WO2008049760A2

WO2008049760A2 - An electrical conductive substrate having a porous coating layer filled with a inherently conductive polymer

Info

Publication number: WO2008049760A2
Application number: PCT/EP2007/061008
Authority: WO
Inventors: Johan Vanbrabant; Rudi Werckx; Hilde Delrue
Original assignee: Nv Bekaert Sa
Priority date: 2006-10-24
Filing date: 2007-10-16
Publication date: 2008-05-02
Also published as: WO2008049760A3

Abstract

The invention relates to electrically conductive substrates coated with a coating layer having a superior corrosion and/or wear resistance. The coating layer has pores and these pores are at least partially filled with a material comprising an electrically non-conductive polymerized material formed by polymerization of inherently conductive monomers. The invention further relates to a method to seal the pores of a porous coating.

Description

An electrically conductive substrate having a porous coating layer filled with a material

Field of the invention. The invention relates to conductive substrates coated with a coating layer having a superior corrosion and/or wear resistance. The invention further relates to a method to seal the pores of a porous coating.

Background of the invention.

During the last years thermal sprayed ceramic coatings have been widely used because of their excellent wear resistance.

However, ceramic coatings obtained by thermal spraying typically show a porous structure. This porous structure is mainly the result of the pores created between the lamella of a thermal sprayed coating. Also micro-cracks present in the deposited coating contribute to the porosity of the coating.

In the porous structure of a thermal sprayed coating direct paths are created that interconnect the underlying substrate or the underlying coating with the environment. In corrosive environments this may lead to corrosion problems.

Thermal sprayed coatings are therefore unable to prevent the occurrence of corrosion.

As the pores negatively affect the corrosion properties of thermal sprayed coatings, large efforts have been made to reduce the porosity level by optimizing the thermal spraying conditions. However, a drawback of high density ceramic coatings is their high brittleness reducing the mechanical performance of the coatings.

An alternative way to reduce the porosity level of ceramic coatings is by sealing the pores, for example by using a resin or a molten metal as sealant. Sealing is a post-spraying treatment which means filling or closing of the near-surface pores and cracks. A major limitation of this technique is that only the pores close to the surface of the coating are sealed as the sealant can only penetrate in the pores in a limited way.

Although the corrosion protection is initially increased by the sealing treatment, once the coating is worn out to a certain degree or once the sealant is leached out, open pores will come in contact with the surface and the substrate is no longer protected against corrosion.

Inherently conductive polymers (ICP's) are known in the art. Generally,

ICP's are organic polymers that have poly-conjugated π electron systems (e.g. double bonds, aromatic or heteroaromatic rings or triple bonds). ICP's are able to conduct an electrical current due to a specific conjugated structure in the molecule. ICP's are for example used to impregnate porous materials to make these materials electrically conductive.

WO2004/062020 describes a porous material having an electrically conductive coating comprising an inherently conductive polymer.

Summary of the invention.

It is an object of the present invention to provide a coating layer having a superior corrosion and/or wear resistance.

It is another object of the present invention to provide a porous coating layer having its pores sealed.

It is a further object of the present invention to provide a method to seal the pores of a porous coating layer.

According to a first aspect of the present invention an article comprising an electrically conductive substrate and a coating layer having pores is provided. The pores of the coating layer are at least partially filled with a material. The material comprises a polymerized material formed by polymerization of inherently conductive monomers. The inherently conductive monomers are electrically conductive, however, once deposited the polymerized material is electrically non- conductive.

The electrically non-conductive material has preferably an electrical resistivity higher than 10⁴ ohm. cm and more preferably higher than 10⁵ ohm. cm or even higher than 10⁶ ohm. cm.

The electrical resistivity ot the inherently conductive monomers is preferably lower than 10¹ ohm. cm.

The pores and cracks present in the coating layer create channels towards the substrate. These channels are interconnecting the substrate with the outer environment.

The inherently conductive monomers penetrate in the channels preferably all the way to the substrate and the channels pores are at least partially filled with this material.

According to the present invention, the channels are filled at least at the interface of the substrate and the coating layer.

In one embodiment the channels are completely filled from the substrate to the outer surface of the coating.

Preferably, the channels and/or pores are at least partially filled with the material by electrochemical deposition of the inherently conductive monomers. The electrochemical deposition is preferably followed by polymerization.

The electrochemical deposition of inherently conductive monomers preferably comprises the application of a charge, preferably a positive charge to the substrate. As the charge is applied the inherently conductive monomers penetrate in the channels, preferably all the way the substrate. Once the inherently conductive polymers reach the surface oxidation takes place and the polymerization of the inherently conductive monomers starts to form a polymerized material. The -A-

polymerized material starts to fill the pores thereby starting to fill the channels at the interface of the substrate with the coating layer and continuing to fill the pores towards the outer surface of the coating.

