Low alloy steel panels, exposed in marine atmosphere for 4 years, formed a compacted rust layer which could suppress the penetration of corrosive electrolyte and lower the corrosion rate. But in practical applications, the protective reliability is limited by the mechanical properties of the rust layer under the action of various loads. This paper presented the first attempt to evaluate the variation of mechanical properties of the protective rust layer with the exposure term by means of micro-indentation testing. Based on the analysis of energy release during cracking, the fracture toughness and the adhesion strength of the rust layer were evaluated. Results showed that the fracture toughness and adhesion strength of the rust layer were improved with the prolongation of the exposure term, the adhesion strength of the rust layer was higher than the fracture toughness of the rust layer itself.