The hot dip galvanizing process was developed in the 18th century when it was observed that the reaction of molten zinc with steel results in a metallurgical bonded protective coating having superior durability against any other coating for atmospheric exposure. The 1st hot dip galvanized product, galvanized iron sheeting, were imported to Australia in the 1850s.
During early days, hot dipped galvanized items were manufactured by dipping them in batches in a molten zinc bath. Furtherance of technology has led to different engineered ways being developed to more efficiently applying zinc-based coatings to continuously manufactured products such as wires, sheets, tubes, pipes, etc.
Steel segments or products of varying chemistry are delivered at the galvanizing plant. These items have differing surface conditions, may be coated with preconstruction primers or other applied protective coatings. For this reason, the pre-treatment of the steel segments or products delivered is critical to remove grease, oil, paint and rust.
Obtaining the cleanest possible steel surface by removing all the oxides, residues and other contaminations is of paramount importance in the HDG process. The first and the most important step is surface preparation and is accomplished by first hanging the steel using chains, wires, or specially designed dipping racks to move the parts through the process.
There are three cleaning steps to prepare the steel for galvanizing.
Acid Based Degreasing/Caustic Cleaning : First the steel is immersed in an acid degreasing bath solution in order to remove the dirt, oil and grease from the steel surface.
Acid Pickling : Then the steel is immersed in an acid tank filled with a dilute solution of heated sulphuric acid and hydrochloric acid which removes oxides and mill scale. Once all oxidation has been removed from the steel, it is again rinsed with water.
Fluxing : The final stage of surface cleaning serves the purpose of cleaning all the remaining oxides or the oxidation developed since the pickling of the steel and to create a protective coating to prevent the steel from any oxidation prior entering the galvanizing kettle containing molten zinc. One type of flux is contained in a separate tank and contains a combination of zinc chloride and ammonium chloride.
After being immersed in the degreasing, pickling, and fluxing tanks, the surface of the steel is completely free of any oxides or any other contaminants that might inhibit the reaction of the iron and liquid zinc in the galvanizing kettle.
Once the steel has been completely cleaned, the material is completely immersed in a bath of liquid zinc. The galvanizing kettle contains zinc which is specified by ASTM B6, and requires at least 98% pure zinc maintained at 815-850o F (435-455o C).
The steel products are immersed into the galvanizing kettle until the temperature of the steel has reached the temperature required to form a hot-dip galvanized coating. Once the inter-diffusion of the iron constituent of the immersed steel and zinc is completed, the steel product is withdrawn from the zinc kettle. The entire dip usually lasts less than ten minutes, depending upon the thickness of the steel.
When the steel is removed from the galvanizing kettle, it may receive a post- treatment to enhance the galvanized coating. One of the most commonly used treatments is quenching. The quench tank contains water & the other chemicals which help in bluing, a passivation process, to create a passivation layer that protects the galvanized steel during storage and transportation. Other finishing steps include removal of zinc drips, or icicles, by grinding them off.
This section will review a number of possible surface defects visible on the galvanized coating. Some of these surface defects are turned down, as they will seriously lower the corrosion protection, while others have little or no effect on the corrosion performance and are acceptable. Other attributes such as coating thickness, surface condition, thickness, uniformity, adherence, etc. are also taken into consideration and inspected for any defects.