The Corrosion Mechanism Of Hot Dipped Steel In Marine Environment

The good corrosion resistance, cathodic protection and low cost of Zn(GI), Zn-5%Al-0.2%RE(GF) and Zn-55%Al-1.6%Si(GL) is the reason for its widespread used in many applications like architecture, mashine, auto mobile, bridge ,etc. With the development of marine, more and more hot dipped coated steel have a wide application in marine constructions. But the corrosion behavior and mechanism of these coatings in marine environment is very few and not systemic.The electrochemical impedace spectroscopy(EIS), EIS-polarization-relaxation, galvanic test was employed to study the corrosion behavior and mechanism of three coatings(GI,GF,GL) in marine enviroment.Atmospheric corrosion testing results indicated that the corrosion progress of coatings attribute to the relative humidity(RH) and temperature. The higher relative humidity was, the faster corrosion rate was on the coating surface. When the RH was below critical relative humidity of corrosion, the effect of temperature can be ignored, on the contrary, the effect of temperature is very remarkable. The corrosion rate of GI and GF coatings was equal and larger than GL. The atmospheric corrosion is a dry-moist-wet cylce. Furthermore, we can see that the atmospheric corrosion is an unsteady and complicated process from Rp.The wet-dry simulative testing results confirmed that the coating surface was moist and wet in three wet-dry conditions, namely, the experiment time is equal to the time of wetness(TOW). The corrosion rate(Rp-1) was decreased with the test time increased, the local corrosion of GI and GF was observed. The corrosion process of Zinc-riched phase of three coatings was divided into three stages: local corrosion stage(or oxide film corrosion stage)-corrosion devleloping stage-corrosion inhibitor stage. The average corrosion crruent density of GI coating is 1.7~3.1 times than GF coating, and 6.3~9.3 times than GL coating. The longer dry time was, the smaller corrosion rate was on the coating surface, the faster reached steady corrosion stage.The EIS-polarization-relaxation testing results indicated that the corrosion process of three coatings could divide into several stages, the corrosion of each stage was corresponded to different phase respectively. The corrosion process of GI coating:ηphase-ζphase-δphase-Γphase, the optimal corrosion resistance layer wasζphase, theδandΓphase have a good anticorrosion in theory, but didn't perform a good anticorrosion in corrosion process due to the local corrosion. The corrosion process of GF coating: Zinc-riched(crystal buandary) phase-eutectic phase-intermetallic phase, the intermetallic phase present a optimal corrosion resistance, the corrosion behavior of GF coating is uniform corrosion. The corrosion process of GL coating: Zinc-riched phase-Aluminum-riched phase-intermetallic phase. The oxide film has a good pretection to the coating, after Zinc-riched phase corrosion, Aluminum-riched phase present a poor corrosion resistance own to local corrosion. In a word, the corrosion resistance of GL coatings is the best, then GF coating is better, and the worst is GI coating.Galvanic corrosion testing results indicated that the three coatings all had the cathodic protected action with the steel substrate. The polarity reversal didn't happen in three galvanic couples under three different temperature conditions. The potential error and galvanic crruent was very larger in initial corrosion stage, then decreased and maintain a steady state finally. The sacrificial anode protection of GI coatings is the best, then GF coating is better, and the worst is GL coating. The higer temperature was, the smaller potential error was, the larger galvanic crruent.