The Study On Cathodic Protection For Cracking Material Used In The Equipment Of Regenerating System

Many cases of equipment cracking had been found on the equipment of FCC regeneratingsystem in domestic refinery plants at present . The most of equipment cracking usually occurredon welding line and welding point of bud for line. The investigation from the literature reportsand laboratory research showed stress corrosion cracking of the soft steel in the nitrate medium(nitrate embrittlement) was the main cause of equipment cracking in the FCC regeneratingsystem.Stress corrosion cracking was the most dangerous corrosion type. Since the stress corrosioncracking of the welding line on the equipment was a kind of rapid and dangerous crackingprocedure, efficient measures should be actively take to overcome . Now, the protectivemeasures included raising the wall temperature of regenerating equipment, improving ontechnology process , selecting reasonable material, and controlling comprehensive stress level ofthe equipment, etc.Soft steel had a definite lower critical electric potential when it was in the nitrate solution, ifthe external potential was lower than the lower critical potential, there would be no stresscorrosion. Namely cathodic protection could prevent nitrate embrittlement. Cathodic protectioncould be made through adding external electricity, installing sacrificial anode, and sprayingactive metallic coatings. Since the FCC regenerators were very huge and the corrosive mediumswere complicated and unstable, we adopted the method of spraying active metallic coatings toconduct the cathodic protection experiment.Most cracking took place on the welding lines. We selected four-point bendin stresscorrosion samples and combined two samples as a pair. There were welding lines in the middleof the samples, and the samples were loaded with bolts to increase stress. Solution with thecomposition of 60%Ca(NO3)2+3%NH4NO3 was selected to conduct the experiment ofaccelerating the nitrate embrittlement.After treating the sample surfaces by sandblasting, the different active metallic coatings ofzinc or aluminum on different samples were obtained by the method of arc spray, therefore thethickness of the coatings was 0.100mm, 0.200mm and 0.300mm, respectively.Experiments showed that when sprayed to form a proper thickness coating of zinc oraluminum, soft steel could postpone the occurrence of its stress corrosion cracking in the nitratemedium. Under the condition of same thickness, zinc coating could protect the soft steel muchlonger than the aluminum coating. When metallic coating was the same kind, the thicker thecoating is, the longer it can protect the steel.It had been approved that raising the wall temperature of the regenerator to higher than thedew point of flue gas was the most effective anti-cracking measures. But sometimes the walltemperature might be lower than the dew point due to the fluctuation resulted from start-up,shut-down and operation during the course of real production. Though some of the oil refinerieshad raised the wall temperature of their regenerators, there were still some cracks. Since thethickness of active metallic coating was limited, we couldn't use the method of were sprayedalone. Combing the method of both raising the wall temperature and spraying metallic coatingcould greatly prolonged the life span of the regenerators, as was to be further applied in theindustry.