I have an ecoquest fresh air purifier and the end

That corrosion is oxidation for the lug connecting the grid plate.Metal contact fingers make contact on the plates metal grids.The same corrosion exists on those metal contact fingers and the plate itself. Clean the fingers,ensure good connections as the other wire connector spade for the other contact finger is probably compromised. You might de-oxidize the plate by soaking in pure white ammonia.At any rate,I agree about the quality of the machines and you might be faced with a small parts order from eco-quest.Do include the plate, a bulb and the bulb starter. I run 5 of these in a commercial setting and they are fairly enjoyable to work on.