WO2017005582A1

WO2017005582A1 - A surface treatment for enhanced resistance to corrosion and synergistic wear and corrosion (tribocorrosion) degradation

Info

Publication number: WO2017005582A1
Application number: PCT/EP2016/065218
Authority: WO
Inventors: David CULLITON
Original assignee: Dublin Institute Of Technology
Priority date: 2015-07-03
Filing date: 2016-06-29
Publication date: 2017-01-12
Also published as: GB201511739D0; GB2539965A

Abstract

The present application relates generally to the field of coatings and more specifically to providing a coating which provides protection against tribocorrosion. The provision of the coating comprises applying a wear resistant coating to the surface, immersing the surface in a precursor sol- gel solution having an electrolyte, and applying an electrical potential to the surface to cause electrodeposition. The coating of the invention is particularly suitable for use as a conformal coating.

Description

TITLE

A SURFACE TREATMENT FOR ENHANCED RESISTANCE TO CORROSION AND SYNERGISTIC WEAR AND CORROSION (TRIBOCORROSION) DEGRADATION

FIELD

The present application relates generally to the field of coatings and more specifically to providing coatings which provide protection against tribocorrosion degradation.

BACKGROUND

Surface treatments to-date have generally focused on providing either wear or corrosion resistance. Unfortunately, whilst any particular treatment may offer protection against one or the other, they are generally ineffective in relation to the synergistic effect of the two degradation processes of wear and corrosion, i.e. tribocorrosion. Tribocorrosion phenomena involve the interactive effect of tribological and corrosive environments. Modern product design exposes many components to these combined degradation processes, resulting in premature and often catastrophic failure of key structures. In many instances costly procedures are employed to replace failed components, which often involve production shut-downs. In other instances, where such component replacement is de rigueur, companies incorporate costly fail-over systems which also require constant maintenance and additional overhead costs.

It will be appreciated that in order to avoid such costs and for reliability and increased life of products generally, materials when likely to be exposed to such an environment are typically selected in modern design to provide a degree of tribocorrosion resistance. Current techniques employ expensive bulk materials, typically based on stainless steel, nickel- based alloys and high-end carbon-/glass- reinforced composites. These materials are however expensive, both to purchase and to

machine/manipulate.

An alternative approach is to employ a less expensive material with reduced tribocorrosion properties and to provide a coating or other protection on the material to provide a degree of tribocorrosion

resistance.

Traditionally, tribocorrosion resistance has been provided by chromium- based treatments, such as chromating and hard chrome plating. However, their use in industry is diminishing due to continued global pressure from government agencies to eliminate the use of potentially carcinogenic treatments.

An alternative to chromating treatments is described in US 5,932,083 and provides for the electrodeposition of a cerium based coating on

aluminium. Unfortunately to be effective, it was identified that the deposited layer must be less than 1 micron (preferably around 0.3 micron) as values above this were found to be prone to cracking and delamination. Additionally, the pH value of the electrolytic bath is identified to be a key element as too low a pH results in an improper composition and too high a value results in limited, if any, deposition. The ideal pH range is

identified as between 3 and 6. Other treatments involving sol-gels as pre- treatments and sealants also require strong pH control (Mekeridis et al., Adv. Ceram. Sci. Eng. 1 2012 1-10) with highly uniform pores and low molecular-sized precursors (Whelan et al., Surf. Coat. Technol. 235 2013 86-96). Moreover, whilst the coatings indicate a limited improvement in corrosion resistance there is no significant provision for wear resistance.

Thermal Spray coatings are applied for high-wear resistance applications while organic systems, often based on paint with a sol-gel or REM (rare earth metal) pre-treatment, are the preferred treatments for corrosion resistance. Galvanising, anodising and nickel-plating are also used for corrosion and/or wear resistance but are typically confined to use in less aggressive environments. Alternative high-cost solutions include Vapour Deposition coatings and surface laser remelting techniques. However, even with the reduced inherent porosity in these surface treatments corrosion can progress rapidly and catastrophically in the presence of aqueous environments. Accordingly, there is a need for an improved method of treating surfaces prone to tribocorrosion degradation. SUMMARY

One of the reasons why coating solutions such as Thermal Spray coatings, anodising and tin plating are less effective in aggressive environments is that the resulting coatings are normally porous and require additional sealing by another treatment. Unfortunately, these sealing treatments are not infusive (i.e. surface-bound only) and tend to be poor for wear resistance. Accordingly subsequent wear action quickly removes the protective layer, exposing the porous coating underneath.

