Chemical composition and corrosion product of the Northern Song Dynasty Emperor Asoka Pagoda substrate were studied for the first time since it was unearthed from the Changgan Temple underground palace located at Nanjing in 2008. Silver samples based on composition of the Pagoda substrate were simulated by traditional smelting and processing technique. The corrosion potential, corrosion current of the silver sample and the composition, morpholoy, grain size of the corrosion products on the sample in corrosion medium with different concentrations of Na2S were examined by means of electrochemical technique, OM, XRD, XRF, SEM and EDS. The results showed that with the increase of Na2S concentrations from 20 mg/L to 5.0×104 mg/L (i.e. 5% Na2S), the corrosion current density Icorr increased from 0.274 μA/cm2 to 1.056 μA/cm2 and the corrosion potential Ecorr was shifted towards less value by 0.70 V. Thereby, the driving force for corrosion and corrosion rate were enhanced. Silver samples were kept in active state when polarization potential was shifted from -0.31 V to 1.8 V in 20 mg/L Na2S solution. In 5% Na2S solution, the corrosion current density for silver in the active corrosion region increased rapidly from 1.056 μA/cm2 to 1.56×104 μA/cm2, and then silver samples transferred to passive corrosion region and maintained the passivation with higher corrosion current density about 1.56×104 μA/cm2. Corrosion products of silver were mainly Ag2S after 0.3 V (20 mg/L Na2S) or -0.8 V (5% Na2S) constant potential polarization for different time intervals, respectively. The amount of S2- as corrosion active center and the growth rate of Ag2S crystals on the silver sample surface increased with the increase of the Na2S concentrations from 20 mg/L to 5%. However, the grain size of Ag2S crystals was reduced from 20 μm to1.86~5.10 nm.
The corrosion behavior and sulfide induced stress corrosion cracking sensitivity of a sulfide-resistant casing steel P110SS in hyperbaric H2S/CO2 environment were studied by high temperature/pressure reactor which simulated the operating environment of Puguang gas field. The corrosion rate of P110SS in hyperbaric H2S/CO2 environment decreased first and then rose with the increase of temperature and partial pressure of H2S/CO2. Corrosion product peeled off in an environment corresponding to the middle area of the well where the corrosion rate was minimal. The sulfide induced stress corrosion cracking sensitivity of P110SS steel in hyperbaric H2S/CO2 environment was very low revealed by four point bending test for the steel with a load of 90% yield strength, which could be attributed to the uniform tempered microstructure of sorbite with fine grain, high dislocation density and uniform dispersion of carbides of the steel.
The corrosion process of galvanized steel for power transmission tower in media with different concentration of Cl- and HSO3- was investigated by using potentiodynamic polarization test and cyclic wet-dry accelerated laboratory test. Then, the surface morphology and characteristics of the corrosion products were examined by X-ray diffractometer and scanning electron microscopy. The results showed that the galvanized steel was corroded in the presence of NaCl and mixed NaCl and NaHSO3, and the corrosion current density increased with the increasing concentration of Cl- and HSO3-.The corrosion rate of galvanized steel was greater at the early stage of test and then decreased with the extension of test time. In addition, the effect of the combination of Cl- and HSO3- on the corrosion of galvanized steel was more serious than that of Cl-.
The high-temperature oxidation behavior of Ni-Si alloy was investigated at 800 ℃ in air and water vapor, respectively. A two-layered oxide scale formed on the Ni-Si alloy after exposure in air and the inner layer was continuous and rich in Si which could be responsible for enhancing oxidation resistance of the alloy. Water vapor promoted the formation of Si-rich scale on the alloy, while inhibited the formation of NiO during oxidation process. However, water vapor decreased the adherence of the oxide scale remarkably, as a result, heavy spallation of the oxide scale occurred with the increase of time.
The failure analysis for hot water pipes of 20 steel was performed by means of examinations of macro-morphology, metallograph and fractured surface, as well as chemical analysis and mechanical property measurement for the relevant materials. Results showed that the fracture occurred in the weld joints with obvious deterioration in weld area due to creep. The fracture initiated and propagated with typical brittle characteristics. The occurrence of corrosion products on cracking wall and in the cracking tip revealed that stress corrosion cracking (SCC) was the dominant factor of the pipe burst. Creep crack and fatigue characteristics were also found on the fracture, which accelerated the SCC and pipe burst.
