Mechanisms of metal dusting corrosion

In this thesis the early stages of metal dusting corrosion is addressed; the development of carbon expanded austenite, C, and the decomposition hereof into carbides. Later stages of metal dusting corrosion are explored by a systematic study of stainless steel foils exposed to metal dusting conditions.

The lattice parameter of expanded austenite as a function of interstitially dissolved carbon is determined by carburization of thin stainless steel foils. For the first time this dependency is determined on unstrained austenite at room temperature. It is found that a linear relation holds between the lattice parameter, a, and the occupancy of the octahedral interstices, yC.

Furthermore, the maximum solubility of carbon in expanded austenite is determined at two temperatures. From controlled carburization of stainless steel foils it is found that the precipitation of chromium carbides is unavoidable for carbon contents above approximately 3.5wt% and that the maximum carbon content dissolvable in the f.c.c. lattice is 4.4wt% at 693K.

Moreover, from controlled carburization of stainless steel flakes it is found that, the precipitation of chromium carbides is suppressed for carburizing temperatures at 613K and that the maximal dissolvable carbon content in the f.c.c. lattice is 5.3wt% at 613K. The hardness associated with the dissolved carbon, as measured on the metallographic cross sections, is shown to increase linearly with carbon content.

Finally, for a low carburizing temperature it is shown that ferrite in heavily deformed stainless steel flakes is transformed to expanded martensite/austenite during low-temperature carburization. Various experimental procedures to experimentally determine the concentration dependent diffusion coefficient of carbon in expanded austenite are evaluated.

The most promising procedure for an accurate determination is shown to be stepwise gaseous carburization of thin foils in a gaseous atmosphere; the finer the stepsize, the more accurate the approximation of the diusivity. Thermogravimetry was applied to continuously monitor the weight change of thin foils of AISI 316 during carburization in a CO-H2 gas mixture.

It was found that the diffusion coefficient of carbon in expanded austenite increases with carbon concentration. The decomposition of carbon-saturated expanded austenite was studied by systematic differential thermal analysis on compositionally graded AISI 316L stainless steel powders and flakes. The nature of the decomposition products, carbides of the form M23C6 and M7C3, were evaluated by X-ray diffraction, light optical microscopy, scanning electron microscopy and thermodynamic modelling.

The decomposition was found to be dependent on several parameters such as thermal history, carbon content, sample geometry and the presence of precursors and possibly the influence of compressive stresses. A systematic study of AISI 316 stainless steel foils exposed to a mixture of C2H2 and H2 at 823K (550C) is treated.

The applied metal dusting conditions results, fistly, in dissolution of carbon atoms in the surface region by carburization and the formation of carbon expanded austenite, C. Secondly, carbides of the form M23C6 precipitates in the outermost surface region and along grain boundaries. Thirdly, a transformation of the M23C6 carbide layer and simultaneously the supersaturated alloy, into a diverse carbide network.

Finally, the foils turn into metal dust accompanied by a thinning and disappearance of the foils. Investigations of TEM samples, prepared by means of FIB, on the carbide network revealed a lamellar structure with carbides and austenite. Finally, the mutual influence of oxygen and carbon on the metal dusting corrosion is explored.

The results indicate that exposure to metal dusting conditions have a detrimental effect on the resistance against oxidation and, conversely, that exposure to oxidation has a detrimental effect on the resistance towards metal dusting. Consequently, a combination of carburizing and oxidizing conditions has a strong mutual catalyzing effect on the metal dusting corrosion.