Heterogeneous electrode processes and localized corrosion

This book covers the areas of electrochemical heterogeneity and electrode inhomogeneity and their effects on nonuniform electrode processes, in particular, localized corrosion. It covers the fundamentals, experimental methods, and engineering aspects of electrochemical heterogeneity.

Quantifying the efficiency and understanding the mechanism of localised corrosion inhibition using the wire beam electrode

Parameters extracted from the wire beam electrode (WBE) galvanic current maps have been used in conjunction with electrochemical noise patterns to directly quantify the degree of localised corrosion inhibition provided by inhibitors and to understand the mechanism of localised corrosion inhibition. The behaviour of two traditional localised corrosion inhibitors has been assessed by their effects on the maximum anodic current density (imax), total anodic current density (itot), the number of anodic sites (Na) and the localised corrosion intensity index (LCII). Typical experiments are presented to illustrate the application of these parameters in providing useful information on the efficiency and mechanism of localised corrosion inhibition.

On corrosion behaviour of magnesium alloy AZ31 in simulated body fluids and influence of ionic liquid pretreatments

This paper reports on the corrosion of Mg alloy AZ31 in simulated body fluid (SBF) using static immersion tests and electrochemical impedance spectroscopy. A preliminary study on the effect of flowing SBF on the corrosion behaviour of AZ31 has also been carried out. Low toxicity ionic liquids (ILs) trimethyl(butyl)phosphonium diphenyl phosphate P1444DPP and trihexyl(tetradecyl)-phosphonium bis-2,4,4trimethylpentyl-phosphinate [P66614][ i(C8) 2PO2] have been used to provide corrosion protection for AZ31 in SBF. Time dependent immersion tests indicate that under static conditions, AZ31 suffers severe localised corrosion in SBF, with pits developing predominantly beside the Al-Mn intermetallic phase in the α matrix. At longer immersion times, the corrosion product eventually precipitates and covers the entire specimen surface. When exposed to SBF under flowing conditions with a shear stress of 0·88 Pa, more uniform corrosion was observed. The optical profilometry results and electrochemical impedance spectroscopy analysis suggest that both P
1444DPP and [P66614][i(C8)2PO2] pretreatments can increase the corrosion resistance of AZ31 in SBF, in particular by decreasing the number of deeper pits found on the alloy surface. Cytotoxic test shows that the presence of the ILs P
1444DPP and [P66614][i(C8)2PO2] in cell culture media slightly inhibits the growth of human coronary artery endothelial cells in comparison with the good cell viability around the treated specimen. A pretreatment with IL is used in order to improve the corrosion resistance of this alloy in SBF. © 2012 Institute of Materials, Minerals and Mining.

Microstructures, mechanical properties and in vitro corrosion behaviour of biodegradable Mg-Zr-Ca alloys

The microstructures, mechanical properties, corrosion behaviour and biocompatibility of the Mg-Zr-Ca alloys have been investigated for potential use in orthopaedic applications. The microstructures of the alloys were examined using X-ray diffraction analysis, optical microscopy and scanning electron microscopy. The mechanical properties of Mg-Zr-Ca alloys were determined from compressive tests. The corrosion behaviour has been investigated using an immersion test and electrochemical measurement. The biocompatibility was evaluated by cell growth factor using osteoblast-like SaOS2 cell. The experimental results indicate that the hot-rolled Mg-Zr-Ca alloys exhibit much finer microstructures than the as-cast Mg-Zr-Ca alloys which show coarse microstructures. The compressive strength of the hot-rolled alloys is much higher than that of the as-cast alloys and the human bone, which would offer appropriate mechanical properties for orthopaedic applications. The corrosion resistance of the alloys can be enhanced significantly by hot-rolling process. Hot-rolled Mg-0.5Zr-1Ca alloy (wt %) exhibits the lowest corrosion rate among all alloys studied in this paper. The hot-rolled Mg-0.5Zr-1Ca and Mg-1Zr-1Ca alloys exhibit better biocompatibility than other studied alloys and possess advanced mechanical properties, corrosion resistance and biocompatibility, suggesting that they have a great potential to be good candidates for orthopaedic applications. © 2012 Springer Science+Business Media New York.

Rare earth cinnamates and their corrosion inhibition mechanisms for AS1020 steel

Speciation of the inhibitors lanthanum 2-hydroxy cinnamate and lanthanum 3-hydroxy cinnamate in solution has been evaluated and compared to the speciation of lanthanum 4-hydroxy cinnamate. The results have been correlated with corrosion inhibition efficiency for AS1020 steel in an aqueous chloride solution using a combination of analytical tools such as nuclear magnetic resonance (NMR) spectroscopy, electrospray mass spectrometry (ESMS), potentiodynamic polarisation.

