To achieve the long‐term corrosion protection of mild steel under corrosive environment, pure aluminium coatings were cold sprayed (CS) on mild steel with three different powder sizes, namely within the three following classes 85–100, 60–75 and 40–50 µm. The as‐sprayed samples were exposed to the atmosphere for 11 years. The long‐term corrosion behaviour of the sprayed steels was studied by atmospheric exposure, immersion tests as well as electrochemical measurements in 3.5 wt% sodium chloride (NaCl) solution. It was found that the coatings with coarse powder reduced the long‐term corrosion resistance of the samples. The deteriorated corrosion resistance of the sprayed samples was ascribed to the penetration of the corrosive medium (oxygen, water and electrolyte) through the porosity defects of coatings and with coarse powder. These results provide a guide for the design of cold sprayed corrosion‐resistance pure Al coatings for engineering applications.