If memory serves, adding chromium does not have much noticeable effect on rust resistance until you hit something like 8% chromium. Martensitic Stainless Steel is has a minimum chromium level of 12%.
Memory does serve you well.
I believe "true" stainless is at 13% though, as d2 is between 11-12% chromium and still falls just short of being called a stainless.
To elaborate a bit for the good Dr., keeping in mind that even stainless will rust if not cared for, at 13% chromium (and up), the saturation allows the chromium to create a miniscule passive film layer over the steel, which that film is what protects the iron against the elements and rust/corrosion.
Lower levels, like the 7.5% in 3v; 7.25% in cruwear, etc, even 12% in d2 just isn't enough for a full passive layer to form, but it still will resist corrosion better then plain carbon steels, (1045, 1055, 1075, 1095)...
(Oftentimes they'll spot rust before they actually rust)
Low levels like 1% for a2 coupled with a bit of Nickel, or 1.5% for 52100, is still better then nothing I suppose, but even at those numbers the difference is negligible at best, and those additions are more geared for other benefits like heat treat uniformity and such.