B. Heine

The passivity of iron-chromium alloys

Abstract The composition of the passive film formed in 1 N H 2 SO 4 at 25°C on several iron-chromium alloys (0.6, 1.0, 2.3, 6, 10, 13, 15 and 18 at.%) has been determined using XPS. The total enrichment of chromium in the film can be explained quantitatively for the stationary state by a very strong chromium enrichment in the outermost cation layer of the film due to preferential dissolution of iron ions into the electrolyte and a chromium enrichment in the following layers of the film due to a lower mobility of chromium cations compared to iron cations. This model of a compositional change within the passive film is a direct consequence of the equation of continuity and it is supported by various experimental observations. For example, the removal of a small fraction of the film by sputtering results in a pronounced decrease of the average chromium concentration. The model is able to explain quantitatively the marked decrease of the stationary current density by small chromium additions and the changes of the passivation potential in Fe/Cr alloys. Conclusions of the model concerning other alloying additions to iron such as molybdenum and nickel will be presented.

Compositional changes of passive films due to different transport rates and preferential dissolution

Abstract During the formation of passive films or during instationary periods the fluxes of the different components of a metallic alloy through the passive film into the electrolyte are expected to have divergencies at the metal/film and film/solution interfaces and within the film giving rise to an enrichement or depletion of some of the components. The divergencies are caused by the different mechanisms of transport (i.e. high field transport within the film or charge transfer reactions at the film/solution interface). For the steady state closed solutions can be obtained for the concentrations at the interfaces and within the passive layer. The results of the general model are used to explain experimental findings for iron base alloys, especially FeCr, FeMo and FeAl alloys in 1 N H 2 SO 4 . Predictions for the behavior of nickel and molybdenum in iron base alloys remain speculative due to a lack of reliable experimental results.

Dissolution rates of iron and chromium and FeCralloys in the passive state

Abstract Samples of iron, chromium and FeCralloys were used to determine the dissolution rate in 1 N H 2 SO 4 by Atomic Absorption Spectroscopy (AAS). Special care was taken in reducing the background levels and, therefore, rates could be determined corresponding to 2·10 −8 A/cm 2 . The measured passive current densities in the steady state are in very good agreement with calculated values using the results of the AAS analysis and a valence of 3+ for the chromium and iron ions. The measured fraction of chromium [Cr/(Cr+Fe)] in the electrolyte is equivalent to the chromium fraction in the alloy. The chromium dissolution rate is about the same ((2.4 ± 0.6) · 10 −12 g cm −2 s −1 ) for pure chromium and the alloys with contents as low as 1 at.%. This is considered to be direct evidence that the outermost cation layer of the passive film contains mostly chromium. Before the attainment of steady state the fraction of chromium in the electrolyte is smaller than in the alloy leading to an enrichment of chromium in the passive film. After reaching the steady state the dissolution rates were also measured during galvanostatic transients, where the currents were increased by five times the steady state values. The calculated c.d. using the AAS results were smaller than the total c.d., where the difference is assumed to be used for film formation. The ratio of the c.d. for dissolution and film formation decreases steadily for FeCr alloys by about one order of magnitude from pure iron to pure chromium.

Mechanical behaviour of aircraft relevant aluminium wrought alloys at low temperatures

The mechanical behaviour of the commercial aluminium alloys EN AW-5182, EN AW-6016 and EN AW-7021 is investigated at temperatures ranging from 298 to 77 K and strain rates from 1.7 × 10−3 to 6.6 × 10−2 s−1. A device that allows testing at cryogenic temperatures is developed and demonstrated, where the specimens are subject to uniaxial tensile loads. The influence of a solution heat treatment for precipitation hardenable alloys is shown. The strain-hardening coefficient is determined and mapped in terms of the experimentally investigated uniform elongation. The experimental data of tested aluminium alloys are compared with EN AW-1050A-H14, which is used as a reference. The effect of the Portevin–LeChatelier effect on ductility and strength is discussed. The Ludwik relationship is adapted to describe materials showing a Portevin–LeChatelier effect.

Aluminium Sheet Metal Forming at Low Temperatures

Low-temperature forming technology offers a new potential for forming operations of aluminium wrought alloys which show a limited formability at ambient temperatures. This paper indicates the mechanical behaviour of the commercial aluminium alloys EN AW-5182 and EN AW-6016 at low temperatures. Stress-strain relationships at different temperatures were investigated through tensile testing experiments. Flow curves were extrapolated using an adapted mathematical constitutive relationship of flow stress and strain. A device which allows cupping tests at sub-zero temperatures was specially designed and a limiting dome height was determined.

Werkstoffe und Oberflächenmodifikationen – Schlüssel für eine ökonomische Spritzgießfertigung

Eine okonomische Spritzgiesfertigung umfasst eine kostengunstige Fertigung des Werkzeugs, eine kraftereduzierte Werkzeugbetatigung, eine Ressourcen schonende Produktion mit kurzen Zykluszeiten sowie einen geringen Instandsetzungsbedarf auf Seiten des Werkzeugs. Es zeigt sich, dass kein Werkstoff allen Forderungen gerecht wird. Daher empfiehlt es sich, die Eigenschaften, die von der Beschaffenheit der Werkzeugoberflache bestimmt werden, von einer Beschichtung erbringen zu lassen. Die Volumeneigenschaften dagegen werden vom Grundwerkstoff geleistet. Mit diesem Konzept ist es moglich, ein Leichtbauwerkzeug auszulegen, das uber lange Zeit wartungsfrei betrieben werden kann und geringe Zykluszeiten erlaubt. Es zeigt sich, dass ein Grundkorper aus einer Aluminiumlegierung und die Applikation einer Chemisch-Nickel-Schicht eine zum Ziel fuhrende Losung ist. Materials and surface modifications – Keys to an economical injection moulding An economical injection moulding includes a cost-effective tool production, a free-moving tool, a production which consumes low amounts of resources and runs with short cycle times and last but not least a low need of tool repair all over the life-time of the tool. It can be shown, that no material can satisfy all the mentioned demands. So it is advisable that these properties which are determined by the properties of the tool surface can be performed by a coating. Those properties which are characteristics of the tool volume can be performed by a suitable base material. Following this concept it is possible to configure a light weight tool which is running without maintenance for a long time and makes short cycle times possible. It will be shown that a tool body produced by applying an aluminium alloy finally coated with an electroless applied nickel-plating is one possibility to achieve the aim.