7.6 Korrosion und Korrosionsschutz
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- Pitting corrosion (4)
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Organisationseinheit der BAM
To combat climate change the share of renewable energy production must increase drastically. Offshore energy production – offshore wind and power-to-X – are crucial to achieving climate targets. However, the impact of these offshore constructions – wind turbines and production platforms – on the marine environment remains unclear in many aspects. Potential sources of chemical emissions are corrosion protection systems, remobilization of contaminants from old sediments during construction, the brine of desalination plants and increased shipping for maintenance and transportation. In order to minimize the environmental impact of renewable energies, close monitoring is essential. The goal of this project is the development of a monitoring concept for an offshore power-to-X production platform.
Maritime exposure poses particular challenges for corrosion
protection in plant construction. In addition to the right choice
of materials, special design features that can have an impact
on corrosion resistance must also be taken into account to
ensure long-lasting corrosion protection. In practice, structural
crevice arrangements are unavoidable, and therefore the
crevice corrosion resistances of stainless steels are of
particular importance. These and other influences are being
investigated in this joint project.
In the maritime context of offshore operation, corrosion, in combination with the materials used, poses a particular challenge to ensure safe operation over long indefinite periods and to minimise susceptibility to failure. Until today, only a few corrosion protection systems have proven themselves for the interaction of efficiency, installation, and maintenance in the offshore sector. In addition, available corrosion test methods sometimes have major deficits about conclusions of the durability. Therefore, there is more research and development needed.
The flagship project H2Mare of the Federal Ministry of Education and Research aims to enable the production of green hydrogen and PtX products at high seas. Research is being driven forward by the partners in four individual projects, where BAM is involved in two.
In PtX-Wind, major kinds of corrosion attacks at offshore constructions are characterized and investigated, and suitable corrosion protection measures determined. In TransferWind, attention focusses on transferring scientific results into standardization.
This poster at the H2Mare Conference 2023 presents an overview of the investigation methods and contribution of the department “Corrosion and Corrosion Protection”.
Determination of the corrosion product layer resistance on zinc samples by using gel electrolytes
(2022)
Crash barriers, railings, lamp posts, waste glass containers - these are just a few of the many examples of galvanized surfaces. They withstand wind and weather for decades, which is possible thanks to a protective layer made of corrosion products. When designing the atmospheric durability of galvanized components, a linear corrosion loss is assumed. In the past 30 years, however, the surrounding atmosphere has changed a lot. Due to the restriction of sulfur exhaust air, acid rain completely disappeared in Europe and Germany, which has a direct influence on the type and protective effect of the corrosion products. Under the influence of current atmospheric conditions, zinc layers are significantly more resistant and no longer show any linear corrosion loss. The established test methods cannot correctly map this change in neither short-term nor long-term tests and thus provide incorrect results. In order to solve this problem, a new measurement method was developed at BAM that can assess the protective effect of the formed surface layers in a minimally invasive manner without damaging the sample and in short test times. For this new method, gel electrolytes are used and the so-called corrosion product layer resistance RL, which is made up of various surface resistances, is measured electrochemically. As a single measurement, the surface layer resistance can provide information about the existing corrosion protection, while time-dependent measurements assess the stability of the layer. Thus, it is possible again to evaluate the corrosion protection layer. Furthermore, this method can also be used for field measurements on real components.
Corrosion testing in a laboratory container: Corrosion testing with artificial seawater and sediment
(2022)
Electricity generation by offshore wind turbine generators (WTG) is an important pillar of the energy transition. Corrosion processes on WTGs pose a particular challenge with regard to safe operation over longer periods of time due to the highly corrosive conditions in the offshore area. BAM is therefore conducting research to ensure a safe operation of the turbines over their intended service life to protect people and the environment.
In a joint research project, different corrosion-relevant areas of a WTG foundation structure were specifically simulated in test basins within a laboratory container. The aim of the work was, on the one hand, the gravimetric and electrochemical determination of corrosion rates in the different areas of an WTG that are relevant for cathodic corrosion protection (CCP): Underwater and sediment zone as well as the transition area between these two zones. Also, targeted tests were carried out under CCP. In addition, investigations regarding calcareous deposition during CCP were carried out on idealized specimens under laboratory conditions to better assess its influence on the protection process. The investigations were carried out in a laboratory container with different basins, which were filled with sand as sediment and artificial seawater according to ISO 15711and connected via a closed water circuit.
The poster at EURCORR 2022 provides an overview of the investigations and the most important results. It will be evaluated, if a simulation of offshore corrosion conditions is possible to a certain extent in an idealized system with artificial seawater and sediment.
