Projects

Conversion coatings

Conversion coatings are an important ingredient in modern corrosion protection systems. Together with industrial partners, we investigate traditional phosphating on "difficult" substrates, and modern alternative conversion coatings. [more]
The scanning flow cell enables a high-throughput screening approach that is able to deal with the issue of the complex interrelation between many parameters, for instance the variable ion composition of salts. The fully automated system provides dynamic electrolyte change even during the measurement and thereby allows a comprehensive investigation of the individual effect of different electrolyte constituents with temporally varying concentrations.

Zinc Corrosion

The scanning flow cell enables a high-throughput screening approach that is able to deal with the issue of the complex interrelation between many parameters, for instance the variable ion composition of salts. The fully automated system provides dynamic electrolyte change even during the measurement and thereby allows a comprehensive investigation of the individual effect of different electrolyte constituents with temporally varying concentrations. [more]

Early Stages of High Temperature Corrosion in Steel Processing and Manufacturing

The in situ measurement of mass gain, especially during fast temperature cycles, which is a problem of significant practical importance, remained a problem that was experimentally unsolved. The reason for this is that due to fast changes of temperature in close vicinity to the sample surface, thermal drifts begin to dominate the thermobalance signal, making a clear scientific interpretation of the mass changes impossible. [more]

Microstructure Effects on Hydrogen Embrittlement in Austenitic Steels: A Multidisciplinary Investigation

Hydrogen atoms, which can be absorbed into steel during production and service, often have a detrimental embrittling effect on the mechanical properties of iron and steels. It is meanwhile known that hydrogen embrittlement (HE) is also affected by the microstructure of the material. Consequently, previous indications that hydrogen atoms are trapped by vacancies, dislocations, and grain boundaries led at MPIE to investigations of superabundant vacancy formation, hydrogen-enhanced local plasticity (HELP), and hydrogen-enhanced decohesion (HEDE). [more]

References

1.
Srinivasan Swaminathan and Michael Rohwerder, "Role of Forming Gas Annealing Characteristics on the Selective Surface Oxidation of Fe–Mn–Si–Cr Model Alloy", in Defect and Diffusion Forum, (Trans Tech, Aedermannsdorf, Switzerland, 2011), Vol. 312-315, pp. 918-923.
2.
Srinivasan Swaminathan, Michael Rohwerder, Michael Rohwerder, and Michael Spiegel, "Temperature and dew point dependent segregation of phosphorus and sulfur in Fe–Mn–P–S model alloy," Surface and Coatings Technology 205 (16), 4089-4093 (2011).
3.
Sagar Prabhudev, Srinivasan Swaminathan, and Michael Rohwerder, "Effect of oxides on the reaction kinetics during hot-dip galvanizing of high strength steels," Corrosion Science 53 (7), 2413-2418 (2011).
4.
Huachu Liu, Yanlin He, Srinivasan Swaminathan, Michael Rohwerder, and Lin Li, "Effect of dew point on the surface selective oxidation and subsurface microstructure of TRIP-aided steel," Surface and Coatings Technology 206 (6), 1237-1243 (2011).
5.
Huachu Liu, Fang Li, Wen Shi, Srinivasan Swaminathan, Yanlin He, Michael Rohwerder, and Lin Li, "Challenges in hot-dip galvanizing of high strength dual phase steel: Surface selective oxidation and mechanical property degradation," Surface and Coatings Technology 206 (16), 3428-3436 (2012).
6.
Stefan Evers and Michael Rohwerder, "The hydrogen electrode in the “dry”: A Kelvin probe approach to measuring hydrogen in metals," Electrochemistry Communications 24, 85-88 (2012).
7.
Ceylan Senöz, Stefan Evers, Martin Stratmann, and Michael Rohwerder, "Scanning Kelvin Probe as a highly sensitive tool for detecting hydrogen permeation with high local resolution," Electrochemistry Communucations 13 (12), 1542-1545 (2011).
8.
Michael Auinger, R. Naraparaju, Hans Jürgen Christ, and Michael Rohwerder, "Modelling high temperature oxidation in iron-chromium systems: Combined kinetic and thermodynamic calculation of the long-term behaviour and experimental verification," Oxidation of Metals 76 (3-4), 247-258 (2011).
9.
Michael Auinger, Sergiy Borodin, Srinivasan Swaminathan, and Michael Rohwerder, "Thermodynamic Stability and Reaction Sequence for High Temperature Oxidation Processes in Steels," Materials Science Forum 696, 76-81 (2011).
10.
Michael Auinger and Michael Rohwerder, "Coupling Diffusion and Thermodynamics - Exemplified for the gas nitriding of ironchromium alloys," HTM - Journal of Heat Treatment and Materials 66 (2), 100-102 (2011).
11.
Srinivasan Swaminathan and Michael Rohwerder, "Segregation and selective surface oxidation at the intermediate steps of recrystallization annealing", in Defect and Diffusion Forum, (Trans Tech, Aedermannsdorf, Switzerland, 2011), Vol. 309-310, pp. 203-208.

