GB2125833A - Conversion coatings - Google Patents

Abstract

A corrosion resistant coating is deposited onto the surface of an aluminium brass substrate by contacting the surface with a solution which has a pH 5 to 9 and contains, dissolved therein, inorganic peroxide at a concentration of from 0.25 to 30 gl<-1>, measured as H2O2, and a strong complexant for copper at a concentration of at least 0.05 eq. l<-1>. Preferably, the treatment solution also contains a buffer at a concentration of at least 0.015 molar. The invention is particularly useful for treating marine surface condensers and heat exchangers to reduce the risk of corrosive failure of such equipment.

Description

SPECIFICATION Conversion coatings This invention relates to the treatment of metallic substrates to provide enhanced resistance to corrosion. In particular it relates to the provision of films or coatings on aluminium brasses used in marine surface condensers and heat exchangers, which reduce the likelihood of corrosive failure of such equipment.

Sea water is used as a coolant in many heat exchanger and condenser applications. It has the advantage of being readily available in many situations where the use of fresh water is impractical, but has the disadvantage of being relatively highly corrosive.

The term "marine condensers" is used herein to refer to surface heat exchangers generally, including true condensers, where the coolant is sea water including estuarine water which may have varying salinity.

The invention is applicable to all conventional designs of marine condenser. It is especially applicable to the "tube and shell" type where the coolant (sea water) is passed through a series of pipes (the "tubes") within an outer container (the "shell") through which the fluid to be cooled or condensed is passed. Heat exchange occurs through the walls of the pipes. Condensers for marine steam turbines are commonly of this type.

The desire to maximize the cooling power of marine condensers has led to designs using high coolant stream velocities typically up to 2 ms-'.

Because coolant flow is often, at least partly, turbulent actual velocities can be several times higher than this and may locally be as high as 10 ms-'.

During use in clean sea water circulated at moderate velocities, aluminium brass surfaces in contact with the sea water develop a thin protective film over a period of several weeks. The nature of this film is not known with certainty, but it is believed to be of hydrotalcite or similar material and can be described as mixed basic hydroxycarbonates of zinc, magnesium and aluminium. The film may also contain minor proportions of other metals. The mechanism by which the film provides corrosion resistance is not known, but our work suggests that it acts to buffer the substrate against corrosion. This is aided by the low galvanic potential between the metal substrate and the film.However, because the film forms slowly, there is, after initial commissioning or recommissioning after repair or maintenance, a period in which the marine condenser is vulnerable to corrosive attack which can, particularly with high stream velocities, lead to premature failure. This problem is aggravated by the fact that the water used in commissioning marine condensers e.g. in fitting out basins and harbours, is often heavily polluted especially with sulphide. Sulphide can react with the substrate to give copper sulphide which can mechanically inhibit film formation, or cause disruption of the film, and may cause copper to be incorporated into the film in relatively large concentrations. We have found that the presence of copper in the naturally formed film reduces the corrosion resistance of the film.

Even if a sound natural corrosion resistant film is formed it can be mechanically disrupted at high coolant impact velocities. This problem can be met by dosing the coolant with Fe" which precipitates onto the condenser surfaces to form a film which is resistant to mechanical disruption by high coolant impact velocities. This Fe"' containing film has a relatively short life thus requiring periodic dosing of the coolant, typically once per day. The iron is usually added to the coolant supply as ferrous sulphate which is oxidized by dissolved oxygen to Fell.

In practice, corrosive failure of aluminium brass components in marine condensers is not a frequent event when high quality alloys and construction are used. However, individual failures can be very expensive especially if they fall outside planned maintenance schedules. The improvement of the corrosion resistant qualities of aluminium brasses has been the subject of prior work. Thus, treatment with sodium dimethyldithiocarbamate has been proposed. The protection afforded is qualitatively satisfactory but must be monitored with periodic retreatment as necessary. Cupric aluminate coatings have been suggested but our present work indicates that these coatings can promote corrosion ir certain circumstances.

The present invention is based on the discovery that by treatment with an oxidizing treatment solution, it is possible to deposit corrosion resistant films on aluminium brasses. These films can have low copper contents and are very similar in properties to the naturally occurring film.

Further, over a period of contact with sea water the films are converted to a composition virtually identical to that of the naturally occurring film.

The present invention accordingly provides a method of depositing a corrosion resistant coating on the surface of an aluminium brass substrate which comprises contacting the surface with an aqueous treatment solution having a pH of from 5 to 9 and containing dissolved therein inorganic peroxide at a concentration of from 0.25 to 30 gl-', measured as H202, and a strong complexant for copper at a concentration of at least 0.01 eq. 1-7 whereby a corrosion resistant film is deposited on the surface of the substrate.

