Can you give a few more details about the material you're studying, the purpose of your research endeavour and what do you exactly mean by 'reactivity'?
Without any further information, I recommend marking your ROI with some persistant markers (f.e. hardness indentations) and try a remapping after the reaction took place (Maybe you mean sth like etching).
Afterwards you can align the two mappings with tools like imagej to see what happens to the different orientations in your polycrystal.