Process for the enhanced corrosion protection of valve metals

Abstract

A process for the corrosion protection of metals such as magnesium, aluminium or titanium, where at least two steps are used, including both plasma electrolytic oxidation and chemical passivation. The combination of these two processing steps enhances the corrosion resistance performance of the surface beyond the capability of either of the steps in isolation, providing a more robust protection system. This process may be used as a corrosion protective coating in its own right, or as a protection-enhancing pre-treatment for top-coats such as powder coat or e-coat. When used without an additional top-coat, the treated parts can still retain electrical continuity with and adjoining metal parts. Advantages include reduced cost and higher productivity than traditional plasma-electrolytic oxidation systems, improved corrosion protection, greater coating robustness and electrical continuity.

Claims

1. A process comprising: a plasma electrolytic oxidation step on a surface of a valve metal that comprises surface asperities; a chemical passivation step performed on the surface of the valve metal, wherein the combination of the plasma electrolytic oxidation step and the chemical passivation step forms an electrically insulating coating on the surface of the valve metal; and contacting the electrically insulating coating under high pressure with an adjoining metal component, wherein the electrically insulating coating is sufficiently thin that the surface asperities of the valve metal surface project through the electrically insulating coating to allow galvanic electrical continuity from the surface asperities to the adjoining metal component, and wherein the coating includes an oxide layer impregnated with a chemical passivating agent configured to provide active corrosion protection of the valve metal in an event of physical breach of the oxide layer.

2. A process according to claim 1, wherein the chemical passivation step precedes the plasma electrolytic oxidation step.

3. A process according to claim 1, wherein the plasma-electrolytic oxidation step precedes the chemical passivation step.

4. A process according to claim 1, wherein chemical passivation steps are performed both prior to and after the plasma-electrolytic oxidation step.

5. A process according to claim 3, wherein the plasma-electrolytic oxidation step generates an oxide coating having pores, and wherein the subsequent chemical passivation step does not physically seal the pores of the oxide coating.

6. A process according to claim 1, wherein the chemical passivation step comprises application of a liquid, the liquid not being in the form of a sol gel.

7. A process according to claim 1, wherein some or all of the surface of the valve metal is treated with the plasma-electrolytic oxidation step.

8. A process according to claim 1, wherein some or all of the surface of the valve metal is treated with the chemical passivation step.

9. A process according to claim 1, further comprising a pre-treatment regime of at least one of degreasing, etching or de-smutting to clean the surface of the valve metal prior to the plasma electrolytic oxidation and chemical passivation steps.

10. A process according to claim 1, further comprising a post-treatment regime consisting of at least one of rinses in water, pH-neutralising rinses, primer or sealer solutions.

11. A process according to claim 1, wherein the valve metal comprises at least one of magnesium, aluminium, titanium, tantalum, zirconium, chromium, vanadium, cobalt, hafnium, or molybdenum.

Description

DETAILED DESCRIPTION

(1) Magnesium, aluminium, and titanium, and their alloys are all susceptible to corrosion under certain environmental conditions. Their corrosion resistance can be significantly enhanced by either of the two main categories of surface treatment processes described in the prior art: chemical passivation and plasma electrolytic or micro-arc oxidation. However, each of these surface protection processes has limitations which can be overcome by embodiments of the present invention.

(2) Chemical passivation, using the systems described in the prior art (and in commercial practice, proprietary chemical systems such as Henkel's Alodine™ 5200, or Chemetall's Gardobond™ X4707), provides enhanced corrosion protection by rendering the surface less chemically susceptible to corrosion processes. The chemicals act through a combination of chemical surface conversion and deposition of a thin layer of protective compounds such as chromates, fluorozirconates or phosphates which will react preferentially with any exposed metal surface to provide lasting, active protection of the metal against a corrosive environment. The most efficient chemical passivation treatments are those involving chromates but these are now of limited popularity due to their toxicity, so phosphate and fluoride based passivation systems are now more common though less effective.

