What kind of material is the impeller made of? Cast iron? Steel? As said above, the coolant may have been neglected previously in the car's life and if so, the rust wouldn't be unusual. Iron would be more prone to rust than steel, especially if the coolant was old and lost its anti-corrosion properties at some point in the past.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]