Materials such as iron and steel are prone to react with the air and moisture present in the environment, resulting in rusting.
Hot dip galvanizing protects iron and steel workpieces from rusting and enhances their lifetime.
But what exactly is hot dip galvanizing, and what are the processes involved in performing hot dip galvanization?
Hot dip galvanizing (HDG) is a type of galvanizing process in which iron or steel workpieces are dipped in a molten zinc bath maintained at around 450 °C. The top layer of the base metal reacts with the molten zinc and produces a protective layer over the workpiece.
This article provides a detailed guide on hot dip galvanizing along with its advantages and limitations.
In the end, I've also discussed the difference between hot dip galvanizing and cold galvanizing, and mentioned some of the popular businesses that provide hot dip galvanizing services.
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What is Hot Dip Galvanizing? 4-Step Process
Hot dip galvanizing is a process of adding a protective layer over metal workpieces.
Unlike anodizing steel, where an oxide layer is formed as the final coating, galvanizing steel workpieces involves deposition of a layer of zinc to protect the base metal.
The protective layer formed in hot dip galvanizing then undergoes oxidation to form an oxide layer which later converts to a carbonate layer upon exposure to the environment.
Generally, galvanization of steel workpieces is performed after completing any required heat treatment, such as annealing of steel.
Galvanization can generally be performed by different methods such as electrolysis, thermal diffusion, or a hot-dip method.
Hot dip galvanizing (HDG) is ideal for large-size workpieces and therefore, is popularly used in industrial applications.
The HDG process comprises four stages: surface preparation stage, galvanizing stage, post-treatment stage, and inspection stage.
Step 1: Surface Preparation in HDG
The initial step is surface preparation of the steel or iron objects to be galvanized.
This step consists of various processes such as degreasing, acidic pickling, and fluxing to improve the surface quality of the workpiece, ensuring proper galvanization.
The workpieces are immersed in a degreasing bath, where they are rinsed to remove any dirt, dust, or oil deposits from their surface.
After degreasing, the workpieces are immersed in an acidic pickling bath, where they are rinsed to remove iron oxides and smoothen flaky surfaces (or burrs caused during machining).
Finally, the workpieces are submerged in a flux solution usually containing 30 % zinc ammonium chloride at 65-80°C (338-353 K).
This will eliminate any remaining oxides and apply a protective layer to prevent further oxidation.
It is the most critical stage in surface preparation because thorough cleaning of oxides will ensure a smooth and uniform deposition of zinc over iron or steel workpieces.
Step 2: Galvanizing
After preparing the surface of the workpiece, it is time to begin the galvanization process.
The workpieces are submerged in an all-zinc bath maintained at around 450 °C (723 K). The container that houses the molten zinc bath is called a galvanizing kettle.
Generally, they are immersed in the bath at an angle to allow air bubbles to escape the hollow sections of the workpiece.
When submerged in the galvanizing kettle, the iron or steel reacts with zinc to form multiple inter-metallic zinc-iron layers, with the outermost layer consisting of pure zinc.
This reaction occurs very rapidly in the initial stages, during which the primary coating forms, after which it starts to slow down, and the coating thickness increases gradually.
The immersion usually takes four to five minutes. However, the immersion time increases as the size of the object increases.
After removing objects from the galvanizing kettle, the zinc coating first oxidizes to zinc oxide, which reacts with carbon dioxide in the air to form a zinc carbonate layer, further preventing corrosion.
Step 3: Post Treatment
Post-treatment is an optional stage where the galvanized workpiece is either quenched in water or allowed to cool under forced air through an air-blower.
Quenching provides the fastest cooling rate while enhancing the hardness of the workpiece, whereas air-cooling provides a cheaper alternative with comparatively lower hardness.
The galvanized objects can be painted or covered with an additional coating to enhance their aesthetic value and provide an additional protective layer.
Step 4: Inspection
The inspection step involves visually verifying the galvanized workpieces to determine whether they have been completely galvanized or not.
Apart from visual verification, the workpieces are also inspected by magnetic thickness gauge and non-destructive ultrasonic testing to ensure the uniformity of the coating thickness.
The entire process of hot dip galvanization takes approximately twelve hours to complete.
Benefits of Hot Dip Galvanizing
Hot dip galvanizing offers various advantages, making it one of the most preferable galvanization techniques in various industries.
The protective layer formed during HGD protects iron and steel workpieces from wear and tear caused due to rusting of the surface.
Even if this coating rubs off or scratches, leaving some portion of the iron or steel surface exposed, zinc being more reactive, oxidizes to form a protective oxide layer over the surface.
This phenomenon is known as sacrificial protection, and it keeps the original objects safe from rusting.
HDG is an eco-friendly process with a low carbon footprint, leading to minimal generation of any toxic fumes or harmful waste.
Apart from that, the process enhances the lifetime of the workpiece, leading to reduce waste, and can even utilize recycled iron, steel, and zinc.
The protective coating obtained due to hot dip galvanization has a shiny and smooth appearance, which adds to the aesthetic value of the workpiece.
Commercial structures like lighting poles or building pillars are some of the common examples that use hot dip galvanization to attain a shiny surface with enhanced life.
