Hot-dip galvanizing is a general way that is used on iron or steel for corrosion resistance. In our day-to-day life, galvanized products can be seen everywhere.
Hot-dip galvanizing , short for HDG, is the process of immersing work pieces in a bath of molten zinc to produce a corrosion resistant, multi-layered coating of zinc-iron alloy and zinc metal. Generally, we divide it into three stages, which are surface preparation, hot-dip galvanizing and post-treatment.
When the work pieces are removed from the galvanizing kettle, they may receive a post-treatment to enhance the galvanized coating. One of the most commonly used treatments is quenching. The quench tank contains mostly water but may also have chemicals added to create a passivation layer that protects the galvanized steel during storage and transportation. Other finishing steps include removal of zinc drips or spikes, by grinding them off. Then, they are ready for delivery.
Next the steel is “pickled” in a dilute solution of either hydrochloric or sulfuric acid, which removes oxides and mill scale. Once all oxidation has been removed from the steel, it is again rinsed with water and sent to the final step of the surface preparation.
Finally, the steel is dipped in the flux solution. The purpose of the flux is to clean the steel of all oxidation developed since the pickling of the steel and to create a protective coating to prevent any oxidation before entering the galvanizing kettle. Flux solution is slightly acidic, and contains a combination of zinc chloride and ammonium chloride.
After degreasing, pickling, and fluxing, the surface of the steel is a near white metal clean, completely free of any oxides or other contaminants that might inhibit the reaction of the iron and molten zinc in the galvanizing kettle.
During the process of surface preparation, acid mist and spent fluid will be generated and they are harmful if we don’t take any protective measures for operators and our living environment.
With a enclosure covering on all process tanks of this process while maintaining micro-negative pressure inside, the acid mist generated can be held inside and will be absorbed by scrubber for effective treatment and discharge on standard; and the spent fluid will be also treated by relevant environmental protection equipment for the purpose of recycling.
The purpose of surface preparation in the hot-dip galvanizing process is to obtain the cleanest possible steel surface by removing all of the oxides and other contaminating residues. In order to move the work pieces through the cleaning steps and galvanizing bath, the work pieces are hung using chains, wires, or specially designed dipping racks.
Cleaning steel to prepare for the hot-dip galvanized coating consists of three steps:
First the steel is immersed in an acid degreasing bath to remove organic contaminants such as dirt, oil, and grease from the surface of the steel. After degreasing the steel is rinsed with water.
The galvanizing kettle is heated to a temperature ranging from 438-460℃, at which point the zinc is in a liquid state. The work pieces are lowered into the galvanizing kettle at an angle, and stay in the bath until the steel heats to the bath temperature. Once the diffusion reaction of iron and zinc is complete, the work pieces are withdrawn from the zinc kettle.
Galvanizing fume generated during this process will be collected by a cover installed on the galvanizing kettle and will be treated through bag filter. The surplus heat generated will be utilized for heating the process tanks at pretreatment.