This is no problem. With salt water, however, the pattern of corrosion takes an entirely different direction. Pitting will show as white spots of oxide. Often the spots will appear as raised boils on the otherwise clean surface. If you get rid of the aluminum oxide you'll will reveal severe scarring.
Pitting is an absolute contra-indication to further service.
Old tanks may appear mottled rather than dull gray. This could be a combination of the innocuous surface oxide and residues from poorly filtered air. Whenever a cylinder appears mottled, try to rinse it out. Since the tank probably is very old the mottling could hide more serious problems. If there's the slightest indication of pitting, condemn it.