WO2013083293A1

WO2013083293A1 - Anti-corrosion system for steel

Info

Publication number: WO2013083293A1
Application number: PCT/EP2012/005102
Authority: WO
Inventors: Tapan Kumar Rout; Anil Vilas Gaikwad; Theo Dingemans; Mikhail Zheludkevich; Joao TEDIM; Kiryl YASAKAU
Original assignee: Tata Steel Nederland Technology Bv; Tata Steel Limited
Priority date: 2011-12-08
Filing date: 2012-12-10
Publication date: 2013-06-13
Also published as: EP2788437A1

Abstract

The present invention relates to a method of manufacturing a coated steel substrate which comprises the steps of: (i) providing a steel substrate; (ii) preparing a first coating mixture comprising nanocontainers with nanoscale corrosion inhibitors contained therein; (iii) preparing a second coating mixture comprising a curable organic component; (iv) combining the first coating mixture and the second coating mixture; (v) applying the combined mixture on the steel substrate; (vi) curing the combined mixture so as to produce a dense network structure of the coating for barrier and active corrosion protection of the steel substrate.

Description

ANTI-CORROSION SYSTEM FOR STEEL

FIELD OF THE INVENTION The present invention relates to a method of manufacturing a coated steel substrate, the coated steel substrate thus produced and to the use of the coated substrate in automotive, building or construction applications.

BACKGROUND OF THE INVENTION

It is well known that metal substrates such as steel corrode in oxidising environments and that such corrosion compromises the longevity of the substrate. Metallic coatings may be applied to steel to inhibit corrosion, for instance by hot-dip galvanising, but the application of organic coatings in lieu of metallic coatings is deemed to offer a more cost effective approach.

Polymeric coatings primarily act as an impermeable physical barrier to prevent or at least reduce the underlying steel substrate from coming into contact with corrosive elements and/or compounds such as water, oxygen, corrosive chemicals or ionic salts such as NaCI. However, when the polymeric coating is scratched or damaged, the corrosive entities can enter the damaged site to degrade the exposed steel substrate, and in certain instances accelerate the corrosion process.

To overcome the above disadvantage polymeric coatings may be provided with corrosion inhibitors such as chromates, phosphates or zirconates which form a thin protective layer on the surface of the steel substrate, a so-called passivation layer, which prevents further corrosion by the corrosive entities. However, the use of chromium compounds is now prohibited due to their toxic and carcinogenic nature.

A further disadvantage of providing polymeric coatings with corrosion inhibitors is that the corrosion inhibitors may react with the polymer precursor, i.e. before and/or during curing, which may cause the cured polymeric coating to exhibit reduced barrier properties.

For building and construction applications, organic coated strip products typically comprise a zinc or zinc alloy coated steel strip substrate having a passivation layer thereon, a polymeric coating as a primer and a topcoat. Organic coated strip products comprising conventional primer layers are susceptible to blister formation and filiform corrosion which reduces the longevity of organic coated strip product. Another challenge of such systems is to ensure that the polymeric coating is compatible with the conversion layer and the topcoat otherwise the topcoat and/or the primer will delaminate from the substrate surface. One object of the invention is to provide a polymeric coating on a steel substrate, that when scratched, acts to inhibit the corrosion process.

Another object of the invention is to provide a polymeric coating that is more environmentally acceptable.

A further object of the invention is to improve the corrosion protective properties of polymeric coatings comprising corrosion inhibitors. A further object of the invention is provide a polymeric primer coating that when used in organic coated strip products for building and construction applications reduces the formation of blisters and filiform corrosion.

A further object of the invention is to provide a polymeric primer coating that is compatible with both a zinc or zinc alloy coating and a topcoat.

The first aspect of the invention relates to a method of manufacturing a coated steel substrate for use in construction or automotive applications which comprises the steps of:

(i) providing a steel substrate;

(ii) preparing a first coating mixture comprising nanocontainers with nanoscale corrosion inhibitors contained therein;

(iii) preparing a second coating mixture comprising a curable organic component;

(iv) combining the first coating mixture and the second coating mixture;

(v) applying the combined mixture on the steel substrate;

(vi) curing the combined mixture so as to produce an organic polymeric coating for barrier and active corrosion protection.