The present application provides a coating of a Rare Earth Metal (REM)- based hybrid organic/inorganic material as a pre-treatment or as a sealant layer. The coating of the present invention is highly advantageous for use as a conformal coating. A conformal coating is a coating which is able to "conform" to contours, in particular, the coating of the present invention can advantageously function as a conformal coating, able to "conform" to complex contours such as on a printed circuit board (PCB) so as to protect the copper inlays. Typically, a conformal non-conducting coating is applied to electronic circuitry to act as protection against moisture, dust, chemicals, and temperature extremes which, were the PCB not to be protected by such a coating, are likely to result in damage or failure of the electronics to function. The coating of the present invention is ideal as such a conformal non-conducting coating. (The coating of the present invention is also referred to by the trade mark, /Seal™ ). The coating of the present invention provides a low-porosity treatment and is applied in a way which ensures that the low-porosity treatment is provided as a pre- treatment for subsequent organic treatments (such as on copper patterns on PCBs and flexible circuits) or infuses into the pores of a pre-applied engineering coating, sealing the inherent open, or active, porosity of the underlying wear-resistant coating or bonding layer material (such as on the Thermal Spray coated wear faces of pump impellers). As a result, even when the surface-bound surface-layer coating of the present invention is removed, through wear action, the pores of the underlying wear-resistant coating or bonding layer material remain sealed, maintaining the effective barrier action.

By sealing the inherent active porosity in the coating, initial and long-term corrosion rates are significantly reduced and effective resistance to synergistic wear and corrosion degradation (tribocorrosion) is increased. Accordingly, a first embodiment of the present invention provides a repeatable deposition process as detailed in claim 1. The present invention also provides a method as detailed in claim 1.

Accordingly, in accordance with claim 1, the present invention provides a method of treating a suitably prepared surface for corrosion (with post- applied organic/corrosion resistant treatment) or tribocorrosion (with a pre-applied wear-resistant or hard coating) resistance, the method comprising the steps of:

(a) immersing the surface in a precursor solution of natural pH comprising an electrolyte; and

b) applying an electrical potential to the surface to cause

electrodeposition.

By the term, "natural pH", it is meant that no acid or alkaline medium is intentionally added to the precursor solution. Thus the method of the present invention enables the coating of the invention to be applied as a sealant coating [post-treatment] and/or as a bonding coating [pre-treatment]).

Advantageously, the electrolyte of the precursor solution functions as an electrolyte during deposition and coating build up; but subsequently, functions as an inhibitor. The electrolyte is preferably, REM-based.

More preferably, the electrolyte is a REM salt, ideally, an REM Nitrate.

The present invention also provides a coating as claimed in Claim 15. Advantageous embodiments are provided in the dependent claims.

Brief Description of The Drawings

The present application will now be described with reference to the accompanying drawings in which:

Figure 1 is a schematic of the three-cell electrodeposition configuration Figure 2 is schematic of the cross-section of the final surface treatment on a pre-applied porous wear coating; and

Figure 3 illustrates the performance of the method with reference to experimental results.

DETAILED DESCRIPTION OF THE DRAWINGS

The surface treatment presented herein is based on a near-neutral pH solution with added inorganic elements, which act as a catalyst and enhance electrolytic performance during electro deposition whilst subsequently becoming embedded in the deposited coating and

performing an inhibitive role on exposure to a corrosive environment. By carefully controlling the timing and development of the hydrolysis and condensation reactions, and thus solution viscosity, during electro deposition, substantially increased pore penetration is achieved. In addition, surface thicknesses of the treatment can be strictly controlled in applications where tolerances are low. Thicknesses for this treatment after 15 minutes deposition are typically 30-50nm. Higher deposits (>1μιη) are achieved through solution composition manipulation, variation in applied potential and longer soaking and exposure times.

