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Even though corrosion, e.g., as a process of rusting, seems simple, it surrounds us every day, obviously and less obviously, and the background is actually very complex.
Corrosion can occur when materials come into contact with an electrolyte to form a galvanic element. Moisture and salts are ubiquitous and, in many places, form the electrolyte necessary for an electrochemical reaction. Not only metals and metallic materials that are exposed to environmental influences have to be considered for corrosion properties and protection, but also concrete and rocks, e.g., as building materials or (dental) medical implants are subject to corrosion and are DUTs for corrosion studies.
History of corrosion
Corrosion has been known since the Bronze Age, when mankind began using metallic materials. Over the centuries, people learned more and more about, for example, rusting iron, which turns red accordingly or metals are literally gnawed (Latin corrodere for gnawing). Georgius Agricola also describes the first approaches regarding corrosion protection around the year 1546 and recommends coating with silver or tin. A scientific description of corrosion was not possible until the 19th century with the advent of electrochemistry and so research on and with corrosion processes is closely linked to the development and performance of potentiostats.
Potentiostats are used in many ways for corrosion studies. On the one hand, sometimes very slow corrosion reactions can be accelerated considerably by applying an external voltage or the maximum corrosion potential of a corrosion protection layer can be determined. On the other hand, potentiostats enable the qualitative and quantitative assessment of corrosion processes or products by means of electrochemical analysis, i.e. electroanalytics.
Methods of corrosion analysis
In linear polarisation, the corrosion system is deflected from the equilibrium state by a constant potential change, e.g., by Linear Sweep Voltammetry (LSV) or Cyclic Voltammetry (CV). The obtained current density-voltage curves, also called polarisation curves, can be evaluated via the slope of the so-called Tafel plot as Butler-Volmer fit with respect to corrosion potential (E0), current (I0), polarisation resistance and corrosion rate. The plotting of the Tafel plot and the corresponding Butler-Volmer fit are integrated in the software packages of PalmSens and Ivium so that a simple and quick measurement evaluation is ensured.
Electrochemical impedance spectroscopy (EIS) is widely used to investigate electrochemical systems and thus also for corrosion studies. By analysing the phase and amplitude of the system response to an applied AC signal, processes and changes at surfaces or interfaces can be investigated quickly and non-destructively. EIS is particularly advantageous for corrosion processes with regard to the short measurement time of a few seconds and the minimal disturbance of the system, so that very precise snapshots can be mapped.
Electrochemical noise measurement (ENM) is a stochastic method for the analysis of current or potential fluctuations. In general, the free corrosion potential is measured without current, i.e., without external influence on the electrochemical system, over as long a period as possible. Nucleation processes, crack propagation, metal dissolution and many more cause voltage pulses to be emitted, which are superimposed on the background noise of the potential. With the help of Fourier transformation, the different frequency components can be calculated and therefore evaluated separately from each other. The great measurement challenge in ENM arises from the small amplitude values with a small difference to the background noise, so that a particularly high voltage resolution is necessary and low current measurement technology must be used. Correspondingly high is also the error input due to interference sources, such as incorrect/defective cabling or impurities.
Corrosion measurements are used in the following fields of application:
- Materials science incl. building inspection
- Medicine (implantology and dentistry)
- Corrosion protection
In materials science, knowledge about the environmental and chemical resistance of the manufactured or examined materials is clearly in the foreground. Depending on the question, the focus of the investigations is either on durability or on whether certain, perhaps toxic, components leak out of the material. For example, newly developed alloys are tested for electrochemical stability in a corrosion medium (electrolyte) that is as representative as possible. Structures, such as bridges or pipelines, must be regularly tested for corrosion in order to take timely measures for corrosion protection or to be able to detect impending material fatigue at an early stage.
In the medical sector, the focus of corrosion measurements is on implants, such as stands or dentures, but the corrosion of tissue due to inflammation or chemical burns can also be an issue. Implantation materials and implants themselves are examined in detail before they are used in the human body, among other things in order to be able to recognise possible interactions with tissue or blood/saliva at an early stage. In addition, the implants must of course show sufficient durability in the laboratory corrosion tests and no toxic, highly allergenic or bioactive substances may escape under in-vivo conditions.
Especially essential are corrosion measurements when corrosion inhibitors are being developed. These agents are applied to the corrosive medium and protect it from corrosion by various mechanisms. For example, steel is protected by applying a thin layer of zinc (galvanising) because zinc, being a less noble metal (sacrificial anode), is preferentially oxidised.
Another method of corrosion protection is corrosion inhibitors, which are either added to a coating or to the corrosion medium (surrounding liquid/gas). Through physical or chemical processes, corrosion inhibitors inhibit the corresponding corrosion reactions.
Requirements for measurement setup
As diverse as corrosion occurs, so are the requirements for the measurement setup of a corrosion test.
Small specimens or material samples can be investigated in the classical way in the laboratory, whereas large corrosion bodies such as buildings, pipelines or ships can only be measured in the field. The field application requires correspondingly portable measuring solutions, both on the potentiostat and the measuring cell side. In particular, it must be possible to contact various surfaces flexibly with the measuring cell and/or the electrodes.
For this purpose, top-mounted measuring cells, such as the Ivium MCF cell (Magnetic Corrosion Flat-cell), are recommended. The MCF cell is magnetically attached to the workpiece, filled with electrolyte and connected to a mobile potentiostat, such as the Ivium compactStat. In addition, the highly mobile potentiostats by PalmSens, in particular PalmSens4, are ideally suited for such field measurements as well as for laboratory tests due to their battery operation and Bluetooth connection. For the latter, classic glass cells with sample/workpiece holder, reference electrode incl. Luggin capillary and graphite or platinum counter electrodes are used.