Hot dipping

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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 molten metal adheres to the workpiece and forms a solid metallic coating as it cools.

requirements

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.

history

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 can also be recognized by the archaic-sounding process names (usually with the prefix fire ).

Examples

In the periodic table of elements , a variety of metals can be found. 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. Nevertheless, and above all because of the practical requirements, the number of technically implemented hot-dip processes has remained relatively manageable:

  • The hot-dip galvanizing has become the largest economic and industrial significance of all hot-dip process. Steel parts or strips, cast iron or steel castings are immersed in molten zinc with a few minor alloy components. Value-increasing 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. B. for 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). The hot-dip aluminizing is therefore z. B. used for exhaust pipes.
  • Fire gilding occupies a special position . In this process, the coating metal ( amalgam ) is semi-liquid or thick 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

Depending on the application, hot-dip dipping competes with other surface finishing processes such as coating or electroplating processes. Important market segments for hot dipping are:

  • Hot-dip galvanizing of small, medium and large steel parts and structures for outdoor use.
  • Hot-dip galvanizing of steel strips, sheets and wires in a continuous 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.
  • Depending on the process used, coatings produced by hot-dip dipping achieve higher layer thicknesses compared to electroplating processes and therefore also achieve a correspondingly longer protection period against corrosion.