Effect Of Alloying Elements And Oxiding Anions On The Corrosion Behavior Of Zinc Layer In Simulated Concrete Pore Solution

The corrosion of concrete reinforcement is the main factor affecting the durability of reinforced concrete structures.The erosion of chloride ions and the carbonization of concrete are the main causes of corrosion of reinforcement concrete.Compared with natural black steel bars,hot-dip galvanized rebar have higher chloride threshold and carbonization resistance,thus hot-dip galvanizing is one of the effective methods to improve the durability of reinforced concrete structures.However,some practical problems limit the use of hot-dip galvanized steel bars.For example,since the bending resistance of pure zinc-coated rebar is poor,the pure zinc coating is prone to cracking and peeling when bent in-site;the rapid dissolution of the galvanized layer in the fresh concrete environment will lead to a large consumption of zinc layer,and hydrogen evolution accompanying the rapid corrosion process will increase the porosity of the concrete around the steel bar,and reduce the contact area between the steel and the concrete,which may reduce the bonding force between steel and concrete.Since the bending resistance of Zn-Al,Zn-Mg and Zn-Mg-Al coated steel is better than that of pure zinc-coated steel bars,these Zn-Al,Zn-Mg and Zn-Mg-Al coated steel have potential application value in concrete environment.The corrosion behavior of Zn-Al,Zn-Mg and Zn-Mg-Al coated steel in simulated fresh concrete environment was studied in this paper.In addition,the chromium-free environmentally friendly oxidizing anions NO₃^-?MoO₄^2- and MnO4 was also attempted to add in simulated fresh concrete environment to inhibit the rapid corrosion and hydrogen evolution of zinc layer.It will provide experimental basis and theoretical guidance for the feasibility of applying hot-dip galvanized steel in concrete structures.The main research contents and results for this thesis are as follows:1)The corrosion behavior of Zn-x%Al alloys?x=0.2,1,3 and 6?in saturated Ca?OH?₂ solution with pH about 12.6 and saturated Ca?OH?₂+0.2M KOH solution with pH about 13.2 was studied.The results show that the addition of alloying element Al to zinc caused the corrosion potential to shift negatively.The addition of alloying element Al also reduced the corrosion resistance of zinc layer,and the corrosion resistance gradually increased with the increase of A1 content in the Zn-Al alloy.It was mainly due to the formation of a loose corrosion products calcium aluminate?Ca2Al?OH?7·3H2O?on the surface of the Zn-Al alloy,which affected the formation of the continuous and dense calcium zincate layer?Ca?Zn?OH?3?2·2H2O?on the surface of the zinc layer.However,minor addition of Al?sunch as Zn-0.2%Al alloy?to zinc has little effect on the corrosion resistance of zinc layer.2)The corrosion behavior of Zn-x%Mg alloys?x=0.5,1.5 and 3?in saturated Ca?OH?₂ solution with pH about 12.6 and saturated Ca?OH?₂+0.2M KOH solution with pH about 13.2 was studied.The results show that the addition of alloying element Mg to zinc also caused the corrosion potential to shift negatively.In saturated Ca?OH?₂ solution,the corrosion resistance of zinc was dereased with addition of alloying element Mg.This was mainly due to the fromation of the Mg-containing corrosion product on the surface of zinc layer,which inhibited the formation of the continuous and dense calcium zincate layer?Ca?Zn?OH?3?2·2H2O?and/or the galvanic effect formed between the Zn-Mg eutectic structure and the zinc-rich phase.However,the addition of a small amount of Mg?such as Zn-0.5%Mg alloy?had little effect on the corrosion resistance of the zinc layer.In saturated Ca?OH?₂+0.2M KOH solution,the addition of alloying element Mg was beneficial to improve the corrosion resistance of zinc,the corrosion resistance of zinc gradually increaseed with increase of Mg content.The areas which were not covered by sheet-like corrosion product of calcium zincate?Ca?Zn?OH?3?2·2H2O?could be coverd by Mg-containing corrosion products?such as Mg?OH?