Dear friends:
I am posting this on behalf of Pedro Carvalho; he asked me to do so, so as to shed some light on this discussion. Thank you for taking the time to read this.
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I'm Pedro Miguel Carvalho the material scientist that wrote a post together with Dr Farsalinos.
In that text, there are some less-than accurate statements. I assume the responsibility (and I apologize) because Dr. Farsalinos sent me the text to participate on it, and I didn't read the whole thing with the proper care.
IMO we could move forward and have a great discussion about this theme. Obviously the bulk of resistive wire will change with a heat treatment, increase of grain size, solid solution, crystallization or re-crystallization, etc. In any case, this IMO does not present any concern. What could represent a problem is the formation of oxides layer in the surface of the metal/alloy. The thickness and composition of this layer will depend on the temperature and duration of the heat treatment and type of resistive wire. In some cases like kanthal, a "light" heat treatment can be beneficial, but a longer heat treatment can eliminate the oxide protection layer. This frontier is very narrow, so it is difficult to recommend a specific "light " heat treatment.
Alloys that contain Al or Ti present a oxide layer that protects against deeper oxidation. In the case of nickel chromium wire, the benefits of a cleaning heat treatment should be compared with the possibility of formation of hexavalent chromium. In principle, hexavalent chromium formation requires a higher temperature than 1000 Celsius degrees, but without scientific data, we cannot exclude this possibility.
Any heat treatment that causes the coil to turn red can easily exceed 900 Celsius degrees; this promotes the formation of an oxide layer (inclusively destroying the protective layer as aluminum oxide or titanium oxide). Since we have no scientific data regarding composition/adhesion, or the possible reactions of this oxide layer with the high variety of juices, IMO if its use could be avoided, in general, it would be beneficial.
Soon I will start a start a study to determine the dependence of temperature/duration of the heat treatment on the composition, thickness and adhesion of these oxide layers. I have always believed that is better to be safe than sorry, and given in the lack of scientific data, I would recommend minimizing all heat treatment to
any resistive wire, and avoiding extended and repetitive heat treatment. It will be a pleasure to further discuss this theme. Here is my contact:
pmcbfcarvalho@gmail.com. Kind regards to all.
Pedro