WO1990010095A1

WO1990010095A1 - Polymeric coating compositions for corrosion protection

Info

Publication number: WO1990010095A1
Application number: PCT/AU1990/000075
Authority: WO
Inventors: Gordon George Wallace
Original assignee: Itc Uniadvice Limited
Priority date: 1989-02-24
Filing date: 1990-02-23
Publication date: 1990-09-07

Abstract

A polymeric coating composition including a polymer incorporating a selective, releasable corrosion inhibiting species. A method of inhibiting metallic substrates from corrosion including the steps of incorporating corrosion inhibiting species into a polymer coating, generating the coating onto a metallic substrate, releasing the corrosion inhibiting species on demand during corrosion. The polymer coating is preferably conducting and more preferably (I), where C- are corrosion inhibiting counterions, preferably chromate ions, EDTA, dithiocarbomate, phosphate or oxine, X is -NH, S or O, n is preferably between 2 and 4. A method of preparing (I) by oxidation of pyrrole is also disclosed.

Description

Title : POLYMERIC COATING COMPOSITIONS FOR CORROSION PROTECTION

FIELD OF THE INVENTION

This invention relates to the corrosion protection of metallic substrates and in particular to corrosion protection achieved by the use of polymeric coatings which contain corrosion inhibiting species and a method of preparation thereof.

DESCRIPTION OF THE PRIOR ART

In the past, corrosion protection of metallic substrates has been achieved in a variety of ways. For example, a sacrificial anode may be employed which preferentially corrodes. Alternatively, an inhibitor which causes precipitation of the corrosion product and consequently inhibition of further corrosion may be used. An example of such an inhibitor is chromate ions.

It is an object of the present invention is to provide at least an alternate composition and method for the protection of metal substrates against corrosion. SUMMARY OF THE INVENTION

According to one aspect of the invention, the invention comprises a polymeric coating composition including a polymer incorporating a selective,

releasable corrosion inhibiting species.

The corrosion inhibiting species or "inhibitor" may be an organic or inorganic anion and may be incorporated into the polymer coating as a counterion (Cˉ).

Preferred corrosion inhibiting species include

precipitating agents such as chromate ions (CroO)^2- or other complexing agents capable of reacting with the substrate metal, including ethylenediamine-tetraacetic acid (EDTA) or dithiocarbamate.

When incorporated as counterions (Cˉ), the

species preferably have sufficient mobility to be released from, or redistributed through the polymer. As corrosion commences the species react at sites of corrosion resulting in a blocking of further attack.

The polymer coating of this invention may be synthesised from any suitable monomer. Preferably, the polymer is a conducting polymer. Preferred conducting polymers may be prepared by the process illustrated as follows:-

where Cˉ is a corrosion inhibiting species, Y⁺ is the cation of the supporting electrolyte salt and n is preferably between 2 and 4. The properties of the polymer may be varied by varying the counterion Cˉ. Following polymerization, Cˉ may be released by either chemical or electrochemical reduction of the polymer.

In a second aspect, the invention comprises a method according to the following process for preparing a polymeric coating composition including a polymer incorporating a selectively releasable corrosion inhibiting species:

where Cˉ is a corrosion inhibiting counterion

preferably selected from the group consisting of

chromate ions (CrO₄ ^2-), EDTA, phosphate and oxine;

and X is preferably selected from the group consisting of -NH, S and O.

The coating may be generated either chemically or electrochemically and it is possible to generate the coating on the metal substrate.

The polymers of the present invention are

conducting and may be grown with strong adherent

properties on a variety of metal substrates including tin oxide or other metal oxides, stainless steel and galvanised surfaces.

These polymers are preferably stable and adherent for an extended period after attaching to the

substrate. Some of the conducting polymers (depending on the monomer and counterion employed) are preferably stable to at least 200°C. The conducting polymers are generally flexible materials.

The rate of growth of the polymer material is approximately 1 μm/min. However, in the case of

electrochemical deposition, this can be varied by adjusting the current density or in the case of chemical oxidation by varying the acidity of the solution and/or the concentration. With the onset of corrosion, the corrosion

inhibiting species from the coating is released

according to the process illustrated

as follows:

This ensures that the corrosion inhibiting species is only released at the onset of corrosion and that when corrosion is halted, then the release of the corrosion inhibiting species will cease. Thus, the inhibitor is released on demand and the mobility within the polymer ensures it is delivered to the site of corrosion.

