Transient response analysis of steel in concrete

A new approach is presented for the analysis of galvanostatically induced transients allowing for the rapid evaluation of the corrosion activity of steel in concrete. This method of analysis is based on the iterative fitting of a non-exponential model based on a modified KWW (Kohlrausch–Williams–Watt) formalism. This analysis yields values for the parameters related to corrosion such as the concrete resistance, polarisation resistance, interfacial capacitance and β, the non-ideality exponent.

Biodegradable Mg-Ca and Mg-Ca-Y alloys for regenerative medicine

In this study, Mg-xCa (x = 0.5, 1.0, 2.0, 5.0, 10.0, 15.0 and 20.0 %, wt.%, hereafter) and Mg-1Ca-1Y alloys were investigated as new biodegradable bone implant materials. The compressive strength, ultimate strength and hardness of the Mg-Ca alloys increased, whilst the corrosion rate and biocompatibility decreased, with the increase of the Ca content in the Mg-Ca alloys; higher Ca content caused the Mg-Ca alloy to become brittle. Solutions of simulated body fluid (SBF) and modified minimum essential media (MMEM) with the immersion of Mg-xCa and Mg-1Ca-1Y alloys showed strong alkalisation. The yttrium addition to the Mg-Ca alloys does not improve the corrosion resistance of the Mg-1Ca-1Y alloy as expected compared to the Mg-1Ca alloy. It is suggested that Mg-Ca alloys with Ca additions less than 1.0 wt.% exhibited good biocompatibility and low corrosion rate.

Biodegradable Mg-Zr-Ca alloys for bone implant materials

Mg–Zr–Ca alloys were developed for new biodegradable bone implant materials. The microstructure and mechanical property of the Mg–xZr–yCa [x=0·5, 1·0% and y=1·0, 2·0% (wt-% hereafter)] alloys were characterised by optical microscopy, compressive and hardness tests. The in vitro cytotoxicity of the alloys was assessed using osteoblast-like SaOS₂ cells. The corrosion behaviour of these alloys was evaluated by soaking the alloys in simulated body fluid (SBF) and modified minimum essential medium (MMEM). Results indicated that the mechanical properties of the Mg–Zr–Ca are in the range of the mechanical properties of natural bone. The corrosion rate and biocompatibility decreases with the increase in the Ca content in the Mg–Zr–Ca alloys. The solutions of SBF and MMEM with the immersion of the Mg–Zr–Ca alloys show strong alkalisation. The Zr addition to the Mg–Zr–Ca alloys leads to an increase in the corrosion resistance, compressive strength and the ductility of the alloys, and a decrease in the elastic modulus of the Mg–Zr–Ca alloys.

Factors affecting the determination of electrochemical noise resistance

The similarity between electrochemical noise resistance (Rn) and polarization resistance (Rp) has been confirmed in many laboratories, and a new method for corrosion rate determination has been developed based upon this similarity. However, this method remains controversial. Some researchers have reported experimental results that did not support a similarity between Rp and Rn. Other researchers have found that this similarity does not hold true for every system. To address this controversy, factors affecting the measurement and calculation of Rn were investigated. Several factors - including instrument noise, direct current (DC) trend, noise sampling rates, and noise sampling duration - can affect the accuracy of Rn determination signiftcantly and, in some cases, can cause disagreement between Rn and Rp. It was recommended that a system check be carried out to avoid the influence of these factors.

Conversion coatings of Mg-alloy AZ91D using trihexyl(tetradecyl) phosphonium bis(trifluoromethanesulfonyl)amide ionic liquid

This work reveals the performance of a trihexyl(tetradecyl)phosphonium bis(trifluoromethanesulfonyl)amide ([P6,6,6,14][NTf2]) ionic liquid (IL) conversion coating upon AZ91D. Such conversion coatings represent a novel avenue for chromate replacement. An optimization of coating performance was pursued by careful alloy pretreatment to generate a surface on which the coating performs best, as the AZ91 substrate is distinctly different from pure or dilute Mg alloys. The results reveal that a functional conversion coating can be achieved, retarding anodic dissolution kinetics, causing a significant decrease in corrosion rate. The coating efficacy is closely tied to the pretreatment performed, which dictates both the microstructural and electrochemical heterogeneity of the surface. The resulting coatings were found to contain MgxFx and phosphonium cation related components, the proportions of which were dependent on the pretreatment.

