You will see both, Erosion-corrosion (occurring at the same time) is also a big issue. Depending on flow rates, materials, substances transported, presence of bubbles,... you can have a lot of overlapping issues. You can just as easily have protective layers scratched off by abrasive slurries to reveal a more vulnerable substrate as you can have passive layers attacked by ions to reveal more corrosion prone substrates....then you can even have cavitation.... I recommend taking some NACE International courses. I work in the failure analysis of these materials...and I see both. Chloride damage is the most common...second most (in a close second too) is material compatibility.
Thank you Antonio for your answer. Erosion and Corrosion remain a complicated synergy so difficult to correlate to many interrelated controlled, and uncontrolled, environmental and metallurgical factors.
Restricting to metal pipes, a few observations regarding erosion or corrosion are:
[1]Though both enhance each other (corrosion increases erosion and vice-versa), the dominant process is generally considered to be the effect of corrosion on erosion. Part of erosion is some corrosion. Corrosion is a chemical or electrochemical process while erosion can happen both by physical or chemical means.The conjoint action of erosion and corrosion in the aqueous environments is called erosion –corrosion. In oil and gas production pipes, the effect of sand on erosion –corrosion is a very serious problem. The combined effect of erosion and corrosion is generally higher than when these two processes act separately. This net effect is called synergism.
[2] As only a little work has been reported in the literature on flow loops, it is not possible say it with authenticity as to whether erosion exacerbates the corrosion or vice versa and the problem stands wide open.