Surface preparation is a critical step in the application of any coating. In most instances
where a coating fails before the end of its expected service life, it is because of incorrect or
inadequate surface preparation.
The surface preparation step in the galvanizing process has its own built-in means of
quality control because zinc simply will not react with unclean steel. 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withdrawn from the zinc bath because the unclean areas will remain uncoated, and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.
Degreasing - A hot alkali solution, mild acidic bath, or biological cleaning bath removes organic contaminants such as dirt, paint markings, grease, and oil from the metal surface. Epoxies, vinyls, asphalt, or welding slag, which cannot be removed by degreasing, must be removed before galvanizing by grit-blasting, sand-blasting, or other mechanical means.
Pickling - A dilute solution of heated sulfuric acid or ambient hydrochloric acid removes mill scale and iron oxides (rust) from the steel surface. As an alternative to or in conjunction with pickling, this step can also be accomplished using abrasive cleaning or air blasting sand, metallic shot, or grit onto the steel.
Fluxing - The final surface preparation step, a zinc ammonium chloride solution, serves two purposes. It removes any remaining oxides and deposits a protective layer on the steel to prevent any further oxides from forming on the surface prior to immersion in the molten zinc.
Galvanizing is when the steel is immersed in a bath of molten zinc
The “galvanizing” step of the process occurs when steel is completely immersed in a bath (kettle) of molten zinc.
According to specification, the bath chemistry must be at least 98% pure zinc and maintained at a temperature of
approximately 840 F (449 C).
While immersed in the kettle, the zinc reacts with iron in the steel to form a series of zinc-iron intermetallic alloy
layers. Once the fabricated item reaches bath temperature the coating growth is complete, and the articles are withdrawn
slowly from the galvanizing bath. Excess zinc is removed by draining, vibrating, and/or centrifuging. The metallurgical
reaction will continue after withdrawal from the bath, as long as the article remains near bath temperature. Articles
are cooled either by immersion in a passivation solution or water or by being left in the open air.
The last phase of the process, inspection, is simple and quick. The two properties of the hot-dip galvanized coating
closely scrutinized are coating thickness and coating appearance. There are additional tests outlined for adherence, but
these are typically only administered as a “referee” test, or when an issue is suspected.
A very accurate determination as to the quality of the galvanized coating can be accomplished through a visual
inspection of the material, because as stated earlier zinc will not react with unclean steel. A variety of simple
physical and laboratory tests is also be performed to determine thickness, uniformity, adherence, and appearance to
ensure the coating complies with specification requirements. Products are galvanized according to long-established,
accepted, and approved standards of ASTM.