Prevention of Corrosion

This invention relates to the prevention of corrosion in, generally aqueous, systems which are liable to come into contact with a corrosible metal surface. More particularly, it relates to compositions and methods of inhibiting corrosion in recirculating and other water systems, especially closed recirculating hot water or cooling systems which contain both ferrous and non-ferrous e.g. aluminium, components.

Corrosion in aqueous systems is caused largely by the oxygen and carbon dioxide which is dissolved in it. A large number of corrosion inhibitors have been added to such systems in order to prevent the dissolved gases from attacking the metal surfaces with which the system comes into contact. Amongst these vegetable tannins have been used; these are believed to act by forming an iron tannate protective film.

In boilers and steam condensate lines corrosion of steel is the principal problem. In contrast, in recirculating water systems, for example in household and industrial central heating systems and in cooling systems used in large diesel engines, it is becomming common to incorporate parts made of aluminium in the system. As a result Galvanic corrosion can also occur. Most known corrosion inhibitors including tannins, are largely ineffective in preventing corrosion of non-ferrous metals, even when used in large amounts. It has surprisingly been found, according to the present invention, that if a tannin is used in conjunction with a water-soluble salt of calcium or other Group II metal corrosion, in particular of non-ferrous metal, especially aluminium components, can be reduced markedly.

It will, of course, be appreciated that on an industrial scale and in larger diesel engines the water used is generally deionised so as to reduce scaling and other forms of fouling due to the presence of, for instance, calcium carbonate as well as silicates, sulphates and phosphates. Accordingly, it would not be appropriate to use ordinary hard water to supply the calcium ions.

According to the present invention there is provided a method of inhibiting corrosion in a, generally aqueous, liquid system which comprises incorporating in the system at least one vegetable tannin or a derivative thereof and a water soluble salt of a metal of Group II (A or B) of the Periodic Table.

Tannins are condensed polyphenolic materials of vegetable origin, optionally substituted with solubilising groups such as sulphonic acid groups. A particular tannin which is suitable for use in the present invention is a sulphited extract of naturally occurring catechol-based vegetable tannins.

Typical water-soluble salts which can be used include chlorides, nitrates and acetates but other salts having a solubility of at least about 30 g/l at 20.degree. C. can also be used. The metals of these salts are typically zinc, magnesium and calcium, calcium being especially preferred.

It is, of course, possible to incorporate into the system, either separately or with one or more of the essential components of the composition, other ingredients such as corrosion inhibitors specific for non-ferrous metals such as copper, for instance azoles, especially benzotriazole, and dispersants, including lignin, as well as oxygen and carbon dioxide scavengers, typically hydroxylamines and neutralising amines. Indeed the presence of these scavengers and of the azoles can enhance the beneficial effect obtained. In general, amounts of the hydroxylamine are similar to those of the tannin while the weight ratio of hydroxylamine: amine (if used) is suitably from 0.001 to 10:1, especially about 1:1. The amount of triazole used is typically 3 to 300, especially 5 to 100, ppm and the amount of each of the hydroxylamine and neutralising amine is typically 10 to 5000, preferably from 500 to 3000, ppm.

The hydroxylamine compounds which may be used in the present invention are those having the formula ##STR1## wherein R.sub.1,R.sub.2 and R.sub.3 independently represent hydrogen, alkyl of 1 to 8 carbon atoms, aryl such as phenyl or tolyl or aryl (lower alkylene) such as benzyl, or a water soluble salt thereof. Typical hydroxylamines which can be used in the present invention include N,N-diethylhydroxylamine (DEHA), which is preferred, N,N-dimethylhydroxylamine, N-propylhydroxylamine, N-butylhydroxylamine, O-pentylhydroxylamine, N,N-dipropylhydroxylamine, N-heptylhydroxylamine, O-ethyl-N,N-diethylhydroxylamine, .alpha. and .beta. benzylhydroxylamine, O-methyl N-propylhydroxylamine, N-octylhydroxylamine, N-methyl-N-propylhydroxylamine and N-hexylhydroxylamine. More than one hydroxylamine can, of course, be used.

The neutralising amine used is a volatile amine well known to those in the art. Such amines are conventionally added to react with dissolved carbon dioxide in the water. Typical such amines which can be used include morpholine, cyclohexylamine, diethylaminoethanol, dimethylpropanolamine, 2-amino-2-methyl-1-propanol, methoxypropylamine, 2-aminobutanol, and benzylamine. Further details are to be found in H H Uhlig, "Corrosion and Corrosion Control", pages 252-253, John Wiley & Sons Inc. (1963). Of these, 2-amino-2-methylpropanol (AMP) is preferred. Of course, mixtures of amines can be used.

A particularly preferred composition comprises DEHA, AMP and benzotriazole together with the tannin and calcium acetate, especially so as to provide in the system about 2000, 2000, 40, 2000 and 20 ppm, respectively i.e. a weight ratio of 100:100:2:100:1.

