GALVANIZED COATING THICKNESS REQUIREMENT ASTM A123

ASTM A123/A123M covers individual steel pieces as well as assemblies of various classes of material. The six material categories covered in ASTM A123/A123M include structural shapes, plate, strip and bar, pipe and tubing, wire, and reinforcing bar. A fabrication can have more than one material category such as a frame assembly. Any combination of these products can be assembled into a single fabrication and then can be hot-dip galvanized

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It is the responsibility of the designer and fabricator to ensure the product has been properly designed and built before the hot-dip galvanizing process. The galvanizer should be made aware of any necessary special instructions or requests in advance of shipping the materials to the galvanizing plant. These requests should be stated on the purchase order for the hot-dip galvanizing.

It is the responsibility of the galvanizer to ensure compliance with the specifications as long as the product has been designed and fabricated in accordance with the referenced specifications. However, if the galvanizer has to perform additional work in order to prepare the product for hot-dip galvanizing, such as drilling holes to facilitate drainage or venting, it must be approved by the customer. Once the material has been hot-dip galvanized, it can be fully inspected at the galvanizing plant prior to shipment.

Any materials rejected by the inspectors for reasons other than embrittlement may be stripped, regalvanized, and resubmitted for inspection. The ASTM specifications A143/A143M, ASTM A384/A384M, and ASTM A385 provide guidelines for preparing products for hot-dip galvanizing. The requirements listed in ASTM A123/A123M include coating thickness, finish, appearance, and adherence. These are each defined below.

ASTM A123/A123M Requirements

  • Coating Thickness/Weight – dependent upon material category and steel thickness
  • Finish – continuous, smooth, uniform
  • Appearance – free from uncoated areas, blisters, flux deposits and gross dross inclusions as well as having no heavy zinc deposits that interfere with intended use
  • Adherence – the entire coating should have a strong adherence throughout the service life of galvanized steel

The hot-dip galvanized coating is intended for products fabricated into their final shape that will be exposed to corrosive environmental conditions. Once a product has been hot-dip galvanized, any further fabrication, which very rarely occurs, may have negative effects on the corrosion protection of the coating. All coating thickness requirements in specification ASTM A123/A123M, as seen in the tables below, are minimums; there are no maximum coating thickness requirements in either specification. See “Coating Thickness – Sampling Procedures” in this app for more details.

Structural Shapes

Steel Thickness inches Coating Minimum mils (Grade)
<1/16 1.8 (45)
≥1/16 to <1/8 2.6 (65)
≥1/8 to <3/16 3.0 (75)
≥3/16 to <1/4 3.0 (75)
≥1/4 to <5/8 3.9 (100)
≥5/8 3.9 (100)

Plate

Steel Thickness inches Coating Thickness mils (Grade)
<1/16 1.8 (45)
≥1/16 to <1/8 2.6 (65)
≥1/8 to <3/16 3.0 (75)
≥3/16 to <1/4 3.0 (75)
≥1/4 to <5/8 3.0 (75)
≥5/8 3.9 (100)

Strip & Bar

Steel Thickness inches Coating Minimum mils (Grade)
<1/16 1.8 (45)
≥1/16 to <1/8 2.6 (65)
≥1/8 to <3/16 3.0 (75)
≥3/16 to <1/4 3.0 (75)
≥1/4 to <5/8 3.0 (75)
≥5/8 3.9 (100)

Pipe & Tubing

Steel Thickness inches Coating Minimum mils (Grade)
<1/16 1.8 (45)
 ≥1/16 to <1/8 1.8 (45)
≥1/8 to <3/16 3.0 (75)
≥3/16 to <1/4 3.0 (75)
≥1/4 to <5/8 3.0 (75)
≥5/8 3.0 (75)

Wire

Steel Thickness inches Coating Minimum mils (Grade)
<1/16 1.4 (35)
≥ 1/16 to <1/8 2.0 (50)
≥1/8 to <3/16 2.4 (60)
≥3/16 to <1/4 2.6 (65)
≥1/4 to <5/8 3.1 (80)
≥5/8 3.1 (80)

Reinforcing Bar

Steel Thickness inches Coating Minimum mils (Grade)
≥1/4 to <5/8 3.9 (100)
≥5/8 3.9 (100)

The time to first maintenance of hot-dip galvanized steel is directly proportional to the thickness of the hot-dip galvanized coating. With all other variables held constant, the thicker the zinc coating, the longer the life of the steel. The aim of the finish and appearance requirements is to ensure no coatings have problem areas that are deficient of zinc or have surface defects that would interfere with the intended use of the product. In addition, the coating should have a strong adherence throughout the service of the hot-dip galvanized steel.

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