Industrial Processes

Industrial Processes

Steel surfaces are of particular interest for the department of Interface Chemistry and Surface Engineering. In close collaboration with the steel industry and in many cases funded by European funds, steel related projects cover the full range of surface treatments, which are important in modern steel surface chemistry.

In outdoor applications conversion coatings constitute a fundamental cornerstone for corrosion protection. Traditionally, phosphating is employed. Novel steel surfaces containing oxides, which cannot be attacked by pickling, are difficult to phosphate. On the other hand, passivating oxides may be relevant candidates for providing corrosion protection through grain boundaries in the phosphate coating, which would supersede the use of nickel in the baths. Using electrochemical phosphating, fully phosphate-covered surfaces on Al/Si surfaces could be obtained. Currently, their corrosion protection performance is under investigation. A new project was started on nanoceramic conversion coatings together with Henkel AG & Co. KGaA, investigating the behaviour of the future pretreatment systems under electrochemical polarisation.

Fig.1: Two different stages of the phosphating process of AS steel. Zoom Image
Fig.1: Two different stages of the phosphating process of AS steel.

Moreover, zinc based metallic coatings are of tremendous importance for corrosion protection and are object of research since decades. Besides others, salt spray tests have been used extensively for this purpose by applied researchers, providing however only a partial picture of corrosion processes under actual environmental conditions. The novel high-throughput screening approach based on a microelectrochemical flow cell system is able to deal with the issue of the complex interrelation between many experimental parameters, in particular the variable ion composition of salts. For this purpose the setup has been extended for fully automated switching between electrolytes while still enabling a facile coupling with downstream online analytics. In a strong collaboration with Arcelor Mittal/OCAS this is utilized first of all to increase our understanding of the impact of the electrolyte components on the fundamental corrosion mechanism and kinetics, and shall finally lead to enable tailoring of corrosion protection coatings with respect to the exposure conditions.

Hot dip galvanising is another industrial production step, which is intensely investigated within the department. At the focus in the last two years were co-segregation effects of alloying elements in surface oxide formation [1-3] and effects of oxygen partial pressure not only on the wetting properties of liquid zinc but also on the subsurface structure and its mechanical properties [3-5]. Especially noteworthy is the development of a novel thermo-balance set-up that allows measurements even during fast heating and cooling steps, such as. e.g., during recrystallization annealing before hot dipping. This set-up was developed within the RFCS project “HEAT” where the aim is to grow an ultra-thin iron oxide layer during soaking that serves as diffusion barrier for alloying elements.

Fundamental problems of oxidation and hydrogen uptake during industrial production steps of high strength steel sheet are at the focus of the Christian Doppler Laboratory of Michael Rohwerder [6-10]. Significant advances on developing a novel method for hydrogen detection based on SKP were made [8]. Further, the thermodynamic simulation tool developed within the Christian Doppler lab was applied to different key issues. Hydrogen uptake during high temperature production steps, such as, e.g., recrystallization annealing was investigated in detail. Based on ex-situ [11] and in-situ (ambient ESCA performed at BESSY) analytical investigations we could show that the hydrogen uptake is correlated to the effective oxygen coverage of the surface and that some alloying elements such as Mn can decrease the water dissociation reaction, which results in lower oxygen coverage and higher hydrogen uptake.

 
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