AU2012201976A1 - Treatment of metals - Google Patents

Abstract

A method for the treatment of metals to form conversion coatings thereon wherein a metal surface is treated with an alkaline aqueous solution containing a permanganate salt and an alkaline earth metal salt. The metal is preferably steel, zinc, aluminium or titanium. The alkaline earth metal salt is preferably a strontium salt and the permanganate salt is preferably potassium permanganate. *00 cn C or C tno N, M' CL c o0 Co C M 00) crC.) Ca o 0 CL 0 C5 c, > C.) ( .0 to0 C: +~~C + - c A) S2 (D N e ) U 0 . a ) C C.) C14u + 0 as I-c + E. C - :3 N C% o 7 co + + U)U 0 m) 0) +1 + a

Description

AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT (Original) APPLICATION NO: LODGED: COMPLETE SPECIFICATION LODGED: ACCEPTED: PUBLISHED: RELATED ART: NAME OF APPLICANT: Mark Henderson ACTUAL INVENTOR: Mark Henderson ADDRESS FOR SERVICE: LORD AND COMPANY, Patent and Trade Mark Attorneys, of 4 Douro Place, West Perth, Western Australia, 6005, AUSTRALIA. INVENTION TITLE: "TREATMENT OF METALS" DETAILS OF ASSOCIATED PROVISIONAL APPLICATION NO: Australian Provisional Patent Application Number 2011902015 filed on 24 May 2011 The following Statement is a full description of this invention including the best method of performing it known to me/us: TITLE "TREATMENT OF METALS" 5 FIELD OF THE INVENTION The present invention relates to a method and formulations for treatment of metals. BACKGROUND TO THE INVENTION In industrial applications, metals are often alloyed to improve their physical properties. 10 For example, whilst pure aluminium already has desirable properties including that it is light, non-magnetic and non-sparking, zinc (as a primary alloying element together with magnesium) has been added to yield a 7000 aluminium alloy series with high strength/weight ratio suitable for automotive (7029-T6) and aerospace (7075-T6) application. 15 Alloys are commonly more susceptible to corrosion as compared to unalloyed metal. In the example mentioned above, it is widely known that pure aluminium metal shows excellent corrosion resistance due to the rapid formation (and therefore self-renewing) of an inert, oxide surface layer. The addition of zinc in the 7000 series, however, decreases 20 the corrosion resistance of the alloy, possibly due to the fact that the aluminium oxide layer formed is now not consistent over the surface of the alloy, and the zinc hydroxide corrosion product (i.e. the "white stain") formed is not voluminous, hard and able to self renew as with the aluminium oxide layer. 2 Therefore, chromate- and phosphate based conversion coatings have been employed to protect metal and alloy surfaces from corrosion. Such coatings are formed spontaneously by contacting a metallic surface with a coating solution of specific chemical composition, leading to the formation of a thin inorganic layer on the metal surface. Whilst phosphate 5 coating solutions are primarily used to treat ferrous alloys, chromium coating solutions have been applied to various metal or alloy surfaces, including those of zinc, aluminium and/or magnesium. It has long been known that chromium compounds containing chromium in its hexavalent form Cr (VI), e.g. Cr0 4 2 , Na 2 CrO 4 and Na 2 Cr 2

