GB1595660A - Corrosion protection of metal surfaces - Google Patents

Description

(54) CORROSION PROTECTION OF METAL SURFACES (71) We, STANDARD TELE PHONES AND CABLES LIMITED, a British Company, of 190 Strand, London, WC2R 1 DU, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to corrosion protection of metal surfaces and in particular to provision of such protection by the thermally induced deposition of aluminium from volatile aluminium compounds.

Corrosion is an ever-present problem in situations where metal surfaces are exposed to air and water. Aluminium alloys for example, such as are employed in water faucet fittings, are particularly susceptible as they cannot in general be passivated by conventional anodisation processes.

Similarly, ferrous metals, although they can be plated with corrosion resistant metals more electronegative than iron, are liable to rapid attack as soon as the coating is damaged, the electrolytic action of the coating accelerating the corrosive action.

Our co-pending application No. 22633/78 (Serial No 1594399) describes a process for the pyrolytic deposition of aluminium from tri-isobutyl aluminium (TIBA), the process including passing a stream of dry, oxygen free, argon or nitrogen, through a quantity of liquid TIBA maintained at a temperature below 90"C, so as to entrain a proportion of TIBA vapour, feeding the argon and entrained TIBA to a reaction chamber containing workpieces to be coated in the aluminium, selectively pulsing dry, oxygen free, argon or nitrogen into the reaction chamber so as to provide a substantially uniform TIBA concentration within the vessel, and maintaining the workpieces at a temperature in the range 250-2700C thereby causing pyrolytic deposition of aluminium thereon.

According to the invention there is provided a process for the provision of a corrosion resistant coating on metal workpieces by the pyrolyic deposition of aluminium from tri-isobutyl aluminium (TIBA), the process including passing a stream of dry, oxygen free, argon or nitrogen through a quantity of liquid TIBA maintained at a temperature below 90"C so as to entrain a proportion of TIBA vapour, feeding the argon and entrained TIBA to a reaction chamber containing said workpieces to be coated, selectively pulsing dry, oxygen free, argon or nitrogen into the reaction vessel so as to provide a substantially uniform TIBA concentration within the chamber, the work-pieces being maintained at a temperature in the range 250270 C thereby causing pyrolytic deposition of aluminium thereon, and anodising at least part of the aluminium coating to form a corrosion resistant surface.

Tri-isobutyl aluminium (TIBA) is known to split at a temperature of around 100"C into isobutylene and di-isobutyl aluminium hydride, a compound with a relatively low vapour pressure. It is thought that the problems involved in the prior art thermal deposition process, i.e. poor quality films and poor reproducibility, are caused by condensation of di-isobutyl aluminium hydride on the inner surfaces and the vapour feed lines of the deposition apparatus. In time this condensate reacts with traces of impurity gases, e.g. water and oxygen. Even though such a layer may be very thin it is thought that its catalytic activity changes part of the TIBA vapours on their way to the deposition reactor resulting in the previously observed inferior film quality.

An embodiment of the invention will now be described with reference to the accompanying drawing which is a schematic view of an apparatus for the thermal deposition of aluminium.

Referring to the drawing, the apparatus includes a reaction vessel 11 in which pyrolytic deposition of aluminium is effected and which is maintained at a temperature in the range 250270 C by a furnace (not shown). Tri-isobutyl aluminium vapour is carried into the reaction vessel 11 in a steady stream of dry, oxygen free, argon or nitrogen which is fed through a bubbler 12 containing liquid TIBA and which is maintained at a temperature of 80--90"C by an oven 13.

Supplies of gases to the apparatus are provided via inlet control valves 14, flowmeters 15 and valves 16. A solenoid operable change-over valve 17 couples selectively via a tube 18 to the reactor 11.