Further, release of the corrosion inhibiting species causes the polymeric backbone to become less conductive, thereby halting the corrosion process.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 is a scanning electronmicrograph of

polypyrrole coated zincalum

Fig. 2 is an energy dispersive x-ray (EDX) spectrum of polymer coated zincalum Fig. 3 is an electrochemical cell set-up for an inhibitor release experiment

Fig. 4 is a U.V. - visible spectrum of Cr(VI)

released from a polymer

Fig. 5 is a graph of current v time during the

release of an inhibitor in an electrochemical cell in accordance with Figure 3.

Fig. 6 is an interpretation of Tafel plot

DESCRIPTION OF THE PREFERRED EMBODIMENT

Polymeric coatings according to the present

invention can be grown electrochemically on zincalum substrate. Of the monomers investigated, pyrrole has been found to be the most suitable for

electropolymerisation on zincalum substrate. The visual appearance of the material suggests that zinc is

dispersed throughout the polymer during

electropolymerisation. Polyalanine is another suitable conducting polymeric material.

Surface preparation of the zincalum substrate prior to coating improves the adherent properties of the coating. The inhibitor may usually be incorporated directly during electropolymerisation. Chromium is a preferred inhibitor and can be incorporated directly during electropolymerisation providing a surfactant is present. It is believed that the surfactant inhibits the oxidation of the zincalum substrate during

polymerization. Other suitable corrosion inhibiting species include phosphate, oxine, oxalate and hydroxy quinoline sulfonic acid.

Alternatively chromate containing layers can be grown on top of a surfactant containing polymeric coating. The thickness of such polymer coatings can be controlled by varying the electroplating time and may be air or oven dryed. Preferably, these polymers contain a corrosion inhibiting species in addition to the

surfactant. Preferred surfactants are short chain or amphiprotic.

Potentiodynamic or galvanostatic conditions can be employed during electropolymerisation. Galvanostatic polymer growth is preferred since it has previously been discovered that this results in more consistent polymer growth. Preferable current density is greater than or equal to 0.5 mA/cm². At higher current densities, oxidation of the zincalum substrate occurs.

Zincalum has been chosen as a test case and the ability to initiate polymerisation and chemical

triggering on this substrate have been investigated.

Optimisation of the solution conditions with respect to the monomer dichromate ratio enables direct incorporation of dichromate ions into a range of polymeric materials according to:

Alternatively, it was found that if the polymer was grown with an appropriate counterion such as nitrate, then the dichromate could be incorporated by

electrochemically assisted ion exchange after

polymerisation.

The above reactions were easily achieved using conventional electrode substrates such as gold, platinum or glassy carbon. Polymer thickness and morphology could be controlled using the appropriate

electrochemical conditions. Scanning electron

microscopy (Figure 1) and EDX (Figure 2) where used to investigate the morphology and confirm the incorporation of dichromate. It was established that the release of incorporated dichromate ions could be triggered

electrochemically by considering the EPMA results before and after potential cycling. It was found that after applying negative potentials dichromate was released from the polymer into solution. Initially electrochemical methods of coating were investigated and it was found that using appropriate solution conditions, which included the use of a surface active counterion such as sodium dodecyl sulfate, electropolymerisation on zincalum could be achieved.

Both polypyrrole and polyaniline coatings were

considered. It was found that the pyrrole based

polymers grew more readily and were more mechanically stable. These polymers were investigated further.

By using mixtures of the surfactant and dichromate in the polymerisation solution both counterions were incorporated in the polymer. It is possible that the dichromate loading could be increased by using

amphoteric surfactants which would provide the surface active properties and also some ion exchange to

incorporate dichromate.

Cyclic voltammograms recorded after growth

indicated that the dichromate could be released from the polymer coating electrochemically.

A galvanic cell set up (Figure 3) was used to verify this and the release of dichromate triggered by the onset of corrosion was monitored

spectrophotometrically.

Results summarised in Figure 4 show UV-visible spectra obtained before and after dichromate release from the polymer coating after the onset of corrosion. Figure 5 shows how the current which is proportional to the amount of dichromate released varies with time in sodium chloride and sodium nitrate media. Although the means of electropolymerising directly onto zincalum were established it was found that coatings produced in this way did not adhere well to the zincalum surface.

Presumably this is due to the fact that the zincalum substrate was subject to oxidation using the potentials required during polymerisation.

