Chunan Cao

Effects of NaCl and SO2 on the initial atmospheric corrosion of zinc

The influence of NaCI deposition on the corrosion of zinc in atmospheres with and without SO2 was studied via quartz crystal microbalance. Regularity of the initial corrosion of zinc under these conditions was analyzed. The results show that NaCl can accelerate the corrosion of zinc. Mass gain of zinc increases with the exposure time, which can be correlated by using exponential decay function. The relationship between mass gain and amount of NaCl deposition is well linear at any time in air containing 1 ppm SO2, but follows quadratic function in air without SO2. More amount of NaCI deposition will slow down the corrosion to some extent after exposure for certain time in the presence of SO2. The combined effect of NaCl and SO2 on the corrosion of zinc is greater than that caused by each single component. Fourier transform infrared spectroscopy and X-ray diffraction were used to characterize the corrosion products of zinc. In the absence of SO2, simonkolleite, Zn-5(OH)(8)Cl-2 (.) H2O and zincite, ZnO are the dominant corrosion products, while zinc hydroxysulfate (Zn4SO4(OH)(6) (.) 3H(2)O), zinc chloride sulfate hydroxide hydrate (Zn-12(SO4)(3)Cl-3 (.) (OH)(15) (.) 5H(2)O) and simonkolleite dominate in the presence of SO2. Brief discussion on the mechanisms of atmospheric corrosion under these conditions was introduced

Effect of PVC on the diffusion behaviour of water through alkyd coatings

The diffusion behaviour of water through alkyd coatings with micaceous iron oxide pigment at different pigment volume concentration (PVC) was studied by EIS and SEM techniques. The diffusion coefficients of water through the coatings were also calculated. The experimental results showed that, in the initial diffusion period, the diffusion processes of water through alkyd coatings immersed in 0.5 M NaCl followed the Ficks second law. Within the PVC values which have been studied, the diffusion coefficients of water diffusing through alkyd coatings initially decreased with increasing PVC, reaching a minimum value at 60%, and then increased with increasing PVC. When PVC was less than 60%, the barrier property of pigment was the main factor affecting the water diffusion behaviour. However, the coatings microstructure played an important role when PVC was greater than 60%.

Effects of AC-Modulated Passivation and Post-Treatment on Composition and Stability of Passive Films

Abstract An investigation of the effects of alternating voltage modulated passivation (AC-modulated passivation) and post-treatment on the composition and stability of passive films on 302 and 316 stainless steels by ellipsometry, electrochemistry, and surface Auger electron spectroscopy analysis was carried out. AC-modulated passivation increases the concentration of elements O, and the ratio Cr/ (Cr + Fe) in the passive film, changes the depth composition profile of the film, roughens the surface of the passive film and makes it more favorable to the adsorption of molybdenum-containing ions during post-treatment. Hence, the resistance against the chemical dissolution of the passive film increased and the stability of the passive film improved.

In-situ investigation of atmospheric corrosion behavior of bronze under thin electrolyte layers using electrochemical technique

The atmospheric corrosion behavior of bronze under thin electrolyte layer (TEL) with different thicknesses was monitored using cathodic polarization curves, open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS). Cathodic polarization result indicates that the cathodic limiting current density increases with decreasing the TEL thickness. EIS result shows that the corrosion rate increases with decreasing the TEL thickness at the initial stage because the corrosion is dominated by the cathodic process, whereas after long immersion time, the corrosion degree with the TEL thickness is in the sequence of 150 mu m > 310 mu m > 100 mu m approximate to bulk solution > 57 mu m. The measurements of OCP and EIS present in-situ electrochemical corrosion information and their results are in good agreement with that of physical characterizations.