(3) Among the limitations of the chemical passivation processes is their susceptibility to mechanical damage. The thin chemical conversion coatings present no significant mechanical performance enhancement to the metal surface because they are such thin, soft, layers. Furthermore, they are highly susceptible to the cleanliness of the metal surface onto which they are deposited. Any greases, dye lubricants or mould release agents from metal forming processes, or any significant levels of pre-existing corrosion products such as oxides present a physical barrier to the chemical passivation solutions. In typical applications, particularly on magnesium, a pre-treatment or sequence of pre-treatment steps is performed to sequentially de-grease, etch or “deoxidise” and de-smut the metal surfaces to leave a clean metal surface immediately prior to the chemical passivation step. Examples are given in U.S. Pat. No. 5,683,522. Nevertheless, some alloys such as AE44 magnesium prove particularly difficult to clean sufficiently for typical commercial products and the resulting passivated film is discontinuous and provides limited corrosion resistance.

(4) Convex corners or any sharp radii on parts provide additional problems for chemical passivation processes because, as with many fluid deposition processes, surface tension results in a thinning effect whereby the deposited layer is thinner on such features. This again results in a non-uniform film and in areas of relatively poor protection. These convex corners may be particularly vulnerable to corrosion because they are likely to occur on exposed edges where liquid corrosive agents may accumulate or where mechanical damage to parts is more likely. In corrosion testing (e.g. ASTM B117 neutral salt spray exposure), it is common for corrosion to initiate at such features. Many topcoats applied to the passivated part suffer from the same thinning on edges and corners and this makes matters worse.

(5) The Keronite® plasma electrolytic oxidation (PEO) process (as embodied in U.S. Pat. Nos. 6,365,028 and 6,896,785 for example) is a proprietary process which is widely used in industry to form a relatively thick, hard, protective oxide film by surface conversion of the magnesium, aluminium and titanium into corresponding oxides. On aluminium alloys, for example, alumina is formed, in both amorphous and extremely hard crystalline forms. On magnesium and its alloys, magnesia is formed, sometimes with magnesium aluminium spinels to incorporate any aluminium in the substrate metal. Anomag™ (as embodied in U.S. Pat. No. 5,792,335) is another proprietary process for micro arc oxidation technology, which forms a magnesium phosphate coating on magnesium. They are both electrolytic immersion processes which employ high potentials and high current densities to induce micro plasma discharges which modify the growing oxide film.

(6) Generally speaking, the PEO processes convert the metal surface into an oxide layer which presents a protective barrier against corrosion by isolating the substrate metal from corrosive environments. Because it is a hard yet compliant, semi-crystalline oxide ceramic, the PEO layer provides a level of mechanical protection to the substrate metal. On magnesium and aluminium, for instance, PEO films can significantly out-perform tool steel or hard anodised aluminium in terms of sliding wear or abrasive wear protection as demonstrated by testing equivalent thicknesses of each coating type according to ASTM G99 and G65 respectively.

(7) The surface hardening and other protection is particularly good on edges or sharp convex radii, which naturally result in enhanced electrical field strength on any non-spherical metal component. This enhanced electrical field strength is a preferential state for plasma electrolytic oxidation and accelerates the process, resulting in enhanced growth and oxide layer thickness on such features. Thus, enhanced mechanical robustness is provided for edges and sharp convex radii. This effect can be promoted further by selecting specific processing regimes which enhance the thickness of edges over that of plane surfaces.

(8) The Keronite® or Anomag™ PEO processes also result in a fine network of surface-connected pores which greatly enhance the surface area of the processed part and facilitates liquid impregnation and top-coat adhesion [“Porosity in plasma electrolytic oxide coatings”, J. A. Curran and T. W. Clyne, Acta Materialia 54 (2006) pp 1985-1993]. This is of benefit when using the plasma electrolytic oxide layer as a pre-treatment for powdercoat, e-coat, or the other top-coats described in the prior art, but is also immediately relevant to the treatment of the coating with chemical passivation agents. The plasma electrolytic oxide layer's fine, permeable, pore structure is readily wetted by many liquid systems, including a wide range of known chemical passivation agents (such as zinc dihydrogen phosphate (Zn(H.sub.2PO.sub.4).sub.2), fluorozirconates and chromates and others described in the prior art). As a result, whenever a plasma electrolytic oxidation step is performed on a component's surface prior to immersion in a chemical passivating agent, the quantity of chemical passivating agent retained in the resulting composite layer is significantly higher than for a bare metal surface. The composite layer of plasma electrolytic oxide and chemical passivating agent thus has a significant reserve of chemical passivating agent, which can provide enduring active chemical passivation to the underlying metal whenever a physical breach of the barrier film occurs.