O

7 , are efficient corrosion inhibitors. The chromate (CrO42) species, when incorporated in 1o primers applied to metal surfaces, are found to be particularly effective in protecting aluminium alloys used in aerospace industry (including certain 7000 series alloys). In addition, strontium chromate is considered the bench mark for corrosion prevention in coil coating primers when applied to zinc, zinc-aluminium or similar galvanised coil stock. 15 The superiority of the so called chromate conversion coatings may be attributed to the underlying chemistry of the unique Cr(VI)/Cr(III) combination as explained herein below. When used in a metal treatment process, some of the Cr(VI) species oxidises the metal surface to metal cations with concomitant reduction of Cr(VI) to Cr(III). Hydrogen ions 20 are consumed in the process, causing a rise in the pH resulting in the precipitation of mainly Cr(III) hydrated oxides/hydroxides containing entrapped Cr(VI) species onto the metal surface. Consequently, the metal surface is protected from corrosion (at least to a degree) due to the formation of a thin (0.1-2.0 g/m 2 ) chromate coating. If the coating gets damaged, it is believed that labile Cr(VI) species may be leached from the coating and 3 transported through the corrosive solution to damaged sites where they may be reduced and precipitated to form an insoluble Cr(III) hydroxide barrier. Therefore, the chromate coating has a "self-healing" nature and this affords superior, active protection against corrosion. 5 However, there is strong awareness of the toxicity associated with the Cr(VI) species and ecological risks that require costly environmental control of related industrial effluent. In Australia, the United States and Europe, strict regulatory health and environment laws have been enacted. This has lead to a re-appraisal of chromate chemicals and an 10 extensive search for a non-toxic, effective replacement of Cr(VI). Any suitable alternative would preferably duplicate the desired property of the chromium-based treatment for corrosion inhibition; in this instance, the "self-healing" nature of the Cr(VI)/Cr(III) system. 15 Recently, the broad classes of chromate-free conversion coatings including hydrothermal coatings from alkaline solution, Rare Earth metal and coatings based on the sol-gel technique for aerospace applications were reviewed (Ogle K. and Buchheit R. G., Encyclopedia ofElectrochemistry 2007). However, to date, no chromate-free system has been found to provide the same level of damage protection through "self-healing" of the 20 surface film (containing the Cr(VI)/Cr(III) combination), and many fail to deliver the long duration of outstanding corrosion inhibition as found with a chromate system. The present invention attempts to provide a non-toxic alternative to the known chromium-based treatments. 4 SUMMARY OF THE INVENTION In accordance with a first aspect of the present invention, there is provided a first formulation for treatment of metals to form a coating having a particular 5 surface composition yielding a desired surface chemistry; and a second formulation, optionally applied, for post treatment of the coating formed. The first formulation is preferably an aqueous solution containing a permanganate ion MnO 4 and a counter ion. The counter ion is preferably an alkaline earth metal ion, an 10 alkali metal ion or a transition metal ion. More preferably, the counter ion is an alkaline earth metal ion. Still more preferably, the counter ion is strontium ion Sr 2 . The first formulation further comprises a base to yield a strongly alkaline solution with pH preferably greater than 10. 15 The second formulation preferably contains silane, more preferably a long-chain alkylsilane, which may self-assemble, and cross-link to the coating formed as mentioned hereinabove. 20 In accordance with a second aspect of the present invention, there is provided a method for treatment of metals comprising the steps of: contacting a metal surface or metal object with a first formulation according to the first aspect of the present invention; and optionally further treating the metal surface or metal object with a second formulation 25 according to the first aspect of the present invention. The step of contacting a metal surface or metal object with a first formulation according to the first aspect of the present invention may optionally be carried out under an external potential. 30 In accordance with a third aspect of the present invention, there is provided a surface 5 composition formed upon treatment of metals using the formulations and method described hereinabove, the surface composition comprising precipitated manganese containing solids, more preferably pentavalent manganese Mn (V) containing solids, forming a coating. 5 The surface composition according to the third aspect of the present invention may further comprise a silane layer. The silane layer is preferably a self-assembled silane layer, condensed and cross-linked to 10 the coating comprising manganese containing solids. In accordance with a fourth aspect of the present invention, there is provided a treated metal surface having a surface composition in accordance with the third aspect of the present invention, comprising a coating formed of manganese containing solids, more 15 preferably pentavalent manganese Mn (V) containing solids, and, optionally, a silane layer cross-linked to the coating. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In a preferred embodiment, a first formulation for treatment of metals to form a coating in 20 accordance with a first aspect of the present invention is an alkaline solution with pH > 13. The first formulation is prepared essentially by dissolving in water a permanganate salt (e.g. KMnO 4 ) and an alkaline earth metal salt, preferably a strontium salt (e.g. Sr(N0 3 ).4H 2 0). The basicity of the solution is achieved by adding a base (e.g. KOH) until the solution has reached the desired pH. The solution would therefore contain 25 chemical species such as strontium hydroxide octahydrate Sr(OH) 2 .8H 2 0, which is desired and derived from a reaction between strontium nitrate Sr(NO 3

)

