What is Hot-Dip Galvanizing?
Hot-dip galvanizing has been utilized for over 250 years to protect steel and iron from the destruction of corrosion. The process, which has evolved over the years, entails dipping fabricated steel into a kettle of molten zinc. The iron in the steel reacts with the zinc to form a tightly bonded alloy coating which protects the steel from corrosion.
Our Hot Dip Galvanizing Process
Surface Preparation
- Degreasing: We prepare the surface by first removing any organic contaminants such as dirt, paint markings, grease, and oil from the metal surface with an alkali solution.
- Pickling: We then carefully remove twice any mill scale and iron oxides (rust) from the surface with two separate immersions in diluted solutions of hydrochloric acid.
- Rinsing: We rinse after each step – degreasing and pickling with clean room temperature water.
- Fluxing: Our final surface preparation step is to immerse the product in zinc ammonium chloride solution, which (1) removes any remaining oxides and (2) deposits a protective layer on the surface to prevent any further oxides from forming prior to immersion in the molten zinc.
Galvanizing
The “galvanizing” occurs when items are completely immersed in a zinc bath (kettle) of 99.95% pure molten zinc maintained at a temperature of approximately 840 F (449 C). The items are lowered at an angle best suitable for each shape allowing air to escape any pockets or gaps that may be within the design of the item permitting the molten zinc to displace the escaped air.
Once the fabricated item reaches bath temperature articles are slowly withdrawn from the zinc bath. Excess zinc is removed by draining, vibrating, and/or centrifuging depending on the item. The metallurgical reaction continues even after withdrawal from the zinc bath as long as the item remains near bath temperature. Articles are cooled off by immersing in a passivation solution and water and then left in open air or hot plate for some time to dry (ASTM 123 and 153).
Inspection
- Our QA experts conduct a visual inspection test on randomly selected samples from every batch for the following:
- Ungalvanized or Dark Staining in Weld Areas: Any coating misses on welded areas which results due to the presence of welding slag on the welds.
- Dross Pimples/Inclusions: Dross develops during the process of galvanization in the form of zinc-iron crystals (approximately 95% zinc and 5% iron) as iron/steel has a higher melting point compared to zinc. The presence of dross additions in the coatings is not harmful to the coating’s performance but may give the coating a gritty or rough look.
- White Storage Staining: If galvanized items are stacked or stored wet (sometimes in monsoon season), they react with atmospheric moisture to develop bulky white zinc hydroxide deposits on the surface of the galvanized coating.
- Striations and General Surface Irregularities: Ridges and lines thicker than the nearby galvanized coating are caused by varying rates of reaction of the zinc with the steel surface because of stress areas on the steel surface or the presence of weld areas or weld metal with altered metallurgy to the parent metal. It has no impact on coating performance.
- Runs, Drainage Spikes and Puddling: These defects cannot be avoided during the hot dip galvanizing of regular items and are tolerable as long as they do not affect the assembly or the function of the item or pose a safety risk in handling or service.
- Bare Patches: Uncoated areas on the surface of galvanized work are because of improper surface preparation, pickling, insufficient pretreatment in degreasing or pre-fluxing.
- Rust Staining: Uncoated steel exposed to galvanized coatings will quicken corrosion of the coating and stain the coating brown where contact occurs.
- Delamination: Very heavy galvanized coatings (more than 250 µm in thickness) may be fragile and delaminate from the surface upon impact and may need more cautious handling in transport and erection.
- Black Spots: Scattered black spots are caused by residual galvanizing flux crystallization on the surface and are mostly because of flux-contaminated rinse water or poor rinsing after galvanizing. This flaw is typically seen in galvanizing baths using the “wet” galvanizing process where the flux is on top of the molten zinc. Surplus aluminum in the galvanizing bath can also be the reason for this defect.
Coating Thickness Test
Coating thickness refers to the thickness of the final hot-dip galvanized coating. Two different methods are used to measure the coating thickness of hot-dip galvanized items.
Electronic Gauge
Hot-dip galvanized coatings is evaluated with an electronic gauge by randomly selecting specimen representing every batch. The inspection quantities are determined by the lot sizes as detailed in the ASTM specifications A123/A123M, A153/A153M, and A767/A767M or as per customer request. The ASTM specifications contain detailed information, but a couple of universal practices are recognized e.g.:
- At least five widely dispersed measurements are performed per specimen
- Average for each specimen should not be less than one thickness grade lower than material category or as specified by the customer
Service Life Estimate Test
Salt Spray Test
The purpose of salt-spray is to estimate the service life a particular coating might have in a specific environment. The salt spray test (or salt fog test) is a standardized and popular corrosion test method, used to check corrosion resistance of materials and surface coatings. We conduct Salt Spray Tests as per ASTM B117 and G85 specifications from time to time to determine thickness, uniformity, adherence, and appearance to ensure the coating life.
Adherence Test
- Stout Knife:
- Adherence is usually never a problem but can be tested using a stout knife. To test the zinc coating adherence, we run a stout knife smoothly along the surface of the steel without whittling and gouging, as detailed in the ASTM specifications A123/A123M and A153/A153M. The coating is deemed not adherent if the coating flakes off, exposing the base metal in advance of the knife point.
- Pivoted Hammer:
- We also determine the adherence of the zinc coating by the pivoted hammer test. The hammer used by IronCraft conforms to the drawing shown in IS 2629.1985. The hammer blow is controlled by holding the pivoted base of the handle on a horizontal surface of the galvanized item and allowing the hammer head to swing freely through an arc from vertical position to strike the horizontal surface. The test consists of two or more standard blows forming parallel impressions with 6 mm spacing and a common axis. The specimen is tested at several places throughout its surface.
Preece Test
The official name of the Preece Test is Standard Test Method for Locating the Thinnest Spot in a Zinc (Galvanized Coating on Iron or Steel Articles by the Preece Test (Copper Sulfate Dip) (ASTM 239-95). It is used primarily for hardware items such as nuts and bolts. It is impractical for structural steel items because of the testing solutions and handling that would be required.
Corrective Action
Rejected items are separated and QA team inspects each rejected item and decides action items which could be repair for minor defect by wire brushing, touching etc. or re-galvanizing for defects of considerable size.