The method according to the first aspect of the invention has the advantage that corrosion inhibitors present in the coating do not adversely affect the stability and the barrier properties of the coating. Since the nanoscale corrosion inhibitors are contained in the nanocontainers prior to the step of combining the first and second coating mixtures, the possibility of the nanoscale corrosion inhibitors chemically interacting with the curable organic component and/or other components present in the coating mixture is avoided or at least reduced.

The inventors also found that when the coating of the coated steel substrate is damaged, for instance by abrasion, to an extent that a section of the steel substrate becomes exposed to the external environment, the nanoscale corrosion inhibitors are released from the nanocontainers in response to changes in pH, ionic strength and/or the presence of certain ions in a corrosive solution. The 'intelligent' release of nanoscale corrosion inhibitors thus initiates a self-healing process that inhibits further corrosion of the substrate thereby increasing its working lifetime.

In a preferred embodiment the nanocontainers comprise layered double hydroxides, halloysites, CaC0₃, polymeric containers or mixtures thereof. In the case of layered double hydroxides and halloysites, nanoscale corrosion inhibitors are released in response to a change in local surroundings (pH, ionic strength, corrosive solution), whereas in the case of polymeric containers, the nanoscale corrosion inhibitors are released when the polymeric container itself is damaged. Preferred polymeric containers, for instance in the form of a capsule, comprise polyurethane. The inventors found that loaded LDH nanocontainers provide effective active corrosion protection during the early stages of corrosion, whereas loaded halloysites afford better long term active corrosion protection. Polyetherimides which comprise a mixture of loaded LDH and halloysite nanocontainers are therefore characterised by improvements in both short and longer term active corrosion protection relative to polyetherimides comprising loaded LDH or halloysites alone.

Layered double hydroxides (LDH) are a class of clay that have proven particularly effective as nanocontainers for the controlled release of nanoscale corrosion inhibitors. Layered double hydroxides are layered clay materials comprising positively charged hydroxide layers that are separated by layers of anionic nanoscale corrosion inhibitors and water. When the coating is damaged and upon exposure to a solution containing corrosive anions such as chlorides, an exchange reaction occurs between the nanoscale corrosion inhibitor in the Layered double hydroxide and the corrosive anion, i.e. anionic nanoscale corrosion inhibitors are released into the solution and the corrosive anion is adsorbed into the layered double hydroxide structure thereby inhibiting the further corrosion of the steel substrate.

Halloysites are aluminosilicate clay minerals having a substantially hollow tubular structure, which makes them particularly suitable nanocontainers for receiving and entrapping nanoscale corrosion inhibitors. Halloysites used in accordance with the invention have a length dimension between 20 nm and 25 μιτι and have an internal diameter between 10 and 150 nm. Preferably the halloysite has a particle size dimension between 1 and 15 μιτι.

Nanocontainers may be defined as containers having nanoscale dimensions which are suitable for receiving and confining nanoscale corrosion inhibitors inside the nanocontainer. More specifically a nanocontainer may be defined by its internal diameter which should be in the nanometer range.

In a preferred embodiment the nanocontainers have a particle size dimension between 20 nm and 25 μιη, preferably between 20 nm and 15 μιτι, more preferably between 180 nm and 15 μιτι. The use of nanocontainers, e.g. between 20 nm and 25 μιτι, but particularly between 20 and 180 nm means that active corrosion protection can be obtained over a large surface area at relatively low nanocontainer concentrations.

In a preferred embodiment the steel substrate contains in weight %: 0 - 1.0% C, 0 - 5.0% Mn, 0 - 2.0% Si, 0 - 2.0 % Al and 0 - 1.0 % Cr. Preferably small amounts of other alloying elements may also be present e.g. V, Nb, Ti and B.

