Der Anstrich von NE-Metallen und Überzügen

Coting of non-ferrous metals and platings The base of the coatings of non-ferrous metals and platings may consist, copper, magnesium, Zinc and tin as well as their alloys. The durability of the coatings depends essentially on a structure well suited for coating, cleaning and rust-protection, a careful pre-treatment of the base, or special primers must be adapted to the metal concerned.     

Korrosionsprozesse durch und unter organischen Überzügen

Corrosion proceses through and under organic coatings Areas of metal covered with a sound protective organic coating can serve as cathode to corrosion process occuring at areas of metal exposure. Corrosion currents are carried through the organic coating by ion transport. Factors favouring ion transport result in less resistant coatings. Increase in coating thickness sharply increase the resistance to corrosion currents. Breakdown of the protective system is associated with some form of coating detachment. Of particular concern is an underfilm corrosion process which proceeds from a point of origin and in which the surface metal at the periphery of the growing, detached spot serves as anode. Such corrosion is stimulated by reduction processes taking place directly through the organic coating or at other exposed metals. For tinplate food cans, the tin coating constitutes a permanent anodic surface phase, and the anodic untermining process consists of dissolution of tin from between the steel and the organic coating. For organic- coated aluminium there is no permanent anodic surface phase. The metal at the extremity of the crevice between the organic coating and the aluminium becomes anodic just as does the base of a pit in aluminium. Anodic untermining corrosion, however, occurs preferentially to pitting.     

Optimaler Korrosionsschutz durch Metallüberzüge mit Anstrich und ihr Einfluß auf die Gestaltung der Stahlbauwerke

Optimum corrosion protection by metal coatings and paint coats and their influence on the design of steel structures Systems consisting of metal coatings + paint or plastic coatings are an efficient and economic corrosion protection for steel structures in aggressive atmospheres or soils. The zinc layer should therefore have at least 80 μm in thickness. Paint coats on top of the zinc layer increase the useful life of the latter, provided, however, they are repaired or renewed in time. Although welding such sheet metal or profiles is still difficult, such materials are being used extensively for parts under static load. Their use for parts under dynamic load presupposes a favourable result of fatigue tests which are still in progress. Suitable protection systems for various conditions and sheet thicknesses are listed.     

Inkorporation kohlenstoffhaltiger Fremdstoffe bei der galvanischen Abscheidung von Gold- und Goldlegierungsüberzügen in sauren und alkalischen Cyanidbädern

Incorporation of carbon containing matter during electrolytic deposition of gold and gold alloy coatings from acid and alcaline cyanide baths The incorporation of carbon containing foreign matter may have a strong deleterious influence on the use of noble metal coating e.g. for electrical contacts. The process is a complex one and is governed by changes in electrolyte composition as well as by the differences between the mean electrolyte composition and the composition of the cathodic diffusion layer. In the latter layer reactions may take place which cannot be found at the mean electrolyte composition. From weakly acidic baths only cyanide ions are incorporated, while the incorporation of citrate ions has not been detected. The oxidation of the cyanide at the insoluble anodes as well as polymerization processes have not bearing on incorporation. When the bath contains strong complexing agents for Ni and Co the incorporation depends from the equilibrium between complex cyano-aurates and weakly bonded or free Ni and Co. In alcaline cyanide baths for silver deposition incorporation tends to a stationary limit, irrespective of eventual polymerization reactions at the insoluable cathode.     

Über den Reaktionsmechanismus und die Wirkungsweise von Komplex- und Chelatbildnern bei der Herstellung von Konversionsüberzügen aus Phosphaten und Oxalaten

The reaction mechanism and mode of action of chelating and complexing agents during the preparation of conversion coatings of phosphates and oxalates Complexing and chelating agents act as anodic depolarizers of the primary pickling reaction in phosphating and oxalating solutions. It has been shown on the example of the accelerating effect of fluorides which form soluble and little dissociated ferrous cryolites that fluorides have a double function in zinc phosphate solutions. They dissolve iron at a rate exceeding by a factor of at least ten the rate in any other acid and on the other hand bind and mask the ferrous ions formed, so that complex compounds of the ferrous cryolite type are formed. Reaction with phosphoric acid and the phosphate ions of zinc phosphate in the phosphating bath yields vivianite, phosphophyllite and hopeite; these poorly soluble orthophosphates constitute the phosphate layers and during their formation free hydrofluoric acid is regenerated. This acid thus acts similiar to a catalyst. Other complexing and chelating agents such as pyrophosphates or polyphosphates act in the same way. A reaction mechanism of this same type is at the base of the formation of oxalate coating in the presence of activating fluorides. Optimum reaction conditions for the formation of the coating are simultaneous cathodic and anodic depolarizatioin (by the use of oxidising agents) and the presence of complexing and chelating agents (as accelerators, grains refining agents and activators).     

