GB2104054A - Protective silica coatings - Google Patents

Abstract

This invention relates to a process for depositing a protective silica coating on a preoxidised substrate surface by thermally decomposing an alkoxysilane in contact with the surface at reduced pressure. A carrier gas may optionally be used for vapourising the alkoxysilane.

Description

SPECIFICATION Protective coatings on metallic surfaces The present invention relates to a process for forming protective oxide coatings on metallic surfaces, and especially to protect surfaces exposed to carburising coking and corrosive environments.

Equipment made from various metals and alloys are widely used in industry, where they are exposed to high temperatures and oxidising environments. For instance, equipment used in generating heat and power, such as furnaces, boilers, advanced gas cooled reactors, internal combustion engines, eg diesel engines and gas turbines, and more particularly the equipment used in the combustion of fossil fuels are subject to corrosion, coking and carburisation. If a metal or alloy is employed in any of these uses, whether it be to produce a heat exchanger, a furnace, a boiler or a gas turbine component, it has to withstand a hostile environment, for example high temperatures and oxidising gases, which often results in corrosion, carburisation, coking or ash deposition with a consequent shortening of the life of the equipment and loss of efficiency.In order to improve the resistance of such metals and alloys to such environments, various methods have involved either attempts to minimise the corrosive and oxidative nature of the gases or coating of the surfaces of such alloy substrates with a protective film.

One of the methods used for forming protective coatings on metal or alloy substrate surfaces is claimed and described in our British Patent Specification No. 1483144. According to this method the substrate is first preoxidised and an alkoxy silane is thereafter thermally decomposed from its vapour phase and in a carrier gas in contact with the preoxidised surface to form a silica coating at or above atmospheric pressure.

It has now been found that the performance and efficiency of the alkoxysilane utilisation may be improved if pressures below atmospheric are used during the thermal decomposition of the alkoxysilane for deposition of a protective coating on the substrate surface.

Accordingly, the present invention is a process for forming a protective silica coating on a metallic substrate surface comprising preoxidising the surface and depositing the protective oxide coating by thermal decomposition of an alkoxysilane vapour in contact with the preoxidised surface in a reactor system characterised in that the reactor system after preoxidation is evacuated to bring the pressure below atmospheric pressure and the alkoxysilane vapour is thermally decomposed in contact with the preoxidised surface in the reactor system in which the pressure is maintained below atmospheric pressure.

Examples of alloy substrates which can be protectively coated by the present process include are high alloy steels, low alloy steels and superalloys. Specific examples of these alloys include stainless steel types AISI 304, 310, 316, 321,20/25 niobium steels, 20/25 titanium nitride steels, and the like; stainless steel types AISI 33 5/P2, 335/P5 and 335/P9; and the superalloys such as the Incoloys (Registered Trade Mark) 800, 800H, PE16, lN738 and the low carbon version thereof, IN739, Inconel (Registered Trade Mark) 617 and the Hastelloys (Registered Trade Mark).

The substrate may be cold-worked or machined with residual strain therein or one without any residual strain, it may be suitably annealed at a temperature between 200 and 1 2000C so as to substantially reduce the cold-work strain on the surface thereof. The annealing is suitably carried out in a non-oxidising atmosphere, preferably in vacuum or in the presence of e.g. nitrogen or hydrogen, and most preferably dry nitrogen or dry hydrogen. The annealed substrate is then allowed to cool so as to bring its temperature down to at least 2000C, preferably below 2000C. The cooled substrate surface is thereafter preoxidised by raising its temperature eg from 200-8O00C in the presence of an oxidising gas.

The oxidising gas used for the preoxidation step is suitably carbon dioxide, steam or air, preferably steam. The preoxidation is suitably carried out in a reactor system at atmospheric or sub-atmospheric pressures. If atmospheric pressure is used the system must be evacuated to reduce the pressure in the system to below atmospheric prior to the thermal decomposition of the alkoxysilane in contact with the preoxidised surface. The evacuation step enables the removal of impurities from the reactor system prior to conducting the coating operation.

The substrate surface so pretreated is thereafter ready to receive the protective silica coating. The protective silica coating is thereafter deposited by thermal decomposition of an alkoxysilane from its vapour phase in contact with the substrate surface.