In a preferred embodiment the steel substrate is a carbon steel containing in weight %: 0.04 - 0.30 % C, 1.0 - 3.5 % Mn, 0 - 1.0 % Si, 0 - 1.0 % Al and 0 - 1.0 % Cr, preferably 0 - 0.2% C, 0 - 2.0% Mn, 0 - 0.6% Si, 0 - 1.0% Al and 0 - 0.6 % Cr, the remainder being iron and unavoidable impurities. Other alloying elements such as Mo, P, Ti, V, Ni, Nb and Ta can be present but only in small amounts. Such carbon steels are preferred for building and construction applications.

In a preferred embodiment the steel substrate is a TRIP steel, which contains in weight % 0.10 - 0.30 % C, 1.0 - 3.5 % Mn, 0.2 - 0.8 % Si and 0.5 - 2.0 % Al and 0 -1.0 Cr, preferably 0.10 - 0.20 % C, 1.0 - 2.0 % Mn, 0.2 - 0.6 % Si and 0.5 - 1.5 Al. It is preferred that Si + Al does not exceed 1.5 wt%. Preferably the steel substrate is a TWIP steel that is suitable for use in automotive applications, which contains in weight % between 10 and 40 % manganese, preferably between 12 and 25 % manganese and up to 10 % aluminium. Both of the above TRIP and TWIP steels are preferred substrates for automotive applications.

In a preferred embodiment the first coating mixture comprises 1-50 wt%, preferably 2.5-20 wt%, more preferably 2.5-10 wt% of nanocontainers with corrosion inhibitors contained therein. The inventors found that a nanocontainer (with nanoscale corrosion inhibitors contained therein) content of 2.5-10 wt% was particularly effective at improving the barrier and active corrosion protection properties of the coating relative to coatings where nanocontainers (with nanoscale corrosion inhibitors contained therein) were absent from the first coating mixture. Further improvements in corrosion protection were possible, as the content of corrosion inhibitor loaded nanocontainers was increased up to 50 wt%. However, coating mixtures comprising more than 50 wt % of nanocontainers were difficult to process and resulted in coatings having reduced barrier properties. On the other hand coating mixtures comprising less than 1 wt% of corrosion inhibitor loaded nanocontainers exhibited reduced active corrosion protection.

In a preferred embodiment the nanocontainer is surface modified with a surface modifying component to provide a reactive functional group on the nanocontainer surface. The inventors found that by modifying the nanocontainer surface with a surface modifying component that the problems associated with nanocontainer agglomeration, particularly in water based solutions, were avoided or at least reduced to an extent that the nanocontainers could be uniformly distributed throughout the coating. Uniform distribution of the nanocontainers is preferred since thinner coating layers can be obtained. Moreover, uniform distribution ensures that active corrosion protection is not limited to localised areas of the coating but available throughout the whole of the coating layer. Surface modifying the nanocontainers also has the advantage that the nanocontainer surface can be tailored to improve the interactions between the nanocontainer and the polymeric coating so as to avoid interface failure and improve barrier properties without having to provide an additional top-coat.

In a preferred embodiment the surface modifying component comprises an organofunctional silane and a polyelectrolyte. Organosilanes proved to be particularly effective as surface modifying components to improve the compatibility of the nanocontainers with the polymeric coating. Another approach is to modify the nanocontainer surface with one or more layers of polyelectrolyte to form electrostatically connected layers on the nanocontainer, i.e. to form a polyelectrolyte shell. Using this approach the release properties of the nanocontainers may be tuned in such a way that the nanoscale corrosion inhibitors are only released in response to certain stimuli e.g. changes in pH.

In a preferred embodiment the organosilane comprises alkylsilanes, arylsilanes, alkoxysilanes, aminosilanes or mixtures thereof. Advantageously the surface properties of the nanocontainers can be tailored by selecting organosilanes comprising different reactive functional groups. Organosilanes comprising amine, carboxylic acid, isocyanate, ester and epoxy functional groups are particularly preferred.

In a preferred embodiment the polyelectrolyte comprises polystyrenesulfonate (PSS), poly(allylamine hydrochloride (PAH), poly(anetholesulfonic acid sodium salt) (PAS), poly(4- vinylpyridine) (P4VP), polyethylenimine (PEI), poly(acrylic acid) (PAA). It is particularly preferred that the polyelectrolyte shell comprises alternate layers of positively charged polyelectrolyte e.g. (PAH) and negatively charged polyelectrolyte e.g. (PSS), which may be deposited using the technique of layer- by-layer deposition.

