DE1256993B - Process for applying a chromide coating by contact deposition with, if necessary, additional external EMF on metal bodies - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

EUCHES

PATENT OFFICE

EDITORIAL

Int. Cl .:

C23b

German KI .: 48 a -5/60

Number: 1 256 993

File number: G 37195 VIb / 48a

Filing date: March 4, 1963

Opened on: December 21, 1967

The invention relates to a method for applying a chromide coating by contact deposition with optionally additional external EMF on metal bodies with a melting point of at least 700 ° C and consisting of at least 50 mol percent of at least one metal with the atomic number 26 to 29, 42, 44 to 47 or 74 to 79 containing using a chromium salt molten salt bath at least 700 ⁰ C to below the melting temperature of the metal body in an oxygen-free atmosphere.

A method is already known for applying a chromide coating to steel with a carbon content of 0.10 to 0.20%, in which the steel is embedded in a powder mixture of chromium and aluminum oxide and in a dry hydrogen atmosphere for 3 to 4 hours to 1300 to 1400 ° C is heated. The thickness of the chromide coating depends on the temperature and the duration of the heating .

Further, where a mixture of hydrogen and hydrogen chloride is passed over heated to 900 ⁰ C chromium or ferrochromium to a method for applying a Chromidüberzuges on steel with a carbon content of less than 0.1 ° / ₀ is already known, in which chromium (II) - Chloride is formed, which reacts with iron to form ferric chloride and chromium, which is deposited on the iron. At 900 ° C, however, only very thin chromide coatings are obtained because the reaction is very slow. Higher temperatures are required to produce thicker chromide coatings.

There are also already known methods for depositing chromium on objects made of iron or steel, in which the objects ^{are immersed in a molten salt bath containing a chromium salt at a temperature of 700 to 1200 0} C until between iron and Chromium taking place displacement reaction has formed a chromium coating. If necessary, with these known methods, a direct voltage can also be applied in order to obtain a surface consisting only of chrome.

The chromizing process in which chromium is deposited due to a displacement reaction are not suitable for precisely machined workpieces because of the displacement reaction that takes place the dimensions of the workpieces would change.

Furthermore, phase changes and warping of the workpieces occur at the high temperatures required in the known methods, which is a process for application
a chromide coating
Contact deposition with if necessary
additional external emf on metal bodies

Applicant:

General Electric Company,

Schenectady, N. Y. (V. St. A.)

Representative:

Dipl.-Ing. M. Licht, patent attorney,

Munich 2, Theresienstr. 33

Named as inventor:
Newell Choice Cook,
Schenectady, NY (V. St. A.)

Claimed priority:

V. St. v. America March 5, 1962 (177 196)

is of course undesirable for precisely machined workpieces.

It is also already known to convert iron objects by electrolyzing sodium or potassium fluoride into Presence of sodium silicate using a soluble chromium compound or chromium alloy or to chrome-plate carbon yourself and then coat the precipitated chrome on the objects to melt by heating. However, most of the chromium is in the form of dendritic crystals deposited, which adhere poorly, so that a deficient chromide coating results.

The invention is now based on the object of creating a method, du <xh das | an even, well-adhering, tough, corrosion-resistant chromide coating can be applied.

This object is solved by a method for applying a Chromidüberzuges by contact deposition with optionally additional external emf on metal bodies, having a melting point of at least 700 ⁰ C and consisting of at least 50 mole percent of at least one metal of atomic number 26 to 29, 42, 44 to 47 or 74 to 79 under

709 709/395

Using a molten salt bath containing a chromium salt with at least 700 ⁰ C to below the melting temperature of the metal body in a sauerstofTfreien atmosphere. According to the invention, this method is characterized in that a molten bath of at least one alkali metal fluoride and 0.5 to 50 mol percent, especially 1 to 5 mol percent, of at least one alkali metal chromium fluoride is used and a chromium anode is used at a current density of up to 1 A / dm ² .

Preferably, the salt bath is maintained at a temperature of 900 to 1000 ⁰ C. At lower temperatures there is a risk that chromium will deposit on the surface of the metal body and not diffuse into the metal body to form chromide.

Any alkali metal fluoride and any alkali metal chromofluoride can be used to produce the molten bath be used. Since to avoid dimensional changes of the metal body to be coated a As low a temperature as possible is desirable, a mixture of more than one alkali metal fluoride or more than one alkali metal chromofluoride for a molten bath with a suitably low melting temperature to be used.

