From all I've been able to read/gleen over the years, the most severe form of corrosion (at least) does not necessarily occur immediately, but instead may require some significant duration of time with a somewhat stationary, accumulated air space (for the colonies of bacteria to "do their thing").
With regard to your restated or new question about moving the "air bubble", I know ASCE MOP No. 36, Wastewater Treatment Plant Design (1977) contains the statement, "A velocity of at least 1.2 m/s (4 fps) is required in the pipeline to shear the bubble and to keep it moving down grade." However, the subject of flow in various pipeline services for effectively scavenging, scouring, or shearing air bubbles from high points though without working air valves has been the subject of many other discussions on these blogs and others over the years (I think sometimes suggesting more complicated determinations and more/complex parameters). I suspect with a good search and some of these key words you could find much more information and other viewpoints.