Korrosionsverhalten ausgewählter Werkstoffe in Lithiumbromid‐Lösung für den Anwendungsfall Absorptionskältemaschinen

Corrosion behavior of selected materials in lithium bromide solution for the application absorption chillers Absorption chillers on the basis of lithium bromide‐water are interesting systems for efficient and energy‐saving supply of coldness. The heating energy for the propulsion of the chiller could result ideally from renewable energies, e.g. solar energy, or waste heating energy from industry. The chiller can work as a multiple‐stage process for increasing the energy efficiency. For the three‐stage‐plants, in the generator temperatures of up to 200–220°C are necessary. The factors high temperature, high halogenide concentration (lithium bromide) and the presence of water vapour lead to a very high corrosive exposure of the materials. In that reason the task of the present report was the consideration of commercial materials and inhibitors concerning their suitability for the application in triple‐effect‐chillers. The measurements were carried out under identical conditions, but in unmoved solution and excluding the contact with air. The result of the investigations showed that stainless steel has been identified as a promising material for this application. Furthermore we have found out that the type of the inhibitor and the interaction of different phases have a strong influence on the corrosion processes.     

Korrosionsprognose für den Bereich der Schweißnaht

Forecasting the corrosion susceptibility in the weld zone Forecasting the corrosion susceptibility of a weld seam requires the knowledge of as much data as possible of the material-electrolyte system. Important data are structural changes in the material as a consequence of heat flow during welding. It is therefore important to know the precipitation conditions in materials intended for use in welded structures. A particularly useful tool in corrosion forecasting is the potential probe curve the basic shapes of which and conclusions to be drawn therefrom are described. Its validity is checked using ship steels welded by different methods and being exposed to seawater. It is shown that less noble portions in the potential probe curve are a necessary but not a sufficient condition for selective corrosion. The curves agree with practical experience only when the welded specimens are exposed to flowing seawater. On the other hand, regular shapes of the curve may be taken for a necessary and also sufficient condition for the impossiblility of selective corrosion in the weld zone. As to steels the variety of possible material/electrode combinations may be divided into two large groups: unalloyed steels in aqueous, aerated and near neutral, and stainless steels in neutral and acid media.     

Die Korrosionsprobleme der Klein- und Mittelständischen Industrie - eine Herausforderung für die Gesellschaft für Korrosionsschutz

Corrosion problems in small and medium-sized companies - a challenge for Gesellschaft für Korrosionsschutz The Gesellschaft für Korrosionsschutz (GfKORR) attaches high priority to solving problems related to materials and corrosion encountered in small and medium-sized firms. In this connection historical trends are presented and existing activity in the fields of corrosion and corrosion protection in various institutions is analysed. It is demonstrated that, compared with large-scale industries, small and medium-sized industries have the greatest recourse to service organizations. Statistics, derived from the work of the Karl-Winnacker-Institut and the Bundesanstalt für Materialforschung und -prüfung, indicate the types and the distribution of corrosion projects. Implications for the future work of the society are given. In particular this entails the promotion of production and product-integrated corrosion protection which is applicable during the developmental stage of processes and products. It is intended that this approach to problems should replace additive corrosion protection which only takes effect when the damage has occurred.     

Richtlinien für Werkstoffauswahl und Korrosionsschutz in der chemischen Industrie

Criteria for the selection of materials and corrosion protection in the chemical industry In chemical technology, the technical requirements to be met by materials as well as the protection measures against corrosion become more differentiated. On the other hand, owing to the technical progress in the sphere of materials number of alternative solutions to cope with these specifications is becoming available. The selection process, which has thus become more difficult, must lead to an overall solution representing the economic optimum; but must not become too expensive in the planning of new plant or in the corrosion protection of existing plant. A distinction is made between, and a brief outline given of, several stages in the material selection process, viz. formulation of requirements; selection methods prior to testing; laboratory tests; utilisation of technical testing plants; tests under operating conditions; determining the correct solution. The economic calculations designed to determine the most favourable technical solution call for numerous technical data (e.g., the corrosion rates under different corrosion conditions) as well as economic data (especially the capital and annual costs depending on the material). The calculation and estimating methods suitable for this purpose are described in detail. In order to reduce the costs incurred through corrosion, money must be spent on certain protective measures. This relationship is illustrated by the phenomenon of cost substitution. Finally, a synopsis is given of all the practicable methods suitable for determining the optimum conditions.     

