The contribution of hydrogen to the corrosion of 2024 aluminium alloy exposed to thermal and environmental cycling in chloride media

This work is focused on the role of hydrogen in corrosion damage induced by the cyclic exposure of 2024 aluminium alloy to chloride media with air emersion periods at room and/or negative temperatures. Various analysis and microscopic observation techniques were applied at intergranular corrosion defects. A mechanism involving the contribution of hydrogen to the degradation of the alloy mechanical properties is presented. Several consecutive stress states appear during cycling, resulting from volume expansion of the electrolyte trapped in the intergranular defects during emersion phases at -20°C. These stress states lead to hydrogen diffusion, transport and trapping

Corrosion and tensile behavior of intercritically heat-treated and cathodically polarized prestressing steel wires

The effect of the intercritical heat treatments producing various martensite volume fractions on the corrosion and tensile behavior of the cathodically polarized prestressing steel wires has been investigated. Potentiodynamic polarization and constant extension rate technique (CERT) tests are conducted to clarify these effects and to determine the mode of failure of the prestressing steel wires since the presence of the martensite in the prestressing steel wire microstructure results in the failure and breakage during the corrosion and cold drawing. The electrochemical data measurements reveal that the intercritically heat-treated and cathodically polarized prestressing steel wires containing higher volume fractions of martensite exhibit the lowest corrosion potentials and current densities which indicate higher corrosion rates. A significant decrease in the tensile strength and ductility by the accelerated corrosion tests is observed in the intercritically heat-treated and cathodically polarized prestressing steel wire specimens containing higher martensite volume fractions. The fracture surfaces of the intercritically and cathodically prestressed steel wire specimens containing higher fractions of martensite of 68% exhibit a mixed mode of failure, i.e., quasi-cleavage and microvoid coalescence after potentiodynamic polarization. Moreover, the brittleness due to the residual stresses induced by the formation of martensite and the generation of the galvanic corrosion cell between the martensite and pearlite is noted to be increased with increasing the amount of martensite in the prestressed steel wire specimens

Compressive Strength of Axially Loaded Built-up Sigma Cold Formed Sections Columns

The use of cold formed steel sections in building constructions has lately seen rapid growth due to their light weight and easier erection. Generally, lipped channel sections are used more frequently .Sigma sections while requiring more effort in fabrication, they exhibit high strength to weight ratios compared with the lipped channel sections. This work presents experimental as well as numerical study of the strength of pinned-pinned axially loaded columns utilizing sigma sections. The column section consists of two back to back sigma cold formed sections. The two sections are connected through their webs with connecting fasteners. Eight specimens with variable plate element width to thickness ratio, and having different member slenderness ratios were tested. Residual stresses and geometrical imperfections were recorded. Moreover, the specimens were simulated by a nonlinear finite element model using four node isoparametric shell element that accounts for both geometric and material non-linearities. The measured geometric imperfections and residual stresses were included in the numerical model. The parameters studied include the properties of the cross-section such as the flange, web, and lip dimensions as well as the spacing of the web fasteners. Furthermore, several column heights are considered to study the different modes of failure. Finally, the results are compared with the ultimate strength predicted by the American AISI, the Eurocode-3 and DSM specifications. Results reflect that sectional (local/distortional) buckling governs the failure mode of short columns. Moreover, the AISI, the Eurocode-3 and DSM can reasonably predict the ultimate load capacity of such sections

Numerical Study of Bolted Moment Connections in Cold-Formed Steel Frames

Bolted moment connections for cold-formed steel frames are considered very important and complicated items which need in depth studies. This paper studies the behaviour and strength of eave connections for members that consist of back to back cold-formed double channels in portal (right angle) steel frames. Relatively new configuration is suggested to facilitate the construction and reduce the cost of this type of connections. The connection is comprised of two stubs (welded built-up I shape) connected with two head plates, each stub is welded to its head plate, keeping in mind that the rafter stub is tapered I-shape. The web and flanges of cold-formed members are connected to the connection accessories by bolts. Numerical study is carried out using a finite element software package (ANSYS). The analysis accounts for geometric and material nonlinearities, large deformation and Elastic-plastic behaviour. Different parameters such as width to depth ratios and, width to thickness ratios of web and flange for the connection accessories and the cold-formed channels are considered. Different failure modes, including local yielding, flexural-distortional buckling, bearing failure as well as combined local yielding and bearing failures are observed. Moreover, moment-rotation relationships for this type of connection are deduced. The rigidity of this connection type is compared with EC3 classifications and generally it can be classified as a semi-rigid connectio

Effect of hydrogen charging conditions on the hardness of tin brass heat exchanger tube

Tin brass heat exchanger tube was hydrogen charged under different cathodic charging conditions. The introduction of charged hydrogen into tin brass tube was found to induce hardening on its surface. The severity and the depth of the hardened region was observed to increase with either cathodic current density or charging time. Ageing after charging results in either complete or partial recovery of hardness, depending on the charging conditions applied to heat exchanger tube

Cathodic polarization behavior of the structural steel wires under different prestressing conditions

Cold-drawn structural steel wires prestressed to different levels and cathodically polarized at various potentials were investigated using electrochemical techniques and slow strain rate tests. The potentiodynamic polarization revealed that prestressing enhances the active anodic dissolution of the structural steel wires. The corrosion current density and corrosion potential were observed to vary with prestressing levels applied to the structural steel wire specimens. The structural steel wire prestressed to 80% of its original tensile strength and cathodically polarized at −1500 mV exhibited the highest current densities and lowest corrosion potentials after potentiodynamic polarization, which indicate that the above prestressing and cathodic polarization conditions lower the corrosion resistance of the material. Moreover, the tensile results show that the structural steel wire prestressed to higher levels and cathodically polarized at lower potentials was more susceptible to degradation of the tensile properties. The structural steel wire prestressed to 80% level and cathodically polarized at a potential of −1500 mV exhibits the lowest UTS and ductility. The tensile fracture surfaces of the steel wires prestressed and cathodically polarized under above conditions exhibit mostly quasi-cleavage brittle fracture character. Furthermore, the brittle regions were observed to increase with increasing the prestressing levels and decreasing the cathodic polarization potentials applied to the structural steel wires. Keywords: Structural steel wires, Cathodic polarization, Prestressin