Oh my, what a corroded spade you have there! These will of course corrode over time, but seems strange that one side has gone in such a short period of time, but the other side hasn't. . .
Happy to issue a replacement?
There's a couple of things you can try to prolong the life.
1. Regular cleaning with a soft implement.
2. If it doesn't affect the reading (or you can adjust for the difference), perhaps try putting some grease or 3 in 1 oil on the prongs, just to give them some extra protection? Worth a test anyway. Hammerite would probably affect the reading too much