Repair of H2S stress corrosion cracking in spherical tank by flame spraying copper alloy

       (1) Spraying objects. The storage medium of the spherical tank is liquefied petroleum gas, and the content of H2S in the liquefied petroleum gas is relatively high. After five years of use, the spherical tank was found to have severe cracking, which was determined to be H2S stress corrosion cracking after analysis. The cracks are mainly distributed in the welded joint area, which affects the safety of the spherical tank. In order to prevent and delay H2S stress corrosion cracking, copper alloy powder is sprayed on the welded joint area (weld, fusion line, heat-affected zone) of the inner wall of the spherical tank. The reason is: the coating can isolate the base metal from the liquefied petroleum gas medium. The cathodic evolution (absorption) hydrogen reaction of the sprayed protection zone is controlled by the electrochemical principle; in addition, the residual stress in the welding zone can be reduced by spraying heating.



The material of the spherical tank is 16MnR and the thickness is 24mm.



(2) Spraying process



1) Surface pretreatment. The welds and heat-affected zones that have passed the flaw detection shall be ground with a grinder to remove the rust and reveal the metallic luster. The grinding width is 150~170mm, and then scrub with cotton yarn dipped in acetone for 2~3 times.



2) Preheating. The outer wall of the spraying part is preheated with liquefied petroleum gas flame, and the inner surface temperature of the spherical tank is measured with a surface thermometer, and the preheating temperature is controlled at 250~350℃.



3) Spray the adhesive bottom layer. When the temperature of the inner wall of the spherical tank reaches a predetermined temperature, the nickel-clad aluminum composite powder is sprayed as a bonding bottom layer.



4) Spray the working coat. Immediately behind the spray gun for spraying nickel-clad aluminum composite powder, use the second spray gun to spray copper alloy powder, spraying distance 150~200mm, spraying width 120~150mm, oxygen pressure (0.6~0.8) MPa, acetylene pressure (0.05~ 0.10)MPa. When starting to spray the powder of the working layer, the liquefied gas flame for preheating should be reduced, and the fire should be extinguished when the length of spraying is completed.



(3) Effects. The sprayed spherical tank has been tested for 182 days, and the effect is good, and no H2S stress corrosion cracking is found, which improves the service life of the spherical tank. 

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