BACKGROUND TO THE INVENTION Hydraulic fluids are widely used to transmit power and operate control valves, for example, in factory applications and in sub-sea systems. In the latter systems hydraulic fluids are used, for example, for the remote operation of valves associated with oil production where the well head is beneath the surface of the sea.

Water-based hydraulic fluids, in addition to water, commonly comprise a carboxylic acid and sufficient alkali to give a pH in the range of 7 to 12.

Where low temperature stability is desired, it is known to add a glycol such as ethane 1,2 diol to depress the freezing point of the fluid.

To this basic system are often added other additives, for example, to improve lubricity and/or to reduce corrosion of metals exposed to the fluid. Examples of water-based hydraulic compositions are disclosed in US patents nos.

4434066,4390440,4390439, and in European patent

application no. 0367080, the entire disclosures of which are incorporated herein by reference for all purposes.

The water-based hydraulic fluids known to the art exhibit limited thermal stability. This is a particular problem for hydraulic fluids which may be exposed to high temperatures for long periods of time, for example, in fluids used for operating control valves associated with sub-sea oil production, where there is a requirement for stability at 180 degrees C for several years.

As an example, fluids containing ethane 1,2 diol as a freezing point depressant are subject to oxidation at high temperatures, the glycol producing acids, for example, glycollic acid. Unless the fluid is buffered against a fall in pH, the acids produced can render the fluid corrosive to metals with which it may come into contact. It is therefore known to add amines to the hydraulic fluid in order to provide the necessary buffering effect.

Typically, therefore, a water-based hydraulic fluid can comprise a carboxylic acid neutralised with an amine, the amine being present in excess to provide a slightly alkaline pH and to buffer the fluid against a fall in pH.

It is also desirable to include a vapour phase corrosion inhibitor (VPI) that provides protection against the

corrosion of ferrous metal parts exposed to the vapour of the fluid, but not actually immersed in the fluid.

Certain amines have also been proposed for this role.

Morpholine has been proposed as an amine for use in hydraulic fluids, but such fluids do not have sufficient thermal stability for many purposes and have an undesirably high rate of corrosion of, for example, aluminium.

European patent application no. 0367080 discloses a water-glycol hydraulic fluid comprising a higher aliphatic acid in a ratio of 2 to 15% by weight, alkali hydroxide in an amount less than an equivalent amount of the higher aliphatic acid, and an organic alkaline compound containing nitrogen atoms in an amount sufficient to adjust the pH of the whole fluid to 10 to 12.

Belgian Patent No. 771150 discloses a hydraulic fluid consisting of water, a glycol, a polyoxyalkyleneglycol, a phosphoric ester of a monohydroxyl polyether, and a nitrogenous organic base. The organic base can be an aliphatic amine, an alkanolamine or a cyclic amine such as morpholine or an N-alkylmorpholine, and is added in an amount sufficient to adjust the pH of the fluid to from 7 to 9. Apart from the pH adjustment, there is no suggestion that the organic base performs any other function. The hydraulic fluids of Belgian Patent No.

771150 suffer from the disadvantage that the phosphate

esters are subject to hydrolysis in aqueous systems, forming acids, which can lower the pH, and increase the risk of corrosion. In addition, the partial phosphate esters disclosed, and more particularly phosphates produced by hydrolysis of the phosphate esters tend to form insoluble precipitates with calcium and magnesium ions that are naturally present in sea water. Many hydraulic systems have very small diameter holes and channels which can be easily obstructed by solids.

Precipitates formed by hydrolysis of the partial phosphate esters can render the hydraulic fluid out of specification.

A further disadvantage of the use of phosphate esters is that the loss of the active constituent (the phosphate ester) by hydrolysis will tend to degrade the lubricity of the hydraulic fluid. This effect is particularly important in hydraulic fluids intended to be suitable for use at high temperatures, as the rate of hydrolysis increases rapidly with temperature.

OBJECT OF THE INVENTION It is apparent from the above that there is a need for a water-based hydraulic fluid having improved long term thermal stability, in particular long term stability to temperatures of the order of 180 degrees C, a low rate of corrosion of metals wetted by the fluid, good VPI activity, and good buffering against pH drop, for example, which may be caused by glycollic acid formation.

BRIEF SUMMARY OF THE INVENTION According to the invention, it has been found that the addition of certain analogues of morpholine can provide hydraulic fluids with substantially improved properties.

In a first aspect, the invention provides the use in a water-based hydraulic fluid of an N-alkyl morpholine or a salt thereof as a vapour phase corrosion inhibitor.

In a second aspect, the invention provides a fluid controlled apparatus wherein there is used a water-based hydraulic fluid comprising an N-alkyl morpholine or a salt thereof as a vapour phase corrosion inhibitor.

