Introduction

HOT DIP GALVANIZING is a process in which an adherent, protective coating of zinc and zinc/iron compounds is developed on the surfaces of iron and steel products by immersing them in a bath of molten zinc. The protective coating usually consists of several layers (Fig. 1). Those closest to the basis metal are composed of iron-zinc compounds; these, in turn, may be covered by an outer layer consisting almost entirely of zinc.

Fig. 1 Photomicrograph of typical hot dip galvanized coating. The molten zinc is interlocked into the steel by the alloy reaction, which forms zinc-iron layers and creates a metallurgical bond. See Table 3 for properties of alloy layers. 250x

The complex structure of layers that comprise a galvanized coating varies greatly in chemical composition and physical and mechanical properties, being affected by chemical activity, diffusion, and subsequent cooling. Small differences in coating composition, bath temperature, time of immersion, and rate of cooling or subsequent reheating can result in significant changes in the appearance and properties of the coating.

Hot dip galvanized coatings are produced on a variety of steel mill products, using fully mechanized, mass production methods. This article, however, is concerned primarily with the hot dip galvanizing of fabricated articles in manual or semiautomatic batch operations. For information about continuous coatings, see the article "Continuous Hot Dip Coatings" in this Volume.

ASTM and other standards related to galvanized coatings are given in Table 1.

Table 1 Standards relating to hot dip galvanized materials

ASTM

A 123

Standard Specification for Zinc (Hot Dip Galvanized) Coatings on Iron and Steel Products

A 143

Standard Practice for Safeguarding Against Embrittlement of Hot Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement

A 153

Standard Specification for Zinc Coating (Hot Dip) on Iron and Steel Hardware

A 384

Standard Recommended Practice for Safeguarding Against Warpage and Distortion During Hot Dip Galvanizing of Steel Assemblies

A 385

Standard Practice for Providing High Quality Zinc Coatings (Hot Dip)

A 767

Standard Specifications for Zinc Coated (Galvanized) Steel Bars for Concrete Reinforcement

A 780

Standard Practice for Repair of Damaged Hot Dip Galvanized Coatings

E 376

Standard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy Current (Electromagnetic) Test Methods

AASHTO

M111

Standard Specification for Zinc (Hot Dip Galvanized) Coatings on Iron and Steel Products

M232

Standard Specification for Zinc Coating (Hot Dip) on Iron and Steel Hardware

CSA

G164-M

Hot Dip Galvanizing of Irregularly Shaped Articles

Source: Ref 1

Source: Ref 1

0 0

Post a comment