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I want to build a 4-point bending apparatus for operating within a corrosive environment, using EXCO type corrosion solution as seen in ASTM Standard G46. Thus, in this highly corrosive environment, I need a very stiff material to act as the testing apparatus. Obviously, metals would be preferred.

Can anyone suggest a metal that would be immune to corrosion, and/or an easy method to coat a more generic metal, say tool steel, such that it is immune?

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  • $\begingroup$ Gold is resistant to corrosion - why it is used in jewellery... $\endgroup$ – Solar Mike Feb 14 '18 at 9:18
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    $\begingroup$ chrome or a chrome plated material may be a cost-effective alternative, while also quite corrosion resistant. $\endgroup$ – Bart Feb 14 '18 at 19:40
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    $\begingroup$ In that environment almost any metal is going to suffer. The best you can do, in my view, is extend the life. You could consider austenitic stainless 316 or 317 but your success may come down to the quality of the surface finish - any breaks or damage will cause problems. If using coatings such as epoxy-based options go with fusion bonded rather than liquid applied. This is an expensive option, however. There is also the possibility of zinc dipping. In any case a high quality surface finish will be critical. $\endgroup$ – AsymLabs Feb 14 '18 at 22:34
  • $\begingroup$ Can I actually do any of these things? Are there companies to whom I can send my parts for coatings? $\endgroup$ – User2341 Feb 15 '18 at 19:12
  • $\begingroup$ ceramics maybe. Do you really need to do the test in the solution? You might do exposure, rinse, then test. $\endgroup$ – agentp Feb 17 '18 at 14:37
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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.

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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.

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  • $\begingroup$ NACE can also provide standards, test methods, courses, etc . for any aspect of corrosion . $\endgroup$ – blacksmith37 Feb 19 '18 at 17:12
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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 .

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  • $\begingroup$ Teflon is permeable to some materials. I have seen a number of Teflon coated magnetic stirrers swell and burst in water based corrosion test solutions. $\endgroup$ – blacksmith37 Oct 17 '18 at 22:06
  • $\begingroup$ @blacksmith37: sounds like the Teflon was not applied correctly. You can search companies that apply Teflon for corrosion resistance and see that applying it to metal is a well studied process. $\endgroup$ – William Hird Oct 17 '18 at 22:16
  • $\begingroup$ They were commercial standard lab equipment stirrers. $\endgroup$ – blacksmith37 Oct 18 '18 at 18:54
  • $\begingroup$ Did you do a failure analysis as to why the coating failed? Did you notify the manufacturer as to the failure, what did they say? $\endgroup$ – William Hird Oct 18 '18 at 20:33
  • $\begingroup$ The corrosion tests typically ran 1000 hr : If the molded cover on a stirrer was swelled , it was just thrown away. The teflon was not failing , it was just permeable to a few certain test solutions. $\endgroup$ – blacksmith37 Oct 20 '18 at 3:40
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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

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