1、Designation: A385/A385M 15A385/A385M 17Standard Practice forProviding High-Quality Zinc Coatings (Hot-Dip)1This standard is issued under the fixed designation A385/A385M; the number immediately following the designation indicates the yearof original adoption or, in the case of revision, the year of
2、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 Scope*1.1 This practice covers the precautions that should be taken to obtain high-quality hot-dip galvanized coatings.1.
3、2 Where experience on a specific product indicates a relaxing of any provision, the mutually acceptable change shall be amatter for agreement between the manufacturer and purchaser.1.3 This specification is applicable to orders in either inch-pound units (as A385) or in SI units (as A385M). Inch-pou
4、nd unitsand SI units are not necessarily exact equivalents. Within the text of this specification and where appropriate, SI units are shownin brackets. Each system shall be used independently of the other without combining values in any way.1.4 This international standard was developed in accordance
5、 with internationally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Technical Barriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standar
6、ds:2A123/A123M Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel ProductsA143/A143M Practice for Safeguarding Against Embrittlement of Hot-Dip Galvanized Structural Steel Products and Procedurefor Detecting EmbrittlementA153/A153M Specification for Zinc Coating (Hot-Dip) on Iron
7、 and Steel HardwareA384/A384M Practice for Safeguarding Against Warpage and Distortion During Hot-Dip Galvanizing of Steel AssembliesA563 Specification for Carbon and Alloy Steel Nuts2.2 American Institute of Steel Construction (AISC) Documents:3Steel Construction Manual3. Steel Selection3.1 The pro
8、duction of a galvanized coating has as its basis the metallurgical reaction between the steel and the molten zinc,resulting in the formation of several iron-zinc compound layers, for example, gamma (not always visible microscopically), delta,and zeta in Fig. 1. In addition, a layer of the molten zin
9、c adheres to the surface of the compound layers as the steel is withdrawnfrom the galvanizing bath. Upon solidification, this adherent zinc forms the eta layer.3.2 It is known that the exact structural nature of the galvanized coating, as typified by Fig. 1, may be modified in accordancewith the exa
10、ct chemical nature of the steel being galvanized. Certain elements found in steels are known to have an influence onthe coating structure. The elements carbon in excess of about 0.25 %, phosphorus in excess of 0.04 %, or manganese in excess ofabout 1.3 % will cause the production of coatings differe
11、nt from the coating typified by Fig. 1. Steels with silicon in the range0.04 % to 0.15 % or above 0.22 % can produce galvanized coating growth rates much higher than those The element with the mostsignificant effect on the galvanized coating is silicon. This element has two composition regions that
12、can produce thick and dullcoatings. The first region is silicon concentration between 0.06 % and 0.13 % and is known as the Sandelin region. The secondregion is silicon concentration above 0.25 %. When producing or purchasing steels to be hot-dip galvanized, a good practice isto aim for steels with
13、silicon levels below 0.04 % and between 0.15 % and 0.22 %. a silicon concentration target of 0.15 % to 0.22% and a minimum of 0.13 % and a maximum of 0.25 % to avoid both the Sandelin region and the high silicon region. Recentstudies have shown that even in cases where the silicon and phosphorous ar
14、e individually held to desirable limits, a combined effectbetween them can produce a coating as shown in Fig. 2, which typically would have a mottled or dull gray appearance. The valuesof silicon in the mill test reports of steel can give an indication of the reactivity of the steel in the hot-dip g
15、alvanizing process butare not a guarantee. The best practice is to galvanize test pieces to get a better indication of the reactivity of the steel.3.3 These elements manifest their structural effect as an accelerated growth of the compound layers, particularly the zeta layer,and the virtual eliminat
16、ion of the eta layer. Cosmetically this accelerated growth is seen as a gray matte finished coating as opposedto the usual bright and smooth appearance of galvanized coatings. Sometimes, a large surface may have adjacent areas of mattefinish and bright finish leading to a mottled appearance.3.4 Ther
17、e is some evidence that the coatings resulting from this accelerated growth are more brittle and less adherent thannormal coatings. There is also evidence that these coatings are subject to a premature red staining in atmospheric exposure;however, this staining has been found not to be associated wi
18、th corrosion of the substrate steel.3.5 A problem with steel chemistry is not usually apparent until after an item has been galvanized. Not all combinations ofsilicon, phosphorus, carbon, and manganese can be galvanized successfully. When the steel chemistry is known beforehand,experienced galvanize
19、rs can in some, but not all, instances exercise limited control over the coatings as shown in Fig. 2. Also, thecombination of two different steel types or thicknesses in one item may result in a nonuniform galvanizing finish. The experienceof the steel supplier, designer, manufacturer, and galvanize
