The corrosion research of the department includes aqueous, atmospheric and bio-corrosion processes and their inhibition, covering both fundamental and applied aspects.

In aqueous environments the SFC system with downstream analytics allows electrochemical high-throughput screening and characterization of corrosion properties of material samples. The setup allows the time-efficient investigation of samples by electrochemical methods (OCP measurements, potentiodynamic sweeps, galvanostatic polarization, electrochemical impedance spectroscopy, etc.) on a very small area of about 0.2 mm2. The UV-VIS detector downstream of the electrode analyses in parallel the dissolved amount of an element, and from these dissolution profiles the corrosion current density of the material can be determined directly and compared with electrochemical data. All measurements are fully automated and controlled by in-house programmed LabView routines and the SFC was extended as routine tool for the evaluation of corrosion properties in industry laboratories.

Fig.1: AFM topography images of rough surfaces after corrosion test and corresponding clean surfaces (left corner, with blue edges) of (A) amorphous sample (B) 620°C sample (C) 800°C sample.

Amorphous and nanocrystalline Fe-based materials have been targeted as unique model alloys for fundamental corrosion research. The stainless-type alloy system Fe65-xCrxMo14C15B6 can be reliably produced in the ultimately homogeneous amorphous state as well as, by applying specific heat treatments, in partially (to a varying extent) or fully crystallized versions. The very short length scale of the formed inhomogeneities renders these materials ideally suited for high-resolution techniques such as Atom Probe Tomography and scanning Auger Electron Microscopy, which provide an unprecedented detail of the elemental distribution. The associated well-characterized surfaces serve as the starting point for further unique electrochemical corrosion experiments. In this respect the Scanning Flow Cell ICP-MS setup could provide simultaneous current-voltage measurements and time-resolved individual elemental dissolution behaviour. By combining a unique set of experimental techniques an entire corrosion process could be thus followed from the atomic bulk structure right into the electrolyte.

Stability of metal alloy surfaces and in particular dealloying reactions is another important corrosion-related topic studied in the department. Noble metal binary alloys constitute here the main set of addressed samples. Originally such alloys were studied to understand dealloying-related stress-corrosion cracking in brass materials - nowadays the nanoporous noble metal structures that can be obtained by dealloying have direct technical applications from sensors and actuators to catalysis and batteries. Utilizing a well-studied sequence of surface structural transitions on Cu3Au(111) we could gain a deeper insight in the working mechanisms of the dealloying process [1]. Further control of the surface processes and resulting morphology of the dealloyed layers could be obtained using corrosion accelerators such as halide additives and inhibitors such as plasma-polymer films or applied self-assembled thiol or selenol layers. The obtained knowledge may be very helpful in creating microstructured porosity especially for sensor applications or microreactors. In particular the thiol-modified surfaces opened up completely new views on initial cracking events and thus close back the circle to attempting to understand dealloying and stress-corrosion cracking (Fig.2).

Fig.2: Scanning electron micrographs of a dealloyed Cu3Au polycrystalline sample. (a) Plane view showing grains with different crack densities. (b) Cross-section showing one specific crack that extends into the nanoporous material.

Under practical application of commercial materials in most cases the underlying corrosion mechanisms are very complex, especially during long-term exposure conditions. That is the reason why there is up to now no accelerated test available for really reliable prediction of the corrosion performance under real practical conditions, nor a theoretical simulation tool. One key factor that will play a crucial role in determining the long term performance is the heterogeneous nature of most real life corrosion cases. In the framework of a bilateral project with TKSE and a broader set RFCS project, for instance, strong indications were found that the formation of local cathodes and anodes on the micron and submicron scale and their effect on local pH and concentration of dissolved metal cations play a decisive role for determining the protective properties of the growing corrosion product layer on the alloy, that leads to a certain degree of “passivation” of these otherwise quite reactive materials. For a real fundamental understanding of the underlying mechanisms, however, in situ high-resolution methods are necessary. That is why in addition to the work carried out on SECPM, an STM based potentiometric technique, as described in the last report, further methods were developed, such as a combined SECM and SKP system, that allow to perform in situ electrochemical investigations and ex-situ SKP exactly on the same area. This was at first successfully applied on dedicated aluminium model alloy samples (see [2-4]). Also first steps for the simulation of the surface pH have been carried out [5,6]. Another important project was to investigate the beneficial role of Mg cations on inhibiting oxygen reduction on cathodically polarized iron, in order to understand the reported excellent performance of zinc-magnesium alloy coatings at the cut edge or scratches down to the steel. It could be shown that the significant protective effect found at early stages is not correlated to an improved precipitation layer of corrosion products, but rather to a direct modification of the iron oxide [7].

Fig.3: Growth of native oxide film on zinc as function of applied potential observed by in-situ spectroscopic ellipsometry.

In the understanding of the initial stages of corrosion processes under aqueous environments, the electronic structure of the forming corrosion products is crucial for charge and material transport and the consequent development of the corrosion reaction. A novel analysis scheme for in situ spectroscopic ellipsometry has been developed to enable a model-independent extraction of layer thicknesses and absorption spectrum of the forming, nm-thick layers. Results for zinc in carbonate show the presence of two different time scales in the layer growth, the formation of ZnO is decoupled from the formation of an oxidation layer [8]. Further application to copper in different media shows in situ the formation of oxides in different oxidation stages, depending on the conditions.

Moreover, the electrochemical properties of the oxide layers play a key role in the corrosion behaviour of metals and metal alloys. Within the reporting period it was investigated how different surface treatments of aluminium change the properties, especially the position of the Fermi level [9,10]. For the corrosion performance of zinc alloy coatings at the cut edge, the  properties of the iron oxide on the exposed iron or steel was identified to determine the corrosion rate of the zinc alloy. Strong indications were found that zinc cations exchange with reactive Fe2+ sites in the oxide, thus significantly inhibiting oxygen reduction. This inhibition is synergistically enhanced by the presence of magnesium cations (see [7]).

Fig. 4: Sulfate-reducing bacteria (Desulfopila corrodens strain IS4) in their extracellular polymeric substance on an iron substrate causing anaerobic microbially influenced corrosion.

Biocorrosion of iron that arises in anoxic environments is predominantly ascribed to anaerobic microbially influenced corrosion (MIC), with marine sulfate-reducing bacteria (SRB) being the major contributors [11]. Anaerobic MIC causes serious damages in the oil and gas industry, thus assessing and monitoring of corrosion problems and also elucidating the yet unresolved corrosion mechanism is of great importance. In close cooperation with the Max Planck Institute for Marine Microbiology in Bremen we work on quantifying corrosion rates in-situ and gaining a more detailed insight into the fundamental electron transfer mechanism at the electrode/bacteria interface. Therefore SRB strains with high corrosion activities in comparison to other well-investigated strains are studied in a multidisciplinary approach utilizing electrochemical techniques, surface analytics and molecular biological methods.

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