Coated thermowells

Coatings are used in many areas of the chemical and petrochemical
industry. The most important distinction here is whether the coating is intended primarily to counteract wear protection, ie abrasion by particles, or to have the highest possible chemical resistance.

 

Coatings as wear protection / protection against
abrasion:

If a coating is intended primarily to counteract the effectof abrasion, high velocity oxygen fuel (HVOF) spraying is used nowadays – as is the case in our company. The layers produced with this method have a substantially lower porosity as well as higher adhesive strengths than in conventional powder flame spraying / plasma spraying.

 

Coating by means of plating / welding (WIG – GTAW)

 

A further possibility of applying coatings to increase theservice life is reinforcement by means of deposition welding.

In this case, the desired material is applied by means of a welding filler material (for example, stellites) to the thermal tip involved in the process. As a result, much larger layer thicknesses can be applied and worn out thermal tips can be once again made serviceable.

After the plating, the plate layer is mechanically finished (stripped) and polished, so that the finished surface no longer differs from a new, uncoated tip.

 

 

Coatings to increase chemical resistance:

With many highly aggressive media, such as acids, alkalis, salt solutions, organic solvents or similar, whose chemical aggressiveness is generally enhanced by elevated temperatures and temperature changes, even highly alloyed special materials can reach their limits. Here, a coating with correspondingly stable materials, such as PTFE, is the first choice. PTFE (polytetrafluoroethylene) is a fully fluorinated polymer (known under the trade name Teflon®) which is extremely reactive. This means that even the most aggressive acids cannot attack this coating.
The reason for this is the particularly strong atomic bonding between carbon and fluorine. Add to this the well-known, excellent gliding properties. PTFE is almost completely unwettable, as a result of which – as in the case of the soldering effect – liquids slide off this coating.
Compared to other plastics, PTFE also has a very high temperature resistance. For example, the temperature application range is from -200 °C to + 260 °C (even up to 300 °C for shorter durations). PTFE is available in various designs and alloys, which also influence the electrical conductivity. This makes PTFE the most widely used coating in the chemical industry.

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