Research On Dissolving Behavior Of Dissolvable Mg Alloys In NaCl Solution

Hydraulic fracturing is an effective way of exploiting and stimulating unconventional oil and gas reservoirs.Dissolvable fracturing plugging tools have shown important applications as they can dissolve automatically in underwell saline solution so that those tools are not required to be worn out and no milling scraps are needed to return to the earth surface.The magnesium alloys are considered as ideal materials of making dissolvable packers owing to their low ability of anti-corrosion in saline solution,low cost and good formability.Although the commercial Mg alloys have good mechanical properties,they are not qualified for making dissolvable tools since they corrode slowly in saline solution.Thus,it is necessary to design new magnesium alloys based on traditional ones.In order to obtain good mechanical properties and dissolving performances,Cu,Ni and Fe etc should be added into alloys together with hardening metals(Al,Zn etc)to improve the dissolving performances of alloys.Those additions and their compounds will form corrosion couples with grains to quicken alloy corrosion.So far,the researches about dissolvable magnesium alloys have rarely been reported.It is shortage of systematical investigations on corrosion mechanism and corrosion rule of alloys.Some fundamental issues are worth exploring,including:(1)the effect of distribution and amount of Cu-bearing compounds on the dissolving performances of alloys;(2)the correlations of the types of Cu-bearing compounds with alloy constituents,and the influence of potential difference between Cu-bearing compounds and ?-Mg on the dissolving performances of alloys;(3)the relation of Al and Zn solute contents in ?-Mg with corrosion rate of alloys;(4)the influence of heat treatment on the microstructures and dissolving performances of alloys;(5)the effect of indium on the surface oxide film of Al-bearing magnesium alloys.Focusing on the above issues,several series of Mg-Cu?Mg-Cu-Al?Mg-Cu-Zn?Mg-Al-Cu-In alloys have been designed in this thesis.These alloys have been prepared using casting method,and their microstructures were adjusted by extrusion and annealing,and were investigated by means of XRD,SEM and TEM.The corrosion rates of alloys in saline solution were investigated by H2 generation and weight loss measurement.The Volta potential difference(?VPD)between interfacial phases and ?Mg was measured by AFM/SKPFM.The corrosion performances and mechanism of the alloys were analyzed using electrochemical measurements.The main results are summarized as follows:(1)Mg-Cu alloys are composed of ?-Mg and Mg2Cu phases.The Mg2Cu phases mainly distribute at grain boundaries.With rising Cu content,the alloy corrodes faster.The corrosion rates of alloys are mainly controlled by the volume fraction of Mg2Cu phases,and there is a linear relationship between the corrosion rate of alloy and volume fraction of Mg2Cu.(2)The cast Mg-Cu-Al alloys consist of ?-Mg,Mg2Cu,MgAlCu,Mg17Al12(?)precipitates,and the cast Mg-Cu-Zn alloys consist of ?-Mg,Mg2Cu,MgZnCu precipitates.The type of precipitates in the two kinds of alloys depends on Al and Zn contents,respectively.The atomic ratio of Zn/Cu of MgZnCu phases increases with rising Zn content.(3)The potential differences between interfacial Cu-bearing phase and ?-Mg of Mg-Cu-Al alloys range 877 mV-1034 mV,much higher than that between Mg17Al12 and ?-Mg.The potential of Cu-bearing phase of Mg-Cu-Zn alloys decreases with rising Zn content,about 608 mV-920 mV higher than that of ?-Mg.(4)The cast Mg-Cu-Al and Mg-Cu-Zn alloys corrode initially around Cu-bearing phases.The netlike ? and Cu-bearing phases have a certain hindering effect on corrosion as they distribute continuously at grain boundary.Once the ?-Mg in eutectic consisting of ?-Mg and Cu-bearing phase corrodes completely,the remaining holes become diffusion path of Cl ions,so that other ?-Mg grains corrode further.The volume fraction and ?VPD of Cu-bearing phase,content and distribution of ? phase and content of Al or Zn in ?-Mg grains are key factors to control the corrosion rate of alloys.(5)In does not change the phase constituents of alloys,but reduces Al content in?-Mg,and refines ?-phase of Mg-Cu-Al alloy.As a result,In promotes corrosion of Alcontaining Mg alloys.(6)Compared to those of the cast alloys,the corrosion rates of the annealed alloys reduce significantly,because the Cu-bearing phases become spherical,their volume fractions reduce,and Al or Zn contents in ?-Mg grains increase after heat treatment.In this thesis,the relationship of microstructures and corrosion performances of some dissolvable Mg alloys has been investigated systematically,the corrosion mechanism and key factors of influencing the corrosion rate of the alloys have been explored.The obtained results are expected to provide a certain experimental and theoretical reference for the adjustment of corrosion performances and practical application of dissolvable magnesium alloys.