In a preferred embodiment the nanoscale corrosion inhibitors comprise anionic corrosion inhibitors and cationic corrosion inhibitors. Preferred corrosion inhibitors, all of which are suitable for loading in layered double hydroxides, halloysites and polymeric containers, include aluminium phosphate, sodium gluconate, sodium molybdate Na₂Mo0₄, cerium molybdate Ce2(Mo0 )₃, cerium nitrate Ce(N0₃)₃, calcium nitrate Ca(N0₃)₂, zinc sulfate ZnS0₄, sodium tungstate NaW0₃, sodium phosphomolybdate hydrate Na₃Mo₁₂0₄oP, sodium phosphate Na₃P0₄, sodium hydrophosphate Na₂HP0₄, sodium dihydrophosphate NaH₂P0₄, sodium carbonate Na₂C0₃, sodium polyphosphate NaP0₃x, sodium gluconate, 2- mercaptobenzothiazole, benzimidazole, quinaldic acid, sodium citrate, glycine, 8- hydroxyquinoline, sodium salycilate, sodium benzoate, 1-Hydroxyethylidenediphosphonic acid (etidronic acid) , nitrilo-tris-phosphonic acid , N,N dimethyl amine , di-azo compounds , Cu- thalocyanine , dyes tartrazine (TZ)). In a preferred embodiment the curable organic component comprises polyamic acid or a derivative thereof. When the combined coating mixture is cured the curable organic component (polyamic acid) is converted into a polyetherimide polymer coating, which itself, affords the underlying steel substrate both corrosion and barrier protection. This coating further acts as a matrix to support the uniformly dispersed nanocontainers to maintain the uniformity of the dispersion.

In another preferred embodiment the polyamic acid comprises an aliphatic polyetherdiamine and preferably a polyetherdiamine that contains at least one primary amino group attached to the terminus of a polyether backbone, wherein the polyether backbone is based either on propylene oxide (PO), ethylene oxide (EO), or mixed EO/PO. Particularly suitable first polyetherdiamines include 0,0'-Bis(2-aminopropyl) polypropylene glycol-Woc/ -polyethylene glycol-Woc -polypropylene glycol (J1 ), 4,7, 10- trioxa-1 ,13-tridecanediamine (J2), Polypropylene glycol) bis(2-aminopropyl ether having a molecular weight 230 (J3), Poly(propylene glycol) bis(2-aminopropyl ether having a molecular weight of 400 (J4) and 1 ,2- bis(2-aminoethoxyethane) (J5).

The inventors found that polyetherimide coatings prepared from polyamic acids comprising at least one primary amino group attached to the terminus of a polyether backbone, wherein the polyether backbone is based either on propylene oxide (PO), ethylene oxide (EO), or mixed EO/PO, exhibited comparable or better corrosion resistance and improved flexibility relative to polyetherimide coatings prepared from polyamic acids comprising aromatic anhydrides and aromatic diamines. Moreover, the glass transition temperature (Tg) of such polyamic acids is less than the Tg of polyamic acids comprising aromatic anhydrides and aromatic diamines and therefore lower temperatures may be employed to cure the combined coating mixture comprising the polyamic acid as organic curable component.

In a preferred embodiment the content of the organic curable component in the second coating mixture is between 10 and 25 wt%.