The increased pore-penetration provides extensive long-term protection to the substrate and, dependant on the galvanic nature of the wear coating/substrate system, extremely high corrosion resistances have been measured for up to 2000 hours. The treatment is applied by

electrochemical means. The process will now be described in greater detail with reference to Figure 1.

An initial step in the process is the provision of a wear resistant coating onto a surface to be protected; or to use a surface on which a wear resistant coating has been previously applied. Examples of suitable wear resistant coatings would include, but are not limited to, the examples shown in table 1 with exemplary methods of application shown in the second column.

Exemplary Coating Exemplary Method of

Application

Hot-dip galvanising,

Galvanising

electrogalvanisation

Silicon Carbide, Aluminium Titanium Vapour deposition, including Nitride Coating (AITiN), Chromium Chemical Vapour Deposition, Carbide Coating (CrC), Chrome Nitride, Metal-Oxide Chemical Vapour

Deposition, Physical Vapour Deposition

Powder Metal coatings, including

Powder Metalising

aluminium and bronze powders

Electroless Nickel Electroplating

It will be appreciated that a wide variety of other wear resistant coatings may equally be employed to coat the surface. As explained previously, a problem with applying a coating such as by means of a thermal spray onto a reactive metal such as Aluminium Alloys is that the inherent through, or active, porosity in the coating exposes the substrate at various points over the substrate surface. In the presence of a corrosive environment, this exposure of the substrate at various points can create a (micro) galvanic cell which accelerates the corrosion of the substrate metal and can lead to extensive spalling of the coating.

Although some prior art coatings have reduced porosity, the presence of active pores will undermine the system at the points where the active pores exist and will lead to spalling of the coating.

For example, under controlled conditions, when exposed to aggressive anions (eg CI^" or S^2"), typical Thermal Spray coated metal alloys (eg aluminium alloys, steel, magnesium alloys, bronzes/brasses) begin to corrode within 60 seconds. Even with relatively benign coatings, such as alumina, corrosion of the substrate will still occur, though not as quickly. The standard solution for this is to apply a sealant treatment to the Thermal Spray coating.

This known treatment really has one objective, namely to fill the active pores with an inactive material and prevent ingress of the corrosive media. However, current treatments are applied by painting, spraying or spin-coating and are extremely ineffective as these treatments are surface-bound only and do not promote ingress into the active or open pores.

The present invention uses an electrodeposition process to seal the active pores but by manipulation of the sealant (organic/inorganic) solution and controlling the hydrolysis/condensation reactions (and thus control of the solution viscosity), significant improvements in the corrosion performance have been achieved using the method and coating of the present invention More specifically, the present invention provides a method comprising the step of immersing a substrate in an organic/inorganic solution; this step may be provided as a primary step (pre-treatment for organic coatings); or as a secondary step wherein the immersing step is provided as a post- treatment for pre-applied wear-resistant coatings) in which case, the coated substrate is immersed in the organic/inorganic solution in accordance with the method of the present invention.

Following the step of immersing, after a suitable soaking time has elapsed, a voltage is applied to initiate electrodeposition. The soaking time is suitably less than 10 seconds, more suitably a time in the range generally of 3-5 seconds. Longer periods in excess of 300 seconds may be necessary for large and/or highly complex profiles.

An exemplary sol-gel solution which has successfully been employed as the organic/inorganic solution, in the method of the present invention, is based on a siloxane monomer, for example PTMOS.

However, solutions useable as the organic/inorganic solution, in

accordance with the present invention, may also be based on siloxane monomers with a wide range of molecular weights such as mono-silane systems, (eg MTEOS, TEOS, TMOS, VTMOS, MAPTMS), or bis-silane systems (BTSE and BTESPT).

