Effect Of Molybdate As A Corrosion Inhibitor On Corrosion Resistance Of Reinforcing Steels

Due to good mechanical properties and durability,reinforced concrete structures(RCSs)are widely used in civil engineering.However,premature failure of RCSs caused by chloride-induced steel corrosion can lead to serious safety problems and huge economic losses.Therefore,how to delay and/or inhibit the corrosion process of steels in concrete has become a research hotspot.The addition of corrosion inhibitors is widely considered as one of the most effective and simple methods to inhibit steel corrosion.Furthermore,as an efficient and environmentally-friendly inorganic corrosion inhibitor,molybdate has been widely used in the field of metal corrosion and protection.However,no systematic research on molybdate as a corrosion inhibitor for reinforcing steel in RCSs is available in the literature.Compared with traditional low-carbon steel,corrosion-resistant alloy steel containing a certain amount of elements Cr and Ni has aroused widespread concern and research due to superior corrosion resistance in harsh environments.One of the most representative corrosion-resistant steels is stainless steel,but the application of stainless steel is limited by its high price.To replace stainless steel,a new alloy steel containing 10%Cr is used to reduce cost.However,under the harsh conditions of high chlorides,the occurrence of chloride-induced corrosion can still be found for this new type of alloy steel.Therefore,exploring the synergistic corrosion inhibition mechanism of molybdate and the new alloy steel in concrete has an important scientific significance and engineering application value.To study the effect of molybdate on passivation and chloride-induced corrosion behavior for low-carbon steel(LC)and corrosion-resistance alloy steel(10%Cr)in saturated Ca(OH)₂solution,OPC extract solution and mortar,electrochemical measurements(EIS,M-S and CPP etc.)as well as surface characterization techniques(such as XPS,AFM and SEM)were used.Furthermore,the corrosion inhibition mechanism of molybdate on reinforcing steel was proposed.The main conclusions of the research are as follows:(1)When the natural passivation process of steels is not considered,molybdate can not only react with steels to form protective products,but also compete with chloride ions for adsorption on the surface of steels to improve the pitting resistance of LC and 10%Cr steels after 1 h of immersion in saturated Ca(OH)₂ solution and OPC extract solution.(2)For the passivation stage without chlorides,molybdate can improve the passivation ability of LC steel in saturated Ca(OH)₂ solution and OPC extract solution.In addition,according to the results of XPS and M-S measurements,the increase of Fe oxide/hydroxide layer and Fe²⁺oxide content in the passive film confirms the formation of a thicker and more protective passive film of LC steel,thus promoting the passivation ability due to the addition of molybdate.However,for 10%Cr steel,the complex film formed by the preferential reaction between molybdate and Ca²⁺in the solution will hinder the spontaneous formation of protective passive film,especially the formation process of Cr-enriched passive layer,thereby reducing its passivation ability.(3)For the chloride-induced corrosion stage,the increased E_corr and R_p values as well as the reduced corrosion products can be found for LC steel due to the presence of of molybdate.The reason is that molybdate could improve the formation of passive film,and form a protective film on steel surface to delay the chloride-induced corrosion.Moreover,molybdate ions will compete with chloride ions for adsorption,thereby further improving the corrosion resistance of LC steel.For 10%Cr steel,the formation of passive film will be inhibited by the addition of molybdate,resulting in the lower corrosion resistance in chloride environment.However,according to CPP results,the significant promotion of pitting corrosion resistance of 10%Cr steel can be found due to the presence of molybdate.(4)Regardless of molybdate,much higher passivation ability and corrosion resistance can be obtained for 10%Cr steel compared with LC steel,which is mainly due to the formation of a bilayer passive film consisting of Cr oxide/hydroxide inner layer and Fe oxide/hydroxide outer layer.(5)In OPC mortar,the addition of molybdate can not only participate in the formation of passive film to improve the passivation ability of LC steel,but also compete with chloride ions for absorption on LC steel surface to promote its corrosion resistance.However,the addition of molybdate will result in the decrease of mechanical properties of OPC mortar.The reason may be that molybdate has a possible retarding effect on cement hydration,thereby reducing the mechanical properties of OPC mortar.