Struktur-Wirkungs-Prinzipien von Korrosionsinhibitoren für die Rohöl- und Erdgasförderung

Structure efficiency relationships of corrosion inhibitors for oil and gas production Structure efficiency relationships for quarternary ammonium compounds as corrosion inhibitors for oil and gas production were investigated in the system mild steel/aqueous NaCl solution (5 weight%, CO₂ saturated) at T = 298 K and T = 343 K by Electrochemical Impedance Spectroscopy (EIS) and weight loss measurements. In-situ EIS measurements rapidly gave relatively accurate results on differential inhibitor efficiencies. Composition, thickness and inhibitor content of the inhomogeneous 3-D protecting layers formed on highly pure iron surfaces were determined by XPS measurements. The inhibition mechanism of the inhibitor used could be determined by its distribution within the 3-D protecting surface layer and is interpreted in terms of the interphase inhibition concept.     

Die Kinetik der Auflösung von Aluminium in alkalischen Lösungen

The kinetics of the dissolution of aluminium in alcaline solutions The stationary dissolution rates of rotating aluminium discs were measured in alcaline sodium sulfate solutions as function of the Ph- value and of the rotational speed using the Förster eddy current test and electrochemical methods. The rates are controlled by diffusion or by heterogeneous dissolution of aluminate from the passive, oxide-covered aluminium. If the rate is diffusion-controlled, a secondary layer very probably consisting of hydrargillite is formed on the passive aluminium. The rate of the heterogeneous dissolution grows little with the Ph-value and depends strongly upon the electrode potential at unnoble potentials and weakly at noble potentials. Cathodic evolution of hydrogen increases the diffusion-limited as well as the reaction limited dissolution rates because of the excess of hydroxyl ions formed at the surface.     

Elektrochemische Untersuchung von inhibitorhaltigen Lösungen für zeitweiligen Korrosionsschutz

Electro-chemical examination of solutions containing inhibitors for temporary corrosion protection The products of lubricant type hitherto mainly used for temporary corrosion protection are increasingly superseded by aqueous solutions. Potentiostatic examinations have shown that solutions containing ammonium oxalate, sodium oxalate or sodium nitrite — possibly with an admixture of starch or glycerine — are suitable for this purpose. The action mechanism shows fairly great differences. Whilst the inhibiting effect of sodium oxalate is confined to the cathodic reaction, the effect of ammonium oxalate also extends to the anodic reaction. Ammonium oxalate is much more effective than sodium nitrite. It has not yet been possible to establish with certainty whether the protective effect is due to the formation of an oxa-late film, or to the adsorption of the oxalate ions, or to the reaction with oxygen.     

Zur Frage der anodischen potentiostatischen Phasenauflösung von heterogenen Legierungen

Some question concerning the anodic potentiostatic phase dissolution of heterogeneous alloys. The anodic current-potential curves of bismuth and cadmium and their alloys in 0,1 n oxygen free sulfuric acid solution were measured, using potentionstatic potential control, It is shwon that in the case of these heterogeneous alloys one cannot calculate the rate of anodic dissolution by usual addition of both bismuth and cadmiun polarisation curves. The experimental polarisation curve of the alloy exhibits, instead, in wide potential ranges smaller current densities than it could be assumed from simple summing the single Cd and Bi-anodic curves. An explanation of this phenomenon is proposed based on the observed behaviour of the corresponding model electrodes. Then a possibility of two jointly acting factors is discussed. So may, on one side, the developing passive layer on bismuth act limiting on the anodic dissolutionrate of cadmium by screening partly its grains during polarization, On the other side, must in the course of polarization the progressing anodic dissolution of cadmium also result in the still decreasing surface area, occupied by its crystals. It should be rather obvious, that the first factor can be effective immediately after the bismuth passivation and later on in the whole passive potential range. On the other hand, should the second factor be valid also in the lower potential range, but its role would grow with time of polarization.     

Nichteisenmetalle für die Meerestechnik

Non-ferrous metals for ocean engineering Following a comparative survey of the corrosion behaviour of various non-ferrous metals in sea water the author deals with the individual variables (flow rate, water depth, aeration) and the mechanical requirements such metals have to meet. Possible applications of Cu and Ni alloys are listed in this context, reference is made to recently developed alloys: weldable CuNi casting alloys, high strength CuNi alloys for use in sea water at high flow rates, high strength NiCrMo alloy for ropes which even during extended exposure to sea water must not loose their strength nor be susceptible to crevice corrosion.     