Thus, the alkoxysilanes which may be used to deposit the layer of silica on the substrate surface are suitably mono-, di-, tri- or tetra-alkoxy silanes and the partially hydrolysed or polymerised products thereof. These contain preferably between 1 and 1 5 carbon atoms in the alkoxy group. Of these tetraalkoxy silanes are most suitable and tetra-ethoxy silane is most preferable.

The alkoxysilane vapour may be thermally decomposed to form the protective oxide coating in the absence of any diluent or in the presence of a carrier gas. Examples of carrier gases include gases such as nitrogen, helium and argon which are inert under the reaction conditions, or, oxygenated gases such as carbon dioxide, steam, nitrogen oxides and oxides of sulfur which are mildly oxidising under the reaction conditions. The carrier gas stream may be a mixture of the inert and oxidising gases such as air.

A typical example of such a carrier gas is flue gas. The best mixture will depend on operational constraints, the required rate of coating formation and degree of consumption of the alkoxy silane desired.

The amount of alkoxy silane required for the formation of a layer of silica will depend upon the thickness of the layer required. The concentration of alkoxy silane in carrier gas is suitably less than 50% by volume preferably between 0.05 and 5% by volume. If no carrier gas is used, the concentration of the alkoxysilane will naturally be 100%.

The temperature at which the deposition is carried out may be between 100 and 1 2000C, but it is preferably to carry out the deposition between 250 and 900"C. The deposition is carried out at subatmospheric pressures. This may be achieved by bringing into contact the vapour of the alkoxysilane and the substrate surface to be coated in a reactor system which is maintained under subatmospheric pressures. It is preferable to ensure a constant flow rate of the alkoxysilane through the reactor system in order to achieve a uniform coating of silica on the substrace. The subatmospheric pressure used is suitably between 0.01 and 700 torr, preferably between 0.1 and 200 torr.

Where an oxidising gas such as air, carbon dioxide or steam is used as a carrier gas component, the amount of the oxidising gas required for the deposition may vary between a wide range depending upon the nature of the substrate. It is suitably between 5 and 100% and preferably between 30 and 100% by volume of total carrier gas stream.

The process of the present invention is further illustrated with reference to the following Examples: EXAMPLE 1 Samples to be coated were cleaned in isopropanol in an ultrasonic bath, and then hung in a vertical coating reactor with the following dimensions: Height (total) = 80.0 cm Height (coating zone) = 60.0 cm Diameter (coating zone) = 8.8 cm The reactor was constructed from Inconel 600, and consisted of an outer 'bell' which formed a vacuum housing with an internal 'tower constructed of rings and gauzes from which the samples were hung on nickel wires. The coating gas passed from the top to bottom of the reactor with the inlet tube running the length of the reactor and serving to preheat the gas.

The mode of operation of this reactor was as follows. At the beginning of a run the 'tower' was assembled and the samples hung in place. The outer 'bell' was then fitted and the system checked for leaks by (i) admitting nitrogen at atmospheric pressure, and ensuring that the flow rapidly fell to zero when the reactor outlet was closed and (ii) evacuating the reactor with a rotary vane oil sealed pump to ensure that a good vacuum was achieved (less than 0.1 torr) when nitrogen inlet was shut off. This latter test also served to remove air and contaminants from the system.

The system was then brought up to atmospheric pressure with dry nitrogen and a slow nitrogen flow maintained (1-5 ml/min at atmospheric pressure) while the reactor was then heated to the temperature (approx 200 0 C) for the start of the preoxidation by lowering the hot furnace over it. The steam (approx 1 kg/hr) for preoxidation was then admitted and the nitrogen flow shut off. The reactor was then heated to the final temperature required for preoxidation, which was continued for the specified time at atmospheric pressure.

The steam supply was then shut off, and a slow flow of dry nitrogen maintained (1-2 1/min) while the apparatus was evacuated (using a rotating pump) to remove moisture. Ultimately, the nitrogen purge was shut off and the pressure reduced to less than 0.1 torr to remove final traces of moisture and other impurities. The reactor temperature was adjusted as necessary to that required for coating during this period.

