ASTM A153/A153M Hot Dip Zinc Galvanised Coating on Fasteners
A153/A153M specification covers zinc coatings applied by the hot-dip process on iron and steel fasteners. The hot-dip galvanizing process consists of parts being immersed in molten
zinc for a sufficient time to allow a metallurgical reaction between iron from the steel surface and the molten zinc, resulting in the formation of Zn/Fe alloy layers bonding the coating to the steel surface. A153/A153M specification is
intended to be applicable to fasteners items that are centrifuged or otherwise handled to remove excess galvanizing bath metal (free zinc). Coating thickness grade requirements reflect this. A153/A153M specification is applicable to orders
in either inch-pound units (as A153) or in SI units (as A153M).
- Steel or Iron
- Ferrous articles to be hot-dip zinc coated shall conform to specifications designated by the purchaser.
- The zinc used for the coating shall conform to Specification B6, or Specification B960, or both, and shall be at least equal to the grade designated as “Prime Western.” If a zinc alloy is used as the primary feed for the galvanizing bath, then the
base material used to make that alloy shall conform to Specification B6 or Specification B960, or both. The molten metal in the working volume of the galvanizing bath shall contain not less than an average value of 98.0 % zinc by weight [mass].
NOTE - The galvanizer may choose to add trace amounts of certain elements (for example, aluminum, nickel, bismuth, or tin) to the zinc bath to help in the processing of certain reactive steels or to enhance the cosmetic appearance of the finished
product. The elements can be added to the galvanizing bath as a master feed alloy, or they can be added to the bath by the galvanizer as individual feeds.
- Threaded Articles
- The zinc coating on threads shall not be subjected to a cutting, rolling, or finishing-tool operation, unless specifically authorized by the purchaser. In order to meet overtapping allowances, tapping the threads of nuts or tapped holes after galvanizing
is not prohibited.
- Touch-up and Repair
- Bare spots that are found on parts after galvanizing shall be renovated by use of the methods found in Practice A780 if the following criteria are met. The bare spots shall have an area totalling not more than 1 % of the surface area to be coated
excluding threaded areas of the piece and the bare spots shall not include any threaded areas of the piece. The thickness of the repair shall be equal to the surrounding galvanized coating except for repairs made by paints containing zinc dust in
which case the thickness of the repair shall be 50 % greater than the thickness of the galvanized coating required for the class of material, but shall not be greater than 4.0 mils [100 µm]. Repair thickness measurements shall be made in accordance
with Practice A780. The galvanizer shall make repairs unless directed by the purchaser to deliver items unrepaired for subsequent renovation by the purchaser.
- Minimum Coating Weight [Mass] or Minimum Coating Thickness
- The minimum coating weight [mass] or the minimum coating thickness shall conform to the requirements prescribed in in table below for the material category and thickness of material in which the article belongs.
|Class of Material
||Weight [Mass] of Zinc Coating, oz/ft2 [g/m2 ] of Surface, Minimum
||Coating Thickness, mils [microns], Minimum
|Average of Specimens Tested
||Any Individual Specimen
||Average of Specimens Tested
||Any Individual Specimen
|Class A-Castings-Malleable Iron, Steel Class B-Rolled, pressed, and forged articles (except those which would be included under Classes C and D):
|B-1-3⁄16 in. [4.76 mm] and over in thickness and over 15 in. [381 mm] in length
|B-2-under 3⁄16 in. [4.76 mm] in thickness and over 15 in. [381 mm] in length
|B-3-any thickness and 15 in. [381 mm] and under in length
|Class C-Fasteners over 3⁄8 in. [9.52 mm] in diameter and similar articles. Washers 3⁄16 in. and 1⁄4 in. [4.76 and 6.35 mm] in thickness
|Class D-Fasteners 3⁄8 in. [9.52 mm] and under in diameter, rivets, nails and similar articles. Washers under 3⁄16 in. [4.76 mm] in thickness
- NOTE - Length of the piece, stated in Classes B-1, B-2, and B-3, refers to the finished dimension of the piece after fabrication.
Tests shall be made to ensure that the zinc coating is being furnished in accordance with this specification and as specified for the following :
- Minimum coating weight [mass] or minimum coating thickness
- The minimum coating weight [mass] or the minimum coating thickness shall conform to the requirements prescribed in above Table for the material category and thickness of material in which the article belongs.