This relatively simple solution can deposit protective films on aluminium brasses but the pH of the solution tends to drift up and slow down the formation of the desired film. This effect is particularly noticeable where the ratio of the surface area being treated to the volume of treatment solution is relatively high as would be the case in treating tube and shell condensers. It is, thus, highly desirable that the solution includes a buffer to maintain the pH at or near the chosen operating pH.Accordingly the invention further provides a method of depositing a corrosion resistant coating on the surface of an aluminium brass substrate which comprises contacting the substrate with an aqueous treatment solution containing dissolved therein inorganic peroxide at a concentration of from 0.25 to 30 gl-1, measured as H202, a strong complexant for copper at a concentration of at least 0.01 eq. 1-1 and a buffer at a concentration of at least 0.01 5 molar effective to maintain the pH of the solution within the range 5 to 9 whereby a corrosion resistant film is deposited on the surface of the substrate.

In practice the desired protective films will be deposited on the aluminium brass surfaces of marine condensers in situ in a constructed unit.

Accordingly, the invention includes a method of treating aluminium brass heat exchange surfaces in a marine condenser to increase their resistance to corrosion by seawater which comprises circulating the aqueous treatment solution used in this invention through the coolant circulation path of the condenser to deposit on the aluminium brass surfaces a corrosion resistant film.

The substrates to which this invention is applicable are aluminium brasses. Aluminium brasses are alloys of copper, zinc and aluminium.

Typical of aluminium brasses for use in marine condensers are the alloys specified in British Standard BS 2871/1972 (Part 3) and International Standard I.S.O. 426/11 973(E).

B.S. 2871 specifies aluminium brass under designation CZ1 10 as containing 76 to 78% Cu, 1.8 to 2.3% Al, 0.02 to 0.06% As, impurities 0.3% max remainder Zn. The I.S.O. alloy designated Cu Zn 20A12 has a similar composition. The inclusion of trace amounts of arsenic in such brasses is a well known means of preventing rapid dezincification of such alloys when in contact with sea water. The arsenic can be replaced by similar amounts of antimony and I.S.O. 426 refers to this possibility. The presence of arsenic (or antimony) in aluminium brass substrates is not critical to the deposition of corrosion resistant films according to this invention. However, we expect that this conventional means of avoiding dezincification will be used.

The films are produced by dissolution of the substrate surface, encouraged by the peroxide which depolarizes the reaction, followed by precipitation of hydrous oxides largely of aluminium and zinc to form the protective film.

The presence of high copper concentrations in the film is, as noted above, undesirable because copper tends to promote breakdown of the film.

Since the bulk of the substrate is copper some means for preventing precipitation of copper in the layer is needed. In this invention we accomplish this by including a strong complexant for copper in the treatment solution. The strong complexant binds to the copper and keeps it in solution thus preventing precipitation of copper into the film. A small amount of copper may be trapped in the film e.g. by occlusion from the solution, but this is much less than would be present in the film in the absence of the strong complexant.

The complexants used in the present invention satisfy the following requirements: i) they complex copper, especially Cu", sufficiently strongly in aqueous solution to keep it in solution in the pH range used in the invention; ii) they do not complex strongly with aluminium and preferably they do not complex with aluminium at all; iii) they are stable to peroxide i.e. they are not oxidized by peroxide under the conditions of the invention; iv) they do not otherwise interfere with the deposition of satisfactory films; and v) if, as is described below, metals such as magnesium are included in the treatment solution, for inclusion in the film, they do not complex strongly with such metals.

Preferably, the strong complexants also complex strongly with zinc, but this is not essential.

Among the strong complexants which can be used in this invention are glycine and other similar amino carboxylic acids and polyamines including diamines, such as ethylene diamine, and the polyamine surfactants available under the trade name Miranol. Glycine is the preferred complexant. Examples of ligands which complex or compound strongly with copper but are not strong complexants useful in the present invention include gluconate, which complexes with Al"', polycarboxylic acids, which tend to form insoluble copper salts, and ammonium ion, which, for reasons not yet clear, causes the film deposited to have poor corrosion resistant properties.