₂ and/or MgO?,by which improved the corrosion resistance of zinc layer.3)Based on the results above,the corrosion behavior of hot-dip Zn,Zn-0.2%Al,Zn-0.5%Mg and Zn-0.5%Mg-0.2%Al coatings in simulated concrete pore solution with high pH value was studied.The results showed that the corrosion resistance of Zn-0.2%Al coating is better than that of pure zinc coating at the initial stage of corrosion,which was mainly attributed to the physical barrier effect of the oxide layer on the surface of Zn-0.2%Al coating.The corrosion resistance of Zn-0.2%Al coating was sligh lower when compare to that of pure zinc coating with immersion time due to the dissolution of the oxided layer.Zn-0.5%Mg and Zn-0.5%Mg-0.2%Al coating exhibited better corrosion resistance than pure zinc coating during the whole immersion tests.It was mainly due to the protection of the denser corrosion product layer formed on the surface of the zinc layer,and the corrosion products layer mainly contained Mg?OH?₂/MgO and Zn?OH?₂/ZnO.Therefore,Zn-0.2%Al,Zn-0.5%Mg and Zn-0.5%Mg-0.2%Al coated steel bars have potential application value in concrete environment.4)The inhibition mechanism of NO₃^- ion for hydrogen evolution of zinc layer in simulated concrete pore solution with pH about 13.2 was studied.The results showed that the addition of NO₃^-ion caused the corrosion potential of the zinc layer to shift positively,which could effectively inhibit the hydrogen evolution of zinc layer.The presence of NO₃^-ion accelerated the initial dissolution rate of zinc layer and promoted the formation of calcium zincate?Ca?Zn?OH?3?2·2H2O?on the surface of zinc layer.The reduction products of NO₃^-ion,such as NH3 may be destructive to the corrosion products on the surface of zinc layer.However,the corrosion resistance of zinc layer in the later stage was sligh lower when compare to that of zin layer in the concrete pore solution without NO₃^-ion addition.In the simulated concrete pore solution,NO₃^-ion and its reduction products could adsorpt at the corrosion interface of the zinc layer,which inhibited the corrosion and hydrogen evolution of zinc layer.Therefore,the addition of NO₃^-ion in the concrete would be conducive to inhibit hydrogen evolution of galvanized steel,but it had no inhibitive effect on the rapid dissolution of zinc layer.5)The inhibition mechanism of MoO₄^2- or MnO4-ions for the rapid dissolution and hydrogen evolution of zinc layer in simulated concrete pore solution with pH about 13.2 was studied.The results showed that the inhibition efficiency of the MnO4-ion on zinc layer is higher than that of the same concentration of MoO₄^2- ion.The addition of MoO₄^2- ion has no obvious effect on the corrosion potential of zinc layer,but has a significant inhibitory effect on the rapid corrosion and hydrogen evolution of zinc layer.The reduction of MoO₄^2- ion at the reacting interface may inhibit hydrogen evolution of zinc layer.The rapid dissolution and hydrogen evolution of zinc layer was inhitited due to the reduction product of MoO₄^2- ion co-existing with the corrosion product calcium zincate?Ca?Zn?OH?3?2·2H2O?on the surface of zinc layer to form a protective layer.The addition of MnO4-ion significantly increased the corrosion potential of zinc layer,and had a significant inhibition effect on the rapid corrosion and hydrogen evolution of zinc layer.The rapid dissolution and hydrogen evolution of zinc layer was effectively suppressed due to the reduction reaction of MnO4-ion at the reacting interface and/or the increase of corrosion potential of zinc layer.Ca2+ ion also play an important role in the inhibition effect of MnO4-ions for zinc layer:Ca2+ion and the reduction products of MnO4-ion were co-deposited on the surface of zinc layer to form a dense protective layer.