Cast Iron is carbon rich compared to standard steel. Typically it has a content of about 4% carbon. However, cast iron is not just an alloy of iron and carbon. It contains silicon as well in the type of cast iron we are talking about here, but suffice to say, I won't discuss the different grades of cast iron here.
Anyhow, molten aluminium in contact with the cast iron creates intermetallic compounds in layers at the interface, typically made from Fe2Al5 and FeAl3. The aluminium diffuses into the cast to produce areas that are high in aluminium. An exchange occurs with the silicon moving upwards into the intermetallic compound layers while the aluminium moves down.Now the next part is very complex but suffice to say that aluminium carbide, Al4C3, is produced is produced in the grate bars where the grate surface has been in contact with the aluminium. This aluminium carbide is extremely brittle and degrades readily due to the crystalline structure involved and is the reason the bars become weak and brittle and eventually drop off.