Galvanizing with zinc thermal diffusion - how it works

Zinc thermal diffusion is a process, based on the sherardizing standard, for coating and finishing metal and metal products in a way that is gentle on materials. However, the thermal diffusion process varies from sherardization in a few aspects, e.g. temperature, coating thickness and homogeneity, and measurement methods. Unlike conventional coatings, zinc is not applied to the surface as a coating during thermal diffusion, but rather is directly diffused into the base material to form an ultra-high resistant intermetallic phase. You can find out more about the process of zinc thermal diffusion in this blog post.

Phase 1 – Finding the right coating parameters for surface protection

During a consultation, our experts will discuss with you your surface protection requirements and the options for galvanizing your materials based on those requirements. In an individual analysis, we will create the optimal application for your parts according to the material, geometry, mechanical properties and other special requirements during use.

Before the coating process can be initiated, it must be ensured that the parts are free from any contamination, e.g. corrosion-protection products, cinder, oil or any other coatings. By default, we use sand-blasting to prepare the substrate’s surface for zinc thermal diffusion. Different grit sizes are used depending on the nature of the parts.

Phase 2 – We achieve excellent corrosion protection using this production process

At the start of the coating process, the parts to be galvanized are placed inside a sealed, slowly rotating container whose interior is heated according to a specific time-temperature profile. The process temperatures lie between 320°C and 390°C. During this phase, the parts will come into contact with the special zinc powder mixture.

In this step, the zinc powder reacts with the surface of your parts, forming an intermetallic phase – what is known as the thermal diffusion coating. The thickness of the intermetallic phase can be regulated to suit your requirements. Depending on the application, individual process parameters – e.g. the amount of zinc powder, process time and temperature – can be adjusted to customize the coating thickness, which can range between 4 μm and 25 μm. The relatively low process temperatures ensure that your base material’s mechanical properties are not affected. However, if applicable, the annealing temperature of the parts should be taken into account. The zinc thermal diffusion process is suitable not only for bulk parts, such as screws, nuts and springs, but also for larger, geometrically complex parts. The dry oven process ensures that hydrogen embrittlement can be fully avoided.

Phase 3 – Giving your parts the finishing touch

The container is cooled at the end of the process time. Bulk parts are emptied into a sorting system, where they are separated from any leftover zinc dust and aluminum granules. For rack plating, the rack with the parts will be taken out from the container and the parts subsequently removed. The process materials (zinc powder and aluminum granules) are then fed into the next process cycle to be reused. The entire process is free of chromium VI; the powder and granules used are recyclable and not subject to any disposal regulations. After cooling and separation, the parts are cleaned. At the end of the thermal diffusion process, the coated parts will have a mat-gray appearance:

We offer you customized solutions and a specific finish for every product portfolio. Our experts will be delighted to advise you on our different sealing options and top coats.

Quality of our corrosion protection

We make sure that your thermal diffusion-coated parts meet the quality standards. To achieve this, your parts are analyzed in our in-house laboratory. For example, we perform coating thickness measurements. We use X-ray fluorescence analysis to measure coating thickness for bulk parts. For parts on racks, the magnetic method is applied. At our customer’s request, we can also carry our cross-section microscopy analysis for quality control purposes. All our measurement methods are performed in accordance with ISO standards.

You can find a summary of the zinc thermal diffusion process in this two-page factsheet.