The Stress Corrosion Cracking Behavior Of 7000 Series Aluminum Alloy

The stress corrosion cracking (SCC) behavior of the 7000series aluminum alloy has been studied for many years,but for T5 aging treatment the research is less.pH of solution might be not extremes in the prescription,but the time or service conditions may lead to unexpected SCC failure.However with the coat surface treatment and applied potential protection can effectively reduce the SCC sensitivity of aluminum alloy and prolong the service life of materials. However the researchs of effect of coating on SCC of aluminum is rarely metioned.,in addition,only the appropriate polarization potential can paly a protective role in SCC. Thus, the research of the SCC behavior of aluminum alloy with T5 aging treatment under different conditions can further understand the mechanism of SCC and the protection of both coating and external polarization potential, which can provide a reference for ues of materials in actual life.In this paper,the effect of strain on the electrochemical characteristics of a 7000 sery aluminum alloy of T5 aging in 3.5wt% NaCl solution at different pH was studied by potentiodynamic polarization test.Slow strain rate test (SSRT) have been carried out on the alloy in 3.5wt% NaCl solution at different pH,studied not only the influence of solution pH on the SCC behavior of the aluminum alloy but also the protection of constant potential polarization and coating on SCC in those conditions.The observed SCC behavior of the alloy has been explained with SEM fractographs and the effect of strain on protection of coat was studied by EIS. The main conclusions are as follows:The alloy had different electrochemical behavior in different pH 3.5wt% NaCl solution, moreover, after loading 2% strain the open circuit potential (OCP) shifted negatively and corrosion current increased in all tested pH condition. SSRT results exhibited that pH of the solution has obvious effect on stress corrosion cracking susceptibility in 3.5wt%.The alloy had a high SCC sensitivity in solution pH 2 and decreased to the lowest in solution pH 5,then the SCC sensitivity became more and more severe with increase of pH.The order of the SCC sensitivity in 3.5wt%NaCl solution at different pH is as follow:pH 12>pH 9>pH 2> pH7>pH5.External polarization potential have a significantly influence on SCC sensitivity of aluminum alloy in pH7 3.5wt% NaCl solution:there is a significantly reduce of SCC sensitivity when the polarization potential is -1.01V(vs SEC) and when polarization potential was -0.91V,the sample had the same SCC sensitivity with the not polarized one,meanwhile,at other potential SCC sensitivity would increase.The order of SCC sensitivity of aluminum polarized at potential is as follow:-0.74V>-1.23V>-1.11V>-0.91V>-1.01V.Surface pretreatment has a impact on the SCC behavior of aluminum alloy in 3.5wt% NaCl solution.The SCC sensitivity of aluminum alloy increaces after using etching primer.what is more, different coating have different SCC resistance:SCC sensitivity did not change with only painting zinc chromate primer coating after polished and he SCC resistance could slightly increase after coating a more layer of alkyd paint. SCC sensitivity can decrease after brushing alkyd paint wiht the etching primer paint and The samples with etching primer/zinc chromate primer/alkyd paint coating has no SCC sensitivity in neutral 3.5wt% NaCl.However, there is an obvious relationship between protection of coating with strain:electrochemical impedance values of coat decrease gradually with strain increase.The pH of 3.5wt% NaCl solution also has a significant effect on the SCC sensitivity of samples with etching primer/zinc chromate primer/alkyd paint coating and the order of SCC sensitivity as follow:pH 2>pH 5>pH 12>pH 9>pH 7.