Due to the oxidation at the interface of the substrate with the coating layer oxides such as metal oxides can be formed. The formation of these oxides may help to obtain a good adhesion with the polymerized material.

As inherently conductive monomers all monomers known in the art can be considered. Examples of suitable inherently conductive monomers are aniline, pyrrole, thiophene, phenylenevinylene, diacetylene, acetylene, quinoline, phenylenevinylene, heteroarylenevinylene and derivatives, copolymers and mixtures thereof.

One or more additional components can be added to or incorporated in the material to fill the channels and/or pores for example to influence the properties of the coating layer. Such additional components can for example be used to influence the mechanical, tribological and/or chemical properties of the coating.

Preferred additional components comprise negative groups such as inorganic or organic negative groups.

Examples of inorganic groups comprise phosphates, sulphates, chromates, molybdates, permanganates and nitrates.

Examples of organic groups comprise sulphonates, oxalates, formiates, thiols and organic sulphides.

In order to increase the corrosion resistance of a metal substrate it can be preferred to use negative groups as additional component. These negative groups interact with the metal substrate by increasing the corrosion potential (E_corr) of the metal. The corrosion potential (E_corr) is preferably increased until a passive behavior is reached. For steel substrates preferred negative groups to increase the corrosion protection comprise for example phosphates, chromates or nitrates.

The coating layer preferably comprises an electrically insulating material.

Preferably, the coating layer comprises a sprayed coating layer such as a thermal sprayed coating layer.

Examples of thermal spraying comprise high and low velocity oxyfuel spraying, electric arc spraying and plasma spraying.

The porosity level of the coating layer is preferably ranging between 0.5 and 20 %, for example between 2 % and 10 % such as 4 % or 5 %. For the purpose of this invention the porosity level is defined as the percentage of open surface (voids) of the total surface of a cross- section of the coating.

The porosity level of the coating layer can be influenced by the spraying conditions. For some applications it can be preferred that the porosity level is rather high to allow the material to penetrate in the channels and/or pores all the way to the substrate.

The coating layer is preferably a ceramic coating such as an oxide, a carbide or a doped oxide or doped carbide. Preferred ceramic coatings comprise alumina, titania, chromia and zirconia and mixtures thereof.

As substrate any substrate that is electrically conductive can be considered.

A preferred substrate comprises a metal substrate, such as a steel substrate, a metal alloy substrate or a metallized substrate, such as an insulating substrate provided with an electrically conductive coating. Possibly, an intermediate layer such as an adhesion promoting layer is applied on the substrate before the porous coating is applied. The intermediate layer can be applied by any technique known in the art for example by electrolytic deposition or by spraying.

According to a second aspect of the present invention a method to seal the pores of a coating layer at least partially is provided. The method comprises the steps of providing an electrically conductive substrate; - applying a coating layer on said electrically conductive substrate; said coating layer having pores; - filling said pores at least partially with a material by electrodeposition of inherently conductive monomers followed by polymerization.

As mentioned above, channels interconnecting the substrate with the outer environment are created by the pores.

In the method according to the present invention the inherently conductive monomers penetrate in the channels preferably all the way to the substrate and are deposited at the interface of the substrate with the coating layer by electrochemical deposition followed by polymerization.

By the polymerization reaction the polymerized material fills the channels starting from the interface of the substrate with the coating layer towards the outer surface of the coating layer.

A preferred method to seal the pores of the coating layer comprises immersing the substrate provided with a porous coating layer in a solution. The solution comprises inherently conductive monomers; applying a charge, preferably a positive charge to the substrate. A great advantage of the method according to the present invention is that the channels created by the pores are filled starting from the substrate towards the outer surface of the coating layer. This means that even if the coating layer is worn out to a certain degree, the substrate remains protected against corrosion.

A further advantage of the method according to the present invention is that the properties of the coating layer may be varied by using one or more additional components to the inherently conductive monomers. Additional components can for example be used to influence the mechanical, tribological and/or chemical properties of the coating.

The coating layer according to the present invention is in particular suitable to be used for application requiring a high corrosion resistance and/or wear resistance.

The coating layer according to the present invention can for example be used for offshore applications.

Brief description of the drawings. The invention will now be described into more detail with reference to the accompanying drawings wherein

Figure 1 is a schematic illustration of the process to fill the pores of the porous coating according to the present invention; Figure 2 shows the polymerization reaction of an inherently conductive polymer;

Figure 3 and Figure 4 show two embodiments of equipments to seal a porous coating layer according to the present invention.