The amount of complexant used will depend on the particular circumstances. However, we have found that use of concentrations of less than 0.01 eq. 1-1 results in the solution so rapidly loosing its ability to prevent precipitation of copper that the resulting films are either too thin to be of practical use or contain sufficient copper that they are not corrosion resistant. In normal practice of the invention, the concentration of complexant will be in the range 0.01 to 1 eq. 1-1.

Generally, the optimum is in the range 0.05 to 0.5 eq. 1-1 but may vary somewhat for differing complexants. The concentrations by weight depend upon the molecular weight and complexant functionality of the material used.

Glycine is bidentate in complexing Cu" and thus, the preferred concentration range is 2 to 20 gl-' with the optimum in the range 2 to 10 gl-1. The use of concentrations higher than the upper limits of the ranges quoted generally produce no additional beneficial effect and adds to the cost.

The treatment solution contains inorganic peroxide as an essential ingredient. The term "inorganic peroxide" refers to hydrogen peroxide and alkali and alkaline earth metal peroxides which give rise to hydrogen peroxide or the peroxide ion (or intermediate charged species) in solution at the pH used in the invention. Suitable alkali metal peroxides are sodium and potassium peroxides which give highly alkaline aqueous solutions and will thus necessitate appropriate adjustment of the pH.

Hydrogen peroxide is the preferred source of peroxide in the solution, as it is relatively cheap and readily available in aqueous solutions in the U.K., and 30% wt/vol aqueous H202 is particularly convenient. Where aqueous H202 is not available the use of metallic peroxides may have the technical advantage of relatively long shelf life in the solid forms in comparison to aqueous H202 which is well known to decompose at a significant rate in storage.

The concentration of inorganic peroxide used is from 0.25 to 30 gl-', calculated as H202. At concentrations less than 0.25 gl-' a satisfactory film is not obtained. We believe that this is because the solution is exhausted of peroxide so rapidly that the desired film thickness has not had time to form. At concentrations of from 0.5 to 1 91-1 as H202 the reaction is slower than is usually desirable. At concentrations above 30 all satisfactory films are not formed because the oxidative attack at the substrate surface proceeds so rapidly that any film formed tends to be disrupted particularly by the liberation of oxygen from the solution (generated by decomposition of the peroxide catalysed by Cu2+).

Satisfactory results can be obtained in the concentration range 1 to 1 5 gl-' as H202 and the use of concentrations higher than 1 5 gl-' gives no particular advantage but is more expensive and is likely to involve higher losses by catalytic decomposition. Technically, the optimum concentration range is 1.5 to 10 gl-' as H202, although, where extended treatment times are not disadvantageous, lower concentrations e.g. 1.0 to 1.5 gl-1 may be used to reduce the cost of the solution.

The pH of the treatment solution is in the range 5 to 9. At pHs less than 4, Al"' is soluble and satisfactory films cannot be reliably formed. At pHs less than 5, the films formed are not protective. As the pH increases the rate of oxidation at the substrate surface diminishes thus limiting the rate of film formation. At pHs greater than 9 the rate of film formation becomes vanishingly small and the aluminium oxide may be redissolved as aluminate, particularly since the pH in the surface film of solution will tend to be higher than the bulk of the solution. As is mentioned above it is highly desirable to include a buffer in the solution to stabilize the pH which, in the absence of a buffer, tends to rise thus reducing the rate of film formation.The buffers used in this invention satisfy the following requirements: i) they are effective as buffers at a pH within the range used in the invention; ii) they are stable to oxidation by the peroxide; iii) they do not complex strongly with Al; iv) they do not precipitate insoluble copper salts; v) they do not otherwise interfere with film formation; and vi) they do not precipitate added metals (when present) from the solution.

Examples of suitable buffers include acetate (homologues of acetate can be used but are much more expensive and the copper salts increasingly less soluble with increasing molecular weight), phosphate, carbonate, bicarbonate and citrate.

Acetate is generally preferred on technical grounds especially for operation at about pH 5, but carbonate and/or bicarbonate are effectively nonpolluting and are particularly suitable at higher pHs in the range especially at about pH 8. If phosphate is used then care may be needed to avoid precipitation of cupric phosphate e.g. by using a relatively high concentration of the strong complexant for copper.

The concentration of buffer used will depend on the pH it is desired to maintain. However, the buffering effect is very small at concentrations of less than 0.01 molar.

Typically the concentration will be from 0.05 to 2 molar. By way of example, for acetate as buffer the concentration used will not normally be below 1 gl-' and usually will be at least 2 gl-' (as H Ac).