The stability of the transpassive film on 304 stainless steel with post-treatment

Abstract Investigation on the stability of transpassive film on 304 stainless steel was carried out by ellipsometric measurement, AES and XPS analyses and examination of the breakdown potential, E b , in a Cl − -containing solution and the active dissolution time t a in an acidic medium. It is found that the t a of properly transpassivated film is almost 2 orders of magnitude longer than that of normally passivated film, and that the E b of the film which is post-treated in a molybdate-containing solution is much higher than that of the passive film formed in the passive region with the same post-treatment. The results of ellipsometric measurement show that the thickness of the film on the stainless steel increases with the positive shift of the anodic polarization potential, especially in the transpassive region, and the thickness of the transpassive film increases much more sharply than in the passive region. Moreover, AES and XPS analyses indicate that the transpassivated film consists of much more Fe 3+ and much less Fe 2+ than the passivated one, and this film with proper post-treatment contains much more Mo than the film formed in the passive region with the same post-treatment. It is believed that the increase of stability of the surface film obtained by transpassivation and post-treatment is ascribed to the increase of film thickness, the rise of the ratio Fe 3+ /Fe 2+ and the adsorption of Mo into the film during the post-treatment.

Inhibition Effects of l− and l2 on Stress Corrosion Cracking of Stainless Steel in Acidic Chloride Solutions

Abstract Stress corrosion cracking (SCC) of AISI 321 (UNS S32100) stainless steel (SS) in 0.5 mol/L sodium chloride (NaCl) + 0.5 mol/L hydrochloric acid (HCl) solution and the inhibition effects of...

Electrochemical impedance spectroscopy study on the diffusion behavior of water through epoxy coatings

The diffusion behavior of water through epoxy coatings with micaceous iron oxide (MIO) at different pigment volume concentration (PVC) was studied by EIS techniques. Experimental results showed that, in the initial diffusion period, the diffusion processes of water through epoxy coatings immersed in 0.5 M sodium chloride (NaCl) solution followed Ficks second law. The diffusion coefficients of water through the coatings were also calculated. Within the PVC values that were studied, the diffusion coefficients of water through epoxy coatings decreased with increasing PVC, indicating that the barrier property of the pigment was the main factor affecting the water diffusion behavior.

Characterization of the rust on carbon steels pre-corroded by different gaseous pollutants

Characterization of the rust formed during long-term atmosphere exposure at Beijing station on carbon steels pre-corroded by different gaseous pollutants was performed by means of Fourier transform infrared spectroscopy and X-ray diffraction. Goethite (α-FeOOH), Lepidocrocite (γ-FeOOH) and Feroxyhite δ-FeOOH are identified in the rust formed on all of the samples independent of the different pre-corrosion conditions and the different carbon steels. The results also show that: the rust constituent of the same kind of carbon steel pre-corroded by different pollutants are the same; the rust of carbon steels exposed for one year is some different to that exposed for two years; the phase constituents of the inner rust at the upper surface are the same as that at the downward surface, but those of the outer rust are different. It is concluded that the phase constituents of the rust of the same kind carbon steel after one year are the same under the different pre-corrosion conditions.

On the linear response of a passivated metallic electrode to potential step perturbation

Abstract The transient response of a passivated metallic electrode system to a small amplitude potential step perturbation is analysed, based on a linear model, and a technique for estimating the parameters characterizing the growth and dissolution processes of passive film is proposed. The information which is expected to be obtained from impedance measurements can also be obtained by this technique and it is believed that the latter is superior to the former as it can be carried out much more rapidly and conveniently.

Shot noise analysis on corrosion behavior of zinc alloy (ZnAl4Cu1) under dry-wet cycles

The corrosion behaviors of zinc alloy (ZnAl4Cu1) in 3.5% (mass fraction) NaCl, 7.3% (mass fraction) Na2SO4 and simulated acid rain solutions were investigated using electrochemical measurements. The potential noise during dry-wet cycle was monitored and analyzed by fast Fourier transform (FFT), fast wavelet transform (FWT), shot noise theory and stochastic theory. Cumulative probability curves of event frequency f(n) indicate that the corrosion events in the dry cycles are greater than those in the wet cycles. Uniform corrosion was observed in the NaCl solution compared with more localized corrosion in the Na2SO4 solution, which is evidenced by FWT and SEM. Conditional events generation rate r(t) for diffusion controlled reactions decreases with increasing the time. r(t) values for uniform corrosion and diffusion controlled process are the largest in the wet cycle in 3.5% NaCl solution. The values of r(t) for pitting corrosion in Na2SO4 solution are observed to become large during spraying periods, and r(t) for pitting corrosion has the largest value in the Na2SO4 solution. The intergranular corrosion of zinc is serious in simulated acid rain solution.