Corrosion only occurs at the surface of the material. It doesn't matter if the sample is .01mm or 1mm thick. Material is not lost from the interior of the sample, only the surfaces.
Measure the specimen, immerse in fluid. After specified time measure again.
I am assuming you are looking for a corrosion test coupon, which water treatment companies will provide to very specific sizes and weights for measurement of corrosion rates, and for verification of the effectiveness of a particular water treatment program.
You could contact water treatment chemical companies with your request. You might also try to internet search corrosion consultants to see if they can offer advice where to obtain.
You might also internet search corrosion test standards to determine the dimensional requirements and have a capable shop machine the test coupons to your specific requirements.
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Eventually, one needs to realize that it is far less important to be the smartest person in the room than it is to sit next to that person and make friends.
It is entirely possible that the A106 Gr.B material has already been corrosion-tested with the wetting fluid, which you haven't identified yet (and I am getting tired of this game), and that the results are available elsewhere. Try the Cole-Parmer materials compatibility database. Oh, you haven't specified the temperature either, which can be significant.
I'm off to talk to Michelle of the Resistance instead. At least I can understand what she is trying to achieve. <unsubscribes>