I have resorted to culverts with UT monitoring on some of the main transmission lines I manage, an expensive way to get corrosion rates, but much better than coupons or probes.
I don't know how the question became a discussion on chemical inhibition, but David I have certainly seen my share of corrosion failures on lines that were pigged regularly (twice per month) with no chemical inhibition. I can also say if I stopped applying chemical to most of the sour multiphase pipelines I manage I would see a spike in the number of failures, as this was the case before applying the inhibitor with everything else equal. Like anything else though, you have to know the details of what you are doing rather than just put chemicals in the line and hope for the best.