Long service, brittleness, and hull stress

Did spending longer periods at sea make early iron or steel ships more brittle?

Not usually; the more important concerns were bending moments and the other repeated stresses imposed on the hull by the sea.

Time at sea does not automatically make a metal hull brittle. Bending moments and other stresses imposed by waves and ship motion are the more immediate concern in this answer. Material condition and structural loading are related, but they are not interchangeable problems. Aging-hull questions need to account for actual load paths as well as elapsed time.