Helmholtz Research Field: Information
Institute of Surface Science
Plasma electrolytic oxidation (PEO) coating. Photo: HZG
The Institut of Surface Science develops fault-tolerant anti-corrosion coatings which remain effective even when subjected to mechanical damage as well as coatings for magnesium-based medical implants that dissolve in the body according to a set timetable. Although magnesium is not a precious metal, magnesium-based materials are corrosion resistant when exposed to air. In fact, protection against corrosion is only necessary when they are exposed to aggressive media such as sea water or a saline, humid atmosphere. A special care about protection of magnesium should be taken when it is used as a part of multi-material system being joined — by means of welding, for example — to other, more precious metals. Therefore development of active multi-functional coatings for hybrid-structures is one the main focus of WZK.
In addition to corrosion, mechanical loads can also lead to sudden failure when components crack due to stress corrosion or corrosion fatigue. Such failures, the nature of which the WZK department is currently investigating, are caused by hydrogen, which is the primary product of magnesium corrosion.
Profile
Simulated current distribution around an electrolyte-wetted Mg-Al mixed compound (Al-rivet in a Mg-sheet). Photo: HZG
The Corrosion and Surface Technology department (WZK), headed by Prof. Mikhail Zheludkevich, studies the corrosion mechanisms of different light-weight metallic systems with special focus on Mg alloys. The main goals of WZK are dedicated to development of new alloys with tailored corrosion resistance as well new corrosion protection concepts for Mg components or multi-material structures. A novel concept of flexible functionalization of coatings via introduction of active nanocontainers, which are able to store the active functional agents and release them on demand in the presence of suitable triggers, is followed.
The knowledge gained on the corrosion control of structural systems is transferred to the metallic bio-materials and potential battery application. The corrosion related phenomena are studied using the local electrochemical and analytical techniques at different scales being supported by the multi-scale modelling approaches.
Alloy development
Magnesium surface attacked by corrosion. Photo: HZG
The alloy development is performed in close collaboration with the departments of Mg processing and wrought alloys with the aim to improve the corrosion properties of magnesium alloys further including the development of suitable recycling and degradable alloys.
Contact person: Dr. Carsten Blawert
Coating technology
Calcium phosphate coating on magnesium alloy. Photo: HZG
The work related to the design of new multifunctional corrosion protection systems and concepts including pretreatments such as cleaning and passivation as well as the development of new inorganic and organic coatings and multilayer systems with and without active corrosion protection. Latter should guarantee sufficient corrosion protection even in the case of mechanical damage of the coating.
In the case of degradable implants the coatings should extent and provide the required timescale for dissolution. Additional functionalities can be included such as delivery of drugs, stimulation of cell growth or antibacterial properties.
Contact perons:
Inorganic coatings: Dr. Carsten Blawert
Organic coatings: Dr. Nico Scharnagl
Corrosion inhibition
SVET measurement of AZ91 alloy with or wothout corrosion inhibitor incorporated in PEO layer sealed with sol-gel thin coating. Photo: HZG
Magnesium is prone to corrosion induced damage not only due to its negative electrochemical potential but also due to its susceptibility to noble impurity elements like Fe, Cu or Ni. We perform a systematic study of how inorganic and various groups of organic compounds influence degradation of magnesium and its alloys in varying environments. Identification of substances and mechanisms for accelerating or inhibiting Mg dissolution is the key approach.
Inhibitors are then used to impart active corrosion protection to Mg parts for the applications where the structural integrity of Mg-made parts needs to be preserved for long time (aeronautic and aerospace industries).
One the other hand, the ways for controlled dissolution of Mg are also becoming evident by using the substances accelerating dissolution of Mg. This knowledge finds application in biomedicine and for innovative energy storing devices.
Contact person: Dr. Sviatlana Lamaka
In situ spatially resolved localized measurements
Local current density measured with SVET, distribution of pH and concentration of Mg2+ mapped with micro-potentiometry (SIET). Photo: HZG
Difference in microstructure and presence of impurities causes microgalvanic corrosion which progresses to global scale. Microgalvanic corrosion can be studied in details by local electrochemical techniques. Local (as opposed to global) current density, pH, concentration of dissolved oxygen and other species can be measured with microprobes scanning the area of interest above conductive or non-conductive samples in electrolyte.
O2 local in-situ measurements over the S-phase model cell of aluminium alloy measured by micro-optode fibre optic sensor. Photo: HZG
We use Scanning Vibrating Electrode Technique (SVET) for revealing the local current density distributions, potentiometry with micro-electrodes (also known as SIET-Scanning Ion-selective Electrode Technique) for recording local pH and concentrations of Mg2+, Cl-, Na+, etc. Amperometric microprobes are used for mapping the distribution of Fe2+/Fe3+ or dissolved O2. Recently we have also introduced micro-optode (non-electrochemical fiber-optic sensor) in everyday practice for recording the local concentration of dissolved O2 above actively corroding samples.