The effect of treatment time on the ionic liquid surface film formation: promising surface coating for Mg alloy AZ31

Mg alloys are attractive materials for medical devices. The main limitation is that they are prone to corrosion. A low toxicity surface coating that enables uniform, controlled corrosion at a desired rate (this usually means it must offer barrier functions for a limited time period) is desirable. Phosphate-based ionic liquids (ILs) are known to induce a coating that can reduce the corrosion rate of Mg alloys, Furthermore, some ILs are known to be biocompatible and therefore, controlling the corrosion behaviour of an Mg alloy and its surface biocompatibility can be achieved through adding an appropriate low toxic IL surface layer to the substrate. In this study, we have evaluated the cytotoxicity of three phosphate-based ILs to primary human coronary artery endothelial cells. Among them, tributyl(methyl)-phosphonium diphenylphosphate (P1,4,4,4dpp) shows the lowest cytotoxicity. Therefore, further work was aimed at developing an appropriate treatment method to produce a homogeneous and passive surface coating based on P1,4,4,4dpp IL, with the focus on investigating the effect of treatment time. The results showed that that the formation of IL coating on AZ31 has proceeded progressively, and treatment time plays an important role. An IL treatment at 100 °C with an extended treatment time of 5 h significantly enhanced corrosion resistance of the AZ31 alloy in simulated body fluid. Additionally, the corrosion morphology was uniform and there was no evidence of "localized pitting corrosion" observed. Such a performance makes this ionic liquid coating as a potential surface coating biodegradable Mg-based implants.

Influence of cooling rate on the microstructure and corrosion behavior of Al-Fe alloys

The effect of Fe in Al is technologically important for commercial Al-alloys, and in recycled Al. This work explores the use of the novel rapid solidification technology, known as direct strip casting, to improve the recyclability of Al-alloys. We provide a comparison between the corrosion and microstructure of Al-Fe alloys prepared with wide-ranging cooling rates (0.1. °C/s to 500. °C/s). Rapid cooling was achieved via direct strip casting, while slow cooling was achieved using sand casting. Corrosion was studied via polarisation and immersion tests, followed by surface analysis using scanning electron microscopy and optical profilometry. It was shown that the corrosion resistance of Al-Fe alloys is improved with increased cooling rates, attributed to the reduced size and number of Fe-containing intermetallics.

Chromate replacement in coatings for corrosion protection of aerospace aluminium alloys

Mixed rare earth organophosphates have been investigated as potential corrosion inhibitors for AA2024-T3 with the aim of replacing chromate-based technologies. Cerium diphenyl phosphate (Ce(dpp) 3) and mischmetal diphenyl phosphate (Mm(dpp) 3) were added to epoxy coatings applied to AA2024-T3 panels and they were effective in reducing the amount and rate of filiform corrosion in high humidity conditions. Ce(dpp) 3 was the most effective and characterisation of the coating formulations showed approximately a factor of 5 reduction in both the number of corrosion filaments initiated as well as the length of these. Mm(dpp) 3 appeared to reduce the corrosion growth rate by a factor of 2 although it was the more effective inhibitor in solution studies. Spectroscopic characterisation of the coatings indicated that the cerium based inhibitor may disrupt network formation in the epoxy thus resulting in a coating that absorbed more water and allowed greater solubilisation of the corrosion inhibiting compound.

The corrosion inhibition of aluminium alloy 7075-T651 with cerium and Mischmetal di phenyl phosphate

Previous studies have shown that cerium diphenyl phosphate (Cedpp) 3 is a very effective inhibitor of corrosion of aluminium alloys in chloride solutions. This paper describes the results of further studies using electrochemical and constant immersion corrosion tests to compare the effectiveness of Ce(dpp) 3 and Mischmetal diphenyl phosphate Mm(dpp) 3 as inhibitors of corrosion pitting on AA7075-T651 aluminium alloy. The results shows that both Ce(dpp) 3 and Mm(dpp) 3 are excellent inhibitors of pitting corrosion of this alloy in very aggressive environments of continuously aerated 0.1M and 1.0M sodium chloride (NaCl) solutions. Polarisation tests indicate that these compounds act as a cathodic inhibitors by reducing the rate of the oxygen reduction reaction, which results in a decreased corrosion current density and a separation of the corrosion potential from the pitting potential. This inhibition is thought to be due to the formation of a surface film consisting of rare earth metal oxide, aluminium oxide and a cerium-aluminium organo-phosphate complex. Surface analysis data from scanning electron microscopy and X-ray Energy Dispersive Spectroscopy show the complex nature of this protective film. This work further develops our understanding about the mechanisms through which these complex films form, and how inhibition occurs in the presence of these compounds.

