Humidity electrochemical mechanisms

Metals exposed to humid atmosphere corrode by an electrochemical mechanism due to the formation of a thin electrolyte layer on the metal surface (Chapter 3.1, this volume). This type of corrosion can be controlled by Vapor-phase Corrosion Inhibitors (VCIs), that is, volatile inhibiting substances that allow vapor-phase transport to the corroding surface (examples are amines, benzoates, imidazoles, or triazoles [3]). The vapor pressure should be sufficiently high to ensure a protective surface concentration of the inhibitor, but low enough to prevent premature depletion of... 

Atmospheric corrosion is an electrochemical process and its rate is governed the anodic and cathodic partial reactions taking place at the metal-electrolyte and oxide-electrolyte interfaces. The electrochemical mechanism of atmospheric corrosion resembles that of corrosion in aqueous solution, with two important differences firstly, the corrosion products stay on the surface, rather than being swept away by the electrolyte and, secondly, the electrolyte periodically evaporates during dry periods, then reforms during wet periods, when the metal is exposed to high humidity. 

The analysis by XPS can confirm that the ZnO was always present initially on the surface of zinc although the probe had not been exposed in the ehamber. A very thin film of this oxide forms instantaneously by chemical oxidation on the zine surfaee in contact with clean air at room temperature. This film does not affeet the later eleetroehemical corrosion process [5]. Once the humidity layer is established, zinc hydroxide is rapidly formed on the ZnO film via an electrochemical mechanism. Generally, it is considered that hydroxides are the initial compounds in zinc atmospheric corrosion studies [5, 15]. 

Others observed a pronounced dependence of the field effect on the humidity level. In dry atmosphere almost no change of on the gate voltage was found. i 2 An electrochemical mechanism was proposed to explain the electric field dependence including the dedoping of PEDOT in the presence of water. 

Rusting of iron consists of the formation of hydrated oxide, Fe(OH)3 or FeO(OH), and is evidently an electrochemical process which requires the presence of water, oxygen and an electrolyte — in the absence of any one of these rusting does not occur to any significant extent. In air, a relative humidity of over 50% provides the necessary amount of water. The mechanism is complex and will depend in detail on the prevailing conditions, but may be summarized as ... 

The observed I/U characteristics turn towards the behavior of antiparallel Schottky-type double diodes. A complex behavior of the I/U curves upon humidity, may indicate that two mechanisms might play the key role the mobility of H and OH ions within the fullerite lattice at high voltages and electrochemical corrosion of the fullerite in humid environment, which makes the MOSBIT structure like a galvanic battery [4,5]. 

Shimizu Y etal 1989 The sensing mechanism in a semiconducting humidity sensor with platinum electrodes J. Electrochem. Soc. 136 3868-71 Lukaszewicz J P 1991 Diode-type humidity sensor using perovskite-type oxides operable at room temperature Sensors Actuators B 4 227-32 Anderson RC etal 1990 Investigations of porous silicon for vapor sensing Sensors Actuators A 23 835-9... 

S. Kusanagi, Mechanism of the sensitivity of the planar CO sensor and its dependency on humidity, J. Electrochem. Soc., 1992, 139, 3224-3229 S.B. Lee, A. Cocco, D. Keyvani and G.J. Maclay, Humidity dependence of carbon monoxide rate in a Nation-based electrochemical cell, J. Electrochem. Soc., 1995, 142, 157-160 R.J. Mortimer and A. Beech, AC impedance characteristics of solid-state planar electrochemical carbon monoxide sensors with Nation as solid polymer electrolyte, Electrochim. Acta, 2002, 47, 3383-3387. 

The influence of temperature and humidity affect the corrosion rate through their influence on the electrochemical reactions at the steel/concrete interface and through their influence on ion transport between anodes and cathodes. Although the mechanisms are not fully understood, it appears that the concrete resistivity (or conductivity) is strongly related to the corrosion rate at moderate or low temperature [35-38]. Variation in resistivity due to variation of humidity (at constant temperature) caused an inversely proportional variation of corrosion rate in carbonated mortar and concrete with low amounts of chloride or without chloride. Variation of temperature (at constant humidity) caused a similarly varying corrosion rate. 

There are various improvements that can be made to the presented model, some improvements could be accomphshed. Foremost among these possible future-work directions is the inclusion of nonisothermal effects. Such effects as ohmic heating could be very important, especially with resistive membranes or under low-humidity conditions. Also, as mentioned, a consensus needs to be reached as to how to model in detail Schroder s paradox and the mode transition region experiments are currently underway to examine this effect. Further detail is also required for understanding the membrane in relation to its properties and role in the catalyst layers. This includes water transport into and out of the membrane, as well as water production and electrochemical reaction. The membrane model can also be adapted to multiple dimensions for use in full 2-D and 3-D models. Finally, the membrane model can be altered to allow for the study of membrane degradation, such as pinhole formation and related failure mechanisms due to membrane mechanical effects, as well as chemical attack due to peroxide formation and gas crossover. 

Atmosphere corrosion is the general term for all of corrosion phenomena occurring in air. The vaporized water (humid component) forms very thin water films on materials surfaces and the electrochemical reactions leading to corrosion proceed in the thin water film. There are many environmental factors existing in thin water films and they affect the corrosion mechanism. 

Xie D, Jiang YD, Pan W, Li D, Wu ZM, Li YR (2002) Fabrication and characterization of polyanUine-based gas sensor by ultra-thin film technology. Sens Actuators B 81 158-164 Yasuda A, Doi K, Yamaga N, Fujioka T, Kusanagi S (1992) Mechanism of the sensitivity of the planar CO sensor and its dependency on humidity. J Electrochem Soc 139 3224-3229 Zawodzinski TA, Springer TE, Uribe F, Gottesfeld S (1993) Characterization of polymer electrolytes for fuel cell applications. Solid State Ionics 60 199-211... 

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