The components can of course, be added separately to the system or in a mixture of two or more of the components.

Accordingly the present invention also provides an anti-corrosive composition which is a tannin and a water soluble salt of a metal of Group IIA of the Periodic Table. The combination provides a suitable formulation of low toxicity. In general, the weight ratio of the salt, expressed as metal:tannin is from 0.0025 to 0.1:1, more preferably from 0.005 to 0.05:1.

In general, the level of the tannin in the system should be from 10 to 5,000 ppm, preferably from 500 to 3,000 ppm, while the concentration of the metal salt (as metal) is generally from 5 to 500 ppm, preferably from 10 to 100 ppm, and especially from 20 to 30 ppm.

Typically the formulation is dosed at 2% in base exchanged water. Typical concentrations of the ingredients in the formulation are as follows: tannin, hydroxylamine and neutralising amine 0.05 to 25%, especially 2.5 to 15%, triazole 0.015 to 1.5%, especially 0.025 to 0.5% and metal salt (as metal) 0.05 to 5%, especially 0.1 to 1%, preferably 0.15 to 0.3%, all percentages being by weight, active basis. Accordingly the amounts of tannin, hydroxylamine and neutralising amine are roughly equal while the weight ratio of tannin to triazole is generally from 0.1:3 to 5000:3, especially from 5:1 to 600:1. Thus the preferred formulation mentioned above contains about 10% of the tannin, DEHA and AMP, 0.2% of benzotriazole, 0.38% of calcium acetate and 0.5% acetic acid (to adjust the pH to 8.5) (by weight, active basis).

A further advantage of the method and compositions of the present invention is that they are effective in ethylene glycol/water systems which are, of course, known to be particularly corrosive as well as in other alcoholic systems used as functional fluids.

Suitable alcohols for this purpose are saturated aliphatic hydroxy compounds or mixtures thereof, in particular methyl, ethyl, and propyl alcohols and other monohydroxy alcohols as well as dihydroxy, trihydroxy, and other polyhydroxy alcohols such as ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, other alkylene glycols, and glycerol. The alcohol is usually diluted with water to obtain a mixture of the desired freezing point or other functional property. The alcohol generally comprises from about 10 to about 60 percent by weight of the fluid. A water soluble or miscible alcohol having about 1 to 5 carbon atoms such as methyl, ethyl, or propyl alcohol, ethylene glycol, or propylene glycol is preferably employed. The functional fluids may be used, for example, in the internal combustion engines of automobiles and trucks, liquid-cooled aircraft engines, snow-melting systems, refrigeration systems, diesel locomotive engines, automatic sprinkler systems, brake and other hydraulic systems, heating systems, air conditioning systems, and deicing systems.

The following Examples further illustrate the present invention. In these Examples metal coupons were fixed in a glass tube assembly through which water heated to 80.degree. C. was circulated by means of a pump. The assembly formed part of a closed system provided with an expansion tank open to the atmosphere. The specimens were cleaned and weighed before and after the tests and the average corrosion rate in mils per year calculated from the weight loss recorded.

In all these Examples the tannin used was a sulphited extract of naturally occurring catechol-based vegetable tannins.

Widnes mains water treated with a base exchange unit to provide a total hardness of less than 1 mg per liter, M.alkalinity 110 mg per liter was used. The pH of this water was adjusted to 8.5 with acetic acid. The additives specified in Table 1 below were added in the amounts specified and the corrosion rates determined after three days at 80.degree. C.

These results show the effectiveness of the combination of the metal salt and the tannin. Examples 12-14 show the specific nature of the tannin in combining with the metal salt.

These Examples show the effect of varying the concentration of the metal salt and also the effect of using a magnesium salt.

These examples illustrate further the comparison between calcium and magnesium salt, dosed as the chloride, in combination with the tannin.

In these Examples a 1:1 mixture of the base exchanged Widnes mains water as used in the preceding Examples 1-26 and ethylene glycol was used. Again, the pH of the water was adjusted to 8.5 with acetic acid and the calcium was added as its acetate. Table IV below gives the results obtained after three days at 80.degree. C.

In Example 28 the additives were DEHA, AMP and tannin each 2000 mg/l, benzotriazole 40 mg/l and calcium 20 mg/l.

In the Table the letter "P" signifies that pitting corrosion has taken place. Thus these Examples show that the composition and method of the present invention are extremely effective in reducing not only general corrosion but also in stopping pitting corrosion in ethylene glycol/water mixtures.

Examples 27 and 28 were repeated but using 50:50 mixture of the base exchanged water and methanol. The results obtained after 3 days at 70.degree. C. are shown below.

These Examples clearly illustrate the effectiveness of the formulation in reducing corrosion in a functional fluid.