Study On The Performance Of Protective Coatings For Heat Exchanger Tubes Of Open-frame Heat Exchangers

Aluminum alloy has a good corrosion resistance,but in the salt spray and marine environment aluminum alloy prone to pitting,aluminum alloy heat exchange tubes.In this paper,based on Hubei province natural science foundation of key projects"LNG gasification unit low temperature corrosion mechanism and its surface modification technology",we have studied on the coating performance of high strength aluminum alloy heat exchange tubes which also stay low temperature.In order to simulate the actual low temperature state of ORV aluminum alloy,which is used as heat exchange tubes in the LNG receiving station.Using the principle of semiconductor,we design and make the low temperature condition specimen test platform and the related auxiliary device.We have tested the electrochemical performance aluminum alloy base material and its coating in the low temperature state;The thickness of aluminum alloy coatings on the aluminum alloy substrate have not a reliable nondestructive method to test,we have used eddy current conductivity meter to measure the electrical conductivity of substrate and the different thickness coatings,between the change of the thickness and the current conductivity we build an equation,through testing the electrical conductivity we will know the thickness of the coating,the method solves the problem of the coating thickness control and measurement well,also is applied to practical engineering.Using wire flame spraying process we preparation Zn,Zn85-1,Zn85-2,Al95,Al,Al-RE six kinds of coating on the 6061 aluminum alloy substrate.Using spectrum analyzer,SEM,EDS,XRD to analyze the material and coatings;Test the open circuit potential and Tafel polarization curve of 6061 aluminum alloy substrate and coatings in the normal and low temperature.Results show that:?1?Zn,Zn85-1,Zn85-2,Al95,Al,Al-RE coatings have a good bonding strength,respectively 11MPa,11.6MPa,12MPa,15MPa,24MPa,13MPa,conform to the requirements of the process specification.The results show that with the increase of Al content,the bond strength of coatings are also improved.Phase test showed that there was no alloy phase appear in the Zn-Al alloy coating,mainly rich zinc phase or rich aluminum phase.?2?In the normal state,the open circuit potential of 6061 aluminum alloy is 0.808 V vs SCE,Tafel polarization curve of the coatings show that corrosion potential:E_Al>E_Al95>E_Al-_Re>E_Zn>E_Zn85-1>E_Zn85-2,all the coatings have sacrifice anode protection to 6061aluminum alloy,the order of the coatings,corrosion current is:I_Al<I_Al95<I_Al-RE<I_Zn<IZn85-2<IZn85-1.?3?In the-16±2?state,the open circuit potential of 6061 aluminum alloy is is 0.692V vs SCE,the corrosion potential of the coating shows:E_Al-RE>E_Al95>E_Al>E_Zn>E_Zn85-2>E_Zn85-1,the order of the coatings,corrosion current is:I_Al-RE<I_Al95<I_Al<I_Zn<I_Z85-2<IZn85-1.?4?The open circuit potential of 6061 aluminum alloy under the low temperature state is bigger than in the normal temperature state.The coatings also have an sacrifice anode protection effect on substrate under the low temperature condition,except Al coating other five kinds of coating,the corrosion potential of coatings in the low temperature condition is bigger than under the normal temperature state,that is to say the trend of the corrosion occurring is weaker under the low temperature.No matter in normal state or in low temperature condition,the coating of Al95 and Al-RE can satisfy sacrifice anode protection and slow corrosion rate.The research achievements of this paper have been used in the aluminum alloy surface modification projects such as,Hainan and Guangxi.The paper is limited to study on the electrochemical corrosion of coatings in static salt water environment,it is expected to research on the local damage resistance,the cavitation resistance of coatings in the future.