2 and potassium hydroxide KOH. The temperature of the solution is preferably kept in a range of from 15*C to 100*C, more preferably in a range of from 18*C to 95*C. 30 It should be noted that selection of appropriate counter ion (e.g. Sr 2 ), temperature and pH ensures that, when the first formulation is applied to a metal surface, the manganese 6 containing solids are precipitated, thus forming a coating as desired. The second formulation for post treatment of the coating formed upon application of the first formulation of the present invention may be a homogeneous solution comprising 5 water (5% w/w), ethanol (90% w/w) and silane (5% w/w). Preferably, however, the second formulation is a liquid-liquid dispersion containing only about 0.2% w/w particles of the silane dispersed in an ethanol-water solution, wherein the ethanol content is preferably about 45% of the formulation. This formulation enables 10 spontaneous emulsification based on the "Ouzo Effect" (- an effect that occurs upon adding to water, a mixture of totally water-miscible solvent (exemplified here by ethanol) and an hydrophobic oil (exemplified here by a silane). Such liquid-liquid dispersion is different to other known silane formulations used in the coating industry. Commonly, a long chain alkylsilane such as n-octadecyltrimethoxysilane is used in a silane treatment. 15 Whilst the long chain alkylsilane is able to self-assemble to form a monolayer on a surface, n- octadecyltrimethoxysilane is not soluble in water, a result of the hydrophobic 18-carbon chain present in the molecule. Consequently,> 90% w/w of ethanol in water is required to form an homogeneous solution of about 0.75% w/w of this long chain alkylsilane. In comparison, the silane formulation of the present invention is able to 20 reduce the use of the amount of a flammable substance, ethyl alcohol, thereby reducing the amount of flammable vapour above the flammable liquid while retaining the hydrophobic properties of the silane after its application as a post treatment or secondary coating, for example. 25 Furthermore, when the second formulation is applied as a post treatment upon application of the first formulation of the present invention, the alkaline condition pre-determined or induced by the first formulation is able to catalyse the condensation of the silane. Many silanes may be used to make up the second formulation of the present invention, 30 providing that the silanes are inexpensive, soluble in alcohol but not in water, and may be arranged such that the hydrophobic tails of each molecule are disposed to present a 7 hydrophobic barrier to moisture or a corrodant. Examples of silanes which satisfy these requirements include: n-octadecyltrimethoxysilane 5 n-octadecyltriethoxysilane, n-octadecyldimethylmethoxysilane, n-octadecylmethyldiethoxysilane, n-octadecylmethyldimethoxysilane, dodecylmethyldietoxysilane, 10 7-octenyltrimethoxysi lane, octyldimethoxysilane, n-octylmethyldiethoxysilane, n-octyltriethoxysilane, n-octyltrimethoxysilane, 15 perfluoroalkylethyltriethoxysilane,* perfluorododecyl- I H, 1 H,2H,2H-triethoxysilane-perfluorotetradecyl- I H, I H,2H triethoxysilane,* tridecafluoro- 1,1,2,2-tetrahydro-octyl)triethoxysilane,* tridecafluoro- 1,1,2,2-tetrahydro-octyl)trimethoxysilane,* 20 (heptadecafluro- 1,1,2,2-tetrahydrodecyl)triethoxysilane,* and (heptadecafluro- 1,1,2,2-tetrahydrodecyl)trimethoxysilane,* It should be noted that whilst some fluorinated or partly fluorinated silanes (as marked with *) have been listed, for reasons of low toxicity and lower cost, hydrocarbon-based 25 silanes are preferred. In particular, n-octadecyltrimethoxysilane is preferred. Nevertheless, because of the chemical and thermal stability of the fluorinated or partly fluorinated silanes, a second formulation incorporating such a silane would have enhanced chemical and corrosion resistance as compared to a second formulation incorporating the hydrogenated analogue. 30 With the method of the present invention, it is preferable that before a first formulation is 8 applied to a metal surface, the metal surface is chemically cleaned, preferably by: i) wiping with ethanol and/or immersing in a commercially available "Turco" alkaline solution at a concentration of approximately 30 g/L and a 5 temperature of preferably about 80*C, for a period of approximately 10 minutes; and ii) immersion in about 5 mol/L HN0 3 for a period of from 2 to 15 minutes or immersion in a commercially available "Novolox LF" acid etch for a period of from 10 to 15 minutes at ambient temperature followed by ultrasonic 10 immersion in about 8 mol/L HN0 3 for a period of approximately 2 minutes. Where the metal surface is a steel surface, cleaning may be achieved by: i) wiping with denatured alcohol and immersing in an alkaline solution of KOH 15 at a concentration of approximately 100 g L~' and at ambient temperature for a period of approximately 1 hour, to remove rust layers composed of orange hydrated oxides; followed by ii) immersion in a commercially available 335 g L-' phosphoric acid-based rust remover, "Ranex", for approximately 1 hour in a temperature range of 20 preferably 35 - 45*C, to remove black magnetite. Also with the present method, the step of contacting a metal surface or metal object with a first formulation of the present invention may be carried out under an external potential, imposed by a counter electrode and a reference electrode, for example, on the metal 25 surface or metal object which acts as a working electrode. In the absence of the external potential, a coating in accordance with the third aspect of the present invention is able to form spontaneously simply by contacting the metal surface with the first formulation of the invention, resulting in a type of conversion coating, the formation of which is mentioned hereinabove. In forming the conversion coating, the metal surface may react 30 with the first formulation of the present invention. 