Under the deposition conditions, the alkali metal chromofluoride is in chemical equilibrium with the chromium fluoride and the corresponding alkali metal chromofluoride. Although a higher deposition temperature promotes the decomposition of the alkali metal chromofluoride, at temperatures up to 1000 ^° C. the vapor pressure of the chromium fluoride is not yet so high that it evaporates from the molten salt bath.

Because of this equilibrium reaction, it is possible to dissolve the alkali metal chromofluoride of chromofluoride in the molten bath from the alkali metal fluoride to form in situ. Since, however, chromofluoride is not easily available because it is easy to obtain the corresponding Chromium salt oxidizes, it is advisable to add Chromofluorid, which quickly and easily with reacts with the alkali metal fluoride and forms alkali metal chromifluoride, or the readily available potassium chromifluoride to add. Such a chromium salt is then either reduced to the chromium salt by that one lets the bath assume an equilibrium with the metallic chromium or that one that Electrolyzed bath, using a cathode made of metal chips and a chromium anode and the required Sends an amount of electricity through the weld pool until the chromium ions are reduced to chromium ions.

The amount of alkali metal chromofluoride in the bath is 0.5 to 50 mole percent. However, it is desirable a concentration of 1 to 5 mole percent, which is also commonly used. A lot of less than 0.5 mole percent adversely affects the quality of the chromide coating and the rate of formation. On the other hand, a proportion of more than 50 mole percent, which is also uneconomical, causes the partial pressure of the chromofluoride above the bath is increased.

Careful control of the chemical composition of the weld pool is important. Advantageously the salts to be melted should be as free of water and impurities as possible. The role the impurities could not yet be precisely determined. Oxygen has a negative effect, and the method requires an oxygen-free atmosphere, e.g. B. a neutral gas or a vacuum. Metal connections can also impair the quality of the chromide coatings. Salts of one of the reagents required are preferably used Purity. If the process is carried out in a vacuum, then certain impurities escape and harmful substances (e.g. water). It is also desirable to have the surface of the covering metal parts before immersion in the

ίο Clean the weld pool, e.g. B. to pickle what with or can be done without additional grinding.

To achieve a uniform chromide layer,

the chrome electrode in the bath should be at least 6.2 mm, preferably 25 to 50 mm, from the one to be chrome-plated Metal part must be removed. If an exceptionally large piece is chrome-plated, then possibly a Part of being shielded from the chrome electrode, if only one application is used, so that it is advisable to attach two or more chrome electrodes,

ao which are distributed over the dimensions of the object in such a way that a uniform coating is guaranteed will. Optionally, a perforated, conductive container made of neutral material, for example made of graphite or Monel metal can be used

a5 the one in which the chromium is in the form of smaller ones Pieces.

Is the chrome electrode and the metal body to be coated by an external electrical conductor connected, then current flows through the conductor. The chrome electrode acts as an anode and dissolves in the Melt bath so that electrons and chromo ions are created. The electrons migrate through the outer one Conduct the chromo ions through the weld pool to the metal body, where the electrons get the chromo ions reduce to chromium, which diffuses into the metal body. This creates a very smooth, well-adhering, tough and corrosion-resistant chromide coating. The deposition rate can be changed by changing the resistance of the external circuit or by appropriate dimensioning of the bath exposed surface of the metal part can be controlled.

By applying an additional external voltage to the electrical circuit, the rate of deposition can be increased without the rate of diffusion of the chromium into the metal body being exceeded. Such an additional voltage should not exceed 0.5 V and is usually between 0.01 and 0.03 V. The fact that a higher voltage is required can, for example, be due to a higher resistance at any point in the circuit, the presence of contaminants in the bath or due to the surface of the chrome or metal body electrode. Every external voltage must be connected in such a way that the electrical circuit is connected in series, ie that the additional voltage is added algebraically to the self-generated voltage. In order to obtain a coating of high quality, the current density at the cathode should generally not be more than 1 A / dm ² . The diffusion rate of the chromium in the metal part is different and depends on the composition, the operating temperature and the coating thickness to be formed, so that the upper limit of the current density is also variable. For example, the current density should typically 0.05 A / dm ² not exceed a metal, in which the chromium diffusion is low at 800 ⁰ C, and not

be more than 0.1 A / dm ² for a metal in which there is moderate chromium diffusion and ^{not exceed 0.25 A / dm 2} if a metal with good chromium diffusion is used. If the temperature is increased to 1000 ° C, the current density can be increased accordingly and the 4- to 6-fold amount applied at 800 ⁰ C of. Too high a current density usually results in the formation of elemental chromium, either in the form of non-adherent precipitates or in the form of granular or large crystalline precipitates which result in a rough, undesirable coating that easily flakes off during further electrolysis or on cooling to room temperature.

