Galvanic corrosion | EPFL Graph Search

Galvanic corrosion (also called bimetallic corrosion or dissimilar metal corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte. A similar galvanic reaction is exploited in primary cells to generate a useful electrical voltage to power portable devices. This phenomenon is named after Italian physician Luigi Galvani (1737-1798). Overview Dissimilar metals and alloys have different electrode potentials, and when two or more come into contact in an electrolyte, one metal (that is more reactive) acts as anode and the other (that is less reactive) as cathode. The electropotential difference between the reactions at the two electrodes is the driving force for an accelerated attack on the anode metal, which dissolves into the electrolyte. This leads to the metal at the anode corroding more quickly than it otherwise would and corrosion at the cathode being inhibited. The pre

Heat treatment of carbon anodes for the aluminum industry

Mauriz Lustenberger

Aluminum is produced by the electrolytic reduction of alumina dissolved in molten cryolite. This process requires carbon anodes which are consumed during the electrolysis. The anodes contribute roughly with 15% to the total aluminum production cost. If the anode quality is poor the share can as well reach 25 %. It is known that the anode behavior during aluminum electrolysis is significantly influenced by the anode baking process. However, there is a lack of knowledge to predict the anode behavior through given baking parameters. In addition, process control of industrial furnaces is still to a large degree empirical. Often the emission of industrial furnaces causes problems as deposits can accumulate in the exhaust system which leads to fire incidents. This investigation has three major goals: The prediction of the anode behavior during electrolysis for a given heat treatment. The understanding of the process control of industrial scale open-top anode baking furnaces. The emission control of the open-top anode baking furnace. The baking parameters are defined in this thesis as the anode heat-up rate and the baking level. Through pilot baking it is shown that the baking level is basically determined by the final baking temperature although the soaking duration has some influence too. The anode baking level can be estimated with a calculation for any given baking curve. Therefore the impact of the temperature and soaking duration on the real density anode property needs to be determined through a pilot study. The optimum baking parameters regarding the anode behavior during electrolysis depend on the material. Some anode materials react more sensitive to the heat treatment than others do and are therfore more difficult to handle in the industrial baking process. Through pilot scale anode baking it is shown how to predict the anode behavior during electrolysis. Process control of the anode baking in industrial open-top furnaces is complex since the anode baking is indirectly controlled by the combustion process inside the flues. Through measurements the combustion process is analyzed in this thesis. The results show that a furnace operated at its physical limits is difficult to control regarding to emissions. However, the gained knowledge allows to optimize existing furnace process control systems. The furnace emissions are mainly caused by insufficient combustion of the pitch fumes formed during the pyrolysis of the anode binder pitch. It is shown that emission problems cannot always be solved by a well chosen control system setup. In such a situation combustion control is required in addition to temperature control. A technology has been developed to measure online the combustion situation of each flue. For this purpose 2-color pyrometers, together with an intelligent signal processing, are required. This technology is integrated since 10 months in an industrial baking furnace control system. The results are promising. The developed technology is an important step for furnace operation at maximum productivity and minimum emission. A patent has been applied for the combination of online combustion analysis concurrent with temperature measurement (swiss apply No. 01598/02 by Mauriz Lustenberger and Felix Keller, R&D Carbon Ltd. Sierre).

EPFL2004

Silicon Corrosion in Neutral Media: The Influence of Confined Geometries and Crevice Corrosion in Simulated Physiological Solutions

Roland Hauert, Emilija Ilic, Stefano Mischler, Patrik Schmutz

Silicon (Si) based implantable components are widely used to restore functionalities in the human body. However, there have been reported instances of Si corroding after only a few years of implantation. A key parameter often overlooked when assessing Si stability in-vitro, is the added constricting geometries introduced through in-vivo implantation. The influence of crevices and confined solutions on the stability of Si is presented in this study, considering two simulated physiological solutions: 0.01 M phosphate buffered saline (PBS) and HyClone Wear Test Fluid (WTF). It was found that Si is highly vulnerable to corrosion in confined/crevice conditions. High pitting corrosion susceptibility is found in a crevice, whereas a dissolution rate of ca. 3.6 nm/h at body temperature occurred due to local alkalization within a confined cathodic area. The corrosion rates could be increased by elevating the temperature and yielded linear Arrhenius relations, with activation energies of 106 KJ/mol in 0.01MPBS and 109 KJ/mol in HyClone WTF, corresponding to a phosphorous-silicon interaction mechanism. Phosphorous species favored corrosion and contributed to enhanced Si dissolution, while chlorides were not so influential, and applied anodic potential induced pseudo-passivation. These results highlight the importance geometrical configurations can have on a material's surface stability. (c) The Author(s) 2019. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.

2019

Corrosion Mechanisms of Diamond-like Carbon Coated Interlayers & Interfaces

Emilija Ilic

Diamond-like carbon (DLC) coatings can improve the wear resistance of articulating implants. However despite successful in-vitro testing, some DLC coated joint replacements had to be revised prematurely, mainly due to coating delamination induced by corrosion at the substrate/DLC interface. Body fluid will enter defective coating sites and if the reactively formed interface or interlayer materials are instable in the confined fluid, they with dissolve, resulting in delamination of the DLC and ultimate failure of the implant. The aim of this thesis is therefore to investigate the corrosion mechanisms responsible for delamination at a substrate/DLC interface in body-like media. This is achieved by developing and implementing experimental methodologies addressing/simulating aggravating factors such as confinements/crevices, galvanic coupling, stress and fatigue on the interface to investigate corrosion initiation, propagation, and failure. Firstly, corrosion initiation and galvanic coupling risk are addressed with a methodology for accessing and characterizing the reactivity of buried interlayers. The interlayer is revealed by ion beam polishing, forming a wedge-like profile of the substrate/interlayer/DLC. The composition and chemical state of the interlayer/interface is determined by Auger electron spectroscopy. Reactive interlayers/interfaces, Si, titanium (Ti) and chromium (Cr), formed new phases (carbides and oxides). These new materials are then characterized with a local-electrochemical microcapillary technique. Cr and Si-DLC based interlayers presented good passive behavior, while a Si interlayer corroded in bovine-based wear test fluid (HyClone® WTF). The influence of coating defects on the current response of DLC surfaces was also investigated by varying the exposed working area (cm2 to µm2 range). To address the failure case of a TiAlV/Si/DLC implant, the influence of confinements/crevices on corrosion propagation is investigated. It is shown that Si is highly vulnerable to corrosion in confined conditions. Pitting corrosion susceptibility is found in a crevice, whereas a Si dissolution rate of ca. 3.6 nm/h at 37 °C occurs within a confined area. The corrosion rates increased at elevated temperatures and yielded linear Arrhenius relations, with activation energies of 106 KJ/mol in phosphate buffered saline (PBS) and 109 KJ/mol in HyClone® WTF. Phosphorous species enhanced Si dissolution, while chlorides were not so influential. Galvanic coupling with Ti and applied stress further accelerated the Si dissolution. Finally, corrosion related fracture of a CoCrMo/DLC interface is investigated by applying static and cyclic load. A reciprocating sliding rig is utilized to determine the number of sliding cycles required for interface delamination to occur at a critical load, in PBS at 37 °C. The results followed a Wöhler curve relation. Small O2 contaminations (0.5 and 1.0%) at the interface adversely affected the CF behaviour by lowering the critical load and endurance limit. The fitted Wöhler curves could be extrapolated and were found to be in good agreement with long-term experimental results previously obtained on an implant in a spinal disk simulator.

EPFL2019