Ambient Electrochemical Beheviour Of Aluminum Alloy Affected By The Combination Of Friction Stir Welding And Inhibitors

Friction stir welding （FSW） is a revolutionary jointing technology after the laser welding.During the FSW process, the material undergoes intense plastic deformation, and thisdynamic recrystallization process would influence the electrochemical behaviours andinhibitor adsorption performance for aluminium alloy.Firstly,by gravimetric test, the conventional steady state electrochemical method-potentiodynamic polarization（PDP） curve test, electrochemical impedance spectra （EIS） andscanning electron microscope（SEM） observation, in the ambient aqueous solution, theelectrochemical behavior of5083parent metal and friction stir welding weld werecomparatively investigated; Secondly, the electrochemical behaviours affected by aluminumclad were researched; Then, with the addition of sodium molybdate as inhibitor in thecorrosive medium, by increasing the concentration,adsorption time of inhibitor and thetemperture, the effect of dynamic recrystallization after the welding on the specific adsorptionof sodium molybdate was studied,meanwhile,the interaction mechanism between thecorrosion inhibitor and weld was preliminarily discussed. Finally, by comounding with theorganic inhibitor-thiourea, the synergism between inorganic and organic inhibitors wasstudied. The results indicated:1,In the ambient temperature solution of0.2M NaHSO3and0.6M NaCl, the corrosionpotential of the5083FSW weld,at the tool rotation rate of300r/min, the traverse speed of160mm/min and3otool tilt,was more positive than those of MIG（Metal-Inert-Gas） weld andparent material, meanwhile, the corrosion rate and the corrosion current density were less thanthose of MIG weld and parent material, R_p（polarization resistance） of the FSW weld waslarger than those of MIG weld and parent material. Scanning electron microscope（SEM）observation showed that a few shallow pits presented on the surface of the dissimilar weld,however, a large number of deeper pits emerged on the surfaces of MIG weld and parentmaterial.2, Static weight loss experiment （gravimetric test） indicated that the corrosion rate of7075with Al clad was the least. Scanning electron microscope （SEM） observation showed that afew shallow pits presented on the surface of the FSW weld and7075with Al clad, however, alarge number of deeper pits emerged on the surfaces of7075without Al clad.Thepotentiodynamic polarization curve showed that the corrosion potential of7075with Al cladwas less positive than those of the FSW weld and7075without Al clad, meanwhile, thecorrosion current density were less than those of the FSW weld and7075without Al clad.Friction stir welding weld had more negative corrosion potential and lower corrosion currentcompared with that of7075without Al clad. 3, in the blank corrosion medium of0.2M NaHSO3and0.6M NaCl at room temperature,compared with2024, the corrosion potential Ecorrof FSW weld was more positive, thecorrosion current density Icorrand average corrosion rate was less, but polarization resistanceR_pincreased. After adding sodium molybdate as inhibitor in the acidic corrosive medium, byincreased the concentration or adsorption time of inhibitor, the average corrosion ratedecreased, and inhibition efficiency increased; the Ecorrwas more positive and Icorrwas less,but polarization resistance R_pand charge transfer resistance R_ctincreased, interfacecapacitance C_cand double electric layer capacitance C_dldecreased. When testing temperaturewas given at30oC, the minimum average corrosion rate and maximum inhibition efficiencywere obtained. These results indicated that: during the FSW processing, the slightly elongatedand larger grain structure in2024parent metal was significantly recrystallized and graduallyproduced the refined and equiaxed grains, which made specific adsorption ability on surfaceof FSW weld better than that of2024parent metal. This indicateed that more active points（like grain boundary, triple grain boundary） can be the adsorption positions for the inorganicmolecules.4, On the basis of the sodium molybdate with a given concentration, thiourea was chosen asthe compounding inhibitor for the wide application in industry to study the adsorption modechange between the interface of organic molecule and matal. The results showed that a simplecapacitive loop appeared on the Nyquist plots containing different concentrations of thioureafor2024and FSWed weld. With the increase of concentration of thiourea, inhibitionefficiency got a critical value and the inhibition mechanism was a geometric blockingmechanism. The synergism between inorganic and organic inhibitors influenced the inhibitionefficiency.