9 Application of the formulations of the present invention to a metal surface (for example) may be achieved by immersing the metal surface in a formulation according to the present invention or spraying the metal surface with the formulation. The contact time between a first formulation according to the present invention and a metal surface is in a 5 range of preferably from 0.5 to 30 minutes, more preferably from I to 20 minutes. The contact time between a second formulation according to the present invention and a metal surface is in a range of preferably from 5 min to 30 hours, more preferably from 15 min to 24 hours. 10 It is also preferable that between each step of the method of the present invention (including between the chemical cleaning and application of the first formulation), the metal surface is rinsed with water to remove any excess reagent from the previous treatment. 15 Upon treating a metal surface or metal object with the formulations and method of the present invention, a surface composition comprising precipitated manganese containing solids, preferably pentavalent manganese Mn (V) containing solids, is formed, resulting in a coating on the metal surface. 20 The Mn (V) chemical species, preferably contained in the coating, may undergo disproportionation and in so doing yielding a unique quartet system containing Mn (V)/Mn (IV), Mn (VI) and Mn (VII) chemical species, wherein the system may self replenish and such a system is able to effectively inhibit the corrosion of the underlying metal substrate. The disproportionation of the Mn (V) chemical species may be triggered 25 by water ingress and/or pH change as a result of damage to either the coating itself or damage to an outer silane layer (if applicable). It should be noted that, whilst in the first formulation of the present invention manganese is present in the form of permanganate MnO 4 , the reduction of Mn (VII) as violet 30 permanganate MnO 4 to Mn (VI) as green MnO 4 ~, Mn (V) as blue MnO- and Mn (IV) as brown MnO 2 solid in alkaline solution is well known and is described according to the 10 reduction potential or Latimer diagram shown in Figure 1. It can be interpreted from Figure 1 that, in strongly alkaline solution as set out by the first formulation of the present invention, although the reduction of Mn(VII) and Mn(V) to the lower valence state of Mn(IV) at a metal surface is thermodynamically possible, it is less favourable than in 5 acidic solution. Therefore, in the present invention, the reduction processes are likely to stop at Mn(V), resulting in the formation of a surface composition comprising Mn (V) containing solids. The electron source to enable the reduction of Mn(VII), added as permanganate MnO 4 in the first formulation of the present invention, is likely to be derived from a surface of the metal substrate (e.g. aluminium or zinc panel) to which the 10 present invention is to be applied. In the case of aluminium, for example, the standard reduction potentials as shown in the Latimer diagram in Figure 1 suggest that the reduction of permanganate MnO 4 to manganate MnO 4 2 - and hypomanganate MnO4- by aluminium is thermodynamically favourable. Therefore, when a treatment solution in accordance with the first formulation of the present invention comes into contact with an 15 aluminium surface, the permanganate MnO 4 -may be reduced to hypomanganate MnO4 whilst aluminium may be oxidised to Al 3 . The hypomanganate MnO4~ ion formed may then combine with an alkaline or a transition metal ion present in the first formulation to precipitate the Mn(V) containing solids resulting in the surface composition of the third aspect of the present invention. 20 In a particular embodiment of the invention, wherein the counter ion in the first formulation is Sr 2 and the formulation is applied simply by contacting a metal surface with the formulation (i.e. without imposing an external potential), a conversion coating is formed with the surface composition constituting compounds that contain the 25 hypomanganate ion MnO4- (Mn in a pentavalent state) as crystalline Sr 2 (MnO 4 )OH and/or its hydrate Sr 2 (MnO 4 )OH.2H 2 0. The pentavalent oxidation state of manganese may be verified by performing X-ray diffraction (XRD) analysis on the parent compound Sr 2 (MnO 4 )OH. The Sr 2 (MnO 4 )OH and/or Sr 2 (MnO 4 )OH.2H 2 0 solids formed are not soluble in the alkaline condition of the present invention and do not diffuse away from a 30 metal-solution interface resulting from application of the first formulation of the present invention to a metal surface or object. 11 If the first formulation is applied to an aluminium surface, Sr 3 Al 2

