ASTM A153A153M Standards Guide Hotdip Galvanizing for Fasteners

From bridges and buildings to backyard swings, the steel skeletons of these structures endure relentless exposure to wind and rain yet remain steadfast. The secret behind their durability lies in hot-dip galvanizing technology, with the ASTM A153/A153M standard serving as the cornerstone for this critical anti-corrosion process. This standard guarantees reliable zinc coating protection for everything from small bolts and nuts to large cast iron components.

ASTM A153/A153M is a specification published by ASTM International that governs the hot-dip galvanizing of steel products. The standard meticulously outlines requirements for applying zinc coatings through the hot-dip galvanizing process on various hardware products, including but not limited to castings, fasteners, rolled products, stamped parts, forged items, and threaded products that require centrifugal spinning or other treatments to remove excess zinc. The primary objective is to ensure these products exhibit exceptional corrosion resistance in diverse environments.

Notably, the galvanizing process under ASTM A153 closely follows the procedures specified in ASTM A123. The key distinction is that steel hardware galvanized per ASTM A153 undergoes centrifugal spinning or draining to eliminate excess zinc, ensuring surface smoothness and thread precision.

The standard establishes clear benchmarks for several critical performance indicators of hot-dip galvanized products:

Coating Thickness/Weight: This fundamental measure of corrosion protection varies based on material category and steel thickness. Specific values are detailed in Table 3 below.

Threaded Products: While threaded areas may not meet standard coating thickness requirements, they must maintain adequate corrosion protection.

Surface Quality: Zinc coatings must be continuous, smooth, and uniform, free from uncoated areas, bubbles, flux residues, rough zinc slag inclusions, or excessive zinc buildup that could impair functionality.

Embrittlement Resistance: Special precautions against hydrogen embrittlement are required for high-strength fasteners (tensile strength >150 ksi) and castings.

Adhesion: The zinc coating must bond firmly to the base metal and maintain this adhesion throughout the product's service life.

Certain manufacturing steps may compromise the corrosion resistance of hot-dip galvanized coatings. However, coating damage or peeling resulting from these processes doesn't automatically warrant rejection. Selecting appropriate steel remains paramount for achieving high-quality coatings and surface finishes.

For threaded products, galvanizing typically occurs after manufacturing completion. Subsequent processing may affect the coating's protective properties. An exception exists for nuts: their internal threads should be tapped post-galvanizing to accommodate the increased thread dimensions from the zinc coating on mating bolts. In this configuration, the zinc coating on bolt threads provides corrosion protection for the uncoated internal threads of nuts.

Some manufacturing techniques can introduce stresses that lead to brittle fracture. ASTM A143/A143M provides guidelines for embrittlement prevention. Additionally, selecting steel with appropriate chemical composition helps prevent brittleness in malleable cast iron.