NO801997L - PROCEDURE FOR MAKING PROTECTIVE MOVIES ON A SURFACE OF METAL OR ALLOY - Google Patents

Description

The present invention relates to a method

for the production of protective films on surfaces of metals or alloys.

Constructions of metals or alloys

often used in high temperature reaction systems; in aqueous environments, as reactors in the production of chemicals such as acrylonitrile, phthalic cyanhydride, vinyl chloride monomer and vinyl acetate, as containers for gaseous components under pressure, and as heat exchanger surfaces, e.g. in advanced gas-cooled reactors as well as gas turbine components. Almost inevitably, metal or alloy surfaces exposed to high temperatures, e.g. in thermal cracking processes such as in steam crackers, refining crackers and the like, as well as in distillation units where petroleum feed-

the rial is distilled, be subjected to a coke formation and a charring which results in a limited flow of reaction material and a reduced heat transfer from the metal or alloy surface to the process gases. Structures forward

made of metals or alloys used in aqueous environments will often corrode, especially in heat exchangers.

If metals or alloys are used for the construction of chemical reactors, then often the chemical reactants and their products will be catalyzed so that unwanted secondary products are formed, e.g. by an over-oxidation due to an over-catalytic activity on the reactor walls. When said metals due to alloys are used in containers where gas is under pressure,

then some gases will diffuse through the metal and the alloy, e.g. hydrogen or tritium diffuses through

■steel, nickel or titanium alloys ingest ,■ or produce brittleness

in the metal or alloy. Furthermore: will alloys and metal-

clay used for the manufacture of components in gas turbines is exposed to corrosive forces. When metals and alloys are used to prepare substrates for monolithic catalysts, structures produced from said metals,

and alloys often have problems with regard to the attachment of the catalyst and any catalyst substrate as such when this is used.

Common methods used to reduce

sere coke formation and charring in steam cracking spirals include a steam for treating the surface thereby

to promote a formation of a protective oxide skin. A steam dilution of hydrocarbon vapors has also been used to reduce the level of homogeneous coking. sulfur compound-

se has also been added to the process gases to poison active nickel positions and thereby inhibit coke formation. Nevertheless, the coking rate is very fast under demanding conditions, and driving distances of approx. 30 days has been accepted as normal.

It has been suggested in Japanese Patent No. 4969552 (Furukawa Electric Co.) that coatings resistant to aqueous corrosion can be formed by a . pyrolytic decomposition of tetraisopropyl titanate vapors mixed with a carrier gas in contact with a metal surface, whereby a coating of titanium dioxide is formed on the material. When this technique was used with commercially available titanium compounds and carrier gases, it was found that the coating process was inefficient and that the deposits were irregular and lacked satisfactory adhesion and durability in use.

It is therefore a purpose of the present invention to subject such surfaces to a protective treatment which not only forms attached and permanent coatings, but also increases corrosion; and the carbonization resistance and reduces the harmful catalytic tendency that may be present on the surface, whereby the service life of the surface can be increased without cleaning, without increasing the product yield.

It has now been found that improved protective surface films which are well attached and; smooth, can be formed on the substrate surface by first preoxidizing it and then depositing a metal oxide layer by thermal decomposition of a volatile metal compound.

The present invention thus relates to a method for forming protective films on metal or alloy surfaces which includes first preoxidizing the surface at an elevated temperature and then depositing on the preoxidized surface a film of a high-temperature stable, non-volatile oxide of a metal selected from the group consisting of calcium, magnesium, aluminium, gallium, titanium, zircony<23>" hafnium, tantalum, niobium and chromium by thermal decomposition from the vapor phase of a volatile compound of the metal with at least one metal-oxygen bond in mixture with an inert or an oxidizing carrier gas in contact with the preoxidized surface..