Measurement of potentiodynamic curve and electrochemical impedance spectra (EIS) was used to study the corrosion resistance of a medical 316L stainless steel passivated by citric acid in simulated body fluid. XPS was applied to analyze the effect of surface composition on the corrosion resistance of the studied material. The results showed that after passivation by citric acid, the free corrosion potential shifted to positive side, while the corrosion current density decreased and the polarization resistance and the charge transfer resistance were enhanced, indicating a significant improvement of the corrosion resistance. XPS analysis results showed that the enhancement of the corrosion resistance of 316L stainless steel was attributed to that Fe on the surface of substrate was selectively dissoluted by citric acid and then the enriched Cr was oxidized by H2O2 to form a compact chromium oxide film to protect the substrate from corrosion.
Electrochemical impedance spectroscopy (EIS) was used to study the corrosion behavior of four Al-based alloys commonly used as sacrificial anode for submarine under wet-dry cyclic conditions with various drying periods. The EIS results showed that the failure of the sacrificial anodes was mainly attributed that the coverage effect of corrosion products on the surface of sacrificial anodes, leading to the decrease of the number of active sites on the anode surface. Thus, the further activation of the sacrificial anodes was prevented. After several wet-dry cycles, the severe accumulation of corrosion products on the anode surface resulted in enlargement at the capacitive loops, indicating significant decrease of the active dissolution of the anodes. The performance of the 4 Al-based alloys under wet-dry cyclic environment may be ranked as the following order: Zn-Al-Cd < Al-Zn-In-Cd < Al-Zn-In-Mg-Ti < Al-Zn-In-Mg-Ga-Mn.
Four point bend specimens of 25Cr2Ni4MoV steel were used to investigate the stress corrosion cracking behavior in ultra-high concentration hydrochloric acid (20 mass%). As a result of four point bend experiments, curves of relationship between deflection and time were revealed. The results showed that the life time of stress corrosion cracking of 25Cr2Ni4MoV steel decreased as the concentration of hydrochloric acid increased. The stress corrosion cracking process of 25Cr2Ni4MoV steel under the test condition could be divided into three typical stages: crack initiation, crack propagation and instant fracture. Crack initiation and propagation had a priority along grain boundary. The increase in stress accelerated the stress corrosion cracking initiation and propagation.
The collected dismantled Ni-Cr porcelain crowns in clinic were divided into five groups according to the time for restoration as followed: (1) restored for 1 a, (2) restored for 2 a, (3) restored for 3~4 a, (4) restored for 5~9 a, (5) restored for more than 10 a, and new porcelain crown was set up as control group. The contact angles was measured, the corrosion curve was obtained by an electrochemical workstation, and the surface morphology of inner side of crowns was observed by scanning electron microscope. After then the experimental data was used for statistical analysis. The results showed that the free corrosion potential and self-corrosion current density of group 2 was significantly higher than the others, and the trend was consistent with the results of contact angle measurement. The observation under scanning electron microscope also showed that the corrosion was severer as the restoration time increased. The conclusion was that Ni-Cr porcelain crown suffered from corrosion in oral cavities, which exhibited the fastest corrosion rate at the second year. The corrosion effect was accumulated as time increased. Therefore, dental doctors should take certain measures to avoid this negative effect.
The anti-corrosion coatings for oil tanks were prepared with polyurethane modified epoxy resin, zinc phosphate, aluminum tripolyphosphate, talcum powder, sericite, nano-silica and nano- titania. The performance of the prepared coatings was examined by measurements of mechanical properties, electrochemical impedance spectroscopy (EIS) and resistance to chemical medium. The influence of the mixture of zinc phosphate and aluminum tripolyphosphate on the anti-corrosion properties were investigated. The results showed that mechanical properties and corrosion resistance of the anti-corrosion primer were superior.
Infrared thermal wave technology was employed to detect the corrosion damage under coating, which could not be easily observed and caused safety problem. The infrared thermal images of surface temperature distribution on the tested specimen were captured, as well as the surface temperature history of areas with and without pre-corrosion damages, and there with the temperature difference of the two areas. Analysis of the test results showed that the infrared thermal wave technology was an effective tool in detecting corrosion damage under coating. The optimum detection moment for estimating the coating thickness and position and size of damages was when the temperature difference between corrosion area and sound area reached the maximum. The damages could be quantitatively recognized by images processing, but surface uniformity of the specimen had great impact on the damage identification.