In a preferred embodiment the combined mixture is cured at a temperature between 80 and 250°C , preferably using Infrared, induction, electron beam or radiation heating. In a preferred embodiment the applied combined coating mixture is water based. Such a coating mixture may be prepared in one of two ways. A first approach comprises the step of preparing the first coating mixture and the second coating mixture in a water based solution and mixing the said solutions prior to applying the mixed solution on the substrate. This has the advantage that that the method does not make use of organic solvents, some of which are harmful, toxic and difficult to dispose of and handle. A second approach comprises the steps of preparing the curable organic component in an organic solvent, precipitating the prepared organic curable component from the organic solvent, filtering and drying the precipitate and providing the dried precipitate in water to form a water based second coating mixture comprising the organic curable component. The water based second coating mixture may then be mixed with a water based first coating mixture comprising the nanocontainers and nanoscale corrosion inhibitor contained therein to form a combined water based coating that is suitable for application on the steel substrate. The advantage of preparing the organic curable component in organic solvent is that higher molecular weight organic curable components can be obtained, which once cured, result in coatings that exhibit increased formability and corrosion resistance relative to coatings comprising organic curable components that were prepared in water.

In a preferred embodiment the steel substrate is provided with a corrosion protective coating prior to applying the combined mixture. This has the advantage that the steel substrate is afforded additional corrosion and barrier protection in addition to the active corrosion protection provided by the coating comprising the nanocontainers and the nanoscale corrosion inhibitors. Thus, the working lifetime of the coated steel substrate is prolonged. Preferably the corrosion protective coating comprises zinc or a zinc alloy wherein the zinc alloy comprises Zn as the main constituent, i.e. the alloy comprises more than 50% zinc, and one or more of Mg, Al, Si, Mn, Cu, Fe and Cr. Zinc alloys selected from the group consisting of Zn- g, Zn-Mn, Zn-Fe, Zn- Al, Zn-Cu, Zn-Cr, Zn-Mg-AI and Zn-Mg-AI-Si are preferred and afford additional corrosion protection to the underlying steel substrate. Corrosion protective coatings comprising silanes or zirconium improve the adhesion of the coating to steel substrate and are therefore also preferred.

The second aspect of the invention relates to the coated steel substrate produced according to the method of the first aspect of the invention. The embodiments and the advantages relating to the first aspect of the invention similarly apply to the second aspect of the invention. In a preferred embodiment of the invention the dry film thickness of the coating is between 1 and 25 μιη, preferably between 3 and 10 μιτι and more preferably between 3-6 μιη. Although the nanocontainers of the present invention may have a length dimension up to 25 μητι, it is still possible to obtain a dry film thickness of the coating with the ranges specified above because the nanocontainers align parallel and not perpendicular to the substrate surface.

The third aspect of the invention relates to the use in an organic coated strip product for building and construction applications. The coated steel substrate according to the second aspect of the invention may advantageously be used in an organic coated strip product for building and construction applications. Organic coated steel products typically comprise a steel substrate, a zinc or zinc alloy coating, a passivation layer typically comprising chromium compounds, an organic primer and a top coat on the organic primer. The polyetherimide coating of the invention exhibits very good corrosion protection, flexibility and adhesion properties to both the zinc or zinc alloy and the topcoat meaning that it is no longer necessary to provide an organic coated strip comprising a passivation layer. This has the advantage that the manufacture of organic coated strip products is made easier (less process steps), less expensive and more environmentally acceptable. A further advantage is that the polyetherimide coating comprising corrosion inhibitor loaded nanocontainers reduce both the formation of blisters and filiform corrosion in the organic coated strip product. Moreover, in response to a change in local environment, the nanoscale container releases the nanoscale corrosion inhibitor contained therein for active corrosion protection.

The coated steel substrate according to the second aspect of the invention may also be used in the automotive industry where corrosion protection is of high importance. Specifically the polyetherimide coating may replace zinc or zinc alloy corrosion protective coatings or supplement said zinc or zinc alloy coatings to further increase the level of corrosion protection afforded to the steel substrate.

EXAMPLES Embodiments of the present invention will now be described by way of example. These examples are intended to enable those skilled in the art to practice the invention and do not in anyway limit the scope of the invention as defined by the claims.

Figure 1 shows a graph of polarisation resistance (R_p) as a function of immersion time for undoped polyetherimide coatings (A), polyetherimide coatings comprising 10 wt% sodium gluconate loaded halloysites (B), polyetherimide coatings comprising 10 wt% sodium gluconate loaded LDH that were aged in sodium chloride solution for 3 hours (C) and polyetherimide coatings comprising 10 wt% sodium gluconate loaded LDH that were aged in sodium chloride solution for 72 hours (D). A high R_p is indicative of a reduction in the rate of corrosion.