Description of the preferred embodiments of the invention. The process to fill the pores of the porous coating according to the present invention is illustrated in Figure 1a to 1c. A porous coating layer 1 1 is deposited on an electrically conductive substrate 10. The coating layer 1 1 comprises for example alumina deposited by thermal spraying.

A high number of pores 12 is created in the coating layer 1 1. The porosity level of coating layer 1 1 is preferably higher than 4 % as for example 8 %.

The high number of pores 12 is mainly the consequence of the thermal spraying method.

The porosity can be influenced by the spraying parameters, the gas mixture used for spraying and the distance between the substrate and the spray gun.

At least part of the pores are creating channels 13 interconnecting the substrate with the outer environment.

To fill the pores of the coating layer 1 1 the coated substrate 10 is immersed in a solution comprising inherently conductive monomers 14.

The concentration of the monomers is for example 0.1 molar. Possibly, one or more additional components such as one or more negative groups is added. The concentration of such additional components is preferably between 0.1 and 10 wt% and more preferably between 0.2 and 3% wt% . The concentration depends for example on the polymerization reaction of the monomer.

The solution can be made in water, in an ionic solvent or in an organic solvent such as propylene carbonate, acetonitril, methanol, ethanol, propanol and acetone. The choice of the solvent is dependent upon the application. For carbon steel substrates, water is preferred as solvent. For substrates made of aluminium, titanium, chromium nickel or other alloys such as stainless steel alloys organic solvents such as propylene carbonate are preferred.

A positive charge is applied to the substrate 10. Subsequently, the inherently conductive monomers 14 are migrating through pore 12 and channel 13 towards the substrate 10. The substrate 10 is thereby functioning as an anode. Once the monomer 14 reaches the substrate 10 oxidation takes place and the polymerization of the inherently conductive monomers starts to form a polymerized material 15. The polymerized material 15 is growing starting from the interface of the metal substrate with the coating layer towards the outer surface of the coating layer 1 1.

Although the inherently conductive monomers 14 are electrically conductive, the polymerized material 15 is electrically non-conductive due to the oxidation.

In some embodiments of the present invention, metal oxides are formed at the surface of the substrate 10 due to the oxidation at the surface of the substrate 10. The formation of these oxides allows to obtain a good adhesion with the polymerized material 15.

Figure 2 shows an example of an anodic polymerization reaction of a inherently conductive monomers taking place at the surface of the substrate. step A comprises the electrochemical oxidation of the inherently conductive monomer 22 to form radical 24; - step B comprises the polymerization of the inherently conductive monomer 24 to form the polymer 26 (polypyrrole).

Figure 3 and 4 show two embodiments of equipments to fill the pores of a coating layer according to the present invention.

In Figure 3, a substrate coated with a porous coating layer 32 is immersed partially or completely in a bath 33. The bath 33 comprises a solution 34 comprising inherently conductive monomers and possibly one or more additional components. A power source 35 is negatively connected to a counter electrode 36

(the cathode) and positively connected to the substrate 32 (the anode).

As the power source 35 is switched on the monomers will penetrate into the pores of the monomers will polymerize. In Figure 4, an alternative equipment using a sponge system 43 is used to fill the pores of a coating layer of a coated substrate.

The corrosion resistance of sealed coated substrates according to the present invention was evaluated.

The coating layer comprises AI-Ti-oxide. The solution to seal the sample comprises 0.1 M pyrrole and 0.1 M oxalic acid. The current that is applied is 2 mA/cm².

The corrosion behavior of the sealed coated substrate was analyzed according to the standard procedure: Corrosion tests and standards: application and interpretation, ASTM MNL 20, pp. 75-80, ASTM G3-89, ASTM G5-82, ASTM G15-85a and ASTM STP 727 .

The electrolyte in which the corrosion analysis was performed was 0.05 M sulfuric acid (H₂SO₄) in ethanol water (1/1 ) mixture. The ethanol was added to the sulfuric acid solution in order to obtain a better wettability of the surface allowing the electrolyte to penetrate better in the pores of the coating layer.

The corrosion behavior is analyzed by measuring the corrosion current lco_rr- The higher the value of the corrosion current, the worse the corrosion resistance of the tested specimen will be. Next to the value of the corrosion current l_corr, another parameter, the so-called "inhibition rate" is used to evaluate the corrosion resistance.

