C220 w202 Gasket failure

[Prickleypear]

For the first time in my life i see a car head gasket fail due to the cast iron block rusting away instead of the aluminium head. see picture attached only abour 2 mm surface left for the gasket to seal on at cylinder number 4.

i was driving sunday and stopped to fill up with fuel, about 15 km travelled i suddenly see a cloud of white smoke behind me... my first though was that i got bad fuel but then i realised its not smoke but steam coming from the exhaust since it evaporated quick not hang like smoke... as i pulled off the road the low coolant light came on but the heatgauge was still normal. result a blown headgasket without being caused by overheating... probably the previous owners never bothered with water conditioning but my question is, is this common that the block rather than the head will corrode on these models or was i just the winner of a booby prize..