20、r should determine the steel selection.3.6 In general, galvanized coatings are specified because of their corrosion resistance, not because of their appearance. Therelative corrosion resistance of the normal and abnormal coatings is, for all practical purposes, equal.FIG. 1 Photomicrograph of Normal
21、 Galvanized Coating (X 400)FIG. 2 Photomicrograph of Dull Gray, Thick-Galvanized Coating (X 200)A385/A385M 1723.7 Steels with very low levels of silicon (less than 0.02%) and aluminum-killed steels regularly present a challenge indeveloping a galvanized coating that meets the thickness requirements
22、of Specifications A123/A123M or A153/A153M.Phosphorus (less than 0.020%) can also exhibit low coating thicknesses. For these steels, it may be difficult to meet the coatingthickness requirements of Specifications A123/A123M or A153/A153M. In these cases the galvanizer and the purchaser shouldagree o
23、n a plan of action. Some choices are to accept the lower coating thickness, apply a paint coating over the galvanized coating(Duplex System), blast clean the steel before hot-dip galvanizing to increase the coating thickness, over-pickle the steel in sulfuricacid to roughen the surface and increase
24、the coating thickness, or other possible solutions.4. Assemblies of Different Materials or Different Surfaces or Both4.1 Whenever possible, assemblies should consist of elements of similar steel chemistry and surface condition.4.2 Whenever different analyses of steel or different surfaces of steel a
25、re united in an assembly the galvanized finish is notgenerally uniform in appearance. These differences include: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).4.2.5 Cast steel.4.2.6 Malleable iron.4.2.7 Hot-rolled stee
26、l.4.2.8 Cold-rolled steel.4.2.9 Steel containing chemical elements in excess of those recommended in 3.2.4.3 Where combinations are unavoidable, thorough abrasive blasting of the entire assembly will normally improve galvanizingquality.5. Overlapping or Contacting Surfaces5.1 Overlapping or contacti
27、ng surfaces that have not had all edges seal welded are undesirable.5.2 When the distance between the overlapping surfaces is less than 332 in. 2.38 mm, these surfaces will not normally be wetby molten zinc. Furthermore, cleaning solution compounds that remain on these surfaces volatilize during the
28、 galvanizing processand may interfere with zinc wetting in adjacent areas. Such uncoated surfaces cause a rust staining after exposure to theenvironment. Traditionally however, steel grating has been manufactured without seal welding and when properly executed, thismanufacturing means has permitted
29、the galvanized coating to satisfy the quality requirements of the applicable ASTMspecifications.5.3 When the overlap surface area is large and the edges have been seal welded, air or moisture or both entrapped therein candevelop destructive pressures when the assembly is heated to the galvanizing te
30、mperature, which is nominally 850F 454C. Ventholes or unwelded area around the adjoining surfaces should be provided through one or both sides into the lapped area inaccordance with the following tables.6. Sheet Steel Rolled Over a Wire or Rod Stiffener6.1 All oil or grease should be removed from bo
31、th the sheet steel and wire or rod before rolling (see Fig. 3). Grease or oilbecomes volatile at the galvanizing temperature and will generate gas which will prevent zinc from sealing the contact edges. Allsteel should be degreased before pickling and in the case of folded assemblies, before folding
32、 and assembling (see Fig. 4).7. Weld Flux Removal and Welding Rods7.1 Welding flux residues are chemically inert in normal pickling solutions. Thus, they will not be removed by standardgalvanizing cleaning techniques and are best removed at the time of fabrication by grit or sand-blasting or by a wi
33、re needle gun.7.2 It is desirable to choose a welding rod with a chemical composition as close as possible to the parent metal.7.3 Welding rods high in silicon may cause excessively thick or darkened coatings or both to form in the welded area.TABLE 1 Vent Holes for Overlapped Areas for Steels 12 in
34、. 12.75mm or Less in ThicknessOverlapped Area in.2 cm 2 Vent Holes Unwelded Areaunder 16 103 None None16 103 to under 64 413 One 38 in. 1 cm 1 in. 2.5 cm64 413 to under 400 2580 One 12 in. 1.25 cm 2 in. 5.1 cm400 2580 and greater,each 400 2580One 34 in. 1.91 cm 4 in. 10.2 cmA385/A385M 1738. Flame Cu
35、t Cope Edges Preparation8.1 Flame cut copes on beams can be extremely sensitive to residual stresses in the steel beam and, with the rough surface fromthe flame cutting operation, can be sources of cracking during the thermal cycling of the hot-dip galvanizing process. The steelbeams start near ambi
36、ent temperature then are immersed in the molten zinc and heated to above 800F for usually 5 to 10 minutes.The steel beams are then cooled back to ambient temperature so the thermal cycling can create thermal stresses in the area of thecope.8.2 One method that has had fairly good success at minimizin
37、g the cracking at the edges of the flame cut cope is to weld a beadalong the sides of the cope in the area where the flame cutting was done before hot-dip galvanizing. This welding operation willreheat the area and may relieve some of the residual stress near the cope edges. The weld bead will not e
38、liminate all incidents ofcracking but will greatly reduce the likelihood of cracking.8.3 The weld rod material should be chosen as described in Section 7.9. Cold Forming Before Galvanizing9.1 Refer to the latest revision of Practice A143/A143M.10. Shearing, Cutting and Punching Before Galvanizing10.