(OH)

12 present as 3SrO.A1 2 0 3 .6H 2 0, may also be present amongst the Sr 2 (MnO 4 )OH and/or Sr 2 (MnO 4 )OH.2H 2 0 crystallites. 5 Further to the above, strontianite (SrCO 3 ), formed probably as a result of atmospheric

CO

2 reacting with Sr2+ in the first formulation of the present invention or in the coating formed, may also exist in the surface composition in accordance with the third aspect of the present invention. 10 The pentavalent Mn (V) chemical species, MnO~, contained in the Sr 2 (MnO 4 )OH and/or Sr 2 (MnO 4 )OH.2H 2 0 crystallites may disproportionate to form Mn(IV) and Mn (VI) species as shown in Figure 2 and below: 2MnO 3+ 2H 2 0 =:> MnO 2 (s) + MnO 4 2 + 40H 15 In conditions where the [OH] concentration falls < I mol/L, the Mn (VI) species MnO 4 2 disproportionates to form Mn (IV) as MnO 2 and Mn (VII) as permanganate ion MnO 4 as follows: 3MnO 4 2 + 2H 2 0 => MnO 2 (s) + 2MnO 4 + 40H~ 20 3MnO 4 2 + 4H* = MnO 2 (s) + 2MnO 4 + 2H 2 0 Therefore, should the silane layer protecting Sr 2 (MnO 4 )OH and/or Sr 2 (MnO 4 ).2H 2 0 be damaged, the MnO 2 solids precipitated as a result of the disproportionation of the hypomanganate ion MnO 4 3 and/or manganate ion MnO 4 2 may form a new film, and in 25 so doing replenish the damaged film. Therefore, treatment of metals with the first formulation of the present invention yields a desirable coating, showing the "self-healing" nature of a chromium-based coating, but does not have the toxicity associated with a coating contain Cr(VI). 30 In this particular embodiment of the invention, the coating may contain a mixture of Sr 2 (MnO 4 )OH/Sr 2 (MnO 4 )OH.2H 2 0, 3SrO.Al 2 0 3 .6H 2 0 and (SrCO 3 ). In this instance, co 12 existence of the insoluble oxides and carbonates may prevent Sr 2 (MnO 4 )OH from undergoing immediate disporportionation once formed on the metal surface, thus attributing to the stability of the coating. 5 Further to the above, the coating formed is preferably further protected by post treatment with the second formulation of the present invention, which results in the condensation of the alkylsilane, catalysed by -OH present on the surface of the Sr 2 (MnO 4 )OH and/or Sr 2 (MnO 4 )OH.2H 2 0 crystallites or by hydroxide ion O-. The OH~ is released by disproportionation of Mn (V) chemical species at the coating-silane solution interface and 10 ensures maximum condensation and cross-linking of the silane. Importantly, crystalline Sr 2 (MnO 4 )OH formed displays no change in structure whether hydrated, dehydrated or re-hydrated. Therefore, the coating of the present invention is useful, particularly when film cracking, usually associated with the drying of gel-based 15 coatings, is to be avoided. Also, the structural stability of crystalline Sr 2 (MnO 4 )OH affords the flexibility that the post treatment step using the second formulation of the present invention may be carried out at a much later stage, well after the formation of the conversion coating comprising Sr 2 (MnO 4 )OH. This is important because the second formulation contains ethanol and there are strict safety requirements on storage and use of 20 ethanol. Therefore, once the coating of the present invention is formed, the coated metal substrate may be stored and subsequently transported to another facility suitable for carrying out treatment using the second formulation of the present invention. As the mechanism of deposition of Sr 2 (MnO 4 )OH is an electrochemical one, with the 25 oxidation of Al to Al 3 and the concomitant reduction of Mn (VII) to Mn (V) upon contact of the aluminium surface with the first formulation of the present invention as mentioned hereinabove, the deposition of Sr 2 (MnO 4 )OH may be assisted or achieved using an externally imposed potential. For example, the metal to be treated may be exposed to the first formulation of the present invention in the presence of a counter 30 electrode (e.g. a platinium electrode) imposing a negative potential with respect to a reference electrode (e.g. a mercury/mercury oxide electrode). In this instance, a voltage 13 between approximately -0.6 V and 0.0 V may be applied