The Intelligent Sacrificial Magnesium Alloy Anodes And Their Micro-galvanic Corrosion

As a kind of civil engineering,reinforced concrete has been widely utilized in marine engineering,highway and other large-scale infrastructure constructions.However,the steel embedded in the concrete can be corroded due to the ingression of corrosive medium such chloride ions and carbon dioxide through pores.The corrosion products of the steel further lead to the cracks,delamination and even spallation of the covered-concrete due to their expanded volume,reducing the life span of the reinforced concrete.Corrosion of the reinforcing steel could be suppressed or prevented by the sacrificial anode cathodic protection.Recently,Mg was proved to be a more suitable sacrificial anode than the widely used Al and Zn.As a sacrificial anode in the concrete,the Mg could be used to detect the concrete corrosivity caused by the chloride ingression and automatically provide the sufficient cathodic protection for the steel.Furthermore,the AZ91 Mg alloy was more intelligent when acted as the sacrificial anode than the pure Mg.However,the corrosion mechanism of the AZ91 Mg alloy in the highly alkaline solution has not been revealed yet.Since the Al is an amphoteric metal and it dissolves quickly in the high alkalinity solution.In the AZ91 Mg alloy,the secondary phase formed after the Al was added into the Mg.The secondary phase may act as the anodic sites and dissolve preferentially due to the high activity of the Al in the high alkalinity solution and it may also act as the cathodic sites to accelerate corrosion of the Mg matrix phase.Such role revealing of the Al-rich secondary phase on the corrosion behavior is believed to be beneficial to provide the theoretical guidance for the development of the smarter sacrificial anodes.In this paper,the following studies were carried out to reveal the unknows of the Mg alloys when they act as the smart sacrificial anode:(1)Firstly,the uniform Mg matrix with different Al contents or the Mg-Al intermetallic phase was prepared by magnetron-sputtering.Their corrosion resistance in the various chloride concentration simulated concrete pore solution(SCPS,saturated Ca(OH)2).Results showed that the Al-rich phases and Mg matrix phase showed the better corrosion resistance in the alkaline solution and this result overturned those view that Alrich phases were more susceptible to corrosion in alkaline solution.Besides,key of the Mg alloys acting as the sacrificial anodes lied in the sensitivity of their surface film to the chloride.Only the surface film dissolved or broken by the chloride in the contaminated SCPS,could the Mg alloys substrate provide sufficient cathodic protection current for the reinforcing steel.(2)Based on the results of the(1),the traditional sacrificial anode Zn was successfully intelligentized by Mg-alloying and the Zn-Mg alloys could also act as the smart sacrificial anodes to detect the concrete corrosivity and provide cathodic protection for the reinforcing steel.The intelligence degree of the Zn-Mg alloy acting as the sacrificial anodes also lied in the sensitivity of the Zn-Mg intermetallic compounds surface film to the chloride if these alloys were coupled with the steel.Hence,the intelligence of the Zn-Mg alloys increased with Mg content in general.This indicated that intelligence of the traditional anode Zn could be improved by the Mg alloying.(3)Excepted the chloride mentioned in(1)and(2),the Mg was also sensitive to the concrete carbonation and could be the smart sacrificial anode to detect the carbonation and automatically provide the sufficient cathodic current for the steel.Also,if the Mg was used as the sacrificial anode in the carbonated SCPS,the anodic current efficiency could also be improved.(4)The intelligent sacrificial magnesium alloy anodes were essentially an electrochemical anodic process between substrate and other phases.Hence,according to the results of the(1),to further reveal the secondary phase nanoparticles on the Mg matrix corrosion,the roles of the supersaturated Al and secondary phase nanoparticles on the anodic corrosion of Mg matrix in the weak alkaline solution was also studied.Results showed that corrosion potential of the Mg matrix could be increased by the supersaturated Al and thus the driving force induced by higher potential difference between the Mg matrix phase and the secondary phase could be mitigated.In the Mg based alloys,there was a critical size theoretically for the cathodic sites which was determined by the polarization resistivity of the phase compositions and solution resistivity.When the cathodic sites were larger that the critical size,the overall micro-galvanic corrosion damage could be reduced to some degree if these cathodic sites aggregate together.On the contrary,dispersion of finer cathodic particle sites could mitigate the overall microgalvanic corrosion damage if the cathodic sites were smaller that the critical size.