In attempts to overcome the above problems chemical polymerisation methods were investigated. It was found that using dichromate as the oxidant, in acidic media, the polymer could be prepared on the zincalum surface. It was also established using EDX that the dichromate ions were incorporated. Using this procedure the adhesion problems were overcome.

Principles here have been demonstrated using dichromate as the corrosion inhibitor. Preliminary experiments also indicate that a range of counterions (reagents) can be incorporated using the above

electrochemical or chemical methods. For example complexing groups such as oxalate, dithiocarbamate or sulphonated hydroxyquinoline have been incorporated on to zincalum. Phosphate is another common corrosion inhibitor that has also been incorporated.

Zincalum has been used for demonstration purposes. It has already been shown that such polymers may be coated on to stainless steel and a range of other metal surfaces.

In laboratory experiments, we have produced water and/or organic soluble polymers. These polymers also can incorporate counterions which should be amenable to triggered-release. Such an approach is important if the polymeric corrosion inhibitor is to be painted on to substrates.

Laboratory corrosion tests show that the coated zinc-alum is more corrosion resistant than the

non-coated material.

Alternative polymerisation methods include

U.V.-initiated polymerisation and chemical

polymerisation. It has been established previously by the applicant that chemical oxidants may be used in the formation of conductive polymers. It has been found that a procedure which involves the use of dichromate as the oxidant, resulting in a conducting polymer having dichromate incorporated is particularly suited to use in the present invention. Such polymers are prepared by the process illustrated as follows in the presence of an acidic acetonitrile solvent:

This procedure results in a strongly adherent film.

During electropolymerisation, substrate oxidation may be inhibited by the use of non-aqueous solvents. A preferred solvent for this purpose is acetonitrile with toluene sulfonate as a supporting electrolyte.

An electrocatalyst may also be utilized. A

particularly preferred electrocatalyst is "Tiron". We have found that the presence of catechols such as

"Tiron" facilitate electropolymerisation, reducing the initialisation potential by more than 100 M.V.

Triggered release of corrosion inhibitors can be achieved using conducting polymers of the present invention. Conducting polymers are known to release the incorporated counterion upon reduction of the polymer according to:

This process renders the polymer non-conductive and less porous, ensuring continued mechanical protection for the zincalum substrate. In another embodiment, the corrosion inhibitor is released in two phases. Further controlled release of the corrosion inhibitor is also possible.

Although the invention has been exemplified in the above description, it will be appreciated by those skilled in the art that modifications and/or additions can be made without departing from the broad aspects of the invention hitherto described and embodied in many other forms.

Following coating of zincalum using a chemical polymerization, corrosion tests were carried out:

The corrosion rate was tested by a simple approach i.e. Tafel plot. Scanning potential at low rate (e.g. 5mv/sec) and recording log I vs E, a so called Tafel plot can be obtained. An interpretation of a Tafel plot is shown in Figure 6. Corrosion rate (CR) can be calculated by the following equation:

0.13ι_corr .E_w

CR =

A.d

i_corr - Corrosion current from Tafel;

E_w - Equivalent weight of the oxidized element

A - Surface area of the sample

d - Density of the sample.

The corrosion result of polymer coated samples is shown in Table 1.

TABLE 1

Sample E_corr. (V) cor: (uA/cm²)

Zincalum -0.92 3.3

Zincalum edge-covered -0.91 0.63

Zincalum PP/Cr(VI)-coated -0.86 0.75

Zincalum edge-covered and

PP/Cr(VI)-coated -0.90 0.17 From the data it can be seen that a conducting polymer coated plate could last for 5 years but a bare plate may only last for 1 year.

Claims

1. A polymeric coating composition including a polymer incorporating a selective, releasable corrosion

inhibiting species.

2. A polymeric coating composition in accordance with Claim 1, wherein the polymer is a conducting polymer.

3. A polymeric coating composition in accordance with Claims 1 or 2, wherein the selective, releasable

corrosion inhibiting species are complexing agents such as chromate ions, EDTA or dithiocarbamate.

4. A polymeric coating composition in accordance with any one of the foregoing claims, wherein the species are incorporated as counterions.

5. A method of inhibiting metallic substrates from corrosion including the steps of incorporating corrosion inhibiting species into a polymer coating, generating the coating onto a metallic substrate, releasing the corrosion inhibiting species on demand during corrosion.

6. A method of preparing a polymeric coating

composition including the step of incorporating a releasable corrosion inhibiting species in accordance with the following process:

where Cˉ is a corrosion inhibiting counterion

X is selected from the group consisting of -NH, S and O.