Galvanizing is a relatively simple process of dipping raw or fabricated carbon steel into a bath of molten zinc after which a chemical reaction takes place to form a very thin coating or layer of zinc that chemically bonds to the steel. This process provides a superior corrosion protective coating on steel when compared to other methods or coatings.
As with any coating process, one of the most critical steps in the galvanizing process is surface preparation and cleanliness. Zinc will not bond to dirty steel. Therefore, it is crucial to make sure you clean and prepare the steel in advance of dipping.
Step 1. Visual Inspection & Pre-Cleaning
Inspect the product about to be coated. If there is residual welding slag or any other heavy buildup of dirt or contaminants, it is best to pre-clean the material. This can be done by using abrasive grinding to spot-clean areas or grit or shot blasting if large areas or the entire surface needs to be cleaned.
Step 2. Degreasing/Caustic Cleaning Bath
The steel to be coated is initially placed in a bath of hot alkali solution, mild acid and/or biological cleaning solution to remove contaminants like dirt, paint markings, grease, zinc or oil from the metal surface. It is not uncommon to have to use both a hot alkali solution bath and a mild acid bath when first cleaning steel depending on the surface condition of the steel. After this cleaning process is complete the steel is placed in a rinse bath to clean the steel in preparation for the next process.
Step 3. Pickling Bath
Next the steel is placed in either a heated sulfuric acid or hydrochloric acid bath to remove any remaining mill scale and rust from the surfaces to be coated. If the steel is not immediately processed and comes in contact with air for an extended time, a thin layer of flash rust will form on the steel and this process must be repeated. After this cleaning process is complete the steel is placed in a rinse bath to clean the steel in preparation for the next process.
Step 4. Flux Coating
Applying a flux layer to steel is the last step before the steel is ready to be dipped into the molten zinc bath. Flux is needed to provide a good surface for the zinc to bond to and to remove any remaining oxides.
There are several ways to apply the flux and the decision of which technique to use is typically made at the time of plant construction due to equipment and space requirements. While all flux methods have advantages and disadvantages, the two most common approaches are Dry Flux and Wet Flux. The process known as “Dry Flux” is when the steel is dipped in a bath of zinc ammonium chloride solution and placed into a heated drying area to cure the flux to the steel. The process known as “Wet Flux” is when a layer of zinc ammonium chloride powder is placed on top of the molten zinc bath such that the flux is applied when the steel passes through the flux layer as the steel is dipped into the zinc.
Once properly cleaned and the surface prepared, the steel is ready to be dipped in the molten zinc bath. To ensure a quality coating, the bath chemistry is specified by ASTM B6, and requires at least 98% pure zinc maintained at 815 to 850°F (435 to 455°C).
It is critical in this step that care is taken both when immersing the steel into the bath completely and when removing it from the bath. A slow and steady immersion into the zinc bath enables the zinc to properly bond to the steel to form the important zinc-iron alloy layers that chemically bond the zinc onto and into the steel. Once this is complete, the steel stays in the bath to create the desired amount of zinc coating.
The steel should be removed slowly to allow the excess zinc to drain off and to create an acceptable appearance of the steel product. Additionally, before the steel is removed from the bath, the thin layer of ash impurities (zinc oxide) that forms on top of the zinc bath needs to be removed. This assures that the steel will not pick up the ash – thus avoiding surface imperfections on the steel product – while being removed from the bath.
Due to the high zinc bath temperature and that the zinc bonding process will continue at elevated temperatures after the steel is removed from the bath, it is best to place the steel in a cooling bath of passivation solutions or water. While air alone can also be used to dry the material, it takes longer to cool and inspect the final product.
Inspecting galvanized steel is performed by looking at coating thickness and visual appearance. ASTM specifications A123, A153 and A767 give requirements related to the minimum zinc coating required based on the type of steel being galvanized.
Because of the popularity of galvanizing as a technology to reduce corrosion, there are many industry or governmental standards for coating thickness, surface condition and visual appearance. All appropriate standards should be considered as part of any inspection process to ensure quality product.
For more information about the galvanizing process and the galvanizing industry, please visit the American Galvanizers Association. United Combustion & Services is a long-standing member of the AGA and supports their efforts to promote the hot-dip galvanizing industry.