The siloxane monomer is suitably combined with a low-molarity (molar concentration, mol/dm³) inorganic electrolyte, for example a rare earth metal (REM) nitrate such as Ce(N0₃)₃ or La(N0₃)₃, at molarities <10⁴ but most effective at molarities <10^~6. The present invention is not intended to be restricted, nor is it restricted, to these REM nitrates; and it is to be understood that other REM nitrates including, for example, Y(NOs)3 [Yttrium Nitrate], Sc(NOs)3 [Scandium Nitrate], Nd(NOs)3 [Neodymium Nitrate], can alternatively be used.

As explained above, the method of the present invention uses a near- neutral pH solution. The pH (natural pH) of the solution is suitably in the range between 5 and 7. Better results have been obtained when the pH has been limited to the narrower range of between 5.5 and 6.5. By maintaining the pH of the solution in this near neutral range, it is believed that accelerated hydrolysis of the sol-gel treatment is prevented prior to electrodeposition. As a result, penetration of the sol solution into the pores is enhanced and hydrolysis of the sol-gel is only really initiated upon application of the voltage. This enhances the depth of penetration of the sol-gel treatment and dramatically improves the corrosion performance of the treated systems.

The REM nitrate acts as the electrolyte during deposition and coating build up. It subsequently functions as an inhibitor. However, because of the way it performs these two functions, the REM, such as Ce(lll)/Ce(IV), can become embedded into the sol-gel treatment. The embedded REM then acts as a real-time cathodic and anodic inhibitor during exposure to corrosive environments. Application of low-voltage potentials at or near the measured OCP ± IV of the exposed system but most effective at potentials within the OCP range of ± 0.5V of the exposed system expedites the hydrolysis/condensation reactions thus promoting deposition of the organic/inorganic treatment on the exposed surfaces which seals the active pores.

The step of deposition will now be described in greater detail with reference to an exemplary configuration.

Sealant Deposition

An exemplary solution (sol gel) to provide a volume of 65 - 75 ml for use in the electro deposition suitably comprises:

30ml mono-silane precursor (such as PTMOS), 30ml organic solvent (Ethanol) and 5-15 ml distilled water (higher amounts of water increase the hydrolysis rate during subsequent deposition).

It will be appreciated that the overall volume may be scaled up to produce larger solution volumes for use in industrial scale processes.

The solution (of natural pH) may be obtained by:

1. Mixing the precursor and the solvent. The mixing may use magnetic stirring or suitable stirring process to ensure adequate mixing.

2. Adding the distilled water. Better results have been obtained where the water is added gradually, e.g. dropwise.

3. Adding a REM-nitrate, such as Ce(N0₃)3 or La(N0₃)3, to make a ΙμΜ solution, stirring on a magnetic stirrer at 50-80 rev/min.

Following suitable sample preparation, Electrodeposition (ED) may be carried out, for example, using a three-cell structure arrangement as exemplified in Figure 1. The arrangement comprises a reference electrode (R.E.), a counter electrode (C.E.). The surface onto which the coating is to be deposited acts as the working electrode (W.E.). The C.E. and W.E. have surface area ratios of a minimum of 1:1. The R.E. is suitably Ag/AgCI. The C.E. is suitably platinum and may be in the form of a mesh. A potentiostat (P) functions by maintaining the potential of the working electrode at a constant level with respect to the reference electrode by adjusting the current at the current electrode.

The above process describes a laboratory-scale deposition, with surface areas of approximately 2cm². However, facile scale-up of the technology is built into the process and surface areas typical of alternate

electrodeposition processes, such as electroless nickel and anodising, are easily achievable.

The process of applying the treatment comprises the steps of:

1. Configuring the ED equipment as indicated in Figure 1, with the sample to be coated connected as the Working Electrode.

2. Immerse the sample (eg light metal alloy) in the solution, while

continuing to stir gently.

3. Following a 5 second "soak" or "OCP settling" time, electrodeposition is performed at potentials between -800 to -600 mV (vs Ag/AgCI) but most effective at -755 mV. The performance of the electrodeposition step may depend on number of factors including the surface to be treated. Optimum conditions to obtain optimum results may therefore be determined by experimentation. However, it has been found that a duration for the electrodeposition step to obtain reasonable results is a period of 900 seconds. Alternatively, a two-cell arrangement (working electrode, counter electrode) may be used but the applied electrical potential must be adjusted.

