Pacific Galvanizing’s Guide to Specifying Hot Dip Galvanizing for Structural Steel

Specifying corrosion protection for structural steel is a technical exercise that involves navigating multiple standards, understanding material-coating interactions, and making appropriate choices for the specific environmental conditions and performance requirements of each project. For engineers and specifiers who work with hot dip galvanizing regularly, many of these decisions are second nature. For those who encounter galvanizing less frequently, having a clear guide to the specification process is genuinely useful. Pacific Galvanizing works with specifiers across Northern California on a regular basis and has developed this guide to help navigate the key specification decisions.

Selecting the Right ASTM Standard for Your Application

The first specification decision is which ASTM standard applies to your application. ASTM A123 governs galvanizing on structural steel shapes, plates, and fabricated products — the most common category for structural applications. ASTM A153 governs galvanizing on iron and steel hardware — fasteners, nuts, bolts, and similar items. ASTM A767 governs galvanizing on steel bars used as concrete reinforcement. Each standard has different coating thickness requirements that reflect the different service conditions and handling exposures typical for each product category. Using the wrong standard — for example, specifying A153 for structural steel — can result in a product that is under-specified for its application. Hot Dip Galvanizing specifications should always reference the specific ASTM standard appropriate for the product type being galvanized.

Understanding Coating Thickness Requirements

ASTM A123 specifies minimum coating thickness as a function of the steel category (structural shapes, strip, plate, etc.) and steel thickness, expressed in ounces per square foot or mils (thousandths of an inch). Thicker steel sections require thicker zinc coatings because the steel cools more slowly during galvanizing, allowing more time for the zinc-iron reaction to proceed. The coating thickness specifications in ASTM A123 reflect decades of field performance data establishing the relationship between coating thickness and service life in atmospheric exposure. Specifiers should be familiar with these thickness requirements for the steel categories used in their projects and should verify that the galvanizer is measuring and certifying to these requirements.

Design Details That Affect Galvanizing Quality

Structural steel design has significant implications for galvanizing quality that engineers and detailers should understand. Hollow structural sections and closed assemblies must have vent holes of sufficient size and location to allow air to escape during immersion and zinc to drain during withdrawal. Without proper venting, trapped air creates bare spots in the coating and trapped zinc creates dangerous pressure during withdrawal. Overlapping surfaces in connections should have gaps of at least 3/32 inch to allow acid cleaning solution to reach the interface during pickling — zero-gap lapped connections will have ungalvanized interfaces that are vulnerable to crevice corrosion. ASTM A385 provides specific guidance on design practices for galvanizing and should be referenced in project specifications.

Surface Finish Acceptance Criteria

ASTM A123 includes provisions for surface finish acceptance that specify what visual conditions are acceptable and unacceptable on galvanized structural steel. Bare spots, spangle appearance, coating roughness, and various types of surface inclusions are addressed. For most structural applications, the primary quality concern is coating thickness and bare spot freedom, with appearance being secondary. For architecturally exposed applications, additional surface finish requirements beyond ASTM minimums may be appropriate — Pacific Galvanizing can advise on what finish quality levels are achievable for different preparation methods and can provide samples showing typical finish quality for reference.

Inspection and Documentation Requirements

Project specifications for galvanizing should include clear inspection and documentation requirements. These typically include the requirement for a certificate of conformance to the applicable ASTM standard, coating thickness measurement records, and the name and location of the galvanizing facility. For public works and major infrastructure projects, additional documentation requirements may apply — including facility qualification criteria, inspector qualifications, and traceability documentation linking specific pieces to specific galvanizing records. Pacific Galvanizing’s quality management system produces all standard documentation requirements as a matter of course, and can accommodate additional requirements for specific projects with advance notice.

Common Specification Mistakes to Avoid

Pacific Galvanizing’s team regularly encounters structural steel galvanizing specifications that contain errors, ambiguities, or requirements that are technically incorrect or impractical. Common issues include referencing the wrong ASTM standard for the product type, specifying coating thicknesses that are not achievable or not reflective of actual ASTM requirements, failing to reference ASTM A385 for design requirements, omitting documentation requirements, and failing to address repair procedures for damaged coatings. Submitting draft specifications for review by Pacific Galvanizing before finalizing them is a simple way to catch these common errors before they create procurement disputes or inspection problems during construction.