Our precast plant recently received a shipment of galvanized rebar for a state project. While examining the rebar, our inspector noticed that some bars had a gray coating, a king of mottled appearance. Is the presence of this coating a possible cause for rejection?
According to ASTM A 767 and ASTM A 123, respective specifications requirements for steel bars and assemblies, the inspector should visually inspect bars for process imperfections like bare spots, blisters, flux spots or undercoated areas. These specifications also state that a matte-gray appearance is not a cause for rejection. When galvanizers hot-dip steel bars into the molten zinc bath, the chemical reaction creates a series of zinc-alloy layers that protect the steel from corrosion. The intermetallic, or innermost, layer is about 25% iron. The additional layers' iron-level percentages progressively decrease toward the outer layer, which is pure zinc. The gray coating, whether localized or spread across the entire bar's length may have just been caused by galvanizing host steel with high levels of silicon. By itself, presence of the coating is not cause for rejection. Since coating thickness determines service life, inspectors can use magnetic measuring devices to verify the product's compliance with ASTM A 123 specifications. To help producers with these types of questions, the American Galvanizers Association recently published the free color brochure, "Rebar: A Processing and Inspection Guide for Quality Hot Dip Galvanized Reinforcing Steel." To receive a copy, phone the association at 1-800-HOT-Spec, or e-mail your request to