When we include a salt such as NaCl in solution, it will dissociate into Na+ and Cl- ions. These ions increase the conductivity of the solution (i.e. the ability of electrons to flow). Conductivity plays a role in corrosion since corrosion is a redox reaction that depends on the flow of electrons from one of the metals toward the other. So increased ability of electrons to flow means that corrosion will happen faster. Furthermore, increased concentration of NaCl will increase conductivity, but only until NaCl is saturated in solution (i.e. the NaCl stops dissociating into its ion pairs). I hope this helps.