It may be a statement of the obvious but rust/corrosion of exposed steel is a chemical reaction that begins very very quickly. Check out the brake discs on you car if it is left sitting in damp conditions for a day or so. They will have turned a nice orange rust colour. Leave it for longer and the discs will begin to pit as the corrosion eats into them.
Looking at ships built in WW2 I have read that the build quality was not that of pre-war built ships. That applied across the board, not just to carriers. It has been referred to by various authors over the years. But the pressure was to get ships completed and into service quickly for this war without any real regard to their longevity. So, for example, the warbuilt cruisers of the Dido/Fiji classes did not fair so well as the Town class in the corrosion stakes in postwar years. I have a photo in a book of the WW2 destroyer Vigilant undergoing conversion to a Type 15 frigate (between mid-1951 and mid-1953). About the lowest one third of her underwater hull plates have been removed. That was in a ship laid down in Jan 1942, launched in Dec and completed in Sept 1943, so in the water for less than 10 years.
But severe corrosion was not confined to wartime builds. "Rebuilding the Royal Navy" by DK Brown and George Moore on p180 refers to the frigate Rothesay (built 1956-60) being found to "have fourteen longitudinals on one side and nine on the other so badly corroded as to be virtually useless - it was lucky she did did not break in half." He blames machinery spaces that were warm and moist in steam powered ships and many bilge spaces being almost inaccessible.
There were multiple problems. In some cases steel was not necessarily of the same quality as pre-war, containing less in the way of elements that aid corrosion resistance (eg chromium) due to wartime shortages. There were also issues arising from the quality of construction that varied from yard to yard. I recall reading a long time ago that the first CO of the new cruiser Bellona thought seriously about not taking delivery of the ship in Oct 1943 due to worries about some of the workmanship. With the pressure in the yards undoubtedly corners did get cut. And disgruntled workforces only added to the problems.
With regard to Hermes, note my comment about the metalwork being oiled while she remained suspended. I don't know if the same care was taken with the Ark Royal which was also left sitting on the slip for years. Certainly so long as the steel remained unpainted it remained in greater danger of the dreaded tin worm. And in that respect the other 3 had a distinct advantage. And again my notes about Centaur & Albion getting a bottom clean and repaint before construction restarted.
Brown/Moore note that significant developments in paint and the preparation for painting only begin about 1960 and add to the cost of the ship (2-3%). They point out that in 1982 Falklands War Hermes was the only ship originating in WW2 and was noticably more rusty that any other on her return.
