The corrosion properties of AZ91 alloys are determined by the amount of impurities, enriching the alloy during the recycling process. However within reasonable costs the recycling of magnesium scrap metal results only in AZ91B quality and respectively poor corrosion behavior can be expected. This influence on the corrosion resistance was studied by controlled additions of Fe, Ni, Cu, and Si to AZ91D alloy. The corrosion properties of the gravity permanent mold castings were studied by various corrosion tests and correlated to the observed microstructure and phase composition. Most of the impurities were found as, or in, intermetallic phases. The influence on the corrosion resistance was found to be depending on the solidification behavior of the particular intermetallic phase. Altogether the corrosion test results indicate a much higher tolerance against the impurities than expected. Effects on the possible use of secondary AZ91 alloys are discussed.