Zink in Korrosionsschutzüberzügen

Zink in anti-corrosion coatings Zinc in anti-corrosion coatings is being increasingly used for hot-dip galvanising, electrolytic galvanisation, metal spraying and zinc dust coatings. The author discusses the reasons for this development and the experience gathered with zinc in anti-corrosion coatings. Mention is made of certain specific problems such as the duplex system consisting of hot-dip galvanisation plus coating of freshly hot-dip galvanised surfaces; the question of coatings for insulating materials made of hot-dip galvanised sheeting; the relationship between service life and the thickness of the coating; the penetration of zinc flower with varnishings, etc.     

Elektrochemische Untersuchungen zur Korrosion von Nickelüberzügen

Electrochemical investigations into the corrosion of nickel coatings The corrosion behaviour of pure rolled sheet nickel, electrodeposited nickel and electroless Ni-P-alloy-coatings is investigated by electrochemical methods. Some mechanisms of corrosion and passivation in 0,1 N H₂SO₄ and deionated water - both with addition of several anions - are described.     

Korrosionsbeständigkeit von plasmagespritzten Aluminiumoxid- und Chromoxidüberzügen. Wirkung von NiCr und NiAl als Haftgrund

Corrosion resistance of plasma sprayed aluminia and chromia coatings. Effect of coating sublayers NiCr and NiAl The corrosion resistance of plasma sprayed Al₂O₃ and Cr₂O₃ coatings has been studied in 3.5% NaCl and 10% NaOH solutions. In this context the effect of intermediate coatings (NiCr and NiAl) on the protective efficiency of the ceramic coatings has been evaluated, too. The corrosion rates were determined by gravimetry, corrosion potential and polarisation resistance measurements. The two ceramic coatings afford efficient protection. In the alkaline solutions NiCr is superior, while in the chloride solution NiAl offers better protection.     

Prüfung von Zinküberzügen mit dem Elektronenmikroskop

Testing of zinc coatings by means of the electron microscope Glossy and dull zinc coatings have been compared by means of electron-microscopic photographs of the surface and of the cross-sectional polish of the coating. The surface of the glossy coating has a lamellar structure. An apparent FT-UD transformation of the coating texture was due to the influence of the glossing agent.     

Untersuchung der Eigenschaften von nach einem neuen Verfahren hergestellten Phosphatüberzügen

Investigation into the properties of phosphate layers obtained according layers obtained according to a new process Phosphate coatings were obtained by anodic phosphatizing in sodium phosphate solutions (2.5-5% phosphate, PH 4.2-5.4, current density 2.5-5.0m Amps/cm²). The coatings (coating weight 7.3–20 g/cm₂) consist of ferrous/ferric phosphates, the Fe²⁺: Fe³⁺ ratio depending from bath composition. he hardness of the coatings is lower than the hardness of coatings from Zn and Mn phosphate baths and their absorption capacities for chromic acid, mineral oil and paints are higher because of their fine crystalline structure. With respect to corrosion behaviour they are superior in NaCl and equivalent to Zn and Mn phosphate coatings in moist atmospheres, while the latter seem to be superior in the salt spray seem to be superior in the salt spray test. The minimum coating thickness required for subsequent painting is 8/m² subsequent oil impregnation or cold working require higher weight.     

Eine universelle Einrichtung für die Prüfung der atmosphärischen Korrosion von Metallen und Schutzüberzügen

Universal accelerated weathering cabinet for weathering tests of metals and protective coatings The authors have developed a accelerated weathering cabinet with visual observation of specimens during the test. The temperature can be controlled between 0 and 50°C by a current of warm air and a thermostat, while a humidification device enables the relative humidity to be controlled between 50 and 95%. Aggressive additions such as sulphur dioxide, aerosols, salt spray etc. are added through a spray system. Sulphur dioxide concentrations can be controlled between 0.1 and 10 ppm and 1 to 100 ppm respectively.     

Lochfraß- und Kontaktkorrosion an magnesiumumhülten Spaltstoffelementen

Pitting and contact corrosion in magnesium-canned fuel rods When nuclear fuel elements — containing Mg, graphite and Zircaloy — are stored in water there is the danger of contact corrosion of Mg in contact wit graphite; in partcular when cholorides are present (even in traces) there exists the danger of pitting corrosion. Only at pH values > 11,5 a bydroxide layer is formed which yields a certain protection. The pitting potential in alkaline waters is shifted to more negative values with incrasing chloride contents; this displacement can be prevented by inhiitors. Most efficient is the addition of sodium carbonate which gives rise to the formation of a protective hydromagnesite layer.     

Untersuchungen über die Bildung der Zink-Phosphatüberzüge und ihre Korrosionsschutzeigenschaften

A study into the formation and the protective properties of zinc-phosphate coatings The rupture potential of phosphate coatings increases with the duration of the treatment; this means that extended phosphatation gives rise to an increase of the corrosion resistance. This improvement can be attributed to the hopeite phase. The protective efficiency can be characterized in terms of a coefficient corresponding the ration (dissolved iron) : (zinc-phosphate quantity formed). Phosphate coatings equal in quantity but characterized by higher values of this ratio contain more hopeite and are consequently more resistant.