to a metal substrate, such as a stainless steel or a titanium surface to be coated, for a period of approximately 30 to 300 seconds. The coating formed containing Sr 2 (MnO 4 )OH may be further treated with a second formulation according to the present invention, containing a hydrogenated, 5 fluorinated or partly fluorinated silane. With an externally imposed potential, the scope of the present invention may be broadened to treat any conductive surface to enhance their corrosion resistance, including metals that are considered either noble (e.g. gold) or already have good corrosion resistance (e.g. titanium, stainless steel). Enhanced corrosion protection is particular important for steels used in oil and gas industries, in 10 which the problem of "sour gas" (i.e. gas containing highly corrosive H 2 S) is extreme. Using the method and formulations of the present invention, Sr 2 (MnO 4 )OH may be deposited electrochemically at ambient conditions onto the steel surface, followed by post treatment with a second formulation in accordance with the present invention. 15 It is likely that the coating formed via electrodeposition is more pure than the conversion coating formed simply by contacting the metal surface with the first formulation of the present invention. In the case of the conversion coating, the host metal matrix is not passive in the alkaline solution of the first formulation, but reacts with the alkaline solution and re-deposits as 3SrO.Al 2 0 3 .6H 2 0 in the conversion coating; and as 20 mentioned above, SrCO 3 may also be present in the conversion coating. The likely absence of the oxides and carbonates in the coating formed via electrodeposition may compromise the stability of Sr 2 (MnO 4 )OH with respect to undergoing disproportionation. In this instance, post treatment with the second formulation of the present invention is important in order to induce a silane layer scaling the coating containing Sr 2 (MnO 4 )OH, 25 thus preventing water ingress triggering the disproportionation of Sr 2 (MnO 4 )OH. In the case of coated steel, in addition to the Mn (IV), Mn (VI) and Mn (VII) chemical species formed by disproportionate, the alkaline environment produced from the OH~ byproduct would help to maintain a passive film on any exposed steel surface and neutralise the effects of acids such as carbonic acid from dissolved CO 2 and/or sulfuric acid from H 2 S. 30 It should be noted that whilst Sr 2 is used to exemplify a suitable counter ion, other metal 14 ions that are considered non-toxic and have similar chemical properties/reactivities to Sr 2 may also be used. The pentavalent manganese Mn(V) induces blue/green coloured compounds, leading to a 5 blue/green coloured coating (comprising Sr 2 (MnO 4 )OH, for example) which can be aesthetically appealing. Therefore, the formulations and method of the present invention may be used to prevent metal corrosion and/or to colour-decorate a metal surface. For example, the electro-reduction of Mn (VII) to Mn (V) as Sr 2 (MnO 4 )OH may provide a method to decoratively colour stainless steels as an alternative to a commonly used 10 anodizing process involving surfuric/chromic acid mixtures. In the latter case, the colour is a result of optical interference based on the thickness of the grown chromium hydroxide oxide film. However, because the Sr 2 (MnO 4 )OH coating of the present invention is not formed by oxidation of a steel that is highly alloyed with chromium (as with the known process), but rather is deposited by a cathodic process, the deposition is 15 not limited to steels having a certain chromium content but any conductive surface, including carbon steel, low-alloy seel, cast iron, or titanium where corrosion protection and/or aesthetic decorative qualities are required. The present invention will now be illustrated with the following examples. Whilst these 20 examples demonstrate at least some aspects of the present invention, they are not to be taken in any way as limiting the scope of the present invention. EXAMPLE 1 Illustrating the surface coating solution make-up procedure 25 To a stirred solution of Sr(N0 3