The reactor pressure was then adjusted to that required for coating, using a water ring pump. The coating gas flow was then adjusted as required and dry nitrogen employed as carrier gas., The alkoxysilane was added to the carrier gas in an evaporator which was heated to slightly above the temperature for the saturated vapour pressure required. A reflux condenser with a thermostat was used to finally adjust the concentration. The flows were established while passing the gases to vent, and at the start of the coating period the gas was switched to the reactor through a needle valve which reduced the pressure to that of the reactor.

At the end of each coating period, the evaporator nitrogen supply was shut off, the reactor purged with dry nitrogen, and finally raised to atmospheric pressure and cooled by removing the furnace.

Coatings of silica (Example 1 ,Table 1) were deposited in this way, using as feedstock tetraethylorthosilicate. Conditions and resulting coating rates (estimated using sample weight gain) in each case are detailed in Table 1.

EXAMPLE 2 Similar coatings of silica were produced on a larger coating unit of the type used in Example 1. The dimensions were; Height (total) = 70.0 cm Height (coating zone) = 40.0 cm Diameter (coating zone) = 15.0 cm The method used was the same as that described in Example 1 and the respective conditions and coating rates are detailed in Table 2.

TABLE 1 SILICA COATING RUNS (INCLUDING PREOXIDATION) CONDUCTED UNDER EXAMPLES 1 & 2 INCOLOY 800 SUBSTRATE, PREOXIDISED IN STEAM (ATMOSPHERIC PRESSURE) FOR 1 HOUR AT 850 C

Coating Nominal Gaseous Coating Zone Weight Gain Estimation Run Time Temp Pressure Conc Total Gas Flow Residence Time Internal Temp. of Coating Rate Number (h) C (Torr) (% v/v) (lit/min STP) (sec) ( C) ( m/h) T M B T M B (a) 4 775 25 0.2 11.0 0.19 775 765 600 1.64 1.38 1.34 (b) 4 775 25 0.2 11.0 0.19 772 768 630 0.98 1.15 1.17 T = Top Coating Zone M = Middle Coating Zone B = Bottom Coating Zone TABLE 2 SILICA COATING RUNS (INCLUDING PREOXIDATION) CONDUCTED UNDER EXAMPLES 3 & 4 INCOLOY 800 SUBSTRATE, PREOXIDISED IN STEAM (ATMOSPHERIC PRESSURE) FOR 1 HOUR AT 850 C

Coating Nominal Gaseous Coating Zone Weight Gain Estimation Run Time Temp Pressure Conc Total Gas Flow Residence Time Internal Temp. of Coating Rate Number (h) C (Torr) (% v/v) (lit/min STP) (sec) ( C) ( m/h) T M B T M B (a) 4 760 50 0.2 22 0.37 761 757 763 1.32 1.61 1.26 (b) 3 780 50 0.2 22 0.36 782 778 784 4.23 3.74 3.81 (c) 4 735 100 0.2 22 0.76 739 736 745 1.40 1.30 1.39 (d) 4 760 50 0.2 22 0.37 764 762 770 - 1.28-1.69 (e) 6 760 50 0.2 22 0.37 762 757 766 - 1.16 -1.65 (f) 2 775 50 0.5 22 0.37 777 773 787 - 3.28-4.67 T = Top Coating Zone M = Middle Coating Zone B = Bottom Coating Zone EXAMPLE 3 Samples of annealed Incoloy 800 were ultrasonically cleaned in acetone, weighed and preoxidised in steam at temperatures around 8000C for periods between 11 2 hours.

The preoxidised samples were subsequently weighed and then individually hung in the vertical reactor of a low pressure CVD unit. This unit had the following dimensions: Reactor Height (total) = 53.0 cm Coating Zone length = 10.0 cm Coating Zone diameter = 4.0 cm The coating reactor was fabricated from Inconel 600. This was placed in a resistance heated furnace with reactant gases being introduced via the reactor top and extracted through a tube at the reactor base. Temperature measurement within coating zone of the reactor was carried out by means of mobile thermocouple in a stainless steel thermowell.

The mode of operation of this reactor was as follows.