The chromide coating can also be deposited on a metallic surface, which in turn is deposited on another metal surface has been applied, e.g. B. on an iron base, which is electroplated with nickel became.

The method according to the invention can, as a tough, well-adhering and corrosion-resistant chromide coating can be used in a variety of ways. It is used for the production of vessels for chemical reactions, the manufacture of turbine blades for gas and steam turbines, the withstand the corrosive and erosive influences of the gaseous drive medium, the manufacture of gears, bearings or similar objects that require a hard and abrasion-resistant surface and for protecting metals, such as molybdenum and tungsten, against oxidation when they are on elevated temperatures are brought.

The invention can be better understood and explained in more detail with reference to the following exemplary embodiments will be explained, in which Example A is used both for illustration and for comparison.

Example A.

A salt mixture made from 150 g of potassium chromium fluoride and 1600 g of a ternary eutectic of 45 mole percent potassium fluoride, 45 mole percent lithium fluoride and 10 mole percent sodium fluoride was placed in a stainless steel container having a Monel liner and was provided with a glass bell, the two openings for the electrodes and one third for a thermocouple and a connection to the evacuation device. A chrome rod from 27.5 mm long and about 4.5 mm in diameter and an iron strip 18 x 2 x 0.1 cm each attached to a nickel rod. The nickel rods were inserted through rubber tubes into the appropriate Electrode openings inserted airtight. These two electrodes were outside by a suitable Head connected together.

After evacuating the container, the salt mixture was melted by heating to 670 ° C. The electrodes were then lowered so that about 50 mm of each electrode was immersed, thereby a current was generated which fluctuated between 0.5 and 0.8 A at a potential of 0.03 V. Became the external electrical circuit was open, then a voltage of 0.21 V when the circuit was open available. After closing the external circuit again, the current fluctuated between 0.35 and 0.8 A at a voltage of 0.04 V. It was noted in this connection that the current was higher increased than 1 A when the cathode (the iron strip) was shaken, but decreased when the cathode was touched let come to rest.

An external voltage was then applied to increase the voltage to 0.05 V and at the same time the temperature of the bath was increased to 680 ° C. The current that flowed now had a strength of 0.35 A. If the iron cathode was immersed a total of 8 cm deep, the current strength increased 0.5 A. A deeper immersion of the chrome electrode in the salt bath, however, did not cause any noteworthy Increase in current or voltage. If the external circuit was opened again, then set it was found that the voltage was still 0.21V. The external circuit was closed again, but no external voltage applied, then the cell generated a current of 0.5 A at 0.05 V. Up to this point Time had passed about 30 minutes.

The electrodes were then removed from the bath and cooled to room temperature. ao After all adhering molten salt was removed, it was found that the chromium electrode had lost about 0.55 g in weight, while the iron electrode only 1 mg in weight had increased. This result thus proves that »5 there was no noteworthy chrome plating by the weld pool.

These attempts were repeated. Platinum or nickel, niobium, tantalum and molybdenum were used as a cathode and a bath temperature of approximately 700 ° C for 2 hours and 10 minutes and also put an external voltage, so that an average current of 0.3 A in the first 40 minutes and 0.35 to 0.4 A was delivered for the remainder of the time. The niobium and tantalum electrodes were lost in weight, while the weight gain of the other electrodes was only a few milligrams. If chromid formation had occurred, the weight gain should have been approximately 0.6 g. These experiments clearly show that the only reaction which was detectable was reduction of chromium was from chromium to chromium ions and that no noteworthy chromide formation occurs can before the major part of the chromium is not reduced in this way.