In a further aspect, the invention provides a water-based hydraulic fluid comprising an N-alkyl morpholine or a salt thereof and an acid other than a partial phosphate ester, the hydraulic fluid having a pH of 7 or more.

In a still further aspect, the invention provides a fluid controlled apparatus wherein there is used a water-based hydraulic fluid comprising an N-alkyl morpholine or a salt thereof and an acid other than a partial phosphate ester, the hydraulic fluid having a pH or 7 or more.

DETAILED DESCRIPTION OF THE INVENTION Preferred N-alkyl morpholines for use in the present invention are those with alkyl groups comprising from 1

to 10 carbon atoms in the N-alkyl chain, more preferably from one to six carbon atoms, for example, N-methyl morpholine, N-ethyl morpholine and N-propyl morpholine. Substituted N-alkyl morpholines wherein one or more of the methylene groups in the morpholine ring carry alkyl substituents can also be used. These N-alkyl morpholines have been found to provide a substantial improvement over other nitrogenous organic bases such as alkanolamines, in that they have significant volatility, making them excellent vapour phase inhibitors. They also have improved thermal stability and corrosion inhibition properties over morpholine-containing hydraulic fluids.

Preferably the N-alkyl morpholine is present in an amount of greater than 0.1% W/W, more preferably in an amount of from 1 to 15 % W/W.

The hydraulic fluids of the invention preferably also comprise an acid (other than a partial phosphate ester), more preferably an organic acid, and especially a carboxylic acid. Suitable carboxylic acids for use in the invention include, for example, aliphatic and aromatic mono-and di-carboxylic acids. Preferred carboxylic acids are aliphatic carboxylic acids, comprising up to 22 carbon atoms in the alkyl chain, for example, higher aliphatic carboxylic acids comprising 7 to 22 carbon atoms in the alkyl chain for example, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid and

undecanoic acid. Particularly preferred aliphatic carboxylic acids include branched chain aliphatic carboxylic acids, such as, for example, isodecanoic acid.

Other carboxylic acids which can be used include substituted aryl carboxylic acids, such as, for example, benzene N-methylsulphonamido hexanoic acid and toluene sulphonamido hexanoic acid. Preferably the carboxylic acid is present in an amount of from 5 to 15 % W/W. One or more carboxylic acids can be used, as desired.

Suitable salts of the N-alkyl morpholines which can be used include, for example, salts with carboxylic acids, and particularly with aliphatic and aromatic mono-and di-carboxylic acids. Aliphatic carboxylic acids are especially preferred, and very good results have been obtained with aliphate carboxylic acids comprising up to 22 carbon atoms in the alkyl chain, for example, N-methyl morpholine acetate and N-methyl morpholino isodecanoate.

The N-alkyl morpholine salts are preferably present in an amount of from about 5 to 25 % W/W, for example, about 15 %W/W.

For improved low temperature stability, the hydraulic fluids of the invention can comprise a freezing point depressant, for example, a glycol, such as ethane 1,2 diol. The freezing point depressant is preferably present in an amount of from 20 to 60 % W/W, depending on

the temperature performance required, more preferably from 20 to 40 % W/W.

Preferably the hydraulic fluid of the invention has a pH of 7 or greater, more preferably from 7 to 10, more preferably from 7 to 8.5. It has been found that the rate of corrosion of, for example, aluminium, increases at pH values over about 8.5, and for many applications becomes unacceptably high at pH values over 10. A particularly preferred pH is around 8.2.

The pH of the hydraulic fluid can be adjusted by increasing the amount of the N-alkyl morpholine present, but usually it is preferred to add a quantity of a cheaper alkali, for example, an alkali metal hydroxide such as potassium hydroxide. Where the alkali metal hydroxide is added to a salt of the N-methyl morpholine and a carboxylic acid, this can also result in the liberation of the free amine.

Although not essential to the invention, other conventional additives may be included in the hydraulic fluid, for example, other lubricants, corrosion inhibitors, defoaming agents and dyes.

The hydraulic fluids of the invention are preferably substantially free from morpholine and hydroxy alkyl amines.

