| In our country, the most of oil and gas mining have come into the middle and later stage. In many oil and gas fields, highly water, carbon dioxide, hydrogen sulfide and element sulfur have been found. The corrosion problems of petroleum pipe become more and more harsh. At the present time, the preservation technologies of petroleum pipe have many disadvantages, such as highly cost and lowly corrosion resistance. So, research of new preservation technologies of petroleum pipe is very important.Because hot dipped Al and Al-Zn alloy coating have excellent corrosion resistance and lowly cost, the effect of hot dipped Al and Al-Zn alloy coating on properties of non-quenched and tempered N80 steel was studied systematically. And the improvement of properties of hot dipped coated N80 steel also was studied.Firstly, the effects of one bath flux method, two bath flux method and passivating method on quantities and properties of hot dipped aluminum coatings were studied. Meanwhile, the thickness, continuity, corrosion resistance and microstructure of hot dipped aluminum coatings were analyzed and compared. The optimal technology of petroleum pipe was determined by comparing coating quality. The results indicated that the microstructure of hot dipped coatings by 3-kind methods have the same structure, which were composed of aluminum overlay and intermetallic compound layer. But, under the same parameters, the thickness, continuity and corrosion resistance of coatings were different. In terms of thickness, the coating by passivating method was the biggest, the coating by two bath flux method was the second place and coating by one bath flux method was the smallest; In terms of continuity and corrosion resistance, the coating by two bath flux method was the best, coating by one bath flux method was the second place and coating by passivating method was the worst. So, the optimal hot dipped Al technology of petroleum pipe was two bath flux method.The effects of laser technical parameters on cracks and defects of hot dipped Al coating were studied. The optimal laser technical parameters were determined. And the microstructure and properties of laser remelted coating also were studied by means of SEM, EPMA, X-ray diffraction and micro-hardness measure. The results showed that the laser remelted coating treated by reasonable parameters has a compact microscopic structure, it was free from cracks and defects. The phase in the coating was changed from Al-rich phase to Al-poor phase. By laser treating, corrosion resistance of hot dipped Al coating could be improved, and microhardness and brittleness could be reduced.The effects of hot dipped Al process on mechanical properties of non-quenched and tempered N80 steel were studied, and the improvement of mechanical properties of hot dipped Al coated N80 steel was also studied through orthogonal method. The results showed that the strength and plasticity of the N80 steel were reduced by hot dipped Al process, which could not meet requirements of API SPEC 5CT seven edition. The decrease of mechanical properties was due to coarsening of precipitated phase and spheroidization of pearlite. And the ultimate tensile of aluminum coated steel could be rised from 663 MPa to 810～845 MPa by reasonable quenching and tempering or subcritical quenching and tempering treatment, the plasticity and impact toughness could also be improved. But the cost could be increased and coating quality could be reduced. Base on the above experiments, a new hot dipped Al process contained of hot dipped Al in high temperature, water quenching and tempering was put forward. The mechanical properties and coating quality could be greatly improved by the new process, which was of low cost and simple technology.The effects of hot dipped Al coating on corrosion resistance of non-quenched and tempered N80 steel were studied by using of static coupon testing and electrochemical testing, and the further improvement of corrosion and scaling resistance of hot dipped coated N80 steel were also studied. The results showed that hot dipped Al coating could improve the corrosion resistance of non-quenched and tempered N80 steel obviously, which was attributed to the formation of stable Al2O3 covering layer. The corrosion and scaling resistance of N80 steel could be further improved through electroless Ni-P plating or painting epoxy resin on Al coating because the two coatings have smooth surface, good adhesion and high barrier property. And composite coating contained hot dipped Al coating and epoxy resin layer appeared wide application prospect.Hot dipped 55%Al-Zn fluxes for non-quenched and tempered N80 steel were studied. In comparison with other flux methods, authors found out a kind of new flux method that has two steps:electrolytic active flux and 4% K2ZrF6 flux. And better properties of the coatings were obtained with the new flux method. On this base, the application feasibility of hot dip55%Al-Zn technology in petroleum pipe corrosion protection was studied. The results showed that hot dipped55%Al-Zn coating could improve the corrosion resistance of non-quenched and tempered N80 steel obviously. After hot dipped55%Al-Zn alloy, the strength and plasticity of the alloy-coated steels could also be greatly improved. So, using 55%Al-Zn alloy coating to protect petroleum pipe is feasible.
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