Practical Guidelines for the Inspection and Repair of Hot Dip Galvanized Coatings

Information courtesy of: HOT DIP GALVANIZERS ASSOCIATION SOUTHERN AFRICA

Introduction

Hot dip galvanizing is one of the most widely used methods of protecting steel from corrosion.

As a final step in the process, the hot dip galvanized coating is inspected for compliance with specifications. Interpretation of inspection results should be made with a clear understanding of the causes of the various conditions which may be encountered and their effects on the ultimate objective of providing corrosion protection.

This manual has been designed to assist in testing, inspection, and interpretation of test results. It deals with numerous surface conditions, their origins, and their effects on protection from corrosion. The manual also discusses improper conditions before and during hot dip galvanizing and their effects on corrosion protection. Although it is difficult to cover every condition, the manual covers many of the possible conditions, which might be encountered in practice.

To effectively use this manual, inspectors should remember that the purpose of hot dip galvanizing is to protect steel from corrosion. The length of time this protection can reasonably be expected to last is called its "service life or time to first maintenance". This is defined as the time taken for the appearance on an article of 5% surface rust. 

The service life of a hot dip galvanized coating is directly related to the thickness of the protective zinc coating. Corrosion protection is greatest when the coating is thickest. Thus coating thickness is the single most important inspection check to determine the quality of a hot dip galvanized coating.

Coating thickness, however, is only one inspection aspect. Other checks can include the uniformity of the coating, adherence and appearance.

Embrittlement and defects, which arise from an item's design and fabrication, also need to be included in an inspection regime.

While minimum standards must be satisfied in all these considerations, their relative importance varies according to the end use of the finished product. For example, the end use requirement for hot dip galvanized structural steel in a non-visible isolated area is different from that where a product is used in a decorative application. An understanding of the individual requirements of the product concerned and the capability of the hot dip galvanizing process is essential to good inspection.

Inspection of the hot dip galvanized product, as the final step in the process, can be most effectively and efficiently conducted at the galvanizer's plant. Here, questions can be raised and answered quickly, inspection speeded up and, the time saved benefits the overall project.

TABLE 1.

Occasionally, due to rough handling and possible site modifications, which include cutting and welding, coating repair becomes necessary. The typical parameters and suggested materials for proper repair are also included in this manual.

The standards to which reference is made in the following sections are to International and South African Bureau of Standards' specifications.