I performed a DSC analysis of a 7075 aluminum alloy during which the sample was melted. After analysis, the whole sensor was covered with a thin metal layer (probably mainly zinc and aluminum). It is an S-type sensor (Pt/Rh). I tried to etch this deposited layer with HCl and also HNO3, but nothing happened. Does anyone have any recommendations on how to clean the sensor?
Thank you
Both aluminum and zinc dissolve in alkali (e.g. NaOH solution) with evolution of hydrogen, forming soluble aluminates and zincates. You can test a moderate concentration of NaOH (e.g. 2-5%) on a little piece of your alloy and verify that it dissolves.
One thing to check before dipping the sensor in base is whether the sensor is insulated in a material that is sensitive to base (some ceramics are). The manufacturer of your DSC should be able to supply this information if it's not already in the DSC manual.
Pt and Rh will not be affected by base. However, if some Al and/or Zn diffused into the noble metal alloy its temperature readings may change. Therefore you will need to retest the system with standards for temperature reading.
You can read: Nanostructured Sensors for Detection of Heavy Metals: A Review
Article Nanostructured Sensors for Detection of Heavy Metals: A Review
Dear Ondrej, I suggest you try cleaning in an acidic solution diluted in an ultrasound bath.
Emanuel Cooper Thank you for your recommendation. I will definitely try it so we will see
Maria Betânia de Freitas Marques I have already tried to clean it in acid and nothing happened. However, I did not use ultrasound. Thank you, I will try it.
The ultrasound vibration can help you. Try it initially for 10 minutes, in acid solution.
Ondřej Adam I am not pretty much sure that ultrasound with an acidic solution will work. In the case of the use of the acid solution, the Pt/Rh sensor may get corroded or degraded. If the tip of the thermocouple gets reacted with deposited materials and make it weaker while giving acidic cleaning treatment, you might end with big trouble. Let us understand the purpose and its use for temperature sensing. Thermocouples work on the principle of the Seebeck effect, if materials get reacted with acid or some other compositions form surrounded to the tip region, the measured temperature might be different. If nothing reacted with tip, then it is well and good. Although some external materials get deposited on the tip of the thermocouple, you can polish it by using sand paper or diamond filer or something by which you can easily clean it. If it even gives trouble, you can use the soldering removal tools, or heat little the tip using a soldering iron rod. The most important point is after the cleaning treatment, you should read the temperature accurately.
If funding is not a big issue, I recommend replacing the sensor. DSC is a very sensitive technique in which measurement conditions involve temperature and a very small amount of sample is used to study the thermodynamic effect. Any ambiguity might result in bad data collection.
Of course you can try many solutions provided, the only thing you need to keep in mind is that, after the machine is fixed from trouble, you should be able to get the data reproducible on some standard sample. I hope the problem will be solved soon.
Thank you all for the advice. I tried to clean the sensor in alkali and acids, which helped a bit, but it did not work well enough. In the end, I heated the sensor to 1200 °C in an argon atmosphere, and almost all of the metal layer was removed. After testing, the thermocouple seems to work properly
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