The present method is suitable for protecting surfaces of metals or alloys which include one or more metals from the group consisting of copper, aluminium, titanium, zirconium, tantalum, chromium, cobalt, nickel and iron. Specific examples of alloys that can be protected by

using the present method includes brass, cupronickel, various types of steel, e.g. type 335,

stainless steels such as types 321, 304 and 316 which are all alloys, high-alloy steels such as Incoloys (registered trademark) and AISI 310/HK 40 quality materials, super-alloys such as Inconel (registered trademark), Nimonics (registered trademark) and Zircaloys (registered trademark).

These are alloys that are always used in the construction of reactors and structures that are used in the fields that are

mentioned earlier.

To achieve good adhesion and a uniform oxide film is

it is necessary to preoxidize the substrate surface before thermally decomposing the volatile metal compound which. stands in con-

in step with the surface. Usually, the pre-oxidation of the surface can be achieved by a number of different oxidation treatments, such as flame treatment or heating in an oxidizing atmosphere such as air, CC^ or steam, whereby a thin and well-attached oxide film is obtained, or you can use combinations of different oxidation treatments.'

Of the metals which form protective, high-temperature-stable and non-volatile oxide films according to the present invention, it is preferred to use Al, Ti, Zr, Nb, Ta and Cr.

Metal compounds have at least one metal-oxygen bond which can be thermally decomposed into the metal oxides and the compounds can suitably be selected from the group consisting of metal oxides, metal (3-diketonates and metal carboxylates). Specific examples of volatile compounds in this respect are the ethoxides, normal propoxides , isopropoxides,

and normal and tertiary butoxides and the acet<y>lacetonates.

The volatile metal compound is evaporated into a carrier gas which can either be gases which are inert under the reaction conditions or oxygen-containing gases. Thus • the carrier gas can be selected from the group consisting of nitrogen, helium, argon, carbon dioxide, air or steam. The fact that you can easily use available oxygen-containing gases means that the present method is very flexible. In certain cases, steam cannot be used as a carrier gas, and in these cases it is preferred to use an essentially dry gas as carrier gas, i.e. free of moisture. If necessary, the volatile compounds may be dissolved in a compatible solvent or diluted with a compatible diluent to reduce viscosity. Examples of such solvents or thinners are toluene, methanol, isopropanol and tertiary butanol, and toluene and isopropanol are preferred because they can be easily dried.

It has been found that with certain coordinated unsaturated metal compounds the metal decomposition under anhydrous conditions will give optimum results. Examples of such compounds are aluminum isopropoxide, titanium isopropoxide, titanium-n-butoxide, zircon-n-propoxide, aluminum acetylacetonate, chromium acetylacetonate, tantalum oxide and niobium oxide.

With the expression "under substantially water-free conditions" it is understood that the system does not contain more than 10 ppm moisture. A presence of moisture above 10 ppm can be harmful in such systems, because at the present process temperatures some of the latently coordinated unsaturated compounds of the relevant metals could act as a Lewis acid. Such compounds are consequently susceptible to attack by Lewis bases, and water is one such base. In the system, water or moisture can be present in several places, such as the injection site, in preheaters, in the deposition zone or in the starting materials, etc. In order to thus be able

■bring the moisture content below the desired level of

10 ppm, it is not only necessary that the reactor system be cleaned

with an inert dry gas, preferably the carrier gas, but also

be sure that these gases themselves are really dry. This is important because the preliminary pre-oxidation step

may be carried out in the presence of steam. It is also known that commercial grades of gases such as argon, nitrogen, hydrogen, carbon dioxide etc. can contain more moisture than 10 ppm. Thus, before thermally decomposing some of the coordinated unsaturated metal compounds, the system must be made essentially water-free if one is to be able to prevent a loss in deposition efficiency, irregular deposition conditions and uneven and rough depositions.

The amount of the volatile metal compound

which is necessary to form the protective metallo-

the heat will depend on the nature and thickness of the substrate

the film you want to form. The concentration of the fugitive

the metal compound in the carrier gas will suitably be less than 10.0% volume/volume, preferably between 0.01 and 1.5% volume/volume.

