I think the alloying element you added was more electronegative than the steel (the base metal). In this case the charge transfer occurs from the base metal to the alloying element then these electrons accumulated close to the alloying element and the extent of this process increase as the percent of alloying element increase. Now at low percent of alloying element, limited electrons on the based metal shift the potential more positive, which slows the cathodic reaction and speeds up the anodic reaction. This is the case of higher corrosion rate. Now at high percent of alloying element the excess electrons being located near the alloying element and under concentration polarization condition of cathodic reaction the potential of the metal then shift more negative, which slows the anodic reaction and limits the cathodic reaction. This is the case of lower corrosion rate.

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What kind of alloying elements was added ?? As there is not an information about....any discussion is not valid.

In general the alloying elements are characterizing into two groups either more electropositive or more electronegative than the base metal. Now whatever the alloying element was used by Prof. Satendra Kumar , it is the more important thing in this discussion, the alloying element belongs to which group. After the hard comment of Prof. L. Veleva about the answer validity, I did my best to support my idea by published literatures. Finally I found that the abstract of the work by Prof. H.Ezaki etal. is very supportive (try the link blow). Please recommend the answer based on, alloying elements grouping in terms of their electro activity toward the based metal.

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https://www.sciencedirect.com/science/article/pii/001346869385012N

thanks for the information and knowledge sharing.