)

2 (80 g, 0.38 mol) in deionised water (0.5 L) was added a stirred solution of KOH (42 g, 0.75 mol) in deionised water (0.5 L) and optionally containing dissolved finely ground NaSiO 3 .5H 2 0 (0.1 g, 5 mmol). After 1 minute, white crystalline plates of Sr(OH) 2 .8H 2 0 precipitated from solution. The white, cloudy suspension was then heated to 60-70 'C whereupon the solution became almost clear and 30 colourless. KMnO 4 (5 g, 32 mmol) was then added to the hot solution. The clear, deep violet solution was then heated to between 90-95 'C and stirred continuously. 15 Because of the possibility of dissolution of glass-pH electrode in the hot concentrated alkaline solution and consequently an unreliable measurement, an estimate of the pH value (from pOH) of the coating solution was made by considering the activity of the 5 hydroxide ion, aOH~ as follows: aion =fCion. where f, is the activity coefficients for z = Sr 2 +, O-f, K*, NO and Mn0 4 calculated 10 using; logf: = -0.5 1z2 .JI/(1+ 47) and Cion the molar concentration of the each ion present in the solution of ionic strength; 15 I= V 2

C

i z 2 where 1 is the ionic strength of the solution and zi is its charge, the pH of the solution was estimated at 13.6. 20 EXAMPLE 2 Illustrating the formation of a conversion coating in accordance with the present invention at a zinc surface A square zinc eletroplated (-5-10 pim zinc thick) mild steel plate or galvanized steel plate 25 40 millimeters (hereinafter abbreviated "mm") square, and 1.5 mm thick, was chemically cleaned according to the steps of i) and ii) described hereinabove. The plate was heated to 400 *C to reproduce the surface conditions immediately after the hot-dip galvanizing process. The plate was subsequently quenched directly into a solution at ambient temperature containing saturated amount of KMnO 4 and Sr(OH) 2 .8H 2 0) in deionised 30 water (1 L). Immediately, the plate was removed from the solution and rinsed in tap water for 3-5 minutes to reveal a uniform blue surface coating. 16 EXAMPLE3 Illustrating the presence of Mn(V) in material produced at an aluminium surface One rectangular 7075-T651 aluminium plate was immersed for 20 minutes in a solution 5 as described in Example 2 and in accordance with the first formulation of the present invention. The aluminium plate was at ambient temperature prior to immersion in the solution. The plate was used to harvest the green powder for examination by X-ray fluorescence (XRF) and X-ray diffraction (XRD). The partial, semiquantitative XRF analysis (elements sulfur to uranium) was conducted to assist in the phase identification. 10 Relevant analysis results shown below clearly indicate the presence of Sr and Mn. Partial Semiquantitative XRF Chemistry Mn 8.0% Co 1.1% 15 Cu 0.13% Rb 0.21% Sr >>10% Mo 0.9% 20 The XRD pattern is shown in Figure 3. The numerous Bragg reflections in the XRD pattern indicate that the material contains crystalline components. Rietveld refinement of the some of these reflections suggest that at least one component of the green powder contains Sr 2 (MnO 4 )OH i.e., manganese as MnO43 in a pentavalent, Mn 5 state. Another component of the green powder which was identified was Sr 3

AI

2

(OH)

12 as 25 3SrO.Al 2 0 3 .6H 2 0. EXAMPLE 4 Illustrating the formation of a conversion coating in accordance with the present invention at an aluminium surface 30 One 7075 aluminium alloy, in the form of a M6 hexagonal head screw, was treated by immersing the screw for 1 minute in a solution containing KMnO 4 (5 g L~'), 17 Sr(N0 3 ).4H 2 0 (80 g L'), KOH (42.2 g L') and NaSiO 3 .5H 2 0) (0.1 g U') in deionised water (1 L). A uniform protective coating is formed, which also provides the aluminium screw with an aesthetic appeal with the dark-green overall coating. 5 EXAMPLE 5 Illustrating the effectiveness of a conversion coating in accordance with the present invention in protecting an aluminium surface from corrosion Two 7075 aluminium alloy panels covered with a conversion coating in accordance with the present invention were tested for corrosion resistance by exposure to a salty 10 environment for 168 hours. The test, conducted in a salt spray chamber at 35 ± 2 *C, was based on the operation requirements of the ASTM Standard B 117, Standard Practice for Operating Salt Spray (Fog) Apparatus and performed by an independent research group. The test report provided by the independent research group indicated that the two 15 aluminium panels experienced no significant weight loss and the results estabilish that the coatings performed well from a corrosion protection viewpoint. EXAMPLE 6 Illustrating a post-treatment make-up (liquid-liquid particle dispersion) containing 55 wt 20 % water Octadecyltrimethoxysilane (0.4g) was dissolved in ethanol (90g) to give a clear and colourless solution. Water (109.6g) was poured into the ethanol octadecyltrimethoxysilane solution to produce the dispersion. The dispersion was left undisturbed for I day and then agitated by stirring prior to use as a post-treatment bath. 25 EXAMPLE 7 Illustrating a method offorming a surface coating at a steel surface under an external potential according to the present invention 30 18 3 6 5 300 e -0 The meanings of the reference numerals in the diagram above are explained below: I represents a working electrode of duplex stainless steel round bar having a 5 dimension of 25.4 mm (diameter) x 300 mm (length); 2 represents a counter-electrode of 316/316L stainless steel pipe (300 mm in length); 10 3 represents the electrical connection of working and counter electrodes to a 0 19 30V/0-2A power supply; 4 represents a treatment solution in accordance with the first formulation of the present invention for electrolytic deposition to form a surface coating (or surface 5 composition) in accordance with the third aspect of the present invention; 5 represents heat supplied to a polypropylene vessel containing the reaction solution in accordance with the first formulation of the present invention; and o 6 represents a thermometer. In the method illustrated in the diagram hereinabove, the duplex stainless steel round bar (1) was first pre-cleaned by a solvent wipe with methylated spirits, and further cleaned by immersion in an aqueous KOH solution at a concentration of approximately 100 g L-1 and 15 at ambient temperature for a period of approximately 1 hour. The duplex stainless steel bar was then rinsed by immersion in water at ambient temperature for approximately 3 minutes. This was followed by an acid clean using commercially available phosphoric acid-based rust remover, "Ranex", in which the duplex stainless steel round bar was immersed for approximately 1 hour in a temperature range of 35 - 45 *C. 20 The electrodeposition took place in a chromate-free treatment solution in accordance with the first formulation of the present invention. The I L treatment solution comprises 20 g U' KMnO 4 , 80 g L' Sr(N0 3