Pipe 19 supplies argon to the bubbler 12 via an expansion chamber 20. In order to reduce the rate of conversion of TIBA into DIBAH, which is probably catalysed by the surfaces of the vapour feed lines of the apparatus, it is necessary to maintain the TIBA vapour temperature below 900C and preferably within the range 84 to 860C. The amount of TIBA fed into the reaction vessel 11 is determined both by the temperature and the flow rate of argon through the bubbler. In the arrangement described an argon flow rate of 7 litres/min at a TIBA temperature of 85"C was found to be suitable.

Pyrolytic deposition of aluminium on to metal work pieces 13 is effected in the reaction vessel 11 which vessel is maintained at a temperature preferably between 250 and 270"C. We have found that at temperatures below 250"C little deposition takes place, whilst at temperatures above 270"C the film quality becomes poor.

While not in operation the apparatus is purged with purified nitrogen fed via the tube 18 to the reaction vessel 11.

Furthermore, through all periods when the reactor 11 is cold or at a reduced temperature, isobutylene is passed through the bubbler at a rate of about one bubble per second in order to convert any DIBAH contamination in the gas lines back into TIBA.

We have found that this technique of purging the apparatus with isobutylene results in superior quality deposited fibres.

It should be noted that when the apparatus is in use the isobutylene purge is turned off.

For deposition the work pieces 13 are loaded through a door 19a at one end of the reactor 11. The door is closed against gasket 20 and the reactor 11 is purged with nitrogen. After the work pieces have had sufficient time to reach the temperature of the reactor the nitrogen purge is switched off and argon is supplied to the bubbler so as to introduce TIBA to the reactor 11.

Uniform distribution of the gas within the reaction is ensured by supplying periodic pulses of dry, oxygen free argon or nitrogen via the pipe 18 and solenoid valve. The valve may be controlled advantageously by a timer 21, a suitable pulse rate being l second on in every 20 seconds. Typical operation conditions are as follows:- Oven temperature 850C Reactor temperature 250--2700C Argon flow rate 7 litre/minute Argon pulse rate 1 second in 20 seconds.

This results in a deposition rate of the order of 0. I microns/minute.

The deposition process is terminated by switching off the carrier gas and purging the reactor with purified nitrogen for a further two minutes.

The work-pieces thus coated are provided with an inherently corrosion inhibiting surface as the aluminium is self passivated by its thin oxide coating. It should be noted that aluminium is less electronegative than conventionally employed metals and will not therefore induce electrolytic corrosion.

In a particularly advantageous application of the process non-anodisable aluminium alloy piece parts, such as are employed in water faucet fittings may be provided with an aluminium coating by the process previously described, which coating is then anodised by a conventional anodisation process. This provides the alloy with an adherent corrosion resistant coating.

WHAT WE CLAIM IS: 1. A process for the provision of a corrosion resistant coating on metal workpieces by the pyrolytic deposition of aluminium from tri-isobutyl aluminium (TIBA), the process including passing a stream of dry, oxygen free, argon or nitrogen through a quantity of liquid TIBA maintained at a temperature below 900C so as to entrain a proportion of TIBA vapour, feeding the argon and entrained TIBA to a reaction chamber containing said workpieces to be coated, selectively pulsing dry, oxygen free, argon or nitrogen into the reaction vessel so as to provide a substantially uniform TIBA concentration within the chamber, the work-pieces being maintained at a temperature in the range 250--270"C thereby causing pyrolytic deposition of aluminium thereon, and anodising at least part of the aluminium coating to form a corrosion resistant surface.

