Greetings
An unprotected metal hull would be rendered unseaworthy in about 5 years. That would mean that critical parts of the hull would be sufficiently weakened or embrittled enough to threaten to break in strong weather. It is for this reason that ships are regularly maintained, as I will discuss further a bit later in this post.
Of course, there are other ways corrosion can cause problems as well, such as in the decomposition of seals (responsible for the loss of one ex-soviet vessel sold to India), and portals jamming shut due to expansion or freezing. If soldiers lives depended on keeping thier equipment functioning, however, I am sure you would see a marked improvement in the maintainance work done by soviet sailors, so there would be less of a difference there than you might think.
If looking to work out the life span of a metal ship by looking at corrosion of the hull alone, you would have to figure in the following factors:
-Availability of Zinc electrodes. Usually refitted every couple of years, without these sacrificial anodes around the hull, corrosion will be much faster.
-Surface protection, ie paint (but not always. Aluminium forms it's own protective but brittle coating, Aluminium Oxide, which protects the metal underneath). This needs replacing quite often, depending on many conditions. For most warships, the crew do this on down time about once per year, depending on how much action they see. Repairs make this needed more often, as do temperature extremes (artic or pacific heat can remove paint pretty well). Pesticides and copper in pait on the underside can reduce the damage barnicles will cause to the paint, but such paints will be harder to obtain.
-Temperature of the water. Warm water will be more corrosive than cold water, and there will also be more fouling. On the other hand, ice will be much more abrasive.
Most of these rely on regularly taking the boat out of the water every couple of years. Never a problem in peace time, but if all the major drydocks are out of commission, and they would be major naval targets, then the lifespan of the ships will drop remarkably. It should be noted that most smaller vessels (ie with thinner hulls, etc) from World War II were decomissioned before the
1980's, despite regular maintainance. If vessels were not used (not really applicable in this scenario) they might last for 20 years without proper maintainance (ie a drydock), but in action, the hulls are unlikely to last more that 10-15 years (most protections will be reduced in 5 years), and many smaller vessels (50-500t) will most likely not last that long. Fast attack craft, of which the Soviets have heaps, can be pulled out of the water by trucks, and thereby maintained. Indeed, you may even find some still hidden on thier trucks...
By the way, the Australian Wheat board has been taking tips from thier US counterparts who demand fair trading in Australia while putting up hugh protection for US farmers in the US. Ironic we get better trading terms for our wheat from China than the US, despite fighting beside US forces in most wars since the second world war...guess money does show who your friends are!
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