Investigation Into New Phenomena About Localized Corrosion Of Some Kinds Of Metal Materials

A micro-electrode is used to measure the potential on the metal surface of brass-16Mn steel galvanic couple corroding in NaCl aqueous solution of various concentrations , and the current distribution on the surface is calculated by employing potential distribution in conjunction with potentiodynamic polarization curves for the first time. It is found that the potential and current distribution change with the conductivity of solution and the distance from brass-16Mn steel couple joint, and they distributes more symmetrically when the conductivity of solution is higher. The potential polarizes greatly and the current density is larger near the couple joint. The results from potential and current distribution are substantiated by the surface observation of the corroded sample after immersion test. Experimental results show that corrosion would focus around the joint if the conductivity of solution is low, and it would distributed more symmetrically with the increase of the conductivity of solution. The method of calculating the current distribution is reliable.Brass wedge in 16Mn steel tube corroding in seawater is simulated by the experiment of brass-16Mn steel with high anode/ cathode area ratio corroding in 0.600 mol/L NaCl solution. The experimental results show that the anodic current density is very low which should not affect the long-term corrosion behavior of steel tube and brass wedge.If 16Mn steel coupled with brass wedge, the area ratio of cathode/anode is very high. The electrochemical parameters of brass-16Mn steel couple with high cathode/anode chang with immersion time monotonously and steadily. Intense anodic reaction doesn't influence the anodic surface state, and the reversal of galvanic potential does not happen. The corrosion rate of anode increases with increasing of the area ratio of cathode/anode, 16Mn steel exposuring in seawater will corrode badly.The influence of chloride on the corrosion behavior of nanocrystallized bulk 304 stainless steel (nano304ss) and cast 304 stainless steel (cast304ss) is invesgated by potentiodynamic polarization curves, cyclic polarization curves, EIS, potentiostatic polarization and Mott-Schottky curves.Compared to cast304ss, nano304ss showes a higher free corrosion potential , a lower corrosion current density and a lower polarization current density in Na2SO4 solution, the corrosion behavior is improved due to nanocrystallization. But in the solution with some aggressive anions (Cl- or Br-), nano304ss has a lower breakdown potential and a lower pitting corrosion resistance than cast304ss.In 0.5mol/L H2SO4 solution, nano304ss has a lower passivation current density than cast304ss . In 0.01 mol/L HCl solution , nano304ss showes a narrower passive range and a higher passivation current density. This indicates that passive film formed on nano304ss in HCl solution is unstable because of nanocrystallization. There was a little difference between polarization behavior of nano304ss and cast304ss in 0.1 mol/L HCl solution, but the polarization current density of nano304ss is higher.The corrosion behavior of nano304ss and cast304 treated by 30%HNO3 and 50% HNO3 in 3.5%NaCl is improved. Nano304ss showes a higher breakdown potential . Cast304ss showes a lower corrosion current density, a lower polarization current density and a high free corrosion potential. The corrosion resistance of the steels treated by 30%HNO3 is higher than ones treated by 50% HNO3. Nano304ss and cast304ss treated by 30%HNO3 have a higher free corrosion potential, lower corrosion current density and polarization current density in 0.01 mol/L HCl with respect to untreated ones, the breakdown potential of nano304ss shifts to more positive value. But the corrosion behavior of nano304ss is worse than nano304ss when they are treated by the same way.A critical chloride concentration which is equal to 0.55mol/L for the passive behavior of nano304ss is determined. Above the critical concentration, the comparison between nanocrystallized and cast 304 stainless steel showes a drastic decrease in the protective properties of the passive film on the nano304ss. This behavior is correlated with passive film's properties. When the chloride concentration achieves to 0.55mol/L, the passive film on nano304ss becomes porous, the n-type behavior does not transform to p-type behavior in the pitting-susceptible potential region and the carrier density drastically increases.