| In this paper, focusing on the submerged zone, the corrosion of steel in marine was discussed from two aspects:material property and marine environment. From the point of view of corrosion interface between substrate and corrosion products, the corrosion mechanism of two Q235 steel was investigated by weight-loss method, open circuit potential (OCP), electrochemistry impedance spectroscopy (EIS), metallographic microscope, Scanning Electron Microscopy (SEM) and Energy-Dispersive Spectroscopy (EDS), taking corrosion types, formation and extension of corrosion and affect of corrosion products into account. At the same time using pitting potential test and microbial culture method, the influence of the marine natural biofilms on the corrosion of Q235steel and 316L,304 stainless steel was studied, and related to the corrosion-restrained in submerged zone.For the first time, combining corrosion interface with electrochemistry test systematic, we investigate the corrosion driving force. The results show that the galvanic effect between cementite and ferrite is the most important driving force for the whole process. Normalizing sample has better corrosion resistance than the annealing one for the galvanic effect between free spherical cementite and ferrite is higher. At beginning period, driving force also include grain boundary, later period it become rust. Corrosion diffuses along the grain boundary in annealing Q235 steel, especially small and slim grain. So rests which are 30-40μm appear in the corrosion interface, and 20-30μm pitting arises on the matrix. Corrosion diffuses faster in the small grain.But for normalizing sample, in addition to grain boundary, pearlite is also a diffusion channel. Small arcs which are about 60μm long and 15-20μm width appear, this conform to the pearlite. So, there is more localized corrosion in normalizing sample. The pearlite corrosion area contains more oxygen than other area, than reaches to 68%. Pearlite is more corrosive.The form and conversion of rust is observation systematic, and then test there effect to corrosion. Found that rust can accelerate the corrosion of Q235 steel, especially the inner rust. Due to the increase of ferrite, corrosion resistance of matrix becomes better after that rust is completely removed. Q235 steel rust is independent nucleation. There are 1μm separate sphere on annealing sample, and they are not dense, just like the distribution of spherical cementite. Some of them grow up to 10μm after 10hours, and become dense after 45days. Inner rust of 90 days is also independent nucleation. Crack encloses 40-50μm rounds, and there are 10-20μm nucleation core on it, which are transformed from big spheres of 45dsys. Because to the pearlite, normalizing sample rust has two morphology:1μm rounds with 0.3μm nucleation core,2μm flakes with 0.5μm nucleation cores which spread like sector. There is uniform inner rust after 45days and the outer inner has transform to flakes completely.In an innovative way, grain boundary, inclusion, grain form, galvanic effect of cementite are integrate with electrochemistry test, founding that grain boundary and cementite are foremost corrosion driving force when oxygen is enough. Silicate inclusions can easily induce pitting. Pitting that shows 50-60μm is induced by 15-20μm silicate inclusions, obviously larger than the 5μm pitting induced by alumina, calcium salt inclusions. Then small and slim grains are corroded first. One month later due to the lower of oxygen, grain boundary is weakening to corrosion; driving force becomes inclusions, cementit and rust, especially inclusions. Corrosion diffuses in big grain, so, corrosion rate slow down.There are lots of bacterial in the rust and biofilms, including aerobic bacteria (SOB) and anaerobic bacteria (SRB). Using comparative test, marine natural biofilms are found to inhibit the aerobic corrosion and promote the anaerobic corrosion. The Rp of natural sample is about 1000-1100Ω·cm2, and after 100days it reduces to 600Ω·cm2 quickly, showing anaerobic corrosion appears and rust contains 9.63% sulfur. But the sterile one maintains at 600-700Ω·cm2, so the sterile sample shows higher corrosion rate than natural one, about 1.4 time. This indicates than biofilms is an important reason for the corrosion-restrained in submerged zone.In Qingdao seawater, pitting potential of 316L stainless steel is 0.551V,304 is 0.249V.316L SS is 0.3V higher than 304. The corrosion resistance of 316L SS increased after exposure, on the contrary,304 SS decreased. In high frequency of electrochemistry impedance spectroscopy, Z of 316L is about 1.62 higher than 304 at the third day, it extends to4.25 time at 35th day. What is more, Alumina inclusions have induced 5μm pitting on 304 SS. The proportion of Cr, Ni, Mo/Fe in 316L SS matrix became higher, and manganese disappeared, that both enhance the corrosion resistance, especially pitting resistance.
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