(9) A limitation of PEO surface treatments in terms of corrosion protection is that, like any barrier film protection, they are vulnerable to corrosion whenever the barrier film is breached. This is where the presence of passivating chemical compounds in the layer can offer continued, lasting protection.

(10) This is one of the benefits of embodiments of the present invention, examples of which combine the barrier film effect and the mechanical robustness of the Keronite® or Anomag™ micro arc oxide layer with the chemical protection afforded by the chemical passivation agent. It is of particular significance that the plasma electrolytic oxidation processes offer enhanced edge protection while this is an area of weakness for many chemical passivation systems. Thus, in each of these cases, one of the treatment systems is enhancing a weakness of the other, thereby providing a surprisingly advantageous technical effect.

(11) Another significant benefit of the duplex process is that the plasma electrolytic oxidation processes, by virtue of their high energy density, are able to electrochemically clean the metal surface, making the duplex coating system less susceptible to surface contamination and to the quality of cleaning pre-treatments.

(12) This is particularly significant on alloys such as the magnesium rare-earth alloy AE44 where even lengthy pre-treatment sequences of de-greasing, etching and de-smutting tend to leave a substantial level of surface contamination on the metal. This inhibits the reaction and adhesion of the chemical passivating agent and leaves areas with relatively poor protection. Although degreasing and other pre-treatment steps are still preferable when using plasma electrolytic oxidation technology, the high energy density of the process, and the resultant plasma discharge conditions, are sufficient to electrochemically clean the surface, and form a protective oxide film on previously greasy or smutty regions of the surface, or even areas where residue of mould-release agents has not been successfully removed prior to treatment. As such, even in one of its simplest embodiments as a two-step process, with either the plasma electrolytic oxidation step preceding the chemical passivation step, or the chemical passivation step preceding the plasma electrolytic oxidation step, embodiments of the present invention result in a more continuous level of surface corrosion protection because there will be no regions where surface contamination has inhibited action of the chemical passivating agent.

(13) Embodiments of the present invention combine benefits of the two protection processes, namely the mechanical robustness of a plasma electrolytic oxide layer, the enhanced protection of convex corners or edges, the insensitivity to metal pre-treatment condition, the excellent base for impregnation or mechanical keying and adhesion of top-coats, the uniformity of the chemical passivation system, and the enduring, active chemical protection against corrosion provided by chemical passivation compounds.

(14) Embodiments of the present invention also enable the use of relatively thin layers of plasma electrolytic oxide coating, as compared to conventional plasma electrolytic oxidation technology, while still maintaining the required corrosion performance. This represents an efficiency gain in terms of the required processing energy and time, but is also of great benefit where electrical continuity is required with adjoining metal parts (for example in electromagnetic shielding applications or where spot welding is to be performed) since this can only be achieved when the thickness of the electrically insulating plasma electrolytic oxide layer is sufficiently low to allow contact between surface asperities. The use of relatively thin layers of plasma electrolytic coating can allow electrical or galvanic continuity through the coating from the underlying metal to an adjoining conductive (e.g. metal) component, even when the coating itself is not electrically conductive. This is achieved by way of sharp surface asperities which may project through the coating, especially where the coated article is in high-pressure contact with another part, which can result in a degree of erosion or displacement of the coating. Ideally, the chemical passivation provides some active corrosion protection even if the physical ceramic coating layer is breached.

(15) A further benefit of embodiments of the present invention is that plasma electrolytic oxidation coatings, since they require an electric field to generate them, have a limitation of throwing power into holes, crevices, recesses and other areas that are electrically shielded. On the other hand, chemical passivation requires only contact of the passivating liquid with the metal so has no such limitations. Therefore, the combination offers enhanced protection in the areas of the part that are shielded from the electric field in the plasma electrolytic oxidation process.

(16) It is anticipated that a wide range of pre-treatments may be used with the process of the present invention. Although the plasma electrolytic oxidation step is relatively insensitive to the pre-condition of the surface, it may still be preferable to use a standard industrial cleaning or de-greasing step in order to minimise contamination of the electrolyte subsequently used. Examples of alkali cleaning stage include aqueous solutions of NaOH or KOH with detergent additives that may be applied either by spraying or immersion. Those skilled in the practice of industrial pre-treatment for metals will recognise viable alternatives. Where the plasma electrolytic oxidation step follows a chemical passivation process, sometimes intermediate rinsing may be required to remove surplus passivation chemicals and sometimes no further intermediate steps may be needed, apart from any rinse or drying specified within the individual chemical passivation process.