Concentrations above about 100 gl-' (as H Ac) provide little further benefit and are more expensive. For operation at pH 5 the treatment solution can be buffered by 4 gl-1 sodium acetate and 1.8 gl-' acetic acid. Since, the treatment solutions are used fairly close to neutral it will usually be most convenient to add the buffer to the solution as conjugate acid; conjugate base components in the required ratio rather than adding one and adjusting the pH with strong acid or base.

For pHs in the range 7.5 to 9, bicarbonate is the most useful buffer, as the dissolved CO2 in the solution forms the acidic component of the buffer.

For pHs above 8, a further addition of hydroxide will be necessary. Typically, a pH of about 8 can be achieved by the addition of from 0.5 to 10 gl-1 of sodium bicarbonate.

As is noted above NH4+ interferes with the deposition of satisfactory corrosion resistant films and, therefore, is not used as a buffer in the invention.

It is generally desirable to keep the composition of the treatment solution as simple as possible.

However, there are two further components which may be included beneficially in the solution. The first is a surfactant. This is useful to wet the substrate and because the oxygen liberated by catalytic decomposition forms bubbles and the presence of a surfactant aids detachment of these bubbles thus preventing masking of the substrate.

The amount used will typically be about 10 ppm with a practical maximum of 100 ppm. As is noted above, the class of substituted imidazoline surfactants sold under the trade name Miranol can be used as strong complexants for copper and may therefore be included at much higher concentrations. In this case it is not necessary to add a separate surfactant. A particularly convenient and effective surfactant is sodium lauryl sulphate.

The second of the two possible further components mentioned above is magnesium which may be included in the solution as a watersoluble salt. At solution pH values of about 7.5 to 8.5, magnesium is incorporated into the precipitated film thus making the initially formed film even more similar to the naturally occurring film. The presence of magnesium in the film appears to have a beneficial effect on corrosion resistance. Typically, magnesium will be included into the treatment solution as the sulphate, nitrate or acetate. Typically, the concentration of magnesium, measured as MgS04, will be 5 to 50 gl-', and especially 10 to 30 gl-1.

In addition to the materials specified as desirable we have found that certain other materials have deleterious effects and are therefore desirably absent. Ammonia is mentioned above as inhibiting the formation of a suitable corrosion resistant film. The reasons for this are not clear but appear to arise from inclusion of ammonium ion in the film. Further, when magnesium is included in the solution the presence of NH3 or NH4+ may encourage precipitation of Mg" containing species from the bulk of the solution.

Another major exclusion is halide ions especially chloride. For reasons which are not altogether clear the presence of significant amounts of chloride encourages the formation of imperfect films and may give rise to rapid pinholing corrosion underneath the film. The maximum tolerable concentration of halide, especially chloride is about 80 ppm, preferably less than 50 ppm. However, where supply water is more brackish, then it may be necessary to deionize or distil it to provide a satisfactory solution water. Other halides are less deleterious than chloride but will normally be restricted to low or zero concentrations.

The other major ion which has a harmful effect is calcium. In a bicarbonate buffered solution the presence of calcium results in the precipitation of calcium carbonate rather than the desired film.

However, even in solutions using other buffers e.g.

acetate at pH 5, calcium has been observed to interfere with the deposition of protective films.

The reasons for this are not understood at present.

The maximum tolerable level of calcium is 50 ppm (as CaCO3) and optimally less than 30 ppm (as CaCO3). Many tap waters in the U.K. have a total hardness in excess of this and, in order to use these in making up coating solutions, it would be necessary to reduce the hardness, for example, by distillation or base exchange softening. In addition, if the chloride ion concentration was also in excess of the maximum then this would have to be lowered, for example by distillation or ion exchange, to an acceptable level.

Aluminium salts are not added to the treatment solution because they will precipitate hydrous alumina and render the solution more acid than desired. Similarly copper and zinc salts are not added because they will use up the strong complexant. As will be apparent, because sulphide is a corrosion promoter, it will not be added although it would rapidly be oxidized in solution to sulphate thus wasting peroxide.

The treatment of aluminium brass surfaces by this invention is conveniently carried out at ambient temperatures which, in the U.K., will normally be in the range of from 5 to 250C. At higher temperatures the reaction proceeds more rapidly and eventually so vigorously as to prevent satisfactory film formation. Temperatures up to 350C can be tolerated but it is preferable to operate at below 3O0C since above this value the substrate may become subject to rapid pitting attack.