2. A process as claimed in claim 1, and which includes purging the reaction chamber with iso-butylene prior to and after aluminium deposition.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. temperature in the range 250270 C by a furnace (not shown). Tri-isobutyl aluminium vapour is carried into the reaction vessel 11 in a steady stream of dry, oxygen free, argon or nitrogen which is fed through a bubbler 12 containing liquid TIBA and which is maintained at a temperature of 80--90"C by an oven 13. Supplies of gases to the apparatus are provided via inlet control valves 14, flowmeters 15 and valves 16. A solenoid operable change-over valve 17 couples selectively via a tube 18 to the reactor 11. Pipe 19 supplies argon to the bubbler 12 via an expansion chamber 20. In order to reduce the rate of conversion of TIBA into DIBAH, which is probably catalysed by the surfaces of the vapour feed lines of the apparatus, it is necessary to maintain the TIBA vapour temperature below 900C and preferably within the range 84 to 860C. The amount of TIBA fed into the reaction vessel 11 is determined both by the temperature and the flow rate of argon through the bubbler. In the arrangement described an argon flow rate of 7 litres/min at a TIBA temperature of 85"C was found to be suitable. Pyrolytic deposition of aluminium on to metal work pieces 13 is effected in the reaction vessel 11 which vessel is maintained at a temperature preferably between 250 and 270"C. We have found that at temperatures below 250"C little deposition takes place, whilst at temperatures above 270"C the film quality becomes poor. While not in operation the apparatus is purged with purified nitrogen fed via the tube 18 to the reaction vessel 11. Furthermore, through all periods when the reactor 11 is cold or at a reduced temperature, isobutylene is passed through the bubbler at a rate of about one bubble per second in order to convert any DIBAH contamination in the gas lines back into TIBA. We have found that this technique of purging the apparatus with isobutylene results in superior quality deposited fibres. It should be noted that when the apparatus is in use the isobutylene purge is turned off. For deposition the work pieces 13 are loaded through a door 19a at one end of the reactor 11. The door is closed against gasket 20 and the reactor 11 is purged with nitrogen. After the work pieces have had sufficient time to reach the temperature of the reactor the nitrogen purge is switched off and argon is supplied to the bubbler so as to introduce TIBA to the reactor 11. Uniform distribution of the gas within the reaction is ensured by supplying periodic pulses of dry, oxygen free argon or nitrogen via the pipe 18 and solenoid valve. The valve may be controlled advantageously by a timer 21, a suitable pulse rate being l second on in every 20 seconds. Typical operation conditions are as follows:- Oven temperature 850C Reactor temperature 250--2700C Argon flow rate 7 litre/minute Argon pulse rate 1 second in 20 seconds. This results in a deposition rate of the order of 0. I microns/minute. The deposition process is terminated by switching off the carrier gas and purging the reactor with purified nitrogen for a further two minutes. The work-pieces thus coated are provided with an inherently corrosion inhibiting surface as the aluminium is self passivated by its thin oxide coating. It should be noted that aluminium is less electronegative than conventionally employed metals and will not therefore induce electrolytic corrosion. In a particularly advantageous application of the process non-anodisable aluminium alloy piece parts, such as are employed in water faucet fittings may be provided with an aluminium coating by the process previously described, which coating is then anodised by a conventional anodisation process. This provides the alloy with an adherent corrosion resistant coating. WHAT WE CLAIM IS:

1. A process for the provision of a corrosion resistant coating on metal workpieces by the pyrolytic deposition of aluminium from tri-isobutyl aluminium (TIBA), the process including passing a stream of dry, oxygen free, argon or nitrogen through a quantity of liquid TIBA maintained at a temperature below 900C so as to entrain a proportion of TIBA vapour, feeding the argon and entrained TIBA to a reaction chamber containing said workpieces to be coated, selectively pulsing dry, oxygen free, argon or nitrogen into the reaction vessel so as to provide a substantially uniform TIBA concentration within the chamber, the work-pieces being maintained at a temperature in the range 250--270"C thereby causing pyrolytic deposition of aluminium thereon, and anodising at least part of the aluminium coating to form a corrosion resistant surface.

2. A process as claimed in claim 1, and which includes purging the reaction chamber with iso-butylene prior to and after aluminium deposition.

3. A corrosion protection process

substantially as described herein with reference to the accompanying drawing.

4. An aluminium alloy body treated by a process as claimed in claim 1, 2 or 3.