleaning Solutions, Fluxes, Air, and Zinc12.1 All fabricated assemblies shall be so designed withvent and drain holes such that no air is trapped during theimmersion of the assemblies into cleaning solutions or moltenzinc. Similarly these holes shall allow all solutions and moltenzinc to drai

34、n freely from the assemblies. Failure to follow thispractice will result in areas that will not galvanize properly, orthat may retain entrapped flux or excessive amounts of zinc.12.2 Free flow of cleaning solutions and molten zinc shallalso be provided for in assemblies of hot-rolled shapes. This is

35、accomplished by cropping the corner to provide an openingwith a minimum area of 0.3 in.2(1.9 cm2) at the corners of allstiffeners (Fig. 5), gussets, or bracing (Fig. 6).12.3 Air or moisture, or both, entrapped within closedfabricated pipework, such as handrail, can develop destructivepressures when

36、heated to the galvanizing temperature. Pipehandrail shall preferably be vented full open internally, asshown in Fig. 7. In addition, there shall be one38-in. (9.5-mm)minimum diameter external hole at each intersection to preventany possible explosions in the event that the fabricator neglectsto prov

37、ide internal venting. Where internal venting is notpossible, external vents shall be provided with one vent hole ineach side of each intersection. The vent openings shall be aminimum of38 in. in diameter or 25 % of the diameter of thepipe that is used, whichever is larger (see Fig. 8).12.4 Figs. 9-1

38、2 show most of the conditions encounteredwith tubular product assemblies. The venting shall openTABLE 1 Vent Holes for Overlapped Areas for Steels12 in. (12.75mm) or Less in ThicknessOverlapped Area in.2(cm2) Vent Holes Unwelded Areaunder 16 (103) None None16 (103) to under 64 (413) One38 in. (1 cm)

39、 1 in. (2.5 cm)64 (413) to under 400 (2580) One12 in. (1.25 cm) 2 in. (5.1 cm)400 (2580) and greater,each 400 (2580)One34 in. (1.91 cm) 4 in. (10.2 cm)TABLE 2 Vent Holes for Overlapped Areas for Steels Greaterthan12 in. (12.75 mm) in ThicknessOverlapped Area in.2(cm2) Vent Holes Unwelded Areaunder 1

40、6 (103) None None16 (103) to under 64 (413) None None64 (413) to under 400 (2580) One12 in. (1.25 cm) 2 in. (5.1 cm)400 (2580) and greater,each 400 (2580)One34 in. (1.91 cm) 4 in. (10.2 cm)FIG. 3 Rolled SurfacesFIG. 4 Folded SurfacesA385083wherever possible. This is the most desirable situation. Ami

41、nimum vent opening of 25 to 30 % of the cross-sectionalarea of tubular structure shall be specified where full-openventing is not possible. For small cross sections, larger percentvent openings are recommended. See attached drawings forspecific recommendations. In box sections (Fig. 9) wheregusset p

42、lates are used, the gusset plates shall be clipped at thefour corners. In addition, a center hole shall be provided so thatthe cumulative area of the vent holes meets the recommendedminimum. Gusset plates shall not be spaced closer than 36 in.(914 mm) apart. In the case of columns with end plates (F

43、ig.10) where the end plate must be closed, the shaft of the columnshall be vented. The vent opening shall be a half circle with itsdiameter at the base plate (D, Fig. 10). This is much superiorto putting a hole with the circumference of the hole tangentialto the base plate. On trusses (Fig. 11 and F

44、ig. 12) where tubularmembers intersect, vent holes are recommended at both sidesof the intersection.13. Moving Parts13.1 When a galvanized assembly incorporates movingparts (such as drop handles, shackles, and shafts) a radialclearance of not less than116 in. (1.59 mm) must be allowed toensure full

45、freedom of movement following galvanizing.13.2 Moving parts such as handles or hinges should begalvanized separately and assembled after galvanizing. It maybe necessary to post heat these parts in order to have themfunction freely. This heating may cause discoloration of thegalvanized coating near t

46、he heated area.14. Marking for Identification14.1 Paint is not removed by pickling and must not be usedwhen marking for identification material to be galvanized.14.2 Satisfactory identification may be provided by weldingthe identifying marks on the material, by embossing theidentifying marks on a st

47、eel tag of no less than No. 12 gage(0.105 in. (2.69 mm) and securing to the material with a heavywire such as No. 9 gage (0.148 in. (3.76 mm), or by diestamping the identifying marks into the material with charac-ters12 in. (12.7 mm) high and a minimum of132 in. (0.79 mm)deep.14.3 All markings shall

48、 remain legible after galvanizing.15. Galvanized Steel in Concrete15.1 Galvanized steel can be placed in direct contact withconcrete as the galvanized coating forms an excellent bondwith the concrete materials. When embedding steel parts inconcrete, the highest quality system is produced when all of

49、 thesteel parts are coated with the same corrosion protectionsystem. If there are galvanized parts in concrete near steel partswith no corrosion protection, there can be active corrosion cellswhenever moisture is present. This will lessen the corrosionprotection offered by the galvanized coating. The highestquality system contains all galvanized steel parts.15.2 Galvanized steel parts when placed in concrete arenormally passivated either by natural weathering prior toplacement, by applying a passivation coating after the hot-dipgalvanizing process, or, effectively,