During my PM, i would normally replace the spark plugs. During my last PM after only less than 3000 km run, the spark plugs are all burned as in black with brown in the tip (parang corrosion) my mechanic changed it with a slightly used but it's still sparkling clean. Now after about 2000 kms run, all are burned again with even one of the plugs, loosing the electrode tip ba yun (not the bent portion). Now i replaced it with a brand new Bosch Super 4.
I knew it's not normal caus emy other cars would even last 20K km before i would cahnge spark plugs. Car is CRV 2000 model.

Question is, what would have caused my unusual burning of spark plug?
TIA