The iron cathode was then replaced by a nickel cathode, the salt bath was heated to 700 ° C. and an external voltage was applied which gave a current of about 0.5 A for approximately 16 hours. One Examination of the electrodes showed that the nickel cathode at this point had a coating of chromium in Has the shape of dendrites. The weight of this coating represented only about 9% of the weight loss the chromanode, from which it follows that the essential reaction in this case, too, is the reduction of Chromi to chromo ions existed. At this point, the applied voltage was decreased so that the current was only 0.15 to 0.16 A, and the experiment was continued for a further 24 hours, after which the applied voltage was increased in such a way that the current had a strength of 0.2 A. owned. The experiment was then extended for a further 13 hours. After this time, the Nickel cathode is a thick layer of chromium formed in the form of dendrites, and the chromium inside Schemlzbad had been completely reduced to chromo ions. The resulting bath turned out to be very stable, and the chromium was not oxidized to a chromium compound on storage.

example 1

The molten bath of Example A, in which the chromium had been completely reduced to chromium ions, was placed in an argon atmosphere. A piece of cold rolled steel (15.3 x 1.3 x 0.15 cm), which constituted one of the electrodes was 7.6 cm deeply immersed in the bath, which had been heated to 900 ⁰ C. The other electrode consisted of a chrome rod, which was also immersed in the bath and connected to the outside of the steel electrode. An open circuit voltage of 0.044 V was measured between the electrodes. When the circuit was closed without applying an external voltage, an electrical cell resulted in which the chromium was the anode and the cold-rolled steel was the cathode and which produced a current of 0.06 A at a voltage of 0.022 V. The cell gradually reduced its current output until, after approximately 15 minutes, only a current of 0.015 A at a voltage of 0.004 V could be measured. At this point, an external voltage was applied so that the current was again 0.05 A at a voltage of 0.013 V. Over the course of about 2 hours and 20 minutes, this external voltage was gradually increased to a value of 0.02 V so that the current intensity could be kept at the value of 0.05 A. The current and voltage remained constant for a further 2 hours and 45 minutes. The electrodes were then removed from the salt bath and cooled to room temperature. After washing the cold-rolled steel, it was found that the part of the steel piece which was immersed in the molten bath was provided with a very uniform, corrosion-resistant chromide coating. The thickness of the piece had increased by 0.04 or 0.02 mm on each side, and microscopic examination of the cross-section showed that a diffused chromide layer of 0.046 mm per side had formed. The piece had gained 0.16 g in weight and showed no dendritic chromium deposit on the surface. If the piece was left in the laboratory for 7 months in the open air, the uncoated part rusted, while the part of the surface that was covered with the protective layer of chromide remained unaffected.

Similar results could be obtained with other of the metals listed above. Under the same operating conditions thicker coatings can be formed on some metals in the same time that be classified in the voltage series under the iron, z. B. the coating on nickel is approximately 1.8 times as thick as that on iron, while the coating on platinum is nearly 2.1 times as thick as that on iron.

Example 2

Example 1 was repeated with the difference that a piece of molybdenum (16.7 x 1.3 x 0.15 cm) was used as the cathode. This was immersed 13 cm deep in the molten bath, which was kept at 900 ° C. This cell was operated for 10 minutes, after which time a voltage of 0.025 V was applied so that a current of 0.05 A was achieved. Over the course of approximately 2 hours and minutes, this voltage was gradually reduced so that the voltage was only 0.018 V after the test was complete. After removing it from the bath, the weight of the molybdenum piece had increased by 0.083 g and the thickness on each side by 0.02 or 0.01 mm. Microscopic examination of the cross-section showed that a diffused chromide layer about 0.025 mm thick had formed on each side. Was the piece to 700 to 800 ⁰ C is heated, the non-coated part of the molybdenum oxidized, while the coated with the chromide remained unchanged.

Claims (4)

Patent claims: 1. A method for applying a chromide coating by contact deposition with, if necessary, additional external EMF on metal bodies with a melting point of at least 700 ° C and consisting of at least 50 mol percent of at least one metal with the atomic number 26 to 29, 42, 44 to 47 or 74 to 79 below Use of a molten salt bath containing a chromium salt at at least 700 ⁰ C to below the melting temperature of the metal body in an oxygen-free atmosphere, characterized in that a molten bath of at least one alkali metal fluoride and 0.5 to 50 mol percent, especially 1 to 5 mol percent, at least one Alkali metal chromium fluoride is used and a chromium anode is used at a current density of up to 1 A / dm ² . 2. The method according to claim 1, characterized in that the salt bath is kept at a temperature of 900 to 1000 ^0C . 3. The method according to claim 1 and 2, characterized in that for the production of an oxygen-free Atmosphere an inert gas is used. 4. The method according to claim 1 to 3, with the additional use of an external EMF, thereby characterized in that an external voltage below 0.05 V is applied. Considered publications:
Italian Patent No. 389 035;
Ref. In Chem. Zentralblatt, 1943/11, p. 2640.