Corrosion inhibition of aluminium alloys by layered double hydroxides: the role of copper

Layered double hydroxides represented by the general formula [M2 2+M3+(OH)6]+X1/n n-.zH2O are being researched as anion-exchange materials with interesting intercalation chemistry that accommodate a wide range of applications, including corrosion resistance. In this work, it is shown that the formation of layered double hydroxides (LDHs) on the surface of copper-rich Al alloys promotes corrosion resistance. For that purpose a LDH of the type [M+M3+ 2(OH)6[An- 1/n].zH2O], where the intercalated cation is mono-valent Lithium is studied. In Aluminium 2024-T3 or Al-Li 8090, corrosion inhibition was achieved as a result of the formation of a LDH film: Al2Li(OH)7.2H2O or Al2Li(OH)62CO3.zH2O according to the precursor solution used. LDHś covered the entire surface of the mentioned alloys, mitigating the galvanic action between the matrix and Cu rich phases, usually responsible for corrosion of the localized type. Inhibition is demonstrated to be under diffusion control. Layered double hydroxides were characterised using Xray diffraction, FTIR and SEM. The role of copper is examined using an approach that includes a study on pure copper sample

Nanoscale layered double hydroxide materials for corrosion resistance

Layered Double Hydroxides (LDHμs), represented by the general formula [MII (1-x)MIIIx(OH)2[An-x/n].zH2O or [MIMIII2(OH)6[An-1/n].zH2O], where MI, MII, MIII are mono-, di- and tri-valent metal cations, are being researched as anion-exchange materials which interesting intercalation chemistry that accommodate a wide range of applications from heterogeneous catalysis to storage and subsequent controlled release of bioactive agents. In this work, layered double hydroxides containing a monovalent (Li+) and trivalent (Al3+) matrix cations, have been synthesized and characterised using X-ray diffraction, FTIR and SEM. LDHμs were prepared by a simple co-precipitation method using metal hydroxides and metal salts in an alkaline solution. Hybrid systems are produced by intercalation which involves a guest molecule introduced into the host structure replacing the existing interlayer ion, without affecting the host structure opening new applications according to desired functionalities namely as thin films in corrosion protection. Li based conversion coatings are easily formed under open circuit conditions on Al surfaces [1-3]. Formation of LDHμs on the metal surface of copper-rich Al alloys were attempted with excellent results. Pitting corrosion was inhibited on Aluminium 2024-T3 with an extensive capability to withstand the presence of high concentrations of chloride ions. Intergranular corrosion was found to be inhibited in Al-Li 8090 alloy by action on copper containing T-phases located at the grain and sub-grain boundaries. The formation of DHLμs is thought to be responsible for inhibition which is demonstrated to be under diffusion control. The action of DLHμs on copper is demonstrated in separated experiments using pure copper samples in similar experimental conditions as for the alloy, in an extensive electrochemical study

Layered double hydroxides for aluminium alloys corrosion resistance

Layered Double Hydroxides (LDHμs), represented by the general formula [MII (1-x)MIIIx(OH)2[An-x/n].zH2O or [MIMIII2(OH)6[An-1/n].zH2O], where MI, MII, MIII are mono-, di- and tri-valent metal cations, are being researched as anion-exchange materials with interesting intercalation chemistry that accommodate a wide range of applications including corrosion resistance. In this work, layered double hydroxides containing a monovalent (Li+) and trivalent (Al3+) matrix cations, have been synthesized and characterised using X-ray diffraction, FTIR and SEM. LDHμs were prepared by a simple co-precipitation method using metal hydroxides and metal salts in an alkaline solution. Formation of LDHμs on the metal surface of Al alloys were attempted with excellent results. Pitting corrosion was inhibited on Aluminium 2024-T3 with an extensive capability to withstand the presence of high concentrations of chloride ions. The formation of DHLμs is thought to be responsible for inhibition which is demonstrated to be under diffusion control. The action of DLHμs on copper is demonstrated in separated experiments using pure copper samples in similar experimental conditions as for the alloy, in an extensiv

Polarity reversal in PEM Fuel Cells by fuel starvation

In this work, the degradation caused by polarity reversal by fuel starvation of a 16 MEA – low power fuel cell is reported. Measuring of the potential of individual cells, while on load, was found instrumental in the location of affected cells which revealed very low or even negative potential. Electrochemical impedance spectroscopy insitu gave insight on the increase in resistance and diffusion processes. Ex-situ analysis of MEA after irreversible degradation by fuel starvation gave as a result delamination of catalyst layers with impacts on fuel cell performance such as development of flooded areas by the pockets created increasing the resistance of reactant transport to the catalyst sites. Striking and thickness variation of the anode layer as well as carbon corrosion were found. The proton exchange membrane is also affected by fluoride depletion

Polarity reversal by fuel starvation in PEM Fuel Cells

In this work, the degradation caused by polarity reversal by fuel starvation of a 16 MEA (membrane-electrode assembly) – low power PEM fuel cell is reported. Measuring of the potential of individual cells, while on load, was found instrumental in the location of affected cells which revealed very low or even negative potential. Ex-situ analysis of MEA, after irreversible degradation by fuel starvation, gave as a result delamination of catalyst layers with impacts on fuel cell performance such as development of flooded areas (in the created gaps by membrane separation) increasing the resistance of reactant transport to the catalyst sites. Striking thickness variations of the anode layers as well as carbon corrosion were found. Also, the proton exchange membrane was found to be affected by fluoride depletion

