Does Spartan armour rust???

All metallic materials oxidize and corrode in some fashion, some are just more resistant to it than others. Basically if there's metal coming into contact with oxygen or an acid there'll be an oxidation layer created where chemical reactions will occur transferring electrons and creating impurities.

If you have a fancy space alloy with platinum or iridium that are corrosion resistant there'd likely not be visible corrosion developed in the lifetime of a suit unless dunked in a strong acid. The only downside to alloys like this are the weight and cost of producing them, a kilogram of platinum(90%)-iridium(10%) is on the order of $100k (check out the International Prototype Kilogram for fancy corrosion resistant alloys). I could see something like this being used to match the description of being incredibly strong, weighing a half tonne when worn and being the cost of a small Starfighter.

You could go the cheaper route of using a steel that's alloyed with a noble metal. This will give you the rust colour eventually if the armour doesn't have a surface coating and isn't cared for. Surface coating of metals is basic preventative maintenance to help keep rust at bay and you see a lot of brightly coloured Spartans running around which means they're protecting their armour and showing team pride. Friction points are the only area that I see surface coatings being removed unless the shielding drops somehow so I'd only expect to see corrosion around joints and overlaps of armour.

Assumptions that I made when doing my suit, maintenance and cleaning is being completed regularly so paint is being either layered or ground down to cover damaged areas. If there is bare metal it is from recent damage from scrapes and scratches, this will leave a bright metal as opposed to a rust because the top surfaces have been removed (where rust/oxidation would form). Space alloys are used and it's likely not a iron based material so you won't see red iron-oxide rust it'll be either a white surface corrosion or a dull-lustre to the surface for oxidation of things like titanium.

 

After the second world war, many Japanese soldiers were left on islands, and some waited DECADES before coming back. In one instance, I forget the guy's name, so take this with a grain of salt, but the soldier hard done his best, stranded on the island, to keep everything in perfect working order. His weapon had rusted from decades of harsh sea air, but his uniform was PERFECT.

Let me interject a personal note, those who worked on halo (bungie, 343i) did it for the money, and honestly I'm not sure they care too super much about consistency or researching what they've done. Not to rag on them, they did a great job. But if you had a person who has been through everything a SPARTAN had been through, in real life, with all the training, respect, drilling, and discipline that lifestyle dictates, along with the materials and tech advances mentioned above, I am not confident you would ever see rust anywhere on a SPARTAN.

Would also not hear Marines complain as frequently, either.