Inhibition of corrosion on AA2024-T3 by rare earths & mercaptoacetate

The use of the aluminium alloy AA2024-T3 has long been associated with a strong vulnerability to localised corrosion. Dealloying and pitting corrosion can occur on and around intermetallic particles when exposed to aggressive environments such as sodium chloride electrolytes. Specific combinations of rare earths and organic compounds have demonstrated strong synergistic inhibition on the AA2024-T3 alloy. This work has focused on rare earths and organic compounds containing thiol functional groups. It is believed that the sulphur in the thiol group can form protective films over the surface of copper-rich intermetallic particles due to the affinity between copper and sulphur. Previous studies with the multiwell tests have identified that solutions containing sodium mercaptoacetate provided strong inhibition at pH 3 and 6. This work presents the initial findings from the polarisation tests and constant immersion corrosion experiments in the presence of sodium mercaptoacetate.

Ionic liquid pretreatment as a means to control the corrosion behavior of magnesium alloys in simulated body fluid

Recently, the use of magnesium alloys as metallic implant materials for biodegradable coronary artery stents has been steadily growing in interest. However, AZ31 magnesium alloys present poor corrosion resistance in the body environment. This work reports on the use of a treatment with low-toxicity IL Trimethyl (butyl) phosphonium diphenyl phosphate P₁₄₄₄DPP, which provides corrosion protection for magnesium alloy AZ31 in simulated body fluid (SBF). Before IL treatment, surface was cleaned by HNO₃ and H₃PO₄ acid pickling solution. The effect of ionic liquid treatment on the corrosion performance of magnesium alloys AZ31in simulated body fluid has been investigated by electrochemical tests and the observation of surface morphology. The results show that this IL treatment succeeded in increasing the corrosion resistance of AZ31 when exposed to SBF.

In vitro studying corrosion behaviour of biocorrodible Mg alloys

The idea of bioabsorbable/biocorrodible stents has gained increasing attention in the last decade. Permanent coronary stents, traditionally made from 316L grade stainless steel, are routinely used for the treatment of blocked arteries. However, these stents can cause complications such as restenosis, thrombosis and the need for the patient to undergo prolonged antiplatelet therapy. Biodegradable metal stents provide an opportunity for the stent to remain in place for a period to ensure restoration of function and then degrade through a carefully controlled bio-corrosion process. Among the number of potentially suitable materials, Magnesium alloys have shown great promise as a stent material due to their non-toxicity [1] and the corrosion rates attainable in biological environments. However, a carefully controlled corrosion process is essential in order to avoid hyper hydrogen generation and the fatal consequences that follow. In addition uniform corrosion is a basic requirement to maintain the mechanical integrity and load bearing characteristics. Work being undertaken in our laboratories focuses on controlling the corrosion behaviour of magnesium in a simulated biological environment in the presence of protein. In the investigation reported here the Mg alloy has been examined using Scanning Electrochemical Microscope (SECM) to visualize the corrosion process and identify the corrosion pattern. Complementary bulk electrochemical techniques (EIS and potentiodynamic polarization) have been used to acquire kinetic and mechanistic information. Early results obtained by SECM have revealed the tendency towards pitting corrosion in the early stages which subsequently develops in to filiform corrosion.

Effect of nanocrystalline structure on the corrosion of a Fe20Cr alloy

The corrosion behaviour of nanocrystalline and microcrystalline Fe20Cr alloys, prepared by high energy ball milling followed by compaction and sintering, was studied in 0.05M H2SO4 and 0.05M H2SO4 + 0.5M NaCl by potentiodynamic polarization. The nanocrystalline alloy exhibited improved passivating ability and pitting resistance as described by passivation potential, critical current density, passive current density and breakdown potential. XPS and SIMS analysis revealed greater Cr content in the passive film formed on the nanocrystalline form of the alloy. The enhanced passivating ability of the nanocrystalline alloy was attributed to the formation of the passive film with higher Cr content.

Effects of high salt concentration and residue on copper and aluminum corrosion

Traditional researches on metal corrosion under salt solutions deposit conditions are usually carried out by visual, electron microscopic observations and simple electrochemical measurement via a traditional one-piece electrode. These techniques have difficulties in measuring localized corrosion that frequently occur in inhomogeneous media. This paper reports the results from the experiments using specially shaped coupons and a relatively new method of measuring heterogeneous electrochemical processes, namely, the wire beam electrode(WBE). Preliminary results from copper and aluminum corrosion in highly concentrated sodium chloride solutions with and without solid deposits show that the method is useful in simulating and studying corrosion especially localized corrosion in pipelines.