Effect Of Intergranular Corrosion Of High Magnesium Alloy In Simulated Tropical Ocean Environment

Aluminum alloy has a good development future in the shipbuilding industry due to its light weight,excellent mechanical properties and good corrosion resistance.Aluminum alloys are used in more and more ships as hull structural materials in various countries.However,some of its corrosion problems have restricted its usage in the shipbuilding industry.High-magnesium aluminum alloy is one of the commonly aluminum alloys used on ships.However,the material is prone to intergranular corrosion when it is used in tropical waters.With the deterioration of the external environment,the intergranular corrosion of aluminum alloys becomes more and more serious,and the tendency to develop toward stress corrosion cracking gradually jeopardizes the normal navigation of the ship.Aiming at solving this problem,the thesis studies the intergranular corrosion behavior of high-magnesium aluminum alloy in the tropical ocean atmosphere and provides a theoretical basis for the corrosion protection of aluminum vessels and the development of new marine aluminum alloy materials.The effects of intergranular corrosion behavior of high magnesium aluminum alloy in tropical ocean atmosphere were studied by PANDAT software,ImageJ-pro software,intergranular corrosion test,metallographic microscope and scanning electron microscope(SEM,EDS).The results show that the higher the Mg content is,the more ? phase(A12Mg3)with electrochemically active properties is formed by thermodynamic equilibrium simulation analysis.When the temperature is from room temperature to 80?,the percentage of corrosion area of high-magnesium aluminum alloy rises sharply.When the temperature is between 80?and 100?,the percentage increase of corrosion area becomes slow.Under the same conditions,aluminum alloys with higher magnesium content are more prone to severe intergranular corrosion than that with lower magnesium content.After sensitization of the aluminum alloy,Mg and Mn elements segregate to the vicinity of the grain boundary and form a micro-couple,which aggravates the intergranular corrosion of the aluminum alloy.The intergranular corrosion sensitivity of high-magnesium aluminum alloy increases with the increase of sensitization temperature and sensitization time.The former has a more significant influence on its sensitivity than the latter.The critical temperature(CT)is 80?.If the high magnesium aluminum alloy is sensitized at a temperature more than CT,the intergranular corrosion sensitivity is significantly improved.The electrochemical corrosion behavior of high-magnesium aluminum alloy after thermal exposure treatment and its effect on corrosion resistance were studied by using dynamic potential polarization curve,cyclic polarization curve and constant potential test.The results show that under the same exposure conditions,the aluminum alloy with higher magnesium content is more prone to intergranular corrosion than that with low magnesium content.At the same exposure temperature,the self-corrosion potentials of the two high-magnesium aluminum alloys shift negatively with increasing exposure time,and the self-corrosion currents become larger as the exposure time increasing.Under the same exposure time,the self-corrosion potentials of the two high-magnesium aluminum alloys decrease first and then increase with the increase of temperature.Their self-corrosion currents increase first and then decrease with the increase of temperature.When the other conditions are constant,as the retrace current density increases,the breaking potential Eb of the high-magnesium aluminum alloy gradually increases,and the protective potential Ep gradually decreases.At a constant retrace current density as the exposure temperature increasing,the breaking potentials of the high-magnesium aluminum alloy don't change significantly,and the protective potentials moves negatively,and the potential difference also increases.The results indicates sensitivity of intergranular corrosion is enhanced.Electron probe,scanning electron microscope(SEM)and self-contained EDS elemental analysis,corrosion weight loss test were conducted to study the composition of corrosion products of high magnesium aluminum alloy after being exposed in the simulating tropical marine atmospheric.The results show that the corrosion products of high-magnesium aluminum alloy are floc in the simulating tropical ocean atmosphere,and the main component of the corrosion product is Al2O3.The high-magnesium aluminum alloy after the intergranular corrosion test,the main elements are C,O,Mg,Al,Mn.The Mg and Al elements in the intergranular corrosion cracks are less than those in other regions,indicating that the(3 phase(A13Mg2)is formed or Mg is segregated near the grain boundaries.Therefore,corrosion occurs preferentially in the regions with higer Mg and results in intergranular corrosion.The potential difference between the Mn-rich region and aluminum is bigger than the(3 phase(Al3Mg2),and has a stronger driving force to cause corrosion and results in more serious intergranular corrosion.The corrosion weight loss rate increases when exposed at the temperature from room temperature to 80?,and decreases significantly at 80? and 90?,and rises again when exposed between 90? and 100?.The corrosion weight loss rate of aluminum alloy is proportional to the Mg content of the aluminum alloy.