(17) Whenever a chemical passivation step is used prior to a plasma electrolytic oxidation step, a more extensive pre-treatment, (including, for example deoxidation in an acid solution wherever magnesium is being pre-treated) is still preferred, in order to maximise the effectiveness of the chemical passivation. All commercial chemical conversion coatings include recommendations for pre-treatments for particular alloy systems and it is expected that these would be used. Again, those skilled in the practice of chemical conversion treatment will recognise many suitable variants for the pre treatment of different metals for chemical passivation.

(18) Some chemical passivation treatments require post-treatment rinses and/or drying, while others do not. Again, it is anticipated that the recommendations of the individual process be followed.

(19) Wherever chemical passivation follows the plasma electrolytic oxidation step, intermediate rinses may be used to remove residual electrolyte from the surface and pore structure of the oxide layer. These may include a town water rinse, followed by a Dl water rinse, or pH-neutralising rinses.

EXAMPLES

Example 1

(20) A multi-part magnesium case for electronic components made from AZ91D where electrical continuity (<5Ω contact resistance) is required between treated parts (in order to maintain electromagnetic shielding) and corrosion performance is to be sufficient to endure 96 hours of corrosion in salt for (ASTM B117) with less than 10% corroded area. #

(21) Three different processing regimes were evaluated. For each, the pre-treatment regime was a typical industrially used sequence of commercially available proprietary chemicals: i) 3 minute dip in Henkel Ridoline 305 caustic alkaline cleaner (˜0.4% KOH and 0.1% anionic surfactant in Dl water) ii) Town water rinse iii) Dl water rinse iv) 3 minute etch in Henkel HX 357 sulfuric acid etch (˜0.8% H.sub.2SO.sub.4, 0.15% H.sub.2SiF.sub.6 in Dl water) v) Town water rinse vi) Dl water rinse
Immediately following this pre-treatment, parts were processed in three distinct ways: a) 1 minute phosphate-permanganate chemical conversion coating as described, for example, in WO 2004/022818 b) Plasma electrolytic oxidation for 30 seconds, but in other respects, as per example in WO 03/083181 c) Treatment (a) followed by an intermediate Dl water rinse and then treatment (b) for 20 seconds, but in other respects, as per example in WO 03/083181
Process (c) is thus an embodiment of the present invention, while (a) and (b) are examples of prior art.

(22) The ohmic resistance of the three samples was evaluated using a resistance meter, with one probe in contact with a bare part of the substrate metal and a 20 mm diameter brass disc as the other contact point. The parts were then subjected to 120 hours of salt fog exposure (ASTM B117) and then inspected for corrosion and re-evaluated for contact resistance.

(23) The results were as follows:

(24) TABLE-US-00001 Ohmic Ohmic resistance Corrosion after resistance Coating before 96 hours B117 after variant: corrosion: salt fog exposure: corrosion: a) Conversion <1 Ω >75% area: >20 MΩ coating only general corrosion b) Plasma <5 Ω ~15% area: 50-20 MΩ electrolytic heavy corrosion oxide only c) Combined <3 Ω <5% area: <5 Ω coatings mild corrosion

(25) Thus, variant (c), which is one of the embodiments of the present invention, provides significantly better corrosion performance than either the conversion coating or the plasma electrolytic oxide coating in isolation, while also allowing adequate electrical continuity both before and after testing.

Example 2

(26) Magnesium components made from AM50A alloy requiring a chrome-free pre-treatment for polyester powdercoat Akzo Nobel MN204E to meet or surpass the corrosion performance of an existing chromate based conversion coating.