39、1 Refer to the latest revision of Practice A143/A143M.11. Warpage and Distortion11.1 Refer to the latest revision of Practice A384/A384M.12. Design Recommendations for Providing for the Free Flow of Cleaning Solutions, Fluxes, Air, and Zinc12.1 All fabricated assemblies shall be so designed with ven
40、t and drain holes such that no air is trapped during the immersionof the assemblies into cleaning solutions or molten zinc. Similarly these holes shall allow all solutions and molten zinc to drainfreely from the assemblies. Failure to follow this practice will result in areas that will not galvanize
41、 properly, or that may retainentrapped flux or excessive amounts of zinc.TABLE 2 Vent Holes for Overlapped Areas for Steels Greater than12 in. 12.75 mm in ThicknessOverlapped Area in.2 cm 2 Vent Holes Unwelded Areaunder 16 103 None None16 103 to under 64 413 None None64 413 to under 400 2580 One 12
42、in. 1.25 cm 2 in. 5.1 cm400 2580 and greater,each 400 2580One 34 in. 1.91 cm 4 in. 10.2 cmFIG. 3 Rolled SurfacesFIG. 4 Folded SurfacesA385/A385M 17412.2 Free flow of cleaning solutions and molten zinc shall also be provided for in assemblies of hot-rolled shapes. This isaccomplished by cropping the
43、corner to provide an opening with a minimum area of 0.3 in.2 1.9 cm2 at the corners of allstiffeners (see Fig. 5), gussets, or bracing (see Fig. 6).12.3 Air or moisture, or both, entrapped within closed fabricated pipework, such as handrail, can develop destructive pressureswhen heated to the galvan
44、izing temperature. Pipe handrail shall preferably be vented full open internally, as shown in Fig. 7. Inaddition, there shall be one 38-in. 9.5-mm minimum diameter external hole at each intersection to prevent any possible explosionsin the event that the fabricator neglects to provide internal venti
45、ng. This hole shall be located as close as possible to the weld beadjoining the two steel pieces and the edge of the hole shall be not more than 0.5 in. 12 mm from the edge of the weld bead. Whereinternal venting is not possible, external vents shall be provided with one vent hole in each side of ea
46、ch intersection. The ventopenings shall be a minimum of 38 in. 1 cm in diameter or 25 % of the diameter of the pipe that is used, whichever is larger (seeFig. 8) and shall be located as described above.12.4 Figs. 9-12 show most of the conditions encountered with tubular product assemblies. The venti
47、ng shall open whereverpossible. This is the most desirable situation.Aminimum vent opening of 25 to 30 % of the cross-sectional area of tubular structureshall be specified where full-open venting is not possible. For small cross sections, larger percent vent openings are recommended.See attached dra
48、wings for specific recommendations. In box sections (see Fig. 9) where gusset plates are used, the gusset platesshall be clipped at the four corners. In addition, a center hole shall be provided so that the cumulative area of the vent holes meetsthe recommended minimum. Gusset plates shall not be sp
49、aced closer than 36 in. 914 mm apart. In the case of columns with endplates (see Fig. 10) where the end plate must be closed, the shaft of the column shall be vented. The vent opening shall be a halfcircle with its diameter at the base plate (D,Fig. 10). This is much superior to putting a hole with the circumference of the holetangential to the base plate. On trusses (see Fig. 11 and Fig. 12) where tubular members intersect, vent holes are recommendedat both sides of the intersection.13. Moving Parts1
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