The inhibition rate is defined in "Compendium of Chemical

Terminology", IUPAC Recommendations, Blackwell Scientific

Publications, 1987, p. 198 as :

I=(V₀ -V) / V₀

wherein I represents the rate of corrosion inhibition (in percent); V₀ represents the corrosion rate of a non-treated specimen with

V o~ lcorrj

V represents the corrosion rate of a treated specimen V=l_corr.

An inhibition rate of 50% corresponds to a lifetime multiplied by 2, an inhibition rate of 67% corresponds to a lifetime multiplied by 3, an inhibition rate of 90% corresponds to a lifetime multiplied by 10, an inhibition rate of 99% corresponds to a lifetime multiplied by 100, etc. The lifetime expectancy is an asymptotic approach towards 100% which would mean infinity and as such cannot be reached or is irrelevant.

The corrosion results are summarized below in table 1.

The corrosion potential is measured versus a standard hydrogen electrode (SHE).

Table 1

From the corrosion results it can be concluded that the lifetime of the coating layer has been doubled by sealing the coating layer according to the present invention.

Claims

1. An article comprising an electrically conductive substrate and a coating layer, said coating layer having pores, said pores being at least partially filled with a material, characterized in that said material comprises an electrically non-conductive polymerized material formed by polymerization of inherently conductive monomers.

2. An article according to claim 1 , whereby said pores create channels towards said substrate and whereby said channels are filled with said material at least at the interface of said substrate with said coating layer.

3. An article according to claim 2, whereby said channels are at least partially filled with said material by electrochemical deposition.

4. An article according to claim 3, whereby said electrochemical deposition is followed by polymerization.

5. An article according to any one of the preceding claims, whereby said inherently conductive monomers are selected from the group consisting of aniline, pyrrole, thiophene, phenylenevinylene, diacetylene, acetylene, quinoline, phenylenevinylene, heteroarylenevinylene and derivatives, copolymers and mixtures thereof.

6. An article according to any one of the preceding claims, whereby said material further comprises at least one additional component.

7. An article according to claim 6, whereby said additional component comprises at least one negative group selected from the group consisting of phosphates, sulphates, chromates, molybdates, permanganates, nitrates, sulphonates, oxalates, formiates, thiols and organic sulphides.

8. An article according to any one of the preceding claims, whereby said coating layer comprises a thermal sprayed coating layer.

9. An article according to any one of the preceding claims, whereby said coating layer has a porosity level ranging between 0.5 % and 20 %.

10. An article according to any one of the preceding claims, whereby said coating layer is a ceramic coating selected from the group consisting of oxides, carbides, doped oxides and doped carbides.

1 1. An article according to any one of the preceding claims, whereby said coating layer is selected from the group consisting of alumina, titania, chromia, zirconia and mixtures thereof.

12. An article according to any one of the preceding claims, whereby said substrate comprises a metal substrate, a metal alloy substrate or a metallized substrate.

13. A method to seal the pores of a coating layer, said coating layer being deposited on an electrically conductive substrate, said method comprising the steps of providing an electrically conductive substrate; applying a coating layer on said electrically conductive substrate; said coating layer having pores; - filling said pores at least partially with a material by electrodeposition of inherently conductive monomers followed by polymerization.

14. A method according to claim 13, whereby said pores create channels towards said substrate and whereby said channels after electrodeposition and polymerization are filled with said material at least at the interface of said substrate with said coating layer.

15. A method according to claim 13 or 14, whereby said inherently conductive monomers are selected from the group consisting of aniline, pyrrole, thiophene, phenylenevinylene, diacetylene, acetylene, quinoline, phenylenevinylene, heteroarylenevinylene and derivatives, copolymers and mixtures thereof.

16. A method according to any one of claims 13 to 15, whereby said material further comprises at least one additional component.

17. A method according to claim 16, whereby said additional component comprises at least one negative group selected from the group consisting of phosphates, sulphates, chromates, molybdates, permanganates, nitrates, sulphonates, oxalates, formiates, thiols and organic sulphides.

18. A method according to any one of claims 13 to 17, whereby said coating layer is applied on said substrate by thermal spraying.

19. A method according to any one of claims 13 to 18, whereby said coating layer has a porosity level ranging between 0.5 % and 15 %.

20. A method according to any one of claims 13 to 19, whereby said coating layer is a ceramic coating selected from the group consisting of oxides, carbides, doped oxides and doped carbides.

21. A method according to any one of claims 13 to 20, whereby said coating layer is selected from the group consisting of alumina, titania, chromia, zirconia and mixtures thereof.

22. A method according to any one of claims 13 to 21 , whereby said substrate comprises a metal substrate, a metal alloy substrate or a metallized substrate.