Assessing cell polarity reversal degradation phenomena in PEM Fuel Cells by electrochemical impedance spectroscopy

The mechanisms of fuel cell degradation are multiple and not well understood. Irreversible changes in the kinetic and/or transport properties of the cell are fostered by thermal, chemical and mechanical issues which constrain stability, power and fuel cell lifetime. Within the in-situ diagnostics methods and tools available, in-situ electrochemical impedance spectroscopy (EIS) is within the most promising to better understand and categorize changes during fuel cell ageing. In this work, the degradation phenomena caused by cell polarity reversal due to fuel starvation of an open cathode 16 MEA (membrane-electrode assembly) –low power PEM fuel cell (15 W nominal power) is reported using EIS as a base technique. A frequency response analyzer from Solartron Model 1250 was used connected to an electrochemical interface also from Solartron, Model 1286. The range of covered frequencies spans from 37000 Hz to 0.01Hz. Hydrogen is supplied from a metallic hydride small reactor with a capacity of 50 NL H2 at a pressure of 0.2 bar. Measuring the potential of individual cells, while the fuel cell is on load, was found instrumental in assessing the “state of health” of cells at fixed current. Location of affected cells, those farthest away from hydrogen entry in the stack, was revealed by the very low or even negative potential values. EIS spectra were taken at selected break-in periods during fuel cell functioning. The analysis of impedance data is made using two different approaches: using an a priori equivalent circuit describing the transfer function of the system in question -equivalent circuit elements were evaluated by a complex non-linear least square (CNLS) fitting algorithm, and by calculating and analyzing the corresponding distribution of relaxation times (DRT) -avoiding the ambiguity of the a priori equivalent circuit and the need for provision of the initial fitting parameters. A resistance and two RQ elements connected in series are identified as describing the impedance response of the cell during normal functioning. A constant phase element (CPE) was chosen to describe the impedance observed behavior. The quality of the fit was evaluated by analysis of the residuals between the fit result and the measured data at every single point. Consistency and quality of the impedance data were established by Kramers-Kronning validation. With continuous operation, using a reduced hydrogen flow, an inversion of polarity was observed in the 16th cell of the stack, evident in the potential measurement of individual cells as a result of insufficient hydrogen to reach the last cells. EIS data analyses suggest that water electrolysis happens at the anode judging by the appearance of an intermediate semicircle associated to a marked change in resistance and capacitance values. The presence of an inductive loop at low frequencies is now evident, which cannot be explained by the relaxation of reaction intermediates involved in the oxygen reduction reaction [1]. It is to be noticed that when the incursion into the negative potential values is not too marked the phenomenon is partially reversible, so it is suggested that the relaxation is due to intermediates in the water electrolysis process. The anode potential rose to levels compatible with the oxidation of water. Once the phenomenon is made irreversible and when water is no longer available, oxidation of the carbon support is favored accelerating catalyst sintering. Ex-situ MEA cross section analysis, under a scanning electron microscope, confirmed it. Electrode thickness reduction and delamination of catalyst layers were observed as a result of reactions taking place during hydrogen starvation. Carbon corrosion and membrane degradation are analyzed, according to evidence by SEM

Penetration of hydrogen technologies: study on the environmental impact of road transport in Portugal

Road traffic is one of the transportation sectors with faster growth and also one of the most important emitters of greenhouse gases (GHGs). In this work, an analysis of the environmental benefits resulting from the introduction of hydrogen on road transport in Portugal is made. Impact is analyzed mainly looking at the pollutant emissions provided by road transport at the point of use. Emissions associated to road transport have been estimated using the software COPERT (version 4), since it provides a detailed methodology for each specific pollutant related to the vehicle fleet of a region or country, as well as the driving conditions and fuel consumption. Passenger cars, light duty vehicles and public transport buses are the vehicles categories in which the hydrogen technology is foreseen. The hydrogen penetration rates (moderate and high) are extracted from the European Project HYWAYS. Two trends are then considered, which give penetration rates of 40.0 % and 74.5 % in 2050 for the moderate and high scenarios respectively

Fuel Starvation: irreversible degradation mechanisms in PEM Fuel Cells

PEM fuel cell operates under very aggressive conditions in both anode and cathode. Failure modes and mechanism in PEM fuel cells include those related to thermal, chemical or mechanical issues that may constrain stability, power and lifetime. In this work, the case of fuel starvation is examined. The anode potential may rise to levels compatible with the oxidization of water. If water is not available, oxidation of the carbon support will accelerate catalyst sintering. Diagnostics methods used for in-situ and ex-situ analysis of PEM fuel cells are selected in order to better categorize irreversible changes of the cell. Electrochemical Impedance Spectroscopy (EIS) is found instrumental in the identification of fuel cell flooding conditions and membrane dehydration associated to mass transport limitations / reactant starvation and protonic conductivity decrease, respectively. Furthermore, it indicates that water electrolysis might happen at the anode. Cross sections of the membrane catalyst and gas diffusion layers examined by scanning electron microscopy indicate electrode thickness reduction as a result of reactions taking place during hydrogen starvation. Catalyst particles are found to migrate outwards and located on carbon backings. Membrane degradation in fuel cell environment is analyzed in terms of the mechanism for fluoride release which is considered an early predictor of membrane degradation