Analysis Of7050Aluminum Alloy Grain Boundary Characteristics And Its Effects On Alloy Corrosion Behavior

In this paper, transmission electron microscopy (TEM) was chosen toobserve and analyse the characteristics of the alloy intragranular precipitates, thegrain boundary precipitates and the grain boundary of the precipitate free zonecharacteristics of7050aluminum alloy treated by isothermal and non-isothermalaging crafts. In particular, observation of distribution of grain boundary regionscomponents, EDS analysis of the grain boundary regions and semi-quantitativeresearch on the distribution of different alloying elements in the grain boundarywere carried out by the HADDF-STEM mode of TEM.The surface potential of alloys in different aging treatments treated bychemical etching was tested by the use of Scanning Kelvin Probe Microscopecroscopy, and detailed analysis was speially carried out on the surface potentialof grain boundary region. Relationship between the component of the grainboundary region and surface potential was studied. Grain boundary precipitateswere made up of a certain percentage elements of Cu, Mg, Zn; the three kinds ofalloying elements in grain boundary area of alloy treated by different agingtreatments were in different proportions. The intensity ratio of the three elementsincreased as the non-isothermal aging time extends. In the grain boundaryprecipitate free zone location, contented of the three elements dilutes and theintensity ratio was lower than the matrix.Analysis of the pitting morphology and incentives which caused the pittingpotential varies was carried out by ways of measuring polarization curves,characterizing the pitting potential and corrosion current of each aging statealloy.The EBSD analysis was carried out on potentiodynamic polarization testsamples to exlpore the relationship between pitting location and grain boundary,the correlation between pitting behavior and grain boundary characteristics. Itwas found that the pitting often occured preferentially at grain boundaries, thenexpanded into the grains.The EXCO solution immersion method was used to analyze theintergranular corrosion and exfoliation corrosion behavior of7050aluminumalloy in various aging states. Scanning electron microscopy (SEM) was used to observe the corrosion morphology of alloy removed of corrosion products.Thefollowing main result was drawn from these experiments. Corrosion developmentprocess of the alloy in different aging treatment was similar, but the corrosiondeveloped at different rates. With the immersion time entended, corrosion of thedevelopment process was that intermittent pits appeared at the grain boundaries,then the pits constantly grew and developed towards the depth, intermittentcorrosion pits gradually putted together forming intergranular corrosion. As thesoaking time continued, the existing corrosion product provided external force,developing into overt exfoliation corrosion.