![Shocked :shock:](./images/smilies/icon_eek.gif)
Have any other steel parts suffered the same?
Do you have an inlet manifold/heater take off? that's steel on my GEMS engine. Could check to see if it is as corroded as badly your core plugs?
Maybe you could do a test with some of your antifreeze in a bowl and drop in some steel parts to see if they corrode?
Could this be down to electrolysis? maye part of the core plug was uncoated and in direct contact with the aluminium block?
Tom.