Laser welded aluminium alloy exposed to 0,05M NaCl electrolyte for 12 hours. The feature in the central left side of the 0h optical image is the SVET probe. Photo: HZG
Difference Viewer Imaging Technique (DVIT) provides in situ non-invasive observations of minor changed in visual appearance of surface and adjacent electrolyte. It is also used for time-lapse microscopy.
Contact person: Dr. Sviatlana Lamaka
Mechanically induced corrosion
Surface damage evolution in the magnesium alloy. Photo: HZG
Corrosive environments can have a severe influence on the mechanical properties of magnesium alloys. Components and weldments can suffer from stress corrosion cracking and corrosion fatigue at loads well below the yield stress. The crack formation and growth in corrosive media as well as the related hydrogen embrittlement are studied and analyzed.
Modelling and simulation as well as the experimental investigation of the material degradation due to electro-chemical corrosion and mechanical loading are the additional focus of the research. Corrosion can be observed as a “driving force” of defect formation in materials. The defect evolution in the corrosive material structures has many important engineering applications. The problem of the corrosion fatigue in lightweight multi-material systems can be successfully studied in details with help of experimental and numerical analysis.
Contact persons: Dr. Natalia Konchakova
Active corrosion protection
Photo: HZG
The development of basic strategies for “smart” corrosion protection for active metals and multi-material combinations in new “green” aircraft designs is the main objective of the research subject. In this way the work is driven by an industrial demand but has an extremely important fundamental research component on a scientific level.
The investigation of multi-functional surfaces with high level of self-healing ability on the basis of Layered Double Hydroxide (LDH) is the focus of the research. The main idea is based on “smart” triggered release on demand for functional anionic compounds intercalated into nanocontainers. The functionality is achieved via growth of LDHs on Al, Mg and Zn based alloys and their loading with functional anions such as corrosion inhibitors, biocides, drugs, or hydrophobic agents. The release of anions occurs only on demand when the respective functionality is triggered by the relevant external stimuli (like presence of anions or local pH change).
Photo: HZG
The development of the novel active corrosion protection systems for multi-material combinations, which is used in the “green” aircraft design, is the other important application. The essential stage is finding the synergistic inhibiting combinations which can provide effective suppression of corrosion processes in the case of galvanically coupled dissimilar materials and their introduction to the protective layers (coatings) used in multi-material design of aircrafts.
Contact persons: Prof. Dr. Mikhail Zheludkevich
Dr. Maria Serdechnova
Mg Battery
Photo: HZG
Mg based batteries are promising candidates to enhance energy storage performance. Research and adjacent patenting activity within this field have been accelerated within the last years. Pilot projects for automotive applications have been already accomplished. Nevertheless, a final breakthrough is missing. Owing to a number of detrimental interacting effects related to anode materials, electrolyte issues and also cathode efficiency.
Photo: HZG
Our work is focused on the interface anode - electrolyte by considering anode material- but also electrolyte properties. Running research activities are
- Electrolyte additives for aqueous Mg-Air battery cells
- Mg alloys with optimized discharge behaviour
Future work might also include anode development for secondary (rechargeable) Mg-batteries.
Contact persons: Dr.rer.nat. Daniel Höche
Dr. Darya Snihirova
Modelling and simulation of corrosion processes
Photo: HZG
Corrosion modelling and stepwise also the related predictive service-life modelling is a growing field in a Computer-Aided Engineering. The main issues of the computational description of corrosion are included into the high complexity of corrosion mechanism. Major problems relate to a multiscale nature of corrosion processes and the need to use many model parameters, which limit the “validity window” of simulations. However, the field of potential industrial applications is huge and the progress in industrial oriented corrosion modelling is currently at the beginning.
The horizontal digital strategy of WZK is focused on the extension of corrosion modelling capabilities applicable in different fields. “On-the fly” coupling / modelling action, computational homogenization, linking of discrete and continuum models, coupling of mechanical and electrochemical models, database management, model input and validation data generation and interdisciplinarity are topics in progress. Modelling of corrosion phenomena is developed in the following directions:
- multiphysics / multidisciplinary crosslinking based on computational models (from electronic to macroscale) and detail numerical analysis
- high level experimental validation action (in-situ) and a feedback loop towards new characterization action in order to generate a database regarding corrosion, material degradation and damage prediction
- improved temporal validity and reproducibility
Aimed full-chain corrosion induced failure and damage modelling requires clear definition of test conditions in order to be able to upscale to service-life performance and even prediction. Therefore, vertical approach along the scales assisted by advanced experiments is used.
Contact person: Dr.rer.nat. Daniel Höche
Dr. Natalia Konchakova
The department specialises in:
- Integral and local electrochemical corrosion investigations
- Development of corrosion protective and multi-functional coatings for Mg and hybrid structures including active corrosion protection and intelligent delivery systems
- Clarification of corrosion mechanisms and corrosion-related causes of damage
- Inhibition of corrosion processes on various metallic substrates including multi-material assemblies
- Systematic surface analysis
- Tests on hydrogen embrittlement, stress corrosion and corrosion fatigue according to ASTM, ESA and ISO specifications,
- Exposure tests under corrosive conditions
- Modelling of electrochemical processes