4. After electrodeposition, the surface being coated may be removed

from the electrodeposition solution and allowed to cure. It will be appreciated that curing may require heating the surface, for example by placing the surface in an oven. For the exemplary coating process described above, it has been determined that using a temperature of 100°C for a period of about 30 min results in reasonable results.

The resulting coated surface is extremely durable. Experiments have been performed on treated samples in seawater solutions at room temperature and typical results are illustrated in Figure 3. These treatments were deposited from a treatment solution with 13ml distilled water. As indicated, the reduction in the corrosion rate due to the presence of the sealant treatment was a factor of approximately 10³, where the initial wear coating was provided by High Velocity OxyFuel (HVOF) / Flame Spray, and 10⁵ where the initial wear/hard coating was provided by Atmospheric Plasma Spray (APS) and Anodised coatings compared to that of unsealed samples. As may be seen, at any given immersion time (6 hrs, 1 day, 4 days, ...) the corrosion rate without applying the coating of the present invention and in accordance with the method of the present invention, is higher. Thus the corrosion rate is approximately 10³ (Flame Spray / HVOF) to 10⁵ (APS, Anodised) times higher than the corrosion rate that takes place on a substrate that has been treated using the coating and method of the present invention, for an equivalent immersion period.

It is to be understood that various modifications can be made within the scope of the invention as defined by the appended Claims.

Claims

1. A method of treating a suitably prepared surface for corrosion or tribocorrosion resistance, the method comprising the steps of: a) immersing the surface in a precursor solution comprising an electrolyte; and

b) applying an electrical potential to the surface to cause

electrodeposition.

A method according to claim 1, wherein the electrolyte functions as the electrolyte during the electrodeposition process and

subsequently, functions as an inhibitor by ad-/ab-sorption into the surface treatment.

A method according to claim 1 or claim 2, wherein the electrolyte is REM based.

A method according to claim 3, wherein the electrolyte is a REM salt.

A method according to claim 4, wherein the electrolyte is an REM Nitrate.

6. A method according to any preceding claim, wherein the precursor solution has a pH in the range of 4.5 - 8.5.

7. A method according to claim 6, wherein precursor solution has a pH in the range 5.0-8.0.

8. A method according to any preceding claim, wherein the deposition process is an electrochemical deposition process.

9. A method according to claim 8, wherein the electrochemical

deposition process is configured to apply a potential between a working electrode and a reference electrode.

A method according to claim 9, wherein the applied potential within the range OCP ±1 V

11. A method according to claim 9, wherein the applied potential is within the range OCP ±0.5 V

12. A method according to any preceding claim, wherein the solution is a sol-gel.

13. A method according to claim 12, wherein precursor sol-gel solution comprises a siloxane monomer e.g. a mono-, bis- silane or mixed silane precursor.

14. A method according to any preceding claim, wherein the step of applying the wear resistant and/or hard coating comprises the use of a thermal spray, anodising or galvanising process.

15. A method as claimed in any one of claims 1 to 14 wherein the method is carried out on a surface which does not have any prior coating already applied thereto.

16. A method as claimed in any one of claims 1 to 14 wherein the

method is carried out on a surface to which a coating has previously been applied.

17. A surface having a first layer of wear resistant and/or hard material and an infused second layer of corrosion resistant material wherein the second layer of corrosion resistant material covers the first layer and corrosion resistant material fills pores of the first layer, preferably, wherein the second layer comprises a REM.

18. A surface according to claim 17, wherein the thickness of the

second layer is less than 1.5 microns.

19. A surface according to claim 17, wherein the thickness of the

second layer is less than 1500nm.

20. A method as claimed in claim 1, substantially as herein described in the Examples and with reference to the accompanying drawings.

21. A coating substantially as herein described, with reference to and as shown in the accompanying drawings and in the Examples.