EDITORIAL Thermal zinc
An in-depth look at thermal zinc diffusion
Thermal zinc diffusion, also known as sherardising or thermal diffusion galvanising( TDG), is a well-established process for applying a protective zinc coating to steel components. Unlike electroplating, which deposits a surface layer, thermal zinc diffusion creates a deeper, more permanent bond between the zinc and the steel through a diffusion process at elevated temperatures.
“ This method offers superior corrosion resistance and durability, making it a valuable tool for various industries,” comments International Zinc Association( IZA) Africa Director Simon Norton. A number of South African firms offer zinc thermal diffusion, including Distek in Johannesburg.
The process and benefits of thermal zinc diffusion
At the heart of thermal zinc diffusion lies the concept of diffusion, which allows zinc atoms to penetrate the steel’ s surface at elevated temperatures. It creates a zinc-iron alloy layer that becomes an integral part of the steel, offering superior protection against corrosion. Thermal zinc diffusion is used across a variety of industries due to its exceptional corrosion resistance.
In the automotive industry, it protects fasteners, brackets, and other small automotive components from corrosion. In construction, threaded rods, screws, nails, and other hardware benefit from the longlasting protection provided by thermal zinc diffusion. In appliance manufacturing, components exposed to moisture, such as washing machine parts or refrigerator shelves, can be shielded from corrosion through thermal zinc diffusion. Threaded fittings, enclosures, and other electrical components can also be protected from environmental factors using this process. Components used in harsh offshore environments, such as in the oil and gas industry, benefit from the superior corrosion resistance offered by thermal zinc diffusion.
Application and coating variations
The process begins with thorough cleaning of the steel parts. Any dirt, oil, or contaminants on the surface can hinder the diffusion process, so degreasing, grit blasting, or chemical cleaning is essential.
The cleaned parts are loaded into a tumbling barrel, or a static container filled with a specially formulated zinc powder. The powder plays a crucial role, as its composition can influence the final coating properties. Zinc powder variations include:
• Zinc dust: The most common type, a fine, grey powder produced by grinding zinc metal. Zinc dust offers a good balance between cost and performance.
• Zinc-aluminium mischmetal powder: A blend of zinc and aluminium. The addition of aluminium can enhance the corrosion resistance of the coating and improve its high-temperature performance.
• Pre-alloyed zinc powders: These powders contain specific elements pre-alloyed with zinc, offering tailored properties for specific applications. For example, adding rare earth elements can further enhance corrosion resistance.
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