| The corrosion of reinforcing steel bars embedded in concrete is the main factor affecting the durability of reinforced concrete structure. In general, the alkaline environment of concrete can make the reinforcing steel staying in passive state and not prone to corrosion.However, it is widely accepted that carbonation of concrete and chloride ion are responsible for causing local passive layer on steel bars breakdown and subsequent corrosion of reinforcing bars. The use of galvanized steel bar instead of the ordinary steel bar is an effective method to solve this problem.Extensive researches and applications in have been reported in recent years. ZAM alloy coating is one of high corrosion resistance coatings and it’s properties are much better than those of pure zinc coating.However little research of its application in concrete has been reported. In this paper, we use saturated Ca(OH)2 solution was used as the simulated concrete pore solution,the corrosion behavior of ZAM alloy coatings immersed in simulated solution was studied and the feasibility of this new coating in concrete structure was discussed.The process of hot dip ZAM alloy coating was optimized firstly and the coating was characterized by using various means of detection technology, such as SEM, EDS, XPS.The results showed that the ZAM coating is mainly composed of four different phases, which are the bulk Mg Zn2 phase, the aluminum phase,the binary eutectic consisting of zinc and the hard, intermetallic Laves phase Mg Zn2,and a ternary eutectic consisting of Zn/Al/Mg Zn2.A layer of oxide film was founded on the surface of the ZAM coating, containing Mg O,Al2O3, and a small amount of Zn O. It was proved by Natural Salt Spray test that ZAM coating has higher corrosion resistance than the ordinary pure zinc coating.In this paper, the corrosion behavior of ZAM coating immersed in simulated concrete pore solution with p H value 9~13.5 has been studied and the carbonation resistance of the coating was investigated.The microstructure and composition of the corrosion product formed on the surface of ZAM coating after 15 days and 30 days of immersion has been analyzed by SEM, EDS and XRD analysis.The electrochemical corrosion characteristics of these specimens have been studied by Tafel polarization test and electrochemical impedance.The results of 15 days immersion experiments showed that: the coating surface generated a lot of triangle granular basic zinc carbonate with p H of the immersion solution 9, while a dense film of corrosion product containing of Mg(OH)2,Al(OH)3, and Zn(OH)2 formed on the surface of ZAM coating with p H 11 or 12. When p H is 12.5 or 13,the surface of the coating is mainly covered by acicular Zn O and a small amount of Ca HZn,Ca HAl.When p H is13.5,the corrosion product cracked and the steel substrate was found to be exposed; The ZAM coating layer is under passivate status, and the corrosion current density is less than or close to 0.1μ A/cm2 in the solution with p H 11~12.5among them,while the minimum Jcorr of the coating was abtained when immersing in the solution of p H 11. The results of 30 days immersion experiments showed that the corrosion current density of the coating in solution as similar as that of the samples for 15 days of immersion; It seems that the formation and dissolution of corrosion product has tend to be balanced. It is showed that ZAM alloy coating has excellent carbonation resistance.The chloride threshold of ZAM alloy coating immersed in the simulated concrete pore solution with p H value is 12.5(under normal concrete environment) and 11(under carbonation concrete environment) was investigated.The results showed that the Jcorr of each specimens was greater than 0.1μA/cm2.So the chloride threshold of ZAM alloy coating in normal concrete environment is less than 0.2mol/L. While the microstructure and electrochemical impedance results of the specimens immersed in the solution with p H 11 showed no significant changes on microstructure and electrochemical impedance, and the chloride threshold of ZAM alloy coating in carbonated concrete environment is greater than 1.0 mol/L. |
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