The hot dip galvanization process takes approximately 12 hours to complete (for large workpieces), which is relatively less than other layer deposition processes like powder coating or spray painting.
Flexibility in Construction Applications
Compared to concrete pillars, using galvanized iron or steel pillars offers flexibility in designing infrastructure as they can be easily welded, bolted, or spliced to form complex structures.
The cost of carrying out the hot dip galvanizing process is relatively cheaper when compared to other layer deposition processes like powder coating or electrostatic spraying.
Apart from that, the protective layer prevents the workpiece from harsh environmental conditions, leading to minimal maintenance requirements, and further reducing the overall cost.
Limitations of Hot Dip Galvanizing
Hazardous if not Handled with Care
The HDG process involves handling molten zinc baths, and surface treatment solutions containing corrosive chemicals like acids, and heavy objects and structures.
These chemicals can irritate the respiratory system if inhaled and cause rashes on the skin on contact.
Molten solutions and hot objects can cause burns that may prove fatal, and improper handling of heavy workpieces can cause accidents and endanger the life of the operator.
Therefore, appropriate safety protocols, such as wearing safety gear, keeping a safe distance while handling heavy workpieces, etc., should be implemented when carrying out the HDG process.
Despite being relatively cheaper than many different layer deposition processes, the initial and running costs of hot dip galvanizing are not feasible for DIY users and small-scale industries.
The equipment required for the HDG process occupies significant space and requires certification and training before handling, rendering it impractical for hobbyists and DIY users.
Susceptible to Wear
The galvanized layer can be damaged when subjected to abrasive conditions, leaving the base material prone to damage.
Marine applications are an example, where upon exposure, the original surface can interact with salty seawater, resulting in wear due to corrosion.
The HDG process is limited to iron and steel workpieces.
Using this process for coating other materials will result in a non-uniform layer that can be easily removed.
Therefore, limiting the application of the hot dip galvanizing process.
Difference between Hot Dip and Cold Galvanizing
|Hot Dip Galvanizing||Cold Galvanizing|
|Surface preparation is intensive||Surface preparation is simple|
|Zinc deposition via molten solution||Zinc deposition through electrolysis|
|Zinc layer is more adherent||Zinc layer is less adherent|
|More uniform zinc layer||Less uniform zinc layer|
|Eco-friendly process||Toxic electrolyte waste is generated|
|Comparatively expensive||Comparatively cheaper|
|Applicable on all sized objects||Applicable on small to medium-sized objects|
Difference between hot dip galvanizing and cold galvanizing
HDG involves multiple stages in the surface treatment step, whereas, in cold galvanizing, only acidic pickling and degreasing of the iron or steel object's surface is required.
In cold galvanizing, the structure to be galvanized is placed as the anode in an electrolyte of zinc salt, after which a zinc layer is deposited onto the structure's surface through electrolysis.
On the other hand, hot dip galvanizing involves immersion of the workpiece in a molten zinc bath to form the protective coating.
Hot dip galvanizing results in an adherent and uniform deposition of zinc layer, while the layer deposited during cold galvanizing is comparatively thin and easy to be scratched.
Hot dip galvanizing is environmentally friendly, while the disposal of toxic electrolyte waste generated during cold galvanizing has adverse effects on the environment.
The initial and running costs of cold galvanizing are comparatively lower than hot-dip galvanizing, making it more feasible for DIY users and small-scale industries.
Hot dip galvanizing is generally used for large structures.
Although hot dip galvanizing can also be used for small-sized workpieces, cold galvanization proves to be a comparatively more economical process for small and medium-sized workpieces.
Hot Dip Galvanizing Services
Setting up a hot dip galvanization plant can be highly expensive for applications that occasionally require galvanized parts.
Apart from that, DIY users and small-scale businesses who cannot afford to set up a galvanizing plant can rely on outsourcing their projects to professional galvanizing service providers.
Here are three popular businesses that offer hot dip galvanizing services.
Hot dip galvanization is ideal for enhancing the corrosion resistance properties of iron and steel workpieces, similar to anodizing of aluminum workpieces.
Due to its high initial investment, it is best suited for galvanizing large workpieces, making it ideal for industries such as automobile, ship-building, construction, etc.
If you are looking for a comparatively cheaper alternative for galvanizing small workpieces, cold galvanizing should be your go-to process.
However, hot-dip galvanizing provides various advantages over cold galvanizing, and therefore, you can opt for professional hot-dip galvanizing services to attain a high-quality galvanized product.
Frequently Asked Questions (FAQ)
Can you paint a hot-dipped galvanized steel workpiece?
Yes, you can paint a hot-dipped galvanized steel workpiece. This additional layer of paint will protect against corrosion and rusting while enhancing the aesthetic value of the galvanized workpiece.
What temperature can galvanized steel withstand?
Galvanized steel can withstand temperatures up to approximately 350°C (623 K), beyond which the zinc-iron alloy starts melting.
Does hot dip galvanizing add weight to the workpiece?
Yes, galvanizing generally adds about 4% to 8% of the overall weight of a steel workpiece.