(27) Four processes were assessed—two examples of prior art and two embodiments of the present invention. All started with standard cleaning and etching steps based on commercially available, proprietary chemical systems from Chemetall PLC: i) 2 minutes in alkali Cleaner Gardoclean S5167 (˜0.1% KOH) at 50° C. ii) 30 second tap water rinse iii) 1 minute in Gardoclean S5240 (5 g/l ) at 40° C. iv) 30 second tap water rinse v) 30 second Dl water rinse
This pre-treatment was followed by four different processing variants, two of which—a) and b)—represent prior art, and two of which—c) and d) represent embodiments of the present invention: a) 3 minutes in Henkel Alodine 5200 (˜0.04% H.sub.2TiF.sub.6 with additives in Dl water) b) Plasma electrolytic oxidation for 3 minutes, but in other respects, as per example 2 in U.S. Pat. No. 6,896,785 c) Treatment with Alodine 5200 as per variant a), followed by plasma electrolytic oxidation as per variant b) d) Plasma electrolytic oxidation as per variant b), followed by in intermediate Dl water rinse for 2 minutes, and then treatment with Alodine 5200 as per variant a)

(28) The samples were then dried for 1 hour at 70° C. and powdercoated with a polyester-based powder coat (in this case Akzo Nobel MN204E). The samples were scribed and testing was performed which involved daily cycles of 15 minute immersion in 5% NaCl solution, drying, and 20 hour exposure to 90% relative humidity.

(29) After 10 cycles of this test, variant a) showed signs of corrosion at the edges and in the vicinity of the scribe line. After 30 cycles, much of the powder coat had fallen off at the edges and significant blistering and corrosion creep up to 8 mm from the scribe line had occurred. After 40 cycles of the test, variant b) showed no sign of corrosion at the edges but some blistering and corrosion creep were visible up to 3 mm from the scribe line. On variants c) and d), the corrosion creep was maximum 1 mm from the scribe line.

Example 3

(30) Aluminium 1050 architectural components, requiring pre-treatment to ensure durability of polyester powdercoat in accelerated testing to satisfy architectural lifetime standards for a powdercoat 25 year guarantee.

(31) The aluminium parts were all de-greased for two minutes at 55° C. by immersion in an alkaline solution including an anionic surfactant such as sodium or potassium tartrate (in this case, Chemetall “Gardoclean T5378” at 33 g/l: disodium tetraborate 10-25%, tetrasodium pyrophosphate 10-25%, fatty alcohol polyglycol ether 2.50-10%, and anionic surfactant at 1-2.5%), and rinsed for two minutes in de-ionised water. The parts were then treated according to several different pre-treatment methods prior to rinsing, drying and powdercoating; some (a, b and c) typical of chemical pre-treatment (as per the respective chemical manufacturers' guidelines) alone, one of plasma electrolytic oxidation alone (d), and three consisting of hybrid, synergistic processes representative of the present invention (e, f and g): a) Immersion in an alkaline solution of a silane designed for surface activation and adhesion promotion (10 g/l of Chemetall's “Gardolene D6870” which includes 2.5-10% of 3-aminopropyl triethoxysilane) for 2 minutes at room temperature. b) Immersion in an acidic solution of fluorozirconate (10 g/l of Chemetall's

(32) “Gardobond X4707”, which includes 1-2.5% of hexafluorotitanic acid) for 1 minute at room temperature. c) Immersion in an acidic solution of fluorozirconate (50 g/l of Henkel's “Alodine 5200”) for 1 minute at room temperature. d) Plasma electrolytic oxidation processing for 5 minutes at 1 A per dm.sup.2 in an alkaline electrolyte (as per U.S. Pat. No. 6,896,785). e) PEO processing as per sample d), followed by a 2 minute rinse in de-ionised water and then immersion in an alkaline solution of silane as per sample a). f) PEO processing as per sample d), followed by a 2 minute rinse in de-ionised water and then immersion in an acidic solution of fluorozirconate as per sample b). g) PEO processing as per sample d), followed by a 2 minute rinse in de-ionised water and then immersion in an acidic solution of fluorozirconate as per sample c).

(33) The samples were then dried for 1 hour at 70° C. and powdercoated with a polyester-based powder coat (in this case Akzo Nobel MN204E). Testing was then performed on scribed plates with cut edges, with 2000 hour acetic acid accelerated salt spray testing as per ISO 9227, and 2000 hour cyclic humidity testing as per BS 3900:F2.

(34) The only samples to pass both of the 2000 hour tests (the respective pass criteria applied were corrosion on less than 5% of the surface and blistering on less that 5% of the surface, with undercutting limited to a maximum of 1.5 mm at the scribe or cut edges) were samples e), f) and g).

(35) This confirms the synergistic element of the two technologies, with neither of the independent technologies fully satisfying the test criteria by themselves.

(36) Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

(37) Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. 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. The invention is not restricted to the details of any 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.

(38) 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.