It may not be surprising (and per much literature over many years) that cement lining and eventually the pipe itself could be attacked at “high points”, at least where there could be accumulation of hydrogen sulfide gas for any extended period in any airspace. Unless however a long length of the force main is running downhill (and with no dependable air release and/or flow control device on the end to maintain line in “full” condition), it is possible that significant attack may be somewhat local in nature (depending on topography), and perhaps could lend itself to more localized re-lining or replacement of some pipes with linings/contact more suitable for this service? I guess it should also be realized that some linings can of course also be knocked out by "coupon" removal practices, though it is interestingly is very tightly-adhered in others.
The situations however where damaging, extremely low pH sulfuric acid conditions can occur have of course been explained in many references e.g. ASCE MOP #37 (WPCF MOP #9), “Design and Construction of Sanitary and Storm Sewers” (on or before 1982) and also specifically with regard to force mains, “In some instances where air has been trapped in high places in the main and where the treatment has not been sufficient to keep the main free from sulfide all along its length, damage to the pipe has occurred”, and, “If it is not feasible to construct a system that will maintain self-oxidizing conditions throughout, the engineer should anticipate the results and take the necessary remedial steps. Where substantial concentrations of sulfide will prevail, exposed walls must be protected or be constructed of acid-resistant material.” I believe it also explained further, “Interestingly, those surfaces below the wastewater level are not affected adversely because the presence of sulfides does not produce an acid condition on the exposed portions.” (I think similar information and other preventive strategies were incidentally explained also at about the same time in e.g. ASCE MOP #69, “Sulfides in Wastewater Collection and Treatment Systems”.) [See also e.g.
.]
Also for whatever it is worth, I think it has been explained in other more recent references that attack (due to sufficiently prolonged accumulation of air) in some force mains is some more likely to extend further on the downstream/downhill length of “high points” that than it does on the upstream side (I don’t exactly know the reason for this, unless it is perhaps the flow biasing the air pocket a little downstream?) It is for this reason that special acid-resistant linings have been specified at least some straddling the areas of high points of long force mains, and in some cases with some more length of specified special linings on the downhill side.
As to any other assessment tools/methods (and while I know some folks have looked at in-situ assessment of even cementlined pipes with various types of smart pigs etc.), I am not sure I am aware of the state-of-the-art in that regard or if there are any reputable folks claiming they can feasibly/accurately assess full-length the condition of buried, cementlined pipelines (like you I will await any further response). If you however do not have accurate as-builts e.g. to aid in further determining where the line has not flowed full (I would think the most suspect areas), I guess there might be other ways to determine the existing profile and/if and where there are gas/air pockets on the line (perhaps these might include some sort of traversing video or special types of smart balls or pigs, some of which that can reportedly even plot the spatial coordinates of a pipeline as they traverse same (and that could in turn, if accurate, tell you exactly where there are any other “high points”, now known or unknown). I guess it might also be possible to at least some less intrusively than coupons check the wall thickness anywhere on the line where it could be exposed using various carefully applied and calibrated “ultrasonic” instruments. However, this would of course require exposing at least the top of the pipeline.
About the only other thing I could add is that of course all safety precautions should be employed in all operations, including of course as I suspect you are aware that trapped hydrogen sulfide gas is a toxic gas (I think I remember many years ago a reported incident in which several workers were overcome, and some even on top of the ground outside a trench, by H2S release from I think then a very large concrete pipe forcemain).
As far as stray currents, I have no reason to suspect that this would necessarily be a problem due just to the conditions you describe, based on your coupon observations thus far and information that can be downloaded from DIPRA e.g. at
and
.