Preoxidised samples were suspended in the coating zone of the reactor. The system was then purged with nitrogen at atmospheric pressure for about one minute. The system was then evacuated using a rotary vane oil sealed pump to ensure that a good vacuum was achieved (approx 1.0 torr). The nitrogen flow was subsequently re-established at around 1 litre/min at STP and the pump operated to give a reactor pressure of between of between 10100 torr. The furnace was turned on after low pressure conditions were established in the coating reactor. The furnace setting was adjusted to give a sample temperature of around 7800C (+15"C). The system was then left to heat up and equilibrate (typically 30 minutes).

When the coating temperature of 7800C +1 50C was attained tetraethylorthosilicate (TEOS) vapour was admitted into the reactor. This was preheated in a hot water jacketed saturator system and blended with nitrogen which was used as the carrier gas in these experiments. The blend of TEOS in nitrogen at levels of typically 0.250.85% v/v was admitted into the reactor via a system of needle valves. Care was taken to maintain atmospheric pressure conditions within the saturator itself. At the coating temperature (7800 +150C) the TEOS in the 1 litre/min nitrogen carrier gas stream had a residence time in the reactor coating zone of between 3.12 x 1 02 and 1.03 x 10-1 sec (depending upon the coating pressures used). The coating period was typically about 6 hrs.

At the end of the coating period the saturator nitrogen supply was shut off and the system purged with nitrogen and was finally brought to atmospheric pressure. The cooled samples were subsequently removed and weighed to determine the thickness of coating deposited. Coating conditions and resulting coating rates are detailed in Table 3.

TABLE 3 SILICA COATING OPERATIONS CONDUCTED UNDER EXAMPLES 3 INCOLOY 800 SUBSTRATES PREOXIDISED IN STEAM AT 800 C (ATMOSPHERIC PRESSURE FOR 1 HOUR)

Coating Nominal Gaseous Coating Residence Coating Run Time Temp Pressure Conc Total Gas Flow Time Thickness No (h) ( C) (torr) (% v/v) (lit/min STP) (sec x 10-2) ( m) a 6.0 780 26 0.8 1 6.88 10.7 b 6.0 780 approx 85 0.4 2 1.10 10.22 c 6.5 780 25 0.4 2 3.44 4.87 d 6.0 790 20 0.8 1 6.75 14.7 e 6.0 790 26 0.7 1 6.88 11.11 f 6.0 786 12 0.4 1 3.12 8.27

Claims (10)

CLAIMS:

1. A process for forming a protective silica coating on a metallic substrate surface comprising preoxidising the surface and depositing the protective oxide coating by thermal decomposition of an alkoxysilane vapour in contact with the preoxidised surface in a reactor system characterised in that the reactor system after preoxidation is evacuated to bring the pressure below atmospheric pressure and the alkoxysilane vapour is thermally decomposed in contact with the preoxidised surface in the reactor system in which the pressure is maintained below atmospheric pressure.

2. A process according to claim 1 wherein the metallic substrate is selected from high alloy steels, low alloy steels and superalloys.

3. A process according to claim 1 or 2 wherein the substrate surface is preoxidised using an oxidising gas selected from carbon dioxide, steam and air.

4. A process according to any one of the preceding claims wherein the alkoxysilanes is selected from mono-, di-, tri- and tetra-alkoxy silanes, and the partially hydrolysed or polymerised products thereof.

5. A process according to any one of the preceding claims wherein the alkoxysilane vapour is thermally decomposed to form the protective oxide coating in the absence of any diluent or in the presence of a carrier gas.

6. A process according to claim 5 wherein the carrier gas is selected from the gases nitrogen, helium and argon which are inert under the reaction conditions, and the mildly oxidising gases carbon dioxide, steam, nitrogen oxides and oxides of sulphur.

7. A process according to claim 6 wherein the concentration of alkoxy silane in the carrier gas is less than 50% by volume.

8. A process according to any one of the preceding claims wherein the deposition is carried out at a temperature between 100 and 12000C.

9. A process according to any one of the preceding claims wherein a constant flow rate of the alkoxysilane through the reactor system is maintained during the coating process.

10. A process according to any one of the preceding claims wherein the subatmospheric pressure used is between 0.01 and 700 torr.