Hot Dipped Galvanized Coating

What is Hot Dipped Galvanizing (HDG)?

When it comes to hot dipped galvanized steel pipe in the current market, we have to give a talk about hot dipped galvanizing. It is the process of dipping fabricated steel into a kettle or vat containing molten zinc. The process is inherently simple which provides a distinct advantage over other corrosion protection methods. While the steel is in the kettle, the iron in the steel metallurgically reacts with the molten zinc to form a tightly-bonded alloy coating that provides superior corrosion protection to steel pipe in service.

Technically, the hot-dip galvanizing (HDG) process consists of three basic steps:

￭Surface Preparation

￭Galvanizing

￭Inspection

Surface Preparation

Surface preparation is a critical step in the application of any erw pipe coating.  In most cases, where a coating fails before the end of its expected service life, it is because of incorrect or inadequate surface preparation.  The galvanizing process has its own built-in means of quality control because zinc will not react with an unclean steel surface. Any failures or inadequacies in surface preparation will be immediately apparent when the steel is withdrawn from the zinc bath because the unclean areas will remain uncoated, and immediate corrective action can be taken.

Galvanizing

Galvanizing is the major procedure of hot dipped galvanizing. During the true galvanizing step of the process, the material is completely immersed into a bath of molten zinc.  The bath chemistry is specified by ASTM B6, and requires at least 98% pure zinc maintained at  815-850 F (435-455 C). While immersed in the kettle, the zinc reacts with the iron in the steel to forma series of metallurgically bonded zinc-iron intermetallic alloy layers, commonly topped by a layer of impact-resistant pure zinc. And the metallurgical reaction will continue after the materials are withdrawn from the bath, as long as it remains near bath temperature. Galvanized articles are cooled either by immersion in a passivation solution or water or by being left in the open air.

Inspection

The inspection of hot-dip galvanized steel is simple and quick. The two properties of the hot-dip galvanized coating most closely scrutinized are coating thickness and appearance/surface condition. A variety of simple physical tests can be performed to determine thickness, uniformity, adherence, and appearance.

Sean Brecht: Inspection

Products are galvanized according to long established, accepted, and approved standards of ASTM, the International Standards Organization (ISO), the Canadian Standards Association (CSA), and the American Association of State Highway and Transportation Officials (AASHTO).  These standards cover everything from the minimum coating thicknesses required for various categories of galvanized items to the composition of the zinc metal used in the process.