Research On Technology Of Hot Dipped 55%Al-Zn Coating Of Sucker Rods And Its Properties

With eastern oil fields have entered the late production stage,the oil well production environment continues to deteriorate in the oil,gas,water and all kinds of corrosive medium.The sucker rods are more likely to occur brittle fracture due to corrosion which would cause serious production accidents.Therefore,many people commit themselves to seek a feasible anti-corrosion methods.The method of hot dipping Al-Zn-Si is selected to improve 35 CrMo sucker rod corrosion resistance.Analyzing the ratio of plating auxiliary on the quality of the coating.Researching on the formation mechanism,process parameters and Si content of Al-Zn coating how to affect the organization structure,thickness and corrosion resistance.Exploring the effect of additive Mg and coordination of heat treatment on the corrosion resistance of coating.NH₄Cl,NaF,SnCl₂,ZnCl₂ were used as the constituents of aqueous solution plating aid.The effect of the plating quality was studied by orthogonal test.,and the research indicates that best ratio is: 25wt% NH₄ Cl,0.5wt% NaF,1wt% SnCl₂,30wt% ZnCl₂?In order to prevent the excessive erosion of metal plating solution over 35 CrMo sucker rod and change its dimensional accuracy,the best percent of Si is 1.2 in the plating solution.From the inside to the outside of the coating can be divided into two layers,namely,the intermediate gray compound alloy layer and the outer layer of gray inclusions.The main component of the intermediate compound layer is Fe2Al5,FeAl3 and Al0.5Fe3Si0.5,the outside layer consists of rich aluminum dendrite ? phase and zinc dendrite clearance ? phase through the analysis of XRD.Before and after hot dipping 55%Al-Zn-1.2%Si coating the corrosion weight loss rate is 5.9:1 and the corrosion current density is 3:1,which shows that the coating has excellent corrosion resistanceWith the hot dipped temperature rising and time extending,the thickness of alloy compound layer in the coating increases firstly and then decreases.And the thick alloy layer in the coating means good corrosion resistance of coating.So the parameters of hot dip plating process is 630?×4min,which has the thicker alloy layer than others.And the thickness of intermediate alloy layer is 22.2 ?m,hardness is 352 HV.According to the change trend of thickness of intermediate alloy layer,its growth mode is approximately divided intotwo stages.In the early time,the growth mode is growth without dissolution,and the dynamic equation is x2=4Dt.Then the growth mode changes into growth with dissolution,and the dissolution equation is ???.When the parameter is satisfied with the equation ???,the thickness of intermediate alloy layer reaches the maximum.According to the shape of 35 CrMo sucker rod,the mathematical model of cylindrical pieces under hot dip plating process is set up,and the relationship among lift speed,coating thickness and the radius of plating pieces is obtained,that is???.Different mass fraction of Mg was added in the plating solution for improving the corrosion resistance of the 55%Al-Zn-1.2%Si coating.The study found that additive Mg can strengthen the grain boundary layer and distribute coating well.It also can inhibit the formation of Fe Zn brittle coarse compound at the same time.The corrosion resistance of the coating is the best when adding 2%Mg in the 55%Al-Zn-1.2%Si plating solution,which transforms the static immersion corrosion rate from the 0.33g·m-2·d-1to 0.16g·m-2·d-1 and the corrosion current density from 18.125?A·cm-2 to 10.317 ?A·cm-2.If coordinated processing technology of quenching,hot dipped plating and high-temperature tempering is used,the intermediate compound layer thickness can be increased to 79.49?m that contributes to the improvement of the corrosion resistance.Because the intermediate alloy compound layer belongs to brittle phase which leads to the mechanical properties decreased slightly.Ultimately,the tensile strength of ?b decreases from 832.67 Mpa to 805 Mpa and elongation of ? decreases from 26.6% to 22.6%.