Selection of corrosion resistant material for mechanical testing apparatus

I suggesting contacting a coating company. In the Chicago area a coater is VP Plating, http://www.vpfinish.com

Also, if there any pivot joints you will need bearings. You may want to contact Igus bearings or Pacific Bearing. They various chemical resistant plastics that may hold up in this environment.

I see G -46 only covers evaluation of pitting; not a test media : Pitting is not the objective of 4 point bent beam tests , eg. Shell Sc test. The standard materials for racks in general are Hastelloy C or titanium. But you need to know what the test environments will be. I have seen titanium racks burst into flame when immersed into a test solution without proper evaluation. Contact NACE in Houston , I expect they will give you lists of vendors of corrosion test equipment. ASTM may also be some help ( they used to be on Race St in Philadelphia but I believe they moved.). Your employer has apparently not provided adequate training.

You could have the parts coated with Teflon, which is one of the most chemically resistant materials known to man. During WW2, when we were processing uranium hexafluoride to make weapons grade U235, Teflon was the only material coating that could resist the corrosiveness of the uranium hexafluoride .

I looked at the composition of EXCO solution and it is NaCl 234 g/L, KNO3 50 g/L, 68 percent HNO3 6.8 mL/L.

https://doi.org/10.1108/MMMS-11-2012-0021

Given the composition of the EXCO solution I suggest titanium, maybe anodized if you want to increase its resistance further, as the best option. You can read more about the corrosion resistance of titanium here:

https://www.timet.com/assets/local/documents/technicalmanuals/corrosion.pdf