A novel approach for monitoring pipeline corrosion under disbonded coatings

Underground pipeline corrosion monitoring is a complex technical challenge. Currently there is no corrosion monitoring probe that is able to provide in situ information on corrosion under disbonded coatings. This paper presents a proof of concept of a novel corrosion monitoring probe intended to simulate corrosion under disbonded pipeline coatings and monitor its rate under Cathodic Protection (CP). The probe's capabilities to measure corrosion rates and simulate disbonded coating conditions are illustrated by a typical experiment that involved testing of the probe in 0.1M NaCl at -850mVCSE. Estimated metal thickness losses based on results measured by the probe were compared against corrosion patterns and profilometry measurements of control specimens exposed to the same conditions.

Corrosion behavior of Mg-5Al-xZn alloys in 3.5 wt.% NaCl solution

Five types of Mg-5Al alloys with different weight percentages of Zn ranging from 0 to 4 wt.% were examined using electrochemical techniques and surface analysis. The electrochemical results indicated that the Mg-5Al alloys containing Zn have a lower corrosion and hydrogen evolution rates than the Mg-5Al based specimens with a decrease of value being observed with the decrease in Zn content. Zn addition induced the precipitation of Mg-Al and Mg-Zn phases in the Mg matrix along with grain refinement and increased an interaction of Zn oxide with Mg and Al products serving as a corrosion barrier. © 2014 Elsevier B.V. All rights reserved.

Estimating hydrogen sulphide dissipation rate constant under the influence of different chemical dosing

Sewer odour and corrosion is caused by the reduction of sulphide ions and the release of hydrogen sulphide gas (H2S) into the sewer atmosphere. The reduction of sulphide is determined by its dissipation rate which depends on many processes such as emission, oxidation and precipitation that prevail in wastewater environments. Two factors that mainly affect the dissipation of sulphide are sewer hydraulics and wastewater characteristics; modification to the latter by dosing certain chemicals is known as one of the mitigation strategies to control the dissipation of sulphide. This study investigates the dissipation of sulphide in the presence of NaOH, Mg(OH)2, Ca(NO3)2 and FeCl3 and the dissipation rate is developed as a function of hydraulic parameters such as the slope of the sewer and the velocity gradient. Experiments were conducted in a 18m experimental sewer pipe with adjustable slope to which, firstly no chemical was added and secondly each of the above mentioned chemicals was supplemented in turn. A dissipation rate constant of 2×10^-6 for sulphide was obtained from experiments with no chemical addition. This value was then used to predict the sulphide concentration that was responsible for the emission of H2S gas in the presence of one of the above mentioned four chemicals. It was found that the performance of alkali substances (NaOH and Mg(OH)2) in suppressing the H2S gas emission was excellent while ferric chloride showed a moderate mitigating effect due to its slow reaction kinetics. Calcium nitrate was of little value since the wastewater used in this study experienced almost no biological growth. Thus the effectiveness of selected chemicals in suppressing H2S gas emission had the following order: NaOH ≥ Mg(OH)2 ≥ FeCl3 ≥ Ca(NO3)2.

Experimental and theoretical investigation on corrosion inhibition of AA5052 aluminium alloy by l-cysteine in alkaline solution

The corrosion inhibition of l-cysteine on AA5052 aluminium alloy in 4 mol/L NaOH solution was investigated by hydrogen gas evolution experiment, polarisation curve, galvanostatic discharge, electrochemical impedance spectroscopy measurements and quantum chemical calculations. The adsorption of l-cysteine on aluminium alloy surface obeyed the amended Langmuir's adsorption isotherm. The polarisation curves indicated that l-cysteine acted as a cathodic inhibitor to inhibit cathodic reaction. The inhibition mechanism was dominated by the geometric covering effect. The galvanostatic discharge shows that the additives restrain the hydrogen evolution and increase the anodic utilization rate. Quantum chemical calculations indicated that l-cysteine molecules mainly interacted with on the carboxyl groups on the aluminium alloy surface. A strong hybridization occurred between the s-orbital and p-orbital of reactive sites in the l-cysteine molecule and the sp-orbital of Aluminium.