مجله علوم و مهندسی خوردگی
سال دوم شماره 2 (پیاپی 4، تابستان 1391)

Zn-TiO2 nanocomposite coatings were prepared from a sulfate bath solution by pulse electrochemical deposition. In this method, frequency and duty cycle parameters were investigated. Morphological and corrosion properties of these coatings were investigated by FE-SEM and potentiodynamic polarization in 1M NaCl solution. In case of frequency it was observed that by increasing its value from 10 to 100 Hz, the amount of codeposited nanoparticles increases in the coatings. Moreover, by increasing duty cycle from 10 to 75%, a maximum value of codeposited particles observed at 50%. These changes are function of T on and Toff in pulse current.

Hydroxyapatite coating on Ti substrate is being used more frequently as a bone replacement material. Several methods such as plasma spray, sol-gel, ion beam assisted deposition and electrochemical methods can be used for applying hydroxyapatite on Ti substrate. Controlling microstructure of the coating is one of the advantages of electrochemical methods. In this work hydroxyapatite coatings were applied by electrochemical method on Ti-6Al-4V substrate under a coating voltage of 4V and amperage of 4mAcm-2. The coating bath contained Ca(NO3)2. 4H2O and NH4H2PO4 at a pH of 4.1. The samples were heat treated at 550oC for 4hr. The microstructure and the composition of the coatings were investigated by scanning electron microscope (SEM) and X-Ray diffraction (XRD) methods before and after the heat treatment. The results showed a more compact coating with an increased amount of hydroxyapatite after the heat treatment.

In this study, corrosion rate and mechanism of 1015 carbon steel alloy in the simulated sour electrolyte system was investigated for 21 days by the electrochemical noise and weight loss methods. Changes of the corrosion rate were measured during test period and compared with the results of weight loss. The statistical Distribution type obtained fr om electrochemical current noise data was used to determine corrosion mechanisms. The obtained results were confirmed by surface observations and weight loss results. Distribution of current data showed that localized corrosion must be happened on carbon steel surface during experimental period. It seems that electrochemical noise can be used for corrosion monitoring in sour electrolyte systems.

The effect of thermal oxidation temperature on microstructure, surface chemical composition and corrosion behaviour of commercial pure titanium (CP) are investigated. For this goal and creation of oxide layer with different thickness, the samples of CP tit anium isothermally oxided at temperature range of 400 -800 °C for duration of an hour. Microscopic studies, microhardness measurements, and polarization test in the physiological environment have been applied for characterisation and evaluation of the surfa ce layer. The structure of cross section and oxide layer thickness evaluated by employing optical microscopic examination. Diffusion depth and distribution of oxygen in substrate characterized by glow discharge optical emission spectroscopy (GDOES). Results show that oxide layer thickness and diffusion depth are increased by an increase of oxidation temperature. At oxidation temperature range of 400 -600 °C, increasing oxide layer thickness led to enhancement of corrosion resistance. On the other hand, incre asing of oxidation temperature in the range of 600-800 °C caused decreasing of corrosion resistance due to local spullation phenomenon of oxide layer. Concequently, temperature of 600 ºC was achieved as an optimum temperature for thermal oxidation of commercial pure titanium for corrosion resistance.

In this study, the properties of transpassive film formed on 304 stainless steel in 0.5 M sulfuric acid has been examined using electrochemical impedance spectroscopy (EIS). For this purpose, transpassive films were formed potentiostatically at selected potentials for 3 minutes and then EIS measurements were done. For EIS measurement s, an excitation voltage of 10 mV and an applied frequency ranging from 100 kHz to 10 mHz have been used. Results showed that in the potential rang 0.85-0.90 VSCE, a high frequency capacitive and a low frequency inductive observed. The high frequency capacitive loop reflects the high field assisted migration of defects in the transpassive film, and the inductive loop at intermediate frequencies is due to the relaxation of the negative surface charge formed by the accumulation of cation vacancies at the transpassive film/solution interface. In addition, in the potential rang 0.95-1.00 VSCE, the faradic pseudo capacitance is detected. In this region, by increasing the potential, Rt, RSC and LSC are decreased.

Development of inert anodes in the Hall – Heroult for aluminum production has been one of the major challenges in recent years. In the present study, the use of HSLA-100 steel has been studied for use as an inert metal anode in aluminum production industry. Hence, in order to create a corrosion resistant surface oxide layer, the alloy got surface heat treatment. After surface heat treatment at 1000ºC for periods of 1, 5 and 10 hours on the alloy, the behavior of oxidized samples studied in molten cryolite at 930 ºC for 20 hours. EDS and XRD studies showed that after surface oxidation on the alloy mainly were formed iron phases as Fe2O3 with compounds containing of nickel, chromium, copper and aluminum in the oxide layer. So the oxidized sample for 1 hour that the thickness of oxide layer was about 237μm, has the highest corrosion resistance in corrosive cryolite – alumina environment. Finally, the mechanism of corrosion occurred in the corroded samples were studied in the mentioned environment.

Effect of hypophosphite anion on morphology, deposition rate, composition, and corrosion behavior of trivalent chromium coatings, as a complexing agent in trivalent chromium electrolyte, was investigated. For this purpose, the concentration of hypophosphite anion in trivalent chromium electrolyte was changed from 0-0.75 M. Morphology and composition of the coatings were evaluated by using scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). Corrosion behavior of the coatings was investigated using Tafel extrapolation method and electrochemical impedance spectrometry (EIS) technique in 3.5 wt% NaCl solution. Results show that the deposition rate was decreased by addition of hypophosphite anion to the bath. However, further increasing of its concentration to 0.525 M, results in increased deposition rate. Moreover, the incorporation of phosphorous in the trivalent chromium coatings caused higher micro cracks density and increase in porosity, and thus the corrosion resistances of these coatings were decreased.