The invention is illustrated by the following Example: EXAMPLE Fluids were prepared as follows: Composition % W/W Example 1 2 3 4 5 AMP 75 TEA 85 Morpholine NMM NEM Amine 3.8 5.6 2.8 3.4 3.7 Hostacor H Liquid 4.1 4.1 4.1 4.1 4.1 MEG 40 40 40 40 40 Demineralised Water 52.1 52.5 52.2 AMP 75 Amino Methyl propanol 75%-Product of Angus TEA 85 Triethanolamine 85% W/W commercial quality NMM N-methyl morpholine NEM N-ethyl morpholine MEG Ethane 1,2 Diol Hostacor H Benzene, N-methylsulphonamido hexanoic acid (Product of HOECHST-CELANESE, USA) Some properties were determined as follows: Example 1 2 3 4 5 pH 10.2 8.8 9.2 8.1 8.3 Specific gravity (20 C) 1.059 1.068 1.062 1.060 1.060 Vapour Phase x y x x x Corrosion Inhibition Liquid Corrosion Nil Nil Nil Nil Nil x = pass y = fail

The vapour corrosion test is for VPI activity, and is conducted as follows: beat 100mol fluid to 60'C in wide neck conical flask Place clean mild steel plate over mouth and allow to stand for 15 minutes, check for spots of rust every three minutes.

For examples 1,3,4,5 there was no rust after 15 minutes, which is a PASS.

For example 2, rust was evident after three minutes, FAIL.

The liquid corrosion test is the"iron chip"test according to Institution of Petroleum method 287. All fluids passed.

Further corrosion testing was done with a Galvanic Corrosion Analyser.

Tests performed using the BK Technology Galvanic Corrosion Flow Test Cell Analyser using 5Kg fluid at 25*C with a flow rate of 5 litres/minute. Where appropriate the test was conducted in accordance with IP329.

IP = Institute of Petroleum.

Galvanic Corrosion Analyser Example 1 2 3 4 5 ALL Aluminium 3011 1190 629 3.2 1.1 Copper-2222-466-261-46.6-8.6 Brass-1879-371-72.9 10.7-71 Steel-639-383-180-2.3-15 Zinc 624 36.4 56.9 44.9 175 CastIron-877-659-365-8.1-33 Cadmium 26.6 26.6 2.0 2.0 2.0 Couples Aluminium 1584 879 553 0-2.1 Steel-1865-1014-610-26.7-63 Zinc 254 91.3 22.5 40.4 90 Brass-62.2-12.9-25.7 25.7 21 Cast Iron-30.2 10.4 16.3-10.4-8.1 Copper 2.3 2.3 2.3 2.3 2.3 Cadmium 2.0 2.0 2.0 2.0 2.0 Brass 137 6.4 0 32.2 30 Steel 0 9.3 3.5 0-3.5 Cast Iron-47.6-8.1 0-11.6-8.1 Negative numbers imply cathodic protection and correspond to an insignificant or zero rate of corrosion. The positive numbers are corrosion rates in microns per year calculated from the observed anodic current densities.

It can be seen that examples 4,5 show overall lower corrosion rates, particularly on aluminium.

For many purposes, a corrosion rate in this test of less than 100 microns per year is regarded as acceptable, and using this criterion only the N-alkyl morpholines give acceptable rates on all the metals tested.

BUFFERING 100 grams of each fluid was titrated with 70% glycollic acid and the pH determined after each 0.2ml aliquot.

The results are expressed graphically below: pH buffer curves for alkanolamines and morpholine derivatives pH % glycollic acid added

It can be seen that the N-alkyl morpholines provide excellent buffering characteristics, in particular having a relatively low starting pH. Withouth wishing to be bound by any particular theory, it is believed that the low starting pH may explain the low corrosion rate of the fluids of Examples 4 and 5 on aluminium, which is an amphoteric metal.

THERMAL STABILITY The Fluids were tested for thermal stability by being heated in an autoclave at 220'C for 14 days. 220'C was selected as the test temperature to give an acceleration of ageing compared to an operating temperature of 180'C.

Following the period at high temperature the fluid was examined at 20°C for visible change.

Fluid 1. Considerable quantity of liquid insoluble in fluid ie separate phase.

2. Significant quantity of liquid insoluble in fluid.

3. As 2.

4. Very slight opalescence only.

5. Very slight opalescence only.

Fluids 4,5 are considered to have acceptable thermal stability, for exposure up to 180'C and certainly far superior in this respect to fluids 2,3 which themselves are better than fluid 1.

Fluids 1,2,3 are considered to have insufficient thermal stability for operation at 180'C. the invention also provides the possibility of providing a hydraulic fluid for offshore oil applications where the fluid can be discharged into the sea at the control valve (which may be 10-20 kilometres from the platform).

This avoids the need to provide a return line with the consequent loss of power (back pressure) and the cost of the line. A suitable water based hydraulic fluid in accordance with the invention for this application can comprise 15 % W/W N-methyl morpholino isodecanoate, 25-40 % W/W ethane 1,2 diol, and sufficient potassium hydroxide to give a pH of 8.2. In tests, this fluid gave similar results to those of Examples 4 and 5 above.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.