Synthesis And Inhibitive Properties Of Apigenin Derivatives As Corrosion Inhibitors For Brass In Nitric Acid Medium

Copper and its alloys have found great variety of applications in fabrication of wire,sheets and pipelines in electronic industries,marine industries,power stations,heat exchangers and cooling towers because of their good corrosion resistance properties in many of the environments,superior electrical and thermal conductivity,mechanical workability and malleability.Brass is an alloy of copper and zinc,which is widely used in domestic appliances,industrial and chemical plant process equipments.In general,brass has good corrosion resistance because copper is a relatively noble metal.However,brass is more susceptible to attack by oxidizing acids and other corrosive media that contains chloride,nitride,and ammonia.As a strong oxidant,nitric acid is widely used for industrial cleaning,descaling,chemical and electrochemical etching of copper.Brass is easily soluble in dilute HNO₃ solution.Therefore,adhering to the concept of green environmental protection,it is of great significance to seek a green environment-friendly brass corrosion inhibitor with good anti-rust effect,high corrosion inhibition efficiency,low toxicity and low pollution.In this thesis,apigenin was used as a raw material molecule to modify the Mannich base corrosion inhibitor,which was used for the corrosion protection and inhibition of brass in nitric acid medium.The main contents of this paper include are as follows:（1）Six apigenin derivatives were synthesized through Mannich reaction by using apigenin isolated from Hypericum perforatum as raw material.Their structures were characterized by infrared spectroscopy（IR）,nuclear magnetic（NMR）and mass spectrometry（MS）.（2）The inhibition performance of these apigenin derivatives were investigated as eco-friendly corrosion inhibitors for brass in 1.0 M HNO₃ solution by means of weight loss,potentiodynamic polarization,and electrochemical impedance（EIS）measurements.The obtained results show that the as-synthesized apigenin derivatives act as efficient inhibitors for brass in 1.0 M HNO₃.The inhibition efficiency increased with the increase in inhibitor concentration but decreased with a rise in temperature.The inhibition efficiency higher than 90%for these compounds was found even at a low concentration of 30 mg·L^-1,which is superior to the commonly used synthetic organic corrosion inhibitors for brass in acidic media. Electrochemical studies indicate that the inhibitors are of mixed type but predominantly cathodic in HNO₃ solution.（3）The inhibitor performance depends on the adsorption of the molecules on the metal surface.The thermodynamic parameters of the corrosion inhibition process were calculated according to the statistical model.The results show that the adsorption of these corrosion inhibitors on the surface of brass is a spontaneous process,mainly chemical adsorption.The adsorption behaviour of these apigenin derivatives on the surface of brass was analyzed utilizing SEM,AFM,XPS,and Raman spectroscopy measurements.The results confirmed that the apigenin derivatives prevented corrosion of brass by forming protective layer on its surface.（4）The interaction between apigenin derivatives and brass surface was studied by quantum chemical calculation.The results show that the HOMO orbitals in the molecules are mainly distributed around the nitrogen and oxygen atoms,which can provide electrons to the metals.The empty LOMO orbit is mainly concentrated on the aromatic ring to receive the feedback electrons of copper,and thus the corrosion inhibitor molecules have a strong interaction with copper.