Study On Corrosion In Seawater Of Welded DH36Steel

In the development and the application of ocean resources, steel plays a veryimportant role. A large number of steel will be used in ocean engineering structure ofshipping, offshore platform and port facilities. And welding is an inevitable importantprocess in application. The results of study show the welding seam is a weak locationin corrosion. The local corrosion including stress corrosion, crevice corrosion, pitting,hydrogen brittle, etc will happen in welding seam, in which the galvanic corrosion isthe most common. The welding materials and process will bring different influence onthe welding seam.The present work focused on the electrochemical corrosion behavior andgalvanic corrosion tendency of S-189-1, S-189-2welding seam, heat-affected zoneand base metal by weight-loss, open circuit potential, polarization curve and theelectrochemical impedance spectroscopy（EIS）, metallographic microscopicobservation（MMO）, X-ray Diffraction （XRD） combined with the micro-areameasurement technology. It would provide the theoretical foundation to improve thecorrosion resistance of carbon steel. By analyzing the law of corrosion rate change, itcould give technical assistance for reasonable welding process in practicalengineering. The conclusions were as follows.The self-corrosion potentials of S-189-1and S-189-2with different heat input ofDH36steel changed negatively with time. The self-corrosion potentials reducedmarkedly at the beginning and then became a little and almost stable after20minutes.The self-corrosion potential was stable between-680mV～-730mV. They were all inthe range change of self-corrosion potential of ordinary carbon steels in seawater.Polarization curve showed the corrosion rates of welding materials were smallerthan the corrosion rate of base metal. And the electrochemical impedancespectroscopy indicated that Rctof welding materials were maximum. Rctof base metalof S-189-1was minimum, which was1914W·cm². Rctof the coarse grainedheat-affected zone of S-189-2was minimum, which was1771W·cm². These resultsdemonstrated the welding material was appropriate, which prevented the rise rate ofthe local corrosion. But welding operation had affected the electrochemical propertiesin heat-affected zone of welded sample.The potential distribution demonstrated that open-circuit potential at eachposition of welded sample moved to negative direction with time, but the negativeshift extend was small, and then became stable after14h. Maximum potential difference between each position was small and smaller than20mV. So there wouldbe the galvanic corrosion, but the galvanic corrosion tendency was smaller.Weight-loss results showed the average annual corrosion depth at each positionwas relatively small in seawater. The average annual corrosion depth of weldingmaterial of S-189-1was minimum （5.4823′10-4mm/a）, and that of base metal wasmaximum (8.3321′10^-4mm/a). The average annual corrosion depth of weldingmaterial of S-189-2was minimum (7.2106′10^-4mm/a), and that of heat-affected zonewas maximum (9.8249′10^-4mm/a). These results were consistent with the resultsfrom electrochemical and potential distribution, which meant the welding materialwas appropriate, but welding operation had a certain influence on the welded sample.The analysis of the metallographic structure and corrosion products indicatedthat the metallographic structure of base metal was fine but there were several defects.The compact and homogeneous metallographic structure of welding material washighly seawater corrosion resistant. There must be some changes among coarse andfine grained heat-affected zone after welding. And the structure of fusion line wasdestroyed to some extent. The corrosion product was mainly γ-FeO（OH）.Through comparing the analysis results from electrochemical, potentialdistribution, weight-loss, metallographic structure and corrosion product, we foundout that the law of corrosion of S-189-1（less heating） and S-189-2（more heating）were almost the same. However, the impact on heat-affected zone of S-189-2（moreheating） was larger than that of S-189-1（less heating）.