Expertensysteme für den Korrosionsschutz

Expert systems for corrosion protection technology Corrosion science is a very interdisciplinary special subject, which involves parts of the classic disciplines chemistry, metallurgy and mechanical engineering. Solving of corrosion problems needs the recognition of relations between the different subjects as well as empirical and heuristical knowledge. These are reasons for the loss of 50 billion DM in Germany caused by corrosion damages. By applying existing corrosion control practices 20% per year could be saved. Corrosion experts are very rarely or it is impossible to obtain the required guidence on corrosion. Expert systems are suitable tools for the mentioned problems. In 1985, members of the Laboratory of Corrosion Protection Technologies at the Fachhochschule Hagen started with the development of the expert system CORROS. The domain of CORROS is the corrosion behaviour of corrosion system water/metallic materials.     

Probenhalterungen für elektrochemische Korrosionsversuche

Specimen holders for electrochemical corrosion tests In order to yield reproducible and reliable results in electrochemical testing the apparatus and in particular the specimen holders must make sure, that the following requirements are fulfilled: defined area; identical distance between specimen and reference electrode; good electrical contact between specimen and cable; no admission of electrolyte to metal surfaces outside the specimen. The autors have designed holders which enable experimental setups to be assembled which meet theses requirements. The types of holders include holders for determining current density/potential curves and holders for galvanic corrosion studies.     

Moderne Strategien (test tailoring) in der Prüftechnik für Korrosionsuntersuchungen in der Mikroelektronik

Modern strategies (test tailoring) of environmental tests for corrosion investigations at micro electronic In former times environmental test techniques and simulations contented a summary of extreme conditions and declared a “worst case environment”. The application of these tests led normaly to unrealistic strong efforts for the test specimen or products. Therefore in modern test techniques was envisaged to use or develop optimized environmental tests for well defined applications. This procedure for the estimation of the optimized test parameters and conditions, also called “test-tailoring”, based on the principle of optimal correlation between test results and real efforts during the product life cycle. This gives the security, that the product withstand the expected environment and not only the test standard.     

Beitrag zur Frage der richtigen Behandlung von Zinkprobenoberflächen für Korrosionsuntersuchungen

The correct treatment of the surface of zinc specimens in corrosion investigations The difficulties are emphasised which may arise in connection with zinc corrosion investigations and may be due to the often observed insufficient reproducibility of the results. On the basis of a series of experiments a suitable surface treatment method for the zinc specimens is proposed which ensure a good repoducibility of the results. This is based on anodic polishing of the corrosion specimen until a top metal layer of suitable thickness has been dissolved – following the normal mechanical pretreatment of the surface. The method is especially recommended for short-time experiments and should also be important in studies of the electrochemical behaviour of zinc.     

Qualifizierung metallischer Werkstoffe für die Eindampfung von Abwässern aus der Rauchgasentschwefelung

Qualification of metallic materials for evaporation of waste water from flue gas desulfurization plants The ecologically-minded processing of waste water from the wet scrubbing of flue gases of coal-fired power plants to produce environmentally acceptable products is carried out in a two-step evaporater operating in closed loop mode. The evaporating process leads to high concentration of chlorides in the two evaporation steps: up to about 100 g/l in the 1st step and up to about 350 g/l in the 2nd step. Therefore in case of metallic design of the evaporation equipment materials of construction with exceptional resistance to chloride induced pitting are required. The corrosion resistance of the high-alloyed stainless steel Alloy 31 (X1NiCrMoCu32-28-7 – UNS N 08031) and of the NiCrMo-alloys Alloy C-276 (NiMo16Cr15W – UNS N 10276) and Alloy 59 (NiCr23Mo16Al – UNS N 06059) including their weldments were to be tested for this application both in the laboratory and in field tests. In addition the behaviour of Alloy 59 heat exchanger tubes had to be determined in field tests under heat-transfer service conditions. The critical pitting corrosion temperatures of the 3 materials after having been GTAW welded under uniform conditions with FM 59 (ERNiCrMo–12) filler were determined in potentiostatic tests in model solutions imitating concentrated waste water products as they may occur in practice, using 5 K temperature intervals. As to be expected the critical corrosion resistance limits of the materials lie at 85 °C at chloride concentrations of 100 g/l Cl⁻ for the Alloy 31 and of 300 g/l Cl⁻ for both the Alloy 59 and the Alloy C–276 respectively. Field tests in waste water evaporation units of flue gas desulfurization plants of coal-fired power stations are carried out as immersion tests with the welded materials and as heat-exchange experiments using longitudinally welded tubes of Alloy 59 (2.4605). The immersion tests over a period of 32 months show the Alloy 31 (1.4562) to be a corrosion resistant construction material for tubes and containers in the first evaporation step, whereas the Alloy 59 (2.4605) and the Alloy C–276 (2.4819) have to be used for the second evaporation step, where the chloride contents are much higher. The Alloy 59 is to value as the most resistant material according to its lower tendency to crevice corrosion. The heat-exchange experiments over a test period of 9 months cause to expect the Alloy 59 to be a suitable construction material for heat-exchanger tubes in both evaporation steps in comparison to graphite which is more succeptible to mechanical destroying.     