The duplex stainless steel 1.4062 (X2CrNiN22-2) is used as alternative material to austenitic stainless steels in the construction industry. The corrosion resistance of welded seams is influenced by the base material, the weld filler material, the welding process and also by the final surface treatment. The scale layer next to the weld seam can be removed by grinding, pickling, electro-polishing or blasting depending on the requested corrosion resistance. Blasted surfaces are often used in the industrial practice due to the faster and cheaper manufacturing process compared to pickled or electro-polished surfaces. Furthermore blasting with corundum-grain is more effective than blasting with glass-beads which also lower the process costs. In recent years, stainless steel surfaces showed an unusually high susceptibility to pitting corrosion after grinding with corundum. For this reason, it is now also questioned critically whether the corrosion resistance is influenced by the applied blasting agent. This question was specifically investigated by comparing grinded, pickled, corundum-grain- and glass-bead-blasted welding seams. Results of the SEM analyses of the blasting agents and the blasted surfaces will be presented and correlated with the results of different corrosion tests (KorroPad-testing and pitting potentials).
KorroPad - New Test Method for the Investigation of the Passive Layer Stability of Stainless Steels
(2018)
The KorroPad is a gel-based indicator test designed for the electrochemical determination of the passive layer stability and localization of surfaces of stainless steels threatened by localized corrosion. Different investigations and studies on the performance of the KorroPad as well as field trials by industrial users the have proven the principal suitability of the KorroPad method to assess the passive layer stability of stainless steels. The electrochemical mode of action will be described in detail and also the limitations of the applicability will be demonstrated. The influence of different surface finishes has been investigated and verified by known methods for describing corrosion resistances. As a result, the increased corrosion susceptibility of two ground surfaces has been detected, but also the corrosion resistance of further surface finishes could be confirmed. Furthermore the KorroPad test was used for the investigation of the influence of time and humidity on the passive layer development on different stainless steels.
Hardness and pitting corrosion resistance are the major quality criteria of cutlery. Both are achieved by the heat treatment (austenitization, quenching and tempering) of the normally used martensitic stainless steels. The established quality control method regarding the pitting corrosion resistance is an alternating immersion test in 1 % NaCl solution at 60 °C according to DIN EN ISO 8442. This standard test shows a high deviation, which limits any optimization of the heat treatment process. New approaches for corrosion testing of martensitic stainless-steels were developed and used in the last years to connect the weak pitting corrosion resistance of martensitic stainless-steels with the phenomenon of chromium depletion. The tempering temperatures used in the industrial heat treatment of cutlery are too low to explain the appearance of chromium depletion. For this reason, a systematic investigation of three heat treatment parameters (austenitization time, cooling speed and tempering temperature) were performed on the martensitic stainless-steels X50CrMoV15 (1.4116) to detect their contribution to chromium depletion. The electrochemical potentiodynamic reactivation (EPR), which is very sensitive to any change of the microstructure, was used to quantify the degree of chromium depletion. The KorroPad indicator-test was applied to correlate low pitting corrosion resistance to the presence of chromium depletion. The results of all investigations allow conclusions about the very small process window, which is necessary to achieve cutlery with high pitting corrosion resistance.
The KorroPad indicator test was invented at the Federal Institute for Materials Research and Testing to detect stainless-steel surfaces susceptible to pitting corrosion. The KorroPad indicator test is thus an interesting alternative for manufacturers, processors, users and researchers to time consuming exposition experiments and complex electrochemical investigations, which are normally used to reveal surface-related processing errors resulting in reduced pitting corrosion resistance. Furthermore, the KorroPad indicator test can be used to visualize alloy- and microstructure-related factors causing susceptibility to pitting corrosion. The detection limit of the KorroPad indicator test was characterized in this work using reference alloys with different chromium content (5 % to 18 %). The alloy-specific detection limit was shifted successfully to higher chromium content by increasing the concentrations of NaCl and K3[Fe(CN)6]. The modified KorroPads can now be used to establish an alloy-specific quality control regarding the pitting corrosion resistance of different stainless steel grades.
Nickel-free, nitrogen alloyed austenitic stainless-steels, with about 19 wt.-% Mn and 0,8 wt. % N, are an interesting alternative to classic CrNi austenitic stainless steels due to their superior mechanical properties (Rm > 900 MPa, A5 > 50 %, Av > 350 J) in the solution annealed condition. The formation of chromium-rich nitrides during suboptimal heat treatment, processing or application leads to an inhomogeneous distribution of alloying elements in the microstructure, which reduces the corrosion resistance. Consequently, an accurate knowledge of the sensitization behavior is indispensable for the use of nickel-free, high-nitrogen austenitic stainless steels. The relationship between artificial aging, phase formation and corrosion resistance was investigated on the alloys X8CrMnN18-19 (1.3815) and X8CrMnMoN18-19-2 (1.4456), both alloyed with 0,8 wt.-% Nitrogen, in the present work. The microstructural evolution was studied by LM and SEM while the corrosion resistance was characterized with the electrochemical potentiodynamic reactivation (EPR) and the KorroPad indicator-test. Both alloys showed increased corrosion susceptibility within critical aging parameters. Finally, a sensitization diagram was described successfully for both alloys showing the positive effect of molybdenum.