Stress Corrosion Behavior And Mechanism Of 6082 Aluminum Alloy

With its advantages of low density and high strength,good ductility and strong weldability,6082 aluminum alloy has become the preferred material for lightweight design of automobiles.However,automotive chassis,steering knuckle and other parts made of 6082 aluminum alloy are prone to stress corrosion phenomenon in snow water containing Cl^- snow-melting agent,which threatens road traffic and even people’s life and property safety.At present,there are few researches on the stress corrosion behavior and corrosion mechanism of 6082 aluminum alloy,but it is of great significance to make clear the performance parameters of its related parts and improve the scientific and rationality of its use.Therefore,a three-point loading stress device,combined with electrochemical testing technology,is made in this paper,and systematically carried out stress corrosion research on 6082 aluminum alloy.The main results are as follows:（1）Electrochemical testing of 6082 aluminum alloy under different stress conditions in 1.5% NaCl solution shows that when the applied stress is increased from 0% yield strength to 100% yield strength,the corrosion current density of 6082 aluminum alloy increased by an order of magnitude from 3.422×10^-7A/cm² to 1.377×10^-6A/cm².The corrosion path gradually evolved from the vertical downward direction along the grain boundary to expand around to form stress corrosion cracking.Secondary passivation occurred in the polarization curves of 6082 aluminum alloy under stress,and the phenomenon of secondary passivation is obvious under high stress,which was mainly related to the failure form of passivation film.High stress causes obvious defects in the aluminum alloy passivation film at the initial stage of corrosion.Under the effect of polarization current,the passivation film is corroded,exposing a new aluminum alloy substrate.Thus,the newly exposed aluminum alloy substrate and electrolyte solution are oxidized again.The reduction reaction once again generates a passivation film,which slows down the corrosion process to a certain extent until the electrode potential reaches the breakdown potential of the passivation film,and the passivation film is completely broken down,thus showing two secondary passivation occurs in the polarization curves.（2）The 48-hour constant stress immersion experiment shows that the stress corrosion behavior of 6082 aluminum alloy is closely related to the second phase（AlFeMnSi,Mg₂ Si and elemental Si second phase）in the matrix.The second phase of AlMnFeSi will prevent the movement of dislocations and micro-scale stress concentration at the boundary of the substrate,causing AlMnFeSi itself to be broken or cracking the surrounding aluminum substrate,and the crack direction is perpendicular to the tensile stress.For the second phase of Mg₂ Si,since its self-corrosion potential is significantly lower than that of the aluminum matrix,the formed Mg₂Si-Al galvanic cell system is used as the anode,and causes anodic dissolution,which corrodes itself and generates penetration corrosion cracks along the second phase surface.For the elemental Si second phase,its self-corrosion potential is higher than that of the aluminum matrix.In the formed Al-Si galvanic cell system,the aluminum matrix is more easily corroded as an anode,and corrosion cracks are formed around the second phase.It was found from the segmented impedance test of the sample after 48 hours of immersion that the formation of the double capacitance arc of the Nyquist curve was controlled by the electrode potential E and the surface coverage ratio d of the adsorbed intermediate product.The state variable corresponding to the capacitive reactance arc in the high frequency region is the electrode potential E,which is caused by the charge and discharge relaxation process of the electric double layer capacitor.The state variable corresponding to the capacitive reactance arc in the low frequency region is the surface coverage ratio d of the adsorbed intermediate product.The corresponding process is a relaxation process in which the anode reaction finally generates Al³⁺.（3）In the electrochemical test of constant stress 6082 aluminum alloy in solutions with different pH values,it was found that the stress corrosion mechanism changed with the increase of pH.When the pH value increases from 2 to 7,the stress corrosion crack of aluminum alloy evolves from thick and large to thin and long;the secondary crack significantly reduces,and the stress corrosion sensitivity decreases.Under the condition of low pH value solution,obvious transgranular fracture behavior can be observed at the surface fracture of aluminum alloy,tending to embrittlement,and the corrosion mechanism is hydrogen-induced cracking.When pH increases from 7 to 12,the stress corrosion crack evolves from thin and long to thick and large again,with the maximum crack width up to 5 microns,and the stress corrosion sensitivity increases.Under the condition of high pH value solution,only a large number of dimples are observed on the surface fracture of aluminum alloy,dimples become deeper and tend to be ductile,and the corrosion mechanism is mainly anodic dissolution.