مجله علوم و مهندسی خوردگی
سال چهارم شماره 4 (پیاپی 14، زمستان 1393)

In this paper, the hydrogen embrittlement of the high strength Cr-Si spring steel with after Zn-12%Ni electroplating, have been evaluated. Slow strain rate test with 3.34 × 10-6 sec-1 strain rate was performed, and the hydrogen embrittlement index of the different samples was calculated. The effect of baking after plating and also the effect of applying a nickel interlayer by electroless method, is examined by using the weibull statistical model. The results showed that alloying Zn with nickel considerably reduce the diffusion of hydrogen on the substrate steel structure. So that it increases the average failure time of samples from 4.9 hours for pure Zn to 8.7 hours for Zn-Ni. By applying nickel interlayer, the failure time increased to 11.15 hour. Baking after the electroplating of Zn-Ni samples without interlayer, leaded to an increase in the failure time to 10.15 hours. While baking in samples containing nickel interlayer, showed the opposite effect, reducing the failure time to 8.15 hours.

Chromium (VI) and its alloys used to be widely utilized in industries, because of their great corrosion and wear properties. However, due to their intense toxicities, their uses are currently obsolete in many countries. Nevertheless, it has been shown recently that the best substitute for Cr (IV) is Cr (III), which is not toxic and has similar properties, particularly when it is used as the main constituent or as an alloying element in coatings. In this paper corrosion properties of Cr-Ni coatings applied on St37 steel substrate, obtained from Cr (III) and Ni (II) contained bath, has been investigated. The electroplated coatings have been optimized using their physical appearance such as their surface morphology and uniformity in their thicknesses, etc. They were then subjected to potentiodynamic polarization and EIS (Electrochemical Impedance Spectroscopy) tests in 3.5% NaCl water solutions in order to evaluate their corrosion behavior. Chemical analyses of the coatings have been carried out using EDS (Energy Dispersive Spectroscopy) methods. SEM (Scanning Electron Microscopy) revealed that the predominant surface morphology had a cracked nodular structure while the nodules and cracks grow as electroplating current densities increased.

Corrosion resistance of (1) Fe77Mo5P9C7.5B1.5 glass matrix composite containing α-Fe dendrites and (2) AISI Type 301 stainless steel was assessed in deaerated 1 M HCl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) methods. Results showed that the excessive difference in composition between α-Fe dendrites and glass matrix induced a galvanic effect that causes preferential dissolution of α dendrites. However, no transpassive behavior was observed by anodic polarization up to 1000 mV for composite specimen. In contrast, the stainless steel revealed lower corrosion rate and passive current density. SEM micrographs were also used to confirm these findings.

In this study, it was investigated the effect of modified carbon black Nanoparticles to improve anti-corrosion properties of epoxy-based coatings. SDS surfactant was used for decreasing Van Der Walls force between CB nanoparticles.TEM analysis showed CB nanoparticles were dispersed in epoxy matrix by lower than 50 nm. Coatings with 0.5, 1 and 1.75% CB nanoparticles were prepared and EIS technique was used for evaluating of coatings in 65°C. By adding CB nanoparticles, the resistance of coating to corrosive media increased and the capacitance of coating decreased. Diffusion coefficient of water decreased more than 107 times in 1% CB coating than to neat epoxy. It seems that CB nanoparticles fill micro pores in the coating and limit diffusion of corrosive ions and water.

Magnesium alloys as biodegradable implants have attracted substantial attention since there’s no need to extract them & thus lowering patient’s pains and costs. But their high corrosion rate, makes it inevitable to coat them. In the present research, silicone doped calcium phosphate coating was successfully generated on AZ31 magnesium alloy via pulse electrodeposition, which made it possible to create nano structure morphology. XRD patterns indicated that primary coating was mainly consisted of DCPD, but after immersion in hot NaOH solution, pirmaryDCPD transformed into hydroxyapatite. FESEM images showed sponge-like structure consisting of nano blade crystals. EDS analysis proved the presence of silicone in the hydroxyapatite structure. According to EDS results, Ca/(P+Si) ratio would be equal to 2.35 which by considering the accelerating voltage, formation of siliconedoped hydroxyapatite is approved. Finally, polarization tests in ringer’s solution revealed that silicone-doped hydroxyapatite have greatly reduced corrosion rate. Corrosion rate was reduced from 121.06 μA/cm2 for bare metal, to 4.38 μA/cm2 for sample with silicone-doped hydroxyapatite coating.

Creating of superhydrophobic property in nickel coatings increases the durability of their intrinsic properties such as corrosion resistance in addition to make properties such as anti-fouling and self cleaning. In this study, for investigation of effect of crystal modifier concentration on surface morphology and then superhydrophobicity, Ni coatings were prepared from chloride bath consisted of ammonium chloride and ethylenediammonium dichloride (EDA) by two - step electrodeposition process. The results showed morphology influences considerably by the type and the value of the additives. The coatings prepared of the plating bath with ammonium chloride had micronano cone structures but morphology of the coatings prepared in presence of EDA didn’t have the above mentioned structures. So with storing in ambient, the former coatings could be superhydrophobic, but maximum contact angle of the latter ones were 137º and just could be hydrophobic. Corrosion resistivity of the superhydrophilic and superhydrophobic coatings was evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests and the results showed superhydrophobicity can increase considerably corrosion resistance.