What is a melt cake
Hot dipping is a group of coating processes in which a workpiece made of a higher melting point metal or alloy is completely immersed in a molten bath of a lower melting point metal or alloy. When the workpiece is lifted out of the molten bath, the liquid metal adheres to the workpiece and forms a solid metallic coating as it cools.
For such a procedure to work, two basic requirements must be met:
- The melting points of the two metals must not be too close to one another on the temperature scale, because the molten bath must be at least so hot that it is not about to solidify. On the other hand, the workpiece must not get so hot when it is dipped that it begins to melt.
- The molten metal must adhere to the workpiece, i.e. H. the adhesion must be strong enough that the workpiece is wetted.
In order for such a procedure to be used in practice, the following requirements must be met:
- The coating metal must have any desired, advantageous or value-adding properties that the base material does not have.
- Base material and coating material must be available in sufficient quantities and at a reasonable cost.
- The melting point of the coating metal must absolutely not be too high, since technological difficulties, costs and energy consumption increase disproportionately at very high temperatures.
Immersing a metal part in a molten metal bath is a relatively simple technological process. Therefore, the first hot-dip processes were developed a long time ago. This is also due to the archaic-sounding process names (usually with the prefix Fire-) to recognize.
A large number of metals are listed in the periodic table of the elements. The number of known or theoretically possible alloys is almost unmanageable. Despite the basic prerequisites, one could derive a very large number of conceivable hot-dip processes in this way. Nevertheless, and above all because of the practical requirements, the number of technically implemented hot-dip processes has remained relatively manageable:
- Hot-dip galvanizing has achieved the greatest economic and technical importance of all hot-dip processes. Steel parts or strips, cast iron or steel castings are immersed in molten zinc with some minor alloying components. Value-adding property: Mainly corrosion protection.
- In hot-dip tinning, parts or strips made of steel or copper (alloys) are dipped in molten tin. Value-increasing properties: Mainly corrosion protection, food compatibility, electrical conductivity and solderability.
- Fire leaching is only used if the toxicity of the lead does not interfere, e.g. with oil and fuel tanks made of steel.
- Hot-dip aluminized steel parts have improved corrosion resistance and are more thermally resistant than steel parts with a different coating (e.g. galvanized). Fire aluminizing is therefore used, for example, for exhaust pipes.
- Fire gilding occupies a special position. In this process, the coating metal (amalgam) is semi-liquid or pasty and is distributed on the workpiece. So one can hardly speak of "immersion". This procedure is out of date mainly because of its toxicity.
Competition with other procedures
With the development of electrochemistry, electroplating processes have established themselves as the most serious competition to the hot-dip processes in many areas. Nevertheless, the hot-dip processes were able to defend their market position, especially in the following segments:
- Hot-dip galvanizing of medium-sized and larger steel parts and structures for outdoor use.
- Hot-dip galvanizing of steel strips (Sendzimir process) as a very economical high-speed process.
- Hot-dip tinning of copper strips for the electrical industry.
Advantages and disadvantages
- A particular advantage of hot-dip processes is the formation of alloy layers at the transition from the base material to the coating material. These alloy layers result in extremely good adhesion between the workpiece and the coating. This goes so far that the flaking of the zinc layer is practically impossible on a hot-dip galvanized steel part.
- A potential disadvantage of the hot-dip process is that the layer thickness, and thus the metal consumption and material costs, is relatively high compared to electroplating processes. The disadvantage is partially offset by methods of reducing the layer thickness (spinning, blowing off with air nozzles).
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