DE1796230A1 - Process for the production of coatings from metal oxides and ceramic materials on metals, in particular on iron and steel, as protection against chemical changes to the surface caused by the action of non-metallic attack agents at high temperatures - Google Patents

Description

Process for the production of coatings from metal oxides and ceramic Substances on metals, especially iron and steel, as protection against chemical agents Changes to the surface due to the action of non-metallic attack agents high temperatures. It is known to coat metals with oxides or ceramics To produce substances to protect the metals against corrosion and scaling. The coatings are produced, for. B. by forming. Oxide layers by means of the known burnishing process or by treating the metals with superheated Water vapor, whereby e.g. Fe 0 layers are formed in steel. Also water glass solutions with and without mineral additives are used. At higher temperatures, in particular with iron and steel above 570C, decomposition reactions of the oxides occur, which cause the Greatly reduce the value of the cemented oxide coatings.

It has now been found that surface layers of metal oxides and metal silicates can be produced in a particularly advantageous manner. can, if you coat the metals by dipping or spraying with a dispersion prepared by finely grinding nepheline with water and at the same time adding the metal oxide of the metal to be protected to the dispersion, which corresponds to the highest oxidation level of the scale product of the metal in question in an oxidizing atmosphere, e.g. in case of Metals which can be protected according to the present invention are all metals and their alloys which, unprotected under the conditions usually present in technology, for example in firing processes and other reaction procedures with an excess of oxygen in the reaction gas, form scale systems with multiphase cover layers. The oxide or, in the case of alloys, the mixed oxide of the highest oxidation level of the scale product is used in each case. In the presence of nepheline, the metallic surface does not have a reducing effect on the metal oxides in such a way that the lowest oxidation level, i.e. the area of existence of the FeO phase in the case of iron oxidation, is reached. Since the high rate of oxidation of iron and unalloyed steels above 570 C is due to the rapid rate of formation of the FeO phase, the presence of nepheline greatly reduces the dangerous formation of FeO and thus the scaling. Instead of nepheline, nepheline syenite can also be used, which has a higher Si0 content than nepheline, which can be expressed schematically by the formula x NaAlSi04.y S'02. However, dispersions of nepheline syenite must be added to sodium silicate solutions, preferably sodium metasilicate solutions, in order to obtain the described effect of the nepheline.

The nepheline layers produced from these dispersions continue to adhere particularly firm after baking.

Nepheline layers, which are made from dispersions without the addition of metal oxides, have a permeability for carburizing gases such. B. for C0, but hold back other gases such as C02. They act as a molecular sieve. Another embodiment of the invention consists in protecting metals against internal oxidation. During the oxidation of copper alloys, for example, not only a layer of scale arises on the alloy, which, depending on the oxygen partial pressure, consists essentially of Cu 20 or of Cu 0 with a thin outer layer of CuO, and which we call the outer scale zone, but also an oxidation zone inside the alloy, which we call the inner oxidation zone. The reason for the occurrence of such an inner oxidation zone is the @ solubility of oxygen in the alloy phase and the higher work of formation of the oxides of the base alloy components, which continuously consume oxygen and thus due to the high chemical potential gradient of oxygen between the phase boundaries. outer scale layer and alloy / inner oxidation zone cause a continuous supply of oxygen, which is caused by dissociation internal oxidation.

A special form of internal oxidation that occurs in the Carburization of case-hardened steel occurs, is the so-called edge oxidation in the carburization layer of steel.

For this, too, the oxygen is made available through a dissociation of the oxide, namely through the dissociation: Edge oxidation can occur in all known carburizing processes, in the cyanide salt bath, in powder carburization in the box and also in the gas carbonization furnace. It undesirably changes the strength properties of the steel in the edge zone and, during hardening, the critical cooling rate - by binding the alloying elements with oxygen. As described above, in the presence of nepheline, the lowest oxidation level at the alloy / outer layer phase boundary is not sufficient and FeO, which could dissociate, is not formed. The active oxygen that could diffuse into the alloy is therefore missing. To protect case-hardened steel against edge oxidation, a nepheline layer is used, which is made from a dispersion without metal oxide additives. Since the process takes place in a reducing atmosphere, there is no need for metal oxide additives. Another advantage of carburizing steel with a nepheline coating is the activation of the carburizing process! as a result of preventing the formation of thin passive layers of Fe0.

It was further found that nepheline layers, which are made from nepheline syenite dispersions are made with additives from alkali silicates, a good primer for isolating agents that are used in carburizing to attach the steel surface to the To protect places against carburization that are not covered by the case hardening should. Such isolating agents are compounds of metals that are nobler than Iron, e.g. copper and lead compounds. Be in the reducing carburizing medium these connections are slightly reduced to the metal that covers the surface and thus excludes the diffusion of carbon.

It has proven to be particularly advantageous to use the insulating means mentioned deja add nepheline syenite dispersions and the z. @ insulating parts of the To coat the surface with it. ' For the salt bath carburization with cyanide melts have not yet been found useful isolating agents, because in molten salts the isolating agents are dissolved and the connections are dissolved noble metals adversely affect the carburizing baths. Nepheline is cyanidic Salt bath not dissolved. In the sealing of the primer made of nepheline with chromium oxide Cr 0 has been found to be a useful insulator in salt baths. GEMA of the invention a nepheline dispersion mixed with chromium oxide Cr 0 is used as an insulating agent in cyanidi @ ehen molten salt used.

Application examples. 1. Scaling protection of steel at 1000 ° C in oxidizing The atmosphere: Composition example for the dispersion Nepheline Syenite 28 Iron oxide Fe203 18 Sodium metasilicate Na 2Si03.5H20 18% Water 36 2. Insulation of surfaces that are partially exposed carburization in the cyanide salt bath against carburization protect are: Composition examples for the dispersion Nepheline Syenite 30 Chromium oxide Cr 203 15 Sodium metasilicate Na 2Si03.5H20 20% Water 35 3. Protection against edge oxidation of case-hardening steels the carburization. Composition examples for the dispersion Nepheline syenite 40 f Sodium metasilicate Na 2Si03.5H20 15 Water 45% Nepheline and 2? Epheline syenites are alkali aluminosilicates in which the ratio of alkali to alumina is R 0: A1 @ 0 = 1: 1. Other alkali aluminosilicates can also be used with the same ratio of alkali to alumina, although Tepheline is preferable because of its low SiOa content.

Claims (1)

Claim: Process for the production of coatings from metal oxides and ceramic materials on metals, especially iron and steel, as protection against chemical changes of the surface due to the action of non-metallic attack agents at, high temperatures characterized by a) that metals and alloys against internal and external oxidation in an oxidizing atmosphere with an aqueous alkaline one Dispersion can be coated by dipping, spraying or painting, the alkali aluminosilicates and the oxide of the highest oxidation state of the natural scale product of the one to be protected Contains metal at the same time and is dried on the metal surface. b) that iron and steel for protection against edge oxidation in the carburizing atmosphere be coated with an aqueous, alkaline dispersion made from nepheline in water or from nepheline syenite in alkali silicate solution and on the surface dried to form a layer that is permeable to CO and activates the carburization will. c) that iron and steel for partial protection against carburization in the carburization atmosphere solid and gaseous carbonants with an aqueous alkaline dispersion The alkali aluminosilicates and oxides of metals that are partially coated are nobler than iron, contains and is dried on the surface. d) that Iron and steel for partial protection against carburization in cyanide salt baths are partially coated with an aqueous alkaline dispersion, the alkali aluminosilicates and chromium oxide Cr 203 and is dried on the surface.