Neue Polyolefine für den Apparatebau

New Polyolefines for apparatus I. Polybutene 1 I. Polybutene 1 In comparing its properties with seemingly similar polyolefines, the superiority of polybutene is apparent in three respects: It shows no stress cracking, its creep is very small, and its softening characteristic is favourable. These properties are interpreted with the aid of test results which show considerable advantages compared with other polyolefines. Examples are described of apparatus and piping made of this material. II. Polyolefines of low inflammability The types of high-density polyethylene and polypropylene discussed here are of “low inflammability” as defined in German Standards, DIN 4102, Sheet 3. In this respect, they are, to some extent, comparable to PVC-resin. Their processibility is good and their mechanical, thermal and chemical properties are favourable. For outdoor applications, black polyethylene of low inflammability is suitable because of its high resistance to atmospheric agents. Practical experience now dates back five years. For indoor applications, flame-resistant polypropylene has advantages, e.g. a high thermal stability. Examples of suitability tests and applications are discussed.     

Vorbehandeltes Stahlfeinblech für die Einschichtemaillierung

Pretreated steel sheet for one layer enamelling In order to reduce the amount of work involved in the surface treatment of finished formed parts various pretreated products have been developed which can be used in continous treatment installations. The first step was a nickel coated sheet yielding good enamel adhesion and being suitable for forming operations. The second step in the development was a sheet coated with an enamel frit which can be formed similar to a material with an organic coating. The binder of the enamel frit is then removed by specified heating processes and the frit is fused to give the enamel coat. A methacrylate copolymer curing at 180 to 200°C sheet temperature is used as the vehicle; the dry film thickness is 200 μm, the frit content more than 50 %. Starting at 300 °C the vehicle is depolymerized and the frit melts at 830 °C; the enamel coating thickness is 20 to 150 μ.     

Neue Harze für aggressive Medien

New resins for adverse environments A new family of thermosets have been developed which offer excellent resistance to a range of adverse environments. They have been given the trade name Xylok resins and are the condensation products of aralkyl halides or ethers with phenols and in some instances a range of other aromatic, heterocyclic or organometallic compounds. Composites have been prepared with a wide range of reinforcements, including glass, asbestos and carbon fibres, which are characterised by high mechanical strength and strength retention on prolonged exposure at temperatures up to 25°C. They are, in addition, promising ablative materials and offer high resistance to radiation and attack by many hostile chemicals and solvents. The glasscloth laminates give a good performance as insulating materials under high voltage stress, even after extended ageing at elevated temperatures. Xylok asbestos composites have given encouraging preliminary evaluation results on examination as bearing materials, while Xylok resin moulding powders give the same flow and curing characteristics as phenonolics but the mouldings have significantly better physical properties.     

Zum Verhalten von Natriumsilikat als Korrosionsinhibitor für Aluminium in alkalischen Lösungen

The behaviour of sodium silicate as a corrosion inhibitor for aluminium in alkaline solutions Even at a temperature of 80°C and even in the presence of intensive mechanical stresses on the metal by a sharp spray jet (with a spraying pressure of 30 atmospheres excess pressure at the nozzle), sodium silicate (Na₂O:SiO₂ = 1 : 2,5) is able to prevent corrosion of pure aluminium (99,5 pC) by alkaline solutions (pH 11). For this purpose, very small quantities of adsorbed silicate per unit of area are already sufficient. It is essential, however, that the SiO₂ concentration in the solution does not sink below a certain minimum value, depending on ambient conditions, so that a sufficient quantity of silicate can be adsorbed. With increasing temperatures and spraying pressures, the silicate concentration must be higher if effective corrosion protection is to be attained. The corrosion-preventing film on the metal surface is formed by non-aggregated silicate. Different metals draw different quantities of corrosion inhibitor from the silicate solutions. This behaviour is presumably governed not only by the specific adsorption capacity for silicate ions but also by the corrosion resistance of the different metals in the solution.