Erosion-corrosion is an intricate interaction between mechanical damage process (e.g. solid particle impingement) and electrochemical corrosion process which generally results in severe material loss and premature equipment failure. One of the phenomena that are attracting a lot of attentions is the so-called critical flow velocity (CFV) at which the corrosion state transit from passivation to depassivation. Experimental study indicates that the CFV is a function of sand concentration, sand size, sand density, material under erosion corrosion, and polarization potential. It is envisioned that the CFV and phenomenal laws related to the CFV are keys to elucidate the mechanisms of erosion-corrosion in depth. The first level of the systematical modeling for erosion-corrosion is to define the flow field in the sand impingement apparatus using a CFD simulation tool. The second level is to give a theoretical description of the interaction between sand and the alloy surface (specifically the passivating film) based on the concepts of collision dynamics. The experimental results are analyzed using the model. The third level of the modeling is to conduct a molecular dynamics (MD) simulation to probe the damaging process in a molecular scale. The combination of the CFD and collision dynamics models correctly predicts the pattern of erosion-corrosion of the stainless steel surface under an impingement stream. The dependences of CFV on the concentration of sand and size of the particles are correctly predicted by the model. The dependence of the CFV on the applied potential can also be accounted for by the model. Further analysis indicate that the CFV has no direct relation to the breakdown of the passivating film but correlates with the balance between thinning of the passivating film by impingement and repassivation of the corrosion system. The MD modeling gives a new perspective of the effect of impingement in that the minimum impact causing film thinning by sand is the impact that eliminates the counter charges of the electric double layer (EDL) on the fluid side. The modeling work reconfirms the previous observations that erosion-corrosion is a phenomenon characterized with interactions and cross impacts among mechanical and electrochemical processes.