In the manufacturing industry, hot-dip galvanizing and zinc plating stand out as two prevalent methods for applying a protective zinc coating to steel or iron components.
The primary distinction between the two lies in their application techniques. Hot-dip galvanizing entails immersing the steel component in a vat of hot molten zinc, whereas zinc plating employs electrolysis (electric current) to coat the zinc onto the surface.
This article delves deeper into the disparities between these two processes and examines their impact on the metallurgical properties of the steel component.
When people mention 'galvanizing,' they typically refer to hot-dip galvanizing. In this process, the iron or steel component is immersed in a bath of hot, liquid molten zinc with a temperature of approximately 450 °C (842 °F).
During this immersion, the molten zinc forms a chemical bond with the steel or iron, creating an external protective layer of zinc as it cools and reacts with oxygen in the atmosphere. This reaction results in the formation of Zinc Oxide (ZnO). Subsequently, Zinc Oxide reacts with carbon dioxide, leading to the development of a protective layer of Zinc Carbonate (ZnCO3), imparting the characteristic dull greyish appearance.
To achieve a high-quality galvanizing treatment, the metal surface must be meticulously cleaned. The surface cleaning process involves three primary steps. Initially, degreasing eliminates organic residues, dirt, and oil. Subsequently, pickling with an acidic solution removes mill scales and any rust. Finally, the fluxing process ensures the prevention of oxidation on the base metal before immersion into the molten zinc bath.
2) Drying Process
The component undergoes rinsing between degreasing, pickling, and fluxing to eliminate any lingering traces of oxidation on the surface. Following this, the component is dried before it is ready for immersion into the molten zinc bath. At this stage, it is crucial to ensure the removal of all contaminants from the component's surface, as unclean steel will not react effectively with the zinc.
The cleaned component is immersed in the bath of molten zinc. This step typically lasts around five minutes for small parts but may take longer for larger components.
The galvanized part is cooled and may undergo additional processes, such as painting or powder coating.
Both stainless steel and galvanized components offer excellent corrosion resistance, but galvanizing is often a more cost-effective choice than using stainless steel.
Moreover, it stands out as one of the most environmentally friendly methods for providing corrosion resistance. Any zinc that does not form a coating on the steel or iron components during the hot-dip process remains in the galvanizing bath, ready for reuse.
A crucial consideration in favor of using hot-dip galvanized steel components over stainless steel is the tensile strength requirement for screws, nuts, or bolts. In applications demanding structural bolts for high-stress fastenings and securing heavy objects, manufacturers might prefer using high-tensile steel fasteners.
Although stainless steel components can be manufactured to high tensile strength grades, the significantly higher cost per unit often makes hot-dip galvanized steel components a more appealing option.
Zinc plating, also known as zinc electroplating, is a process that utilizes electric current to disintegrate zinc into ions (small charged particles) and then deposit it onto the metal component. This is achieved by creating a basic electrolytic cell, comprising zinc, an aqueous solution, and the component intended for plating.
In this process, the zinc is designated as the anode (positive charge), while the component to be plated is referred to as the cathode (negative charge). In simple terms, when a direct electric current is applied to the cell, the zinc anode begins to disintegrate into tiny zinc ions.
As the zinc ions carry a positive charge, they are drawn towards the positive terminal (the steel component requiring plating) and flow towards it. The aqueous solution facilitates the movement of these ions. Upon reaching the component, the zinc ions arrange themselves into a neat outer layer of zinc.
Untreated galvanized components typically exhibit a muted grey color with a rough texture. Zinc plated parts, on the other hand, tend to be smoother and shinier, often featuring a colored finish such as blue or yellow.
Zinc plating yields a relatively thin coating, typically ranging from 0.005 mm to 0.025 mm. Achieving a thicker coating with zinc electroplating incurs higher costs. In contrast, hot-dip galvanizing can produce coatings over 0.1 mm in thickness.
The ability to achieve greater thickness with hot-dip galvanizing results in galvanized parts lasting for years or even decades without rusting. The slow rusting process generates zinc oxide, providing an additional layer of protection. Conversely, zinc plated parts, due to their thinner coatings, are less durable than galvanized ones, making them unsuitable for outdoor applications unless post-treatment is applied.
With its thicker zinc coating, galvanized parts boast greater strength and resilience, particularly in outdoor settings. The slow corrosion rate ensures that the coating maintains its strength over an extended period.
This is where zinc plating wins out, being a more readily available metal, it is more cost effective than hot dip galvanising, especially for smaller components. Zinc is also considerably cheaper than other metals such as gold.
Hot-dip galvanizing is applicable to almost any steel or iron component, regardless of size, making it suitable for both outdoor and indoor use.
Zinc plating machine line, on the other hand, is primarily limited to small metal parts like nuts, bolts, and screws, and is better suited for indoor applications. However, zinc plated parts exhibit improved weldability compared to galvanized components.
In conclusion, due to its lower cost per component and relatively lower overall corrosion resistance, a zinc-plated finish is optimal for smaller parts utilized indoors, such as nuts, screws, and bolts.
On the contrary, hot-dip galvanizing generates a thicker zinc coating, resulting in a stronger and more durable finish. This makes it the superior choice for outdoor applications where heightened corrosion resistance is essential.