Example 1 :Loadinq halloysites with corrosion inhibitors

A dispersion of halloysite powder in aqueous solution and a solution of sodium gluconate corrosion inhibitors were added to a reaction vessel and mixed at room temperature by mechanical stirring. The reaction vessel was then evacuated using a vacuum pump to entrap the sodium gluconate corrosion inhibitors inside the halloysite nanocontainer. This solution was centrifuged to remove excess sodium gluconate and subsequently dried. The process was repeated four times to ensure saturation of the inner surface of the halloysite nanocontainer with precipitated sodium gluconate. Example 2: Loading LDH with corrosion inhibitors

The loading of sodium gluconate corrosion inhibitors in LDH nanocontainers occurs via an ion- exchange reaction between an easily displaceable anion (e.g. chlorides, nitrates) and sodium gluconate.

An aqueous solution of sodium gluconate (0.1 M) (120 mL) was prepared and divided into a first portion (60 mL) and a second portion (60 mL). LDH was added to the first portion under a nitrogen atmosphere to form a LDH dispersion and maintained under stirring for several hours (16-24h) at room temperature for the ion-exchange reaction to proceed. The resulting slurry was centrifuged between 2000 and 25000 rpm and subsequently washed with distilled water. The procedure was repeated for the dispersion of LDH into the second portion of sodium gluconate solution. The obtained LDHs were then dried in oven at 60°C for 8-16h and ball- milled to prevent the agglomeration of LDHs into micron-sized particles. Example 3: Preparation of a water based polyamic acid solution

A 100 mL one necked vessel equipped with a nitrogen inlet is charged with 2,2' - (Ethylenedioxy)bis(ethylamine) J5 (3.5 mmol, 0.5187 g), m-phenylenediamine (1.5 mmol, 0.16 g) and NMP (23g). 4,4-Biphthalic anhydride (5 mmol, 1.51 g) is added and this solution is stirred under inert conditions for 8 hrs to form polyamic acid. N-butyldiethanol amine (5 mmol, 0.8g) is added to this stirred solution, which is stirred for an additional hour. The stirred solution is then added to acetone or an acetone/methanol mixture under mechanical stirring causing the polyamic acid to precipitate. The precipitate is dried at 50°C. A 10 wt% solution of the dried precipitate is prepared in water; if necessary 1 wt % of N-butyldiethanol amine may be added to ease the dissolution.

Example 4: Preparation of a water based polyamic acid solution comprising nanocontainers and corrosion inhibitors

A dispersion of sodium gluconate loaded halloysite nanocontainers (30 wt%) was prepared in a water based solution according to the method of example 1. This solution was then added drop-wise to the water based polyamic acid solution under continuous mechanical stirring for a period of five minutes, after which the stirred solution was subjected to ultrasonic agitation for one minute to prevent halloysite agglomeration.

Example 5: Coating application

The water based polyamic acid solution comprising sodium gluconate loaded halloysite nanocontainers was applied on cold rolled steel using either by roll coating or spray coating. The applied coating was then dried at 80 °C for 5 minutes and cured at 250^°C for 5 minutes to produce a 'self healing' polyetherimide coated steel substrate. Experiment 1 - Active corrosion protection

Active corrosion protection properties were determined using electrochemical impedance spectroscopy (EIS). The experimental set up comprises a working electrode (coated steel substrate whose characteristics are to be evaluated), a reference electrode (calomel) and a counter electrode (Ni). The working electrode area was selected using a Teflon holder that exposed a disk of area 12 cm². Impedance measurements were performed using an EG&G PARC 273A potentiostat and a Solartron 1255 frequency response analyzer controlled by a microcomputer running ZPLOT software (Scribner Associates, Charlottesille, VA). Impedance values were determined at five discrete frequencies per decade between 10 and 65 kHz. The experimental data thus obtained was fitted with Randies' equivalent circuit model using ZSIM/CNLS software (Scribner Associates).

Polyetherimide coatings comