Wolfram Fürbeth

Study of initial stages of Al–Mg alloy corrosion in water, chloride and Cu(II) environment by a scanning Kelvin probe and XPS

Abstract Corrosion product layers formed during initial corrosion stages on Al–Mg alloy in water, 3.5% NaCl solution and 3.5% NaCl containing 50 ppm Cu(II) have been studied by scanning Kelvin probe (SKP) and X-ray photoelectron spectroscopy (XPS). The SKP spatio-temporal images reconstructed the solution–air boundary on corroded samples. The SKP sensitivity to detect the corrosion products layer on Al–Mg alloy was demonstrated to be in subnanometer scale. The Volta potential response on pre-corroded surfaces depended upon the immersion time (oxide layer thickness) and a kind of the solution used (oxide layer conductivity). The Volta potentials measured on the surfaces corroded in pure water were higher than those on the surfaces exposed to 3.5% NaCl or 3.5% NaCl+50 ppm Cu(II) solutions. This difference has been explained in terms of higher conductivity of the corrosion product layers developed in the ion-containing media.

Nanoparticle based inorganic coatings for corrosion protection of magnesium alloys

Abstract Inorganic nanoparticle based coatings for magnesium alloys were developed and tested for their performance in corrosion protection. Nanoparticles are characterised by a high sintering activity. This allows to obtain inorganic coatings by a sintering process at rather low temperatures which is suitable for magnesium alloys. Coating sols are based on silicon dioxide and sintering additives such as boron or sodium oxide. One technique is based on aqueous, commercial nanosols which can be applied by dip or brush coating to form layers on AZ31 and AZ91. Another technique is based on the electrophoretic deposition of silicon dioxide nanoparticles which also contain boron and phosphorus oxide. Crack free layers with a thickness of up to several micrometres could be obtained by a two step process including a bimodal particle distribution and polydiethoxysiloxane as adhesion promoter. The composition and surface structure of these novel coatings are characterised by modern analytical methods including SEM and atomic force microscopy and their applicability as protective coatings is investigated by using electrochemical impedance spectroscopy.

Novel protective coatings for steel based on a combination of self-assembled monolayers and conducting polymers

Investigations for a new primer system for iron or low alloyed steel have led to first results. Several special phosphonic acids with thiophene derivatives as head groups have been synthesized. They form stable self-assembled monolayers ( SAMs ) on passivated iron by dipping the substrates into aqueous phosphonic acid solutions. SAM formation was validated by current potential curves and also by contact angle measurements, which showed an intensive hydrophobisation of the iron surface after the dipping process. Finally cyclovoltammetric ( CV ) experiments after SAM formation indicated the successful polymerisation of the immobilised thiophene derivatives.

Novel Steel Corrosion Protection by Microbial Extracellular Polymeric Substances (EPS) – Biofilm-Induced Corrosion Inhibition

Microbially influenced corrosion (MIC) of steel has gained increasing attention in recent years because the damage caused by this process is a significant cost factor for industry. Consequently, inhibition of corrosion and especially the development of corrosion protective films is an important present-day research topic. In this connection, application of microbially produced EPS for mitigating steel corrosion is an innovative idea. However, observations of ”protective” biofilms on metallic surfaces have been previously reported. Their inhibiting effect is generally thought to be caused by oxygen depletion or the formation of passivating layers. In contrast to many conventional corrosion protective methods, EPS or EPS-derived agents would be a cheap and environmentally friendly solution. Extensive research activities are still required, before biofilms or cell-free EPS can be used for corrosion protection on larger scale. In this study, we are developing a novel EPS-based corrosion protection method for unalloyed and corrosion resistant steel in aqueous media, which is based upon the application of microbial metabolic products. EPS of various sulfatereducing bacteria and other microorganisms are investigated for their inhibiting effect. The extent of such inhibition is evaluated in a model test system, in which different steels are subjected to corrosive conditions under sulfate-reducing conditions. To elucidate the protective mechanisms, comparative analyses of the chemical composition of the applied EPS are performed.

Electrochemical Behaviour of Iron in a Third‐Generation Ionic Liquid: Cyclic Voltammetry and Micromachining Investigations

The electrochemical behaviour of Fe in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim](+)Ntf2(-)) and mixtures with Cl(-) is studied with the aim of investigating the applicability of ionic liquids (IL) for the electrochemical machining of iron. Whereas in pure IL iron could not be significantly dissolved, the addition of Cl(-) enables the active dissolution with anodic current densities up to several mA cm(-2). Although several anodic peaks appear in the cyclic voltammograms (CV), the distinct assignment of those electrochemical processes remain difficult. In particular no proof for the formation of FeCl(x) (2-x) complexes during Fe dissolution are deduced from the CV, although such complexes are shown to be stable in the employed electrolyte. In addition, we present electrochemical drilling experiments with short potential pulses, which demonstrate that electrochemical machining of Fe is, in principle, possible in IL based electrolytes, even though hampered by slow machining speed.

Novel corrosion protective coatings for aluminium alloys and steels based on oxidic nanoparticles

Abstract Based on chemical nanotechnology glass-like protective coatings thermally processed at comparatively low temperatures of around 500 °C were developed for aluminium and steel substrates by two different means. Starting from polymeric sols containing a multicomponent oxide of the SiO2 – B2O3 – P2O5 – Na2O system produced by sol-gel technology under acidic conditions, coatings were developed which are thin, hard, transparent, crack-free and corrosion resistant. Electrophoretic deposition of coatings from particulate sols containing multicomponent oxide synthesised under basic conditions proved to be a promising method when very thick coatings are required. Both routes offer the potential of a new type of purely inorganic coatings for corrosion and abrasion protection.

The Influence of Ultrasound Enhancement during Friction Stir Welding of Aluminum to Steel

The innovative joining process of friction stir welding (FSW) offers a wide range of advantages for welding similar as well as dissimilar materials. Even for the field of poorly weldable material combinations like aluminum to steel with their strongly differing physical properties the method of FSW proved its capability for realizing dissimilar joints with tensile strengths up to more than 80 % of the aluminum base material. Trying to improve this value and other properties of the joints several approaches for hybrid friction stir welding processes were tested in the scientific community, whereas the ultrasound enhancement of FSW (USE-FSW) looked as one of the most promising reaching good results. To gain a deeper knowledge of the influence of the ultrasound on the friction stir welds different investigations were carried out in this paper. Therefore the method of USE-FSW was applied on two dissimilar aluminum/steel-joints with varying carbon content of the steel in this work. The material combinations AA6061/SAE1006 and AA6061/SAE1045 were welded successfully with and without additional power ultrasound. Afterwards a comparison between FSW-and USE-FSW-joints was carried out regarding the microstructure of the nugget and interface (IF) by light-microscopy as well as scanning electron microscopy. Furthermore the mechanical properties were characterized in a first step.

Realization of Ultrasound Enhanced Friction Stir Welded Al/Mg- and Al/Steel-Joints: Process and Robustness, Mechanical and Corrosion Properties

As an innovative hybrid joining process ultrasound enhanced friction stir welding (USE-FSW) was successfully applied on Al/Mg-joints and offers a beneficial impact on the resulting microstructure and mechanical properties. Whereas in conventional FSW of Al to Mg continuous band-shaped intermetallic layers of Al3Mg2 and Mg17Al12 are always present, the formation of these layers can be influenced positively by the ultrasound energy that is transferred into the stirred zone in USE-FSW. In this case, the intermetallic phases are spread over the complete nugget zone. Consequently, the tensile strength increases about 25% and the fatigue strength rises by up to 3.5 times. Based on this USE-FSW was investigated with regard to possible advantages for Al/steel-hybrid-joints. Preliminary investigations have shown significant differences in the microstructure of ultrasound enhanced and non-ultrasound enhanced EN AW-6061/DC04-joints. While conventional FSW led to hooks and larger particles of steel in the aluminum, USE-FSW-joints show remarkably smaller and more homogenous distributed steel particles in the nugget. Beside mechanical investigations, non-destructive examinations and corrosion tests were carried out.

Recent Developments for Ultrasonic-Assisted Friction Stir Welding: Joining, Testing, Corrosion - an Overview

Due to the steadily increasing demand on innovative manufacturing processes, modern lightweight construction concepts become more and more important. Especially joints of dissimilar metals offer a variety of advantages due to their high potential for lightweight construction. The focus of the investigations was Al/Mg-joints. Friction Stir Welding (FSW) is an efficient process to realize high strength joints between these materials in ductile condition. Furthermore, for a simultaneous transmission of power ultrasound during the FSW-process (US-FSW) a positive effect on the achievable tensile strength of the Al/Mg-joints was proven. In the present work the industrial used die cast alloys EN AC-48000 (AlSi12CuNiMg) and AZ80 (MgAl8Zn) were joined by a machining center modified especially for Ultrasound Supported Friction Stir Welding. The appearing welding zone and the formation of intermetallic phases under the influence of power ultrasound were examined in particular. In order to identify optimal process parameters extensive preliminary process analyzes have been carried out. Following this, an ultrasound-induced more intensive stirring of the joining zone and as a result of this a considerably modified intermetallic zone was detected. At the same time an increase of the tensile strength of about 25% for US-FSW-joints and for fatigue an up to three times higher number of cycles to failure in comparison to a conventional welding process was observed. Moreover, detailed corrosion analyzes have shown that especially the welding zone was influenced by the corrosive attack. To expand and deepen the knowledge of the US-FSW-process further material combinations such as Ti/Steel and Al/Steel will be considered in future.

Korrosionsschutz durch Bakterien

ZusammenfassungBakterielle extrazelluläre polymere Substanzen wurden als Schutz für metallische Werkstoffe vor (Bio-)Korrosion getestet. Trotz beobachtbarer Schutzwirkungen geben die komplexen Substanzgemische Anlass für den Einsatz von reinen biogenen Substanzen.AbstractBacterial extracellular polymeric substances have been studied with focus on the protection of metals against (bio-)corrosion. Though positive effects are observed, the complex mixtures give rise for the application of single biogenic compounds instead.