Research On Corrosion Behavior And Mechanism Of 2219 Aluminum Alloy Welding Joint In Weak Corrosion Environment

In recent years,China’s rocket storage tank diameter continues to gain breakthroughs,large aluminum alloy storage tank manufacturing to achieve a qualitative leap.After the manufacturing of the completed storage tank is often used deionized water for pressure testing,but after the test in the cleaning residual solution detected the presence of Cl^-and S^2-,ion residues in natural conditions will cause box corrosion,especially in the weld and seam and other parts prone to high concentrations of residue.In-service storage tanks are often welded with 2219 aluminum alloy,in which the tank body is usually welded using Friction Stir Welding（FSW）,and the support structure is often welded using Resistance Spot Welding（RSW）.Therefore,this paper investigates the mechanical properties,corrosion behavior and corrosion mechanism of the FSW and RSW joints of 2219 aluminum alloy in a mixed solution of1 mg/L Na Cl and 0.1 mg/L Na₂S in service.It was found that the precipitated phase in the HAZ of 2219aluminum alloy RSW was aggregated by thermal movement and eutectic organization appeared;deviated precipitation occurred in the FZ and Cu elements were enriched at the grain boundaries.The average shear resistance of RSW joints was 8.73 KN,and two forms of fracture existed:interfacial fracture and button fracture.Immersion corrosion test shows that the initial stage is mainly the seam and the outer surface of the metal dissolution.When the crevice corrosion into the later stages,the crevice oxygen depletion,the crevice outside the oxygen-rich,the formation of"oxygen supply difference battery".Aluminum will continue to dissolve,the charge to maintain equilibrium,Cl^-gradually into the seam,the cathodic process transferred to the seam outside.Periodic infiltration corrosion test shows that the seam is easy to residual solution,and the oxygen supply is sufficient,corrosion intensified.In the gap between the two plates corrosion is serious,the emergence of a thin and brittle layer of corrosion products,can block further erosion of the corrosion solution.By comparing the mechanical properties before and after corrosion,no obvious performance changes,mechanical bonding line tightly combined,resulting in the FZ and the external air is difficult to contact,corrosion solution is not easy to enter,did not cause obvious and substantial impact on the actual connection parts of the joint.It was found that the average tensile strength of 2219 aluminum alloy FSW joints reached 341.95MPa.The electrochemical impedance value in the WNZ was the highest,indicating excellent corrosion resistance.After immersion corrosion and cyclic infiltration corrosion,the WNZ still showed the optimal corrosion resistance,and no large size corrosion pits appeared during the long time corrosion process.The mechanism analysis shows that the corrosion performance is related to the distribution of the precipitated phase in the joint.The closer it is to the center of the WNZ,the greater the heating effect,and the more precipitated phases are dissolved back,which increases the content of Cu in the matrix,making the potential difference between the cathode and the anode decrease,thus reducing the redox reaction efficiency.Macroscopic areas of galvanic corrosion caused a large change in the shape of the BM,Cu²⁺electron precipitation,intensifying corrosion,the phenomenon of large area spalling.By comparing the mechanical properties before and after corrosion,it shows that the welded area is less sensitive to the corrosion of Cl^-and S^2-,and did not have a significant impact on the mechanical properties of the joint.Two types of welded joints exhibited corrosion starting from pitting corrosion after hydrostatic testing.Pitting corrosion pits coalesced,forming larger corrosion pits.Analysis indicated that the main corrosion products were Al₂O₃and Al Cl₃.Overall,the study revealed that a mixed solution of 1 mg/L Na Cl and 0.1 mg/L Na₂S did not significantly affect the mechanical properties of the welded joints.