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Eingeladener Vortrag
- nein (138)
In the late 1980s, a new type of corrosion appeared in drinking water installations where galvanized steel pipes failed forming a deep groove with no corrosion at the other surfaces. The reason was discussed quite fiercely between failure analysts and material suppliers. In the end the discussion lead to the solution: inductive welded pipes of a certain sulphur content were prone to selective corrosion. The history of the arguments is discussed using old communications between Wilhelm Schwenk and Wolfgang Stichel, who later published the results in a paper unfortunately only I german.
Thus the knowledge got lost and new cases based on the same material problems are observed in application of so-called C-Steel piping systems. Failure cases are shown.
The chapter starts with a brief introduction about corrosion, which is defined as the interdependency between a metal, a corrosive environment, and the respective component design. The second section introduces the most important forms of aqueous electrochemical corrosion (uniform corrosion, galvanic corrosion, selective and intergranular corrosion, and finally pitting and crevice corrosion in the case of passive layer forming metals). In addition, electrochemical corrosion under applied mechanical load is introduced (stress corrosion cracking, hydrogen-assisted cracking, corrosion fatigue), as well as special forms of corrosion (erosion, fretting, and microbiologically induced corrosion). The third section of this chapter introduces (mostly dry) chemical corrosion and high-temperature corrosion (oxidation, carburization, high-temperature hydrogen attack, sulfurization, nitriding, halogenation). As in the case of electrochemical corrosion, chemical corrosion can also be superimposed by mechanical loads. Finally, general facts on the testing of corrosion are introduced.
The screening of coatings shows that the modification by adding individual pigment was not sufficient to protect carbon steel even during a short-term exposure, indicated by the discoloration after only seven days of exposure.
Electrochemical tests indicated that the coating cathodically protects carbon steel or slows down the corrosion reaction.
A long-term exposure test confirmed that the PANI/SiO2 modified coating successfully protects the carbon steel in the Sibayak artificial geothermal water at 150 °C for 6 months.
Corrosive geothermal brines are a major challenge to geothermal power-plants. For cost reasons, plant designers favorize low alloyed steels, e.g., carbon steel, which are susceptible to uniform and localized corrosion when exposed to geothermal brines having acidic and saline properties. To solve such problem, coatings or inhibitors would be a protective solution as an alternative to the use of high alloyed materials.
This study investigated a coating system consisting of polyaniline/silicon dioxide basing on locally available resources. Protection against corrosion of carbon steel is shown by long-term exposure and electrochemical tests of coated carbon steels, performed in an artificial acidic and saline geothermal brine, comparable to real conditions at a site in Indonesia.
Therefore, an integrated coating system is presented for corrosion protection, combining the electro-chemical functionality of polyaniline and the physical advantages of silica.
This work presents machine learning-inspired data fusion approaches to improve the non-destructive testing of reinforced concrete. The principal effects that are used for data fusion are shown theoretically. Their effectiveness is tested in case studies carried out on largescale concrete specimens with built-in chloride-induced rebar corrosion. The dataset consists of half-cell potential mapping, Wenner resistivity, microwave moisture and ground penetrating radar measurements. Data fusion is based on the logistic Regression algorithm.
It learns an optimal linear decision boundary from multivariate labeled training data, to separate intact and defect areas. The training data are generated in an experiment that simulates the entire life cycle of chloride-exposed concrete building parts. The unique possibility to monitor the deterioration, and targeted corrosion initiation, allows data labeling.
The results exhibit an improved sensitivity of the data fusion with logistic regression compared to the best individual method half-cell potential.
Half-cell potential mapping (HP) is the most popular nondestructive test (NDT)-method for the localization of corrosion damage in concrete. It is generally recognized, that HP is prone to the environmental factors that arise from salt induced deterioration, such as varying moisture and chloride gradients. Additional NDT-methods are capable to determine distinctive areas, but cannot yet be used to estimate more accurate testing results. We introduce a supervised machine learning (SML) based approach for data fusion to make use of the additional sensor information. SML are methods that explore relations between different (sensor) data from predefined data labels. We use a simple linear classifier named logistic regression to distinguish defect and intact areas. The test performance improves drastically compared to the best single method, HP. In order to generate representative, labeled data we conducted a comprehensive experiment that simulates the deterioration-cycle of a chloride-exposed building part in the lab. Our data set consist of 18 measurement campaigns, each containing HP-, ground-penetrating-radar-, microwave-moisture-, and Wenner-resistivity-data. We detail the challenges that arise with a data driven approach in NDT and how we addressed them.
Recent results from a three-year multi-disciplinary project on Ancient Egyptian gold jewellery revealed that items of jewellery from the Middle Kingdom to the New Kingdom were manufactured using a variety of alluvial gold alloys. These alloys cover a wide range of colours and the majority contain Platinum Group Elements inclusions. However, in all the gold foils analysed, these inclusions were found to be absent. In this work a selection of gilded wood and leather items and gold foil fragments, all from the excavations by John Garstang at Abydos (primarily from Middle Kingdom graves), were examined using Scanning Electron Microscopy-Energy Disperse Spectroscopy (SEM-EDS), X-Ray Fluorescence (µXRF), Particle Induced X-Ray Emission (µPIXE) and Double Dispersive X-Ray Fluorescence (D²XRF). The work allowed us to characterise the composition of the base-alloys and also to reveal the presence of Pt at trace levels, confirming the use of alluvial gold deposits. Corrosion products were also investigated in the foils where surface tarnish was visually observed. Results showed that the differences in the colour of corrosion observed for the foils are related not only to the thickness of the corrosion layer but also to a multi-layer structure containing the various corrosion products.
Adhesion and corrosive delamination of epoxy films on chemically etched ZnMgAl-alloy coatings
(2016)
The effect of alkaline and acidic pretreatment steps on the surface chemical
composition and adhesion properties of ZnMgAl-alloy coated steel was
investigated by means of spectroscopic methods, scanning Kelvin probe (SKP)
and peel test measurements.The spectroscopic results indicate that the surface
film composition can be adjusted by the wet-chemical treatment. To study the
corresponding surface adhesive properties, the samples were coated with an
epoxy amine adhesive. Peel tests under humid conditions indicated an
increased interaction between the acidic pre-treated surface and the adhesive.
The results of the SKP analysis show that the acidic cleaned substrates have the
highest resistance to delamination, which can be explained by the shift of the
interfacial electrode potential.