Weld a plate without root purge and test it with an accelerated corrosion test. Many laboratories that do environmental testing have the equipment needed to perform such tests. The corrosion rate will be dependent on several factors such as the corrodents, concentrations, temperature, etc.
One test assembly can cut into several pieces that can be tested under different conditions. That would give you a sound basis for making some intelligent choices.
You can review the ASTM’s offerings on the subject of corrosion testing. NACE would also be an organization that may provide direction on corrosion testing.
Now, I’m just thinking out loud, but I suspect that if there is no discoloration on the root surface of the weld or adjacent base metal, the corrosion resistance is probably nearly as good as a root that has been purged. Consider for a moment that corrosion inside a pipe will occur if an electrolyte encounters dissimilar metals. The weld deposit is going to be different than the base metal in chemical composition or grain structure or both, either of which can lead to corrosion. It’s a question of whether the corrosion rate is low enough to provide the service life that makes it all worthwhile. Of course, we can’t overlook the material’s that are selected for the particular service. If the wrong material is used, the service life may not be as expected.
Al
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