Molecular Level Studies of the Metal/Atmosphere Interface

Detta är en avhandling från Stockholm : Fysik

Författare: Jonas Weissenrieder; Kth.; [2003]

Nyckelord: Iron; molecular level; in-situ;

Sammanfattning: The chemistry andphysics involved at the metal/atmosphereinterface is interesting both from a fundamental and an appliedperspective. Since iron is the most important of all metalsthis interface is of particular interest. The objective withthis thesis is to obtain new information on a molecular levelof the iron/atmosphere interface with a special emphasis on theinitial atmospheric corrosion.The work presented herein combines a large variety ofdifferent analytical surface science techniques. Both ultrahigh vacuum and ambient pressure investigations were conductedwith single crystals as well as polycrystalline samples.The interaction of segregated sulfur with a Fe(110) surfacewas investigated by means of atomically resolved scanningtunneling microscopy (STM). A large variety of high and lowcoverage reconstructions were reported. Comparable studies ofoxygen adsorption on the same surface were also completed.Similar to the sulfur experiments, oxygen induced a number oflow coverage reconstructions. At higher coverage, oxideformation was observed and ordered oxides could be fabricatedat elevated temperatures.The oxygen interaction with Fe(110) and Fe(100) surfaces wasalso investigated with synchrotron radiation basedphotoelectron spectroscopy. Detailed information of the initialadsorption and subsequent oxidation was obtained. The Fe 2pcore level of the clean Fe(110) surface was subject to furtherinvestigations because of its complicated line profile that wasinterpreted as an exchange split of the final state.Iron exposed to humidified air with low concentrations ofsulfur dioxide (SO2) shows a surprisingly passive behavior. Themeasured mass gain was significantly lower than that of acopper sample exposed in the same environment. In-situtechniques such as atomic force microscopy (AFM), quartzcrystal microbalance (QCM) and infrared reflection absorptionspectroscopy (IRAS) showed little or no corrosion. Initiationof corrosion was observed upon introduction of additionaloxidants. The conclusion drawn challenge the established modelfor formation and growth of sulfate nests. The condition andformation of sulfate nests are discussed in view of thegenerated in-situ observations.During further experiments, iron was exposed to humid airand sodium chloride aerosols. The surface was investigated within-situ techniques, which provided new useful information. Ahigh corrosion rate was observed and the corrosion attacks formfilaments characteristic of filiform corrosion. A schematicmodel for propagation of the corrosion filaments wasproposed.Filiform corrosion was observed on aluminum surfaces aswell. The corroded surfaces were investigated with synchrotronradiation based photoelectron microscopy and scanning over afiliform head revealed different oxidation states within the Al2p spectrum. The microscopy data was interpreted as anenrichment of aluminum chloride containing compounds within thefiliform corrosion head.