)

3 and 42 g L' KOH. 25 The -0.5 V external potential is imposed via the 316/316L stainless steel counter electrode. The electrode deposition was carried out at a temperature of approximately 70 *C for a period of approximately 2 minutes. 30 The surface coating formed was rinsed with an aqueous saturated Sr(OH) 2 .8H 2 0 solution 20 prepared as follows: to a stirred solution of Sr(N03)2 (40 g, 0.19 mol) in distilled water (0.25 L) was added a stirred solution of KOH (21 g, 0.375 mol) in distilled water (0.25 L). After 1 minute, white crystalline plates of Sr(OH)2.8H20 precipitated from solution. The white, cloudy suspension was then heated to 60-70 'C whereupon the solution 5 became almost clear and colourless. On cooling to room temperature, Sr(OH)2.8H20 crystallised from a saturated solution of the same. The surface coating formed proceeded through a post treatment using a solution in accordance with the second formulation of the present invention. The post treatment 10 involved immersion of the duplex stainless steel rod in a liquid-liquid dispersion consisting of water (55.2 w/w %), methylated spirits (44.7w/w %) and octadecyltrimethoxysilane (0.1 w/w %) for a period of 5-6 hours. Modifications and variations as would be apparent to a skilled addressee are deemed to 15 be within the scope of the present invention. 21

Claims (12)

1. A method for the treatment of metals to form conversion coatings thereon, comprising contacting a metal surface or object with a composition containing an aqueous solution of a permanganate salt and an alkaline earth metal salt, the aqueous solution having an alkaline pH.

2. A method according to claim 1, wherein the aqueous solution contains an alkali metal hydroxide salt to achieve the alkaline pH.

3. A method according to claim 2, wherein the alkali metal hydroxide salt is potassium hydroxide.

4. A method according to any one of the preceding claims, wherein the aqueous solution has a pH of at least 10.

5. A method according to claim 4, wherein the aqueous solution has a pH of at least

13. 6. A method according to any one of the preceding claims, wherein the permanganate salt is an alkali metal permanganate, an alkaline earth metal ion permanganate or a transition metal permanganate. 7. A method according to claim 6, wherein the permanganate salt is potassium permanganate. 8. A method according to any one of the preceding claims, wherein the alkaline earth metal salt is a strontium salt. 9. A method according to claim 8, wherein the strontium salt is strontium nitrate. 22 10. A method according to any one of the preceding claims, wherein the solution has a temperature in the range from 15 to I 00 0 C. 11. A method according to claim 10, wherein the solution has a temperature in the range from 18 to 95C. 12. A method according to any one of the preceding claims, wherein the metal is subsequently subjected to a post treatment by being contacted by liquid containing silane which is dissolved or in a suspension. 13. A method according to claim 12, wherein the silane containing liquid contains at least 40% by weight ethanol and the balance water.

14. A method according to claim 12 or 13, wherein the silane is a long chain alkylsilane.

15. A method according to any one of the preceding claims, wherein the metal is cleaned prior to treatment.

16. A method according to any one of the preceding claims in which the metal is contacted with the aqueous solution under an external potential imposed by means of a counter electrode and a reference electrode.

17. A method according to any one of the preceding claims, wherein the treated metal is steel, zinc, aluminium or titanium.

18. A treated metal surface or body produced by the method of any one of the preceding claims, having formed thereon a coating formed of manganese containing solids. 23

19. A treated metal surface according to claim 18, wherein the manganese containing solids comprise pentavalent manganese. 24