Maybe pack some coarse grade steel wool (rock wool) into the socket, grip it with needle nose pliers over the stem and turn it around to scrub as much of the surface corrosion off the stud as you can. Then use spray contact cleaner to flush it clean -repeat the whole process as needed. Now if the stud is badly corroded or pitted then maybe forget it and use this opportunity to treat it to a new cap and rotor, Bosch (Volvo OEM) brand of course preferred by most of us. Purists might want to use some kind of anti-corrosive coating on the stem you just made bare, like a DeoxIT product. Do be sure to seal out moisture so this problem doesn't return using a good wipe of dieletric grease inside the rubber boots to seal to the distributor cap stems and plug ceramics. Dielectric grease is sold as spark plug protector/lubricant, Sil-Glyde being a common brand (avail @Cdn Tire). No Volvo toolkit should be without it.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now