The deposition can take place at temperatures between 20

and 1200°C, but the optimum temperature will depend on.

the type of coating you want. With a suitable choice of metal compounds, the process can be carried out at relatively low temperatures

trips such as .between 200 and 450°C, where special applications require low-temperature depositions or where substrates cannot withstand high temperatures, or at. intermediate temperatures, e.g. in the range from 600 to 800°C when this is appropriate. The deposition can be carried out at atmospheric pressure or at higher or lower pressure.

In some cases, it may be advantageous that

using the present invention, an oxide layer of the same metal is deposited on a substrate surface as - in sub-

street. Suitable examples of this type of combination include titanium oxide on titanium, zirconium oxide on zirconium, and alumina on aluminum. Current procedure

has none of the limitations known from other methods, where either heat treatment in an oxidizing atmosphere or an anodic deposition is used. In the first method, there will be kinetic limitations that affect the thickness of the coating, while the latter method is not suitable for certain geometric shapes, e.g. on the inside of a pipe.

The invention is further illustrated with reference to the following examples.

EXAMPLES 1-9

The surface of tubes (each with an internal diameter of ■ 6.5 mm) of ' the alloys moderate- steel, stainless steel 321 and brass (BS 8-8 5) was preoxidized for one hour in the presence of steam or air at a specified temperature which is reproduced in Table I below. The reactor was then purged with dry nitrogen to remove traces of moisture in the system. Each of the pre-oxidized substrate surfaces with a heating zone of 2 cm was coated with a film of a titanium oxide by thermally decomposing titanium-n-butoxide vapor diluted with isopropanol (75% alkoxide. to 25% for diluent v/v) in a carrier gas (in -containing 0.1% titanium-n-butoxide vapor) under the conditions indicated in Table I below. The reactants and carrier gases were dried over an activated 4 Angstrom molecular sieve to bring the moisture content below 10 ppm. The amount of carrier gas in each case is adjusted so that a residence time of approx. 0.05 seconds in the heating zone at the decomposition temperature. The thickness of the ■protective metal oxide film- . but obtained in each case is shown in Table I. The thickness was measured using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analysis (EDAX) device to determine the elements. A calibration curve for said EDAX was prepared using a pair

■samples where the coating's cross-section was measured both with the help of

by EDAX and by means of SEM.

EXAMPLES 10 - 33

The surface of tubes (each with an internal diameter of 6.5 mm) of stainless steel 321 was preoxidized in the presence of dust at 700°C for one hour. Each of the pre-oxidized substrate surfaces (down a heating zone of between 15 and 20 cm) was coated with a film of a metal oxide by thermally degranulating the corresponding volatile metal compound from its vapor phase using steam or nitrogen as carrier gas under the conditions which is shown in Table II. The flow of carrier

gas was adjusted in each case so that a residence time of approx. 0.05 seconds in the heating zone at the decomposition temperature. The thickness of the protective mixed oxide film is in each case shown in Table II. The thickness was determined using SEM/EDAX together with the calibration curve prepared as described in connection with Examples 1-9. When an oxide other than titanium was used, the thickness value was corrected by multiplying it by the ratio between the oxide densities. The volatile metal compounds used were either liquids or solids at room temperature. Solutions of the solid volatile metal compounds were in some cases prepared by heating the compound in a solvent to promote dissolution. When cooling such . heated solutions, a saturated solution of the volatile metal compound was obtained at room temperature. In cases where the volatile metal compound was a liquid at room temperature, a diluent was sometimes used. The solution and the diluted liquid were then evaporated and mixed with a carrier gas prior to thermal decomposition. The various solutions referred to in Table II had the following concentration of the various volatile compounds.

