Surface modification through friction stir processing of aluminum magnesium alloys occurs through high-strain plasticity, frictional heating, dynamic recrystallization and particle redistribution. In the case of surface nanocomposites involving AA5052-type \AlMg/\TiOtwo\ system, the same microstructure changes leading to hardness enhancement may be linked to decreased tensile continuity and increased corrosion sensitivity toward chlorides. The present investigation involves eight processing conditions; a bare alloy, friction stir processed without reinforcement, and three air processing conditions for \TiOtwo\ loading of 3.1, 6.1 and 9.1 vol.\% at 1200 rpm and 100 mm min$^{-1}$, together with three 3.1 vol.\% \TiOtwo\ contents for 1400 rpm and 50 mm min$^{-1}$ processing under air, water/dry-ice and liquid nitrogen cooling regimes. The measurable variables are: mean grain size, boundary character, hardness, elastic modulus, yield strength, ultimate tensile strength, fracture strength, elongation, corrosion potential, corrosion current density, corrosion rate, solution resistance, constant phase element magnitude and polarization resistance. From these values, the corrosion-mechanical selection index value is computed using normalized mechanical effectiveness, electrochemical efficiency and boundary-safety contributions. Increased \TiOtwo\ content in the range 3.1 to 6.1 vol.\% leads to a higher hardness value in the range 88.0 to 103.0 HV while causing a decrease in elongation from 12.7\% to 3.0\%, yield strength from 253.0 to 146.0 MPa, and increasing corrosion rate from 0.02090 to 0.17330 mm yr$^{-1}$. Controlled heat extraction using the fixed 3.1 vol.\% \TiOtwo\ loading produces a better overall performance. Among these, water/dry ice processing yields the highest value of $S_i=0.721$ based on hardness 131.7 HV, yield strength 154.3 MPa, ultimate tensile strength 263.7 MPa, elongation 16.1\%, corrosion rate 0.00387 mm yr$^{-1}$, and boundary safety measure. Strong mechanical effect occurs under liquid-nitrogen processing, although it is constrained by its high boundary risk due to extreme 0.2 \micron\ mean grain size.

AA5052 alloy,friction stir processing,titanium dioxide,surface nanocomposite,chloride corrosion,electrochemical impedance,mechanical selection,grain-boundary character,desirability analysis