- Finish and appearance
- The zinc-coated articles shall be free from uncoated areas, blisters, flux deposits, dross inclusions, and other types of projections that would interfere with the intended use of the articles, or other defects not consistent with good galvanizing
practice. The zinc coating shall be smooth and reasonably uniform in thickness.
NOTE - Smoothness of surface is a relative term. Minor roughness that does not interfere with the intended use of the part, or roughness that is related to the as-received
(ungalvanized) surface condition of the part, shall not be grounds for rejection.
NOTE - Since this specification is applicable to items that are centrifuged or otherwise handled to remove excess bath metal, irregular coating distribution is
not normally encountered. Drainage problems, which manifest themselves as local excess coating thickness that would interfere with function or as edge tears or spikes that present a safety hazard because of their sharpness, are grounds for rejection
- Embrittlement is a potential condition of steel that is cold-worked, depending on such factors as the steel type (strength level, aging characteristics), thickness, degree of cold work, and galvanizing process. The galvanizer, the designer and the
fabricator shall take precautions against embrittlement. The precautions to fabricate properly and prepare the material for galvanizing to prevent embrittlement are described in Practice A143/A143M.
NOTE - Low service temperatures increase the
risk of brittle failure of all plain carbon steels including those which have been galvanized. This temperature embrittling effect varies with type of steel. The expected service temperature should thus be taken into account when selecting steels
Malleable castings shall be of such composition as will preclude the possibility that they become embrittled by the galvanizing process, or they shall be either cooled from the anneal, or subsequently heat-treated so as to immunize
them against embrittlement.
- Average Weight [Mass] of Coating
- The average weight [mass] of the zinc coating shall be determined by weighing specimens after pickling and drying and again after galvanizing unless the method described in 7.2.2 is used. The number of specimens that are used to determine the average
of an inspection lot shall be derived from below Table.
NOTE - This method does not take into account the weight [mass] of iron reacted from the article that is incorporated into the coating. It will thus underestimate coating weight [mass]
by up to approximately 10 %. Base metal reactivity will affect the extent of underestimation. In the case of materials inspected after galvanizing, the average weight [mass] of coating shall be determined by stripping the number of specimens derived
in below Table in accordance with Test Method A90/A90M, and averaging the results of the individual specimens, unless the method described in Average Weight [Mass] of Coating is used.
- Average Thickness of Coating
- In the case of fasteners such as bolts, nuts, and screws, the determination of the thickness of coating shall be made on a portion of the article that does not include any threads. The average thickness of coating shall be determined by magnetic
thickness gage in accordance with Practice E376 unless the method described in 7.3.3 is used. The thickness shall be measured on at least five widely separated spots on a specimen. No individual spot measurement shall be cause for rejection. If
an individual spot does not provide a coating thickness reading, this spot must be repaired in accordance with 4.5. The five or more individual coating thickness measurements on a specimen must be averaged to determine the specimen average coating
thickness. The average coating thickness for the inspection lot is determined by averaging the specimen average coating thickness values for the number of specimens derived from Table.
The thickness of coating shall be determined by cross section
and optical measurement in accordance with Test Method B487, unless the method described is used. The thickness thus determined is a point value. No less than five such measurements shall be made at locations on the specimen, which are as widely
dispersed as practical, so as to be representative of the whole surface of the specimen. The average of no less than five such measurements is the specimen average coating thickness. The average coating thickness for the inspection lot is determined
by averaging the specimen average coating thickness values for the number of specimens derived from Table.
- Determine adherence of the zinc coating to the surface of the base metal by cutting or prying with the point of a stout knife, applied with considerable pressure in a manner tending to remove a portion of the coating. The adherence shall be considered
inadequate if the coating delaminates in the form of a layer of skin so as to expose the base metal in advance of the knife point. Do not use testing carried out at edges or corners (points of lowest coating adherence) to determine adherence of
coating. Likewise, do not use removal of small particles of the coating by paring or whittling to determine failure.
|Number of Pieces in Lot
|3 or less
|4 to 500
|501 to 1200
|1201 to 3200
|3201 to 10 000
|10 001 and over