EXAMPLES 3 4 - 36

The surface of pipes (each with an inner diameter

of 6.5 mm) of mild steel was preoxidized in the presence of steam at 400°C" for 3 hours. Each of the preoxidized substrate surfaces (with a heating zone of between 15 and 20 cm) was then coated with a film of a metal oxide by thermally decomposing the corresponding volatile metal compound from its vapor phase using nitrogen as the carrier gas under the conditions shown in Table III. The flow of carrier gas was in each case adjusted so as to obtain a residence time of about 0.05 seconds in the heating zone at the decomposition temperature. The thickness of the protective mixed oxide film obtained in each case is shown in Table III below. The thickness was determined using SEM/EDAX together with the calibrating structure prepared as described in connection with Examples 1- 9 and corrected as described in connection with Examples 14-33. Although the volatile metal compounds were liquids at room temperature, a diluent was used in some cases. The solution or the fo the diluted liquid was then vaporized and mixed with the carrier gas prior to thermal decomposition. The various solutions mentioned in Table III had the following concentration of the respective volatile compounds.

EXAMPLES 3 4 - 36

Claims (14)

1. Process for the production of protective films on metal or alloy substrate surfaces, characterized by first preoxidizing the surface at an elevated temperature and then depositing on this preoxidized substrate surface a film of a high-temperature stable, non-volatile oxide of a metal selected from the group consisting of calcium, magnesium, aluminium, gallium, titanium, zirconium, hafnium, tantalum, niobium and chromium by thermally decomposing from the vapor phase a volatile compound of the metal with at least one metal-oxygen bond in mixture with an inert or oxidizing carrier gas in contact with the preoxidized surface. 2. Method according to claim 1, characterized in that the surface on which the protective film is formed is a. metal or an alloy consisting of one or more of the metals from the group consisting of copper, aluminium, titanium, zircon, tantalum, chromium, cobalt, nickel and iron. 3. Method according to claim 2, characterized in that the alloy is selected from the group consisting of brasses, cupronickel, mild steel, carbon steel, low-alloyed steel types, stainless steel, high-alloyed steel types, superalloys and zirconium-containing alloys. ■ 4. Method according to any of the preceding claims, characterized in that the preoxidation of the substrate surface is carried out by flame treatment or by heating in an oxidizing atmosphere selected from the group consisting of air, carbon dioxide and steam in order to provide a thin and well attached oxide film. 5. Method according to any of the preceding claims, characterized in that the volatile metal compound has at least one metal-oxygen bond and is capable of thermally decomposing to a metal oxide selected from the group consisting of metal alkoxides, metal 3-diketonates and metal carboxylates. 6. Method according to claim 5, . characterized in that the volatile metal compound is selected from ethoxides, normal propoxides, isopropoxides, normal butoxides, tertiary butoxides and acetylacetonates. 7. Method according to claim 6, characterized in that the eLe-n-volatile metal for the bond is selected from aluminum isopropoxide, titanium isopropoxide, titanium-n-butoxide, zircon-n-propoxide, chromium acetylacetonate, aluminum acetylacetonate, tantalum oxide and niobium oxide. ■ 8. Method according to any one of the preceding claims, characterized in that the carrier gas stream is selected from nitrogen, helium, argon, carbon dioxide, air and steam. 9. Method according to claim 7 or 8, characterized in that the decomposition is carried out under substantially anhydrous conditions. 10. Method according to any one of the preceding claims, characterized in that the volatile metal compound is either dissolved in a compatible solvent or diluted with a compatible diluent. 11. Method according to claim 10, characterized in that the solvent or diluent is selected from the group consisting of toluene, methanol, isopropanol and tertiary butanol. 12. Method according to any one of the preceding claims, characterized in that the concentration of the volatile metal compound in the carrier gas is less than 10% by volume. 13. Method according to any one of the preceding claims, characterized in that the thermal decomposition of the volatile metal compound is carried out at temperatures between 200 and 1200°C. 14. Method according to any one of the preceding claims, characterized in that the protective oxide film deposited on the substrate surface is of the same metal as in the substrate surface.