As is noted above, decomposition of peroxide gives rise to gaseous oxygen. To aid removal of oxygen bubbles and to prevent stagnation, with possible excessive pH drift, of the treatment solution it is preferably agitated gently during treatment. Where long pipes are treated, as in a tube and shell condenser, a stream velocity through the pipes of from 5 to 100 cm per minute, typically ca. 10 cm per minute is satisfactory.

When treating a condenser this agitation can be carried out using a low power circulation pump between the outlet and inlet boxes of the condensation per unit.

The desired film forms many times more rapidly than the natural film but much more slowly than conversion coatings on more reactive substrates e.g. chromate coatings on Al substrates. In this invention treatment times will typically be at least 1 hour and may be 24 hours or longer in appropriate cases. Times of from 2 to 6 hours are typical of solutions optimised for reactivity but cost considerations may lead to use of more dilute solutions and longer treatment times. It is desirable to provide a minimum thickness of film of at least 1 ,um and preferably at least 2,um for most applications.

The optimum reactive systems described above will typically deposit a film of 0.5 to 1 ,um in 1 hr and 1.5 to 2.5 m in from 3 to 6 hrs.

In operation of marine condensers treated according to the present invention we believe that ion exchange between the film and the sea water coolant leads to the film picking up Mg" and eventually becoming chemically identical to the naturally produced film. As with the natural film the beneficial properties at high stream velocities can be enhanced by regular iron dosing of the input coolant. Our test results indicate that the film produced by the present invention is resistant to corrosion by polluted sea water and reduces the risk of failure in subsequent operation in clean sea water.

The following Examples illustrate the invention: Galvanic potentials are given as the potential between the sample under test and a saturated calomel electrode in flowing sea water.

impingement tests were carried out using the May Jet Test as described in J. Inst. Met.1950, Vol.

77, p. 331. The aluminium brasses used were Cm 1 10 alloys per B.S. 2871/1972.

EXAMPLE 1 An aqueous solution having the following composition was made up: sodium acetate 5 gl-l glacial acetic acid 0.8 cm3 1-1 glycine 5 gl-l 30% w/v aq. H202 10 cm3 1-1 The pH of the solution was 5. Aluminium brass tubes were immersed in the solution under gentle agitation at 250C for 3 hours. At the end of this time the solution had become a deep blue colour.

The tubes were rinsed and dried. On examination a white powdery film was observed on the surface of the tubes. The thickness of the film was measured as about 2.5 Mm. The film on one tube was dissolved in 10% aqueous H2SO4 and the resultant solution analysed by atomic absorption spectroscopy. The film was found to contain ca.

15% Al, ca. 9% Zn and ca. 3.5% Cu. The galvanic potential of another treated tube was found to be -250 mV which rose over 5 days to -225 mV.

A second set of tubes were treated in a similar solution except that the glycine was omitted (pH = 5.0). The film formed was coloured golden and had a thickness of less than 1 ym. The composition of the film was ca.11 Al and ca.

26% Cu. The galvanic potential of one of these tubes was -220 mV which rose to -150 mV over 5 days.

The galvanic potential of an untreated tube was -250 mV which rose to -225 mV over 5 days.

EXAMPLE 2 An aqueous solution having the following composition was made up: glycine 5 gl-' 30% w/v aq. H202 10 cm3 1-1 pH adjusted to 5.0 This solution is similar to that of Example 1, but omitting the buffer. An aluminium brass tube was immersed in the solution with gentle agitation for 3 hours at 250C. After rinsing and drying the tube was examined and a white film, with a measured thickness of 0.9 ym, was detected on the surface.

The solution was modified by addition of: sodium acetate 4 gl-1 glacial acetic acid 0.8 cm3 i-7 .pH adjusted to 5.0 A second identical tube was immersed in the modified solution. After 3 hours this second tube had a film on it approximately 2.7 Mm thick.

EXAMPLE 3 An aqueous solution having the following composition was made up: sodium bicarbonate 591-1 glycine 5 gl-1 30% w/v aq. H202 10 cm3 1-1 magnesium sulfate 20 gl-1 pH adjusted to 8.0 by addition of sodium carbonate Aluminium brass tubes were immersed in this solution with gentle agitation for 3 hours at 250C.

After rinsing and drying a white film, having a measured thickness of 1.5 ssbm, was observed on the surface. A sample analysed as described in Example 1 indicated that the film contained ca.

13.5% Al, ca. 13.9% Zn, ca. 2.5% Cu and ca.

0.5% Mg.

EXAMPLE 4 An aqueous solution having the following composition was made up: sodium bicarbonate 5 gl-' glycine 5gl-' 30%w/vaq.H2O2 10 ml 1-1 CaSO4 142 mgm 1-1 Aluminium brass panels were immersed in this solution with gentle agitation for 3 hrs at 250C.

After rinsing and drying a white film having a thickness of about 1.4 ym was observed. A sample analysed as in Example 1 contained 1.9% Cu; 8.9% Zn and 10.8% Al.

Panels exposed in a similar solution containing no calcium sulphate had a film of 1.7 um thickness containing 2.9% Cu; 18.5% Zn and 12% Al.

EXAMPLE 5 Three series of aluminium brass tubes having a wall thickness of 1.2 mm were treated as follows: A. untreated (control).

B. treated according to the invention as described in Example 1.

C. treated according to the invention as described in Example 3.

Samples of each treatment were corrosion tested by immersion at ambient temperature (ca.

200 C) in stagnant sea water containing approximately 10 ppm H2S for 24 hours, and triplicate samples were prepared from the tubes from impingement testing.

Impingement tests (May Jet Test) were carried out using recirculating sea water with 3% added air at a jet velocity of 10 m5-1 for a continuous period of 4 weeks. The results are set out in Table 1 below. These results include one "rogue" result (specimen 2 treatment A) which showed no impingement attack. After the end of the test it was noted that one of the jets was blocked and it is probable that this corresponded to the "rogue" result although it was not possible (in retrospect) to definitely confirm this. We are aware from our previous use of this test that "rogue" results occasionally happen. This mirrors the unpredictability of failure in equipment in use and emphasises the difficulty of planning maintenance schedules to meet such failures.

TABLE 1

Specimen depth of Treatment No. attack, mm 1 m' 1 oo7 I A 2 0.0 3 ool 1 0.07 B2 0.15 3 0.01 1 0.0 C 2 0.0 3 0.0 1 or means that the test samples was perforated at the end of the test. The time of perforation was not noted.

Claims (10)

1. A method of depositing a corrosion resistant coating on the surface of an aluminium brass substrate which comprises contacting the surface with an aqueous treatment solution having a pH of from 5 to 9 and containing dissolved therein inorganic peroxide at a concentration of from 0.25 to 30 91-, measured as H2O2, and a strong complexant for copper at a concentration of at least 0.05 eq. 1-1 whereby a corrosion resistant film is deposited on the surface of the substrate.

2. A method of depositing a corrosion resistant coating on the surface of an aluminium brass substrate which comprises contacting the substrate with an aqueous treatment solution containing dissolved therein inorganic peroxide at a concentration of from 0.25 to 30 91-, measured as H202, a strong complexant for copper at a concentration of at least 0.05 eq. 1-' and a buffer at a concentration of at least 0.015 molar effective to maintain the pH of the solution within the range 5 to 9 whereby a corrosion resistant film is deposited on the surface of the substrate.

3. A method as claimed in claim 2 wherein the solution has a pH of from 5 to 6 and contains dissolved therein from 1 to 1 5 gl-1 of H2O2, from 2 to 10 gI1 of glycine and from 1 to 100 gl-' of acetate, calculated as acetic acid.

4. A method as claimed in claim 2, wherein the solution has a pH of from 7.5 to 9 and contains dissolved therein from 1 to 1 5 91- of H2O2, from 2 to 10 gl-1 of glycine and from 0.05 to 2 mol í-' of bicarbonate and/or carbonate.

5. A method as claimed in claim 4, wherein the solution additionally contains from 5 to 50 gl-' of Mg", calculated as MgSO4.

6. A method as claimed in any one of claims 1 to 5, wherein the aluminium brass substrate is the heat exchange surface of a marine condenser and wherein the treatment solution is circulated through the coolant circulation path of the marine condenser to deposit on the aluminium brass surfaces, a corrosion resistant film.

7. An aqueous solution for use in treating aluminium brass substrates to provide corrosion resistant coatings on the surfaces thereof, which solution has a pH of from 5 to 9 and contains dissolved therein from 1 to 1 5 91- of H202, from 2 to 10 91- of glycine and from 0.05 to 2 mol l- of a buffer.

8. A solution as claimed in claim 7, having a pH of from 5 to 6, and wherein the buffer is acetate at a concentration of from 1 to 100 91-, calculated as acetic acid.

9. A solution as claimed in claim 7, having a pH of from 7.5 to 8.5, and wherein the buffer is bicarbonate and/or carbonate.

10. A solution as claimed in claim 9, which additionally contains dissolved therein from 10 to 50 gel~' of Mg", calculated as MgSO4.