Hot-dip galvanizing (HDG) is the process of coating steel by immersing it in a bath of molten zinc. There are three basic steps involved in hot-dip galvanizing; Surface preparation, galvanizing, and inspection.
Hot-dip galvanizing is a form of galvanizing. It is the process of coating iron and steel with zinc that when the metal is immersed in a bath of molten zinc at a temperature of about 450°C (842°F), alloy with the surface of the base metal.
In the atmosphere, pure zinc (Zn) reacts with oxygen (O2) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO2) to form zinc carbonate (ZnCO3), a mostly dull gray, fairly strong material that makes up the steel underneath many Protects against further corrosion.
Galvanized steel is widely used in applications where corrosion resistance is required without the cost of stainless steel and is considered to be superior in terms of cost and lifespan. It can be identified by the crystallization pattern on the surface.
1. Surface Preparation
When the fabricated steel arrives at the galvanizing plant, it is hung on wire or placed on a shelving system that can be lifted and moved through the process with overhead cranes. The steel then goes through a series of three cleaning steps; Degreasing, pickling, and flux.
Degreasing removes dirt, oil, and organic residues, while the acidic pickling bath removes mill scale and iron oxide. The final step in surface preparation, the flux, removes any remaining oxides and covers the steel with a protective layer to prevent further oxide formation before galvanizing.
Proper surface preparation is crucial as zinc does not react with contaminated steel.
2. Galvanizing
After surface preparation, the steel is immersed in the molten (830 F) bath containing at least 98% zinc. The steel is lowered into the kettle at an angle so that air can escape from tubular shapes or other pockets and the zinc can flow in, over, and through the entire piece.
When immersed in the kettle, the iron in the steel reacts metallurgically with the zinc to form a series of zinc-iron intermetallic layers and an outer layer of pure zinc.
3. Inspection
The final step is a review of the coating. A very precise determination of the coating quality can be achieved through a visual inspection since zinc does not react with unclean steel, which would leave an uncoated area on the part. In addition, a magnetic thickness gauge can be used to check that the coating thickness meets the specification requirements.
Benefits Of Hot-Dip Galvanizing
Hot-dip galvanizing offers a number of advantages to the steel it protects. The metallurgically bound zinc-iron alloy layers not only form a barrier between the steel and the environment but also protect the steel catholically.
Due to the cathodic protection that zinc offers, the galvanized coating sacrifice itself to protect the underlying steel from corrosion.
The firmly adhering coating with a bond strength of around 3,600 psi is also extremely abrasion-resistant, as the intermetallic layers are harder than the base steel. But even if the coating is damaged, the sacrificial effect of zinc protects exposed steel up to ¼ of an inch away.
In addition to the cathodic protection that hot-dip galvanizing offers, there are a few other properties of the coating that ensure long service life.
First, the reaction in the galvanizing kettle is a diffusion process; which means the coating grows perpendicular to the surface and ensures that all corners and edges are at least the same thickness as flat surfaces. In addition, full immersion in the zinc bath provides complete coverage of the steel, including the interior of hollow structures.
Finally, the zinc coating naturally forms an impermeable layer of corrosion products on the surface, the so-called zinc patina. The patina, the cathodic protection, the complete coverage, and all these other properties give hot-dip galvanized steel a long, maintenance-free service life.
How Long Does The Galvanizing Process Take?
A typical immersion time is around four or five minutes, but it can be longer for heavy objects with high thermal inertia or if the zinc is needed to penetrate indoors.
Upon withdrawal from the galvanizing bath, a layer of molten zinc is removed from over the alloy layer. Often times this cools down to reveal the bright shiny appearance of galvanized products.
Post-Treatment
Treatment after the galvanizing process can include quenching in water or air cooling. Conditions in the galvanizing plant such as temperature, humidity, and air quality have no influence on the quality of the galvanized coating.
In contrast, these are crucial for good painting quality. No post-treatment of galvanized items is required and a paint or powder coating can be applied to improve aesthetics or to provide additional protection in extremely aggressive environments. Chemical conversion coatings and other barrier systems can be used to minimize the occurrence of wet storage stains.
How Can You Use Galvanized Metal?
Galvanized metals are used everywhere!
The bodies of cars and many bicycles are made from galvanized metals. Some drinking water pipes are still made from galvanized steel. Cool rolled sheet metal is also frequently galvanized.
Nuts, bolts, tools, and wires of all kinds are now galvanized because it is a cheap process, and definitely helps boost the metal’s lifespan!
Galvanized steel, in particular, is often what is used in modern “steel frame” buildings. Galvanized steel is also used to create structures like balconies, verandahs, staircases, ladders, walkways, and more.
Galvanized metal is a great choice if your project will live outside after it’s done. Fences, roofs, and outdoor walkways are all great choices for galvanized metal!
Benefits Of Galvanizing Metal
l Lowest first cost. Galvanizing is lower in the first cost than many other commonly specified protective coatings for steel. The application cost of labor-intensive coatings such as painting has risen far more than the cost of factory operations such as galvanizing.
l Less maintenance/Lowest long-term cost. Even in cases where the initial cost of galvanizing is higher than alternative coatings, galvanizing is almost invariably the cheapest in the long term (because it lasts longer and needs less maintenance). And, maintenance causes problems and adds to costs when structures are located in remote areas, and when plant shutdown or disruption to production is involved.
l Long life. The life expectancy of galvanized coatings on typical structural members is far in excess of 50 years in most rural environments, and 20 to 25 years plus, even in severe urban and coastal exposure.
l Reliability. Galvanizing is carried out to Australian/New Zealand Standard 4680, and standard, minimum coating thicknesses are applied. Coating life and performance are reliable and predictable.
l Toughest coating. A galvanized coating has a unique metallurgical structure that gives outstanding resistance to mechanical damage in transport, erection and service.
l Automatic protection for damaged areas. Galvanized coatings corrode preferentially to steel, providing cathodic or sacrificial protection to small areas of steel exposed through damage. Unlike organic coatings, small damaged areas need no touch-up.
l Complete protection. Every part of a galvanized article is protected, even recesses, sharp corners and inaccessible areas. No coating applied to a structure or fabrication after completion can provide the same protection.
l Ease of inspection. Galvanized coatings are assessed readily by eye, and simple non-destructive thickness testing methods can be used. The galvanizing process is such that if coatings appear sound and continuous, they are sound and continuous.
l Faster erection time. As galvanized steel members are received, they are ready for use. No time is lost on-site in surface preparation, painting and inspection. When assembly of the structure is complete, it is immediately ready for use, or for the next construction stage.
l A full protective coating can be applied in minutes; The galvanizing process is not dependent on weather conditions.
Hot dip galvanizing equipment aims to make full use of the galvanizing process. The resettlement plan will be determined according to the construction of the facility, the implementation of the project and the construction area of the facility. Hebei Annuo is a global provider of Hot Dip Galvanizing Plant and Solutions, Specialty General HDG plant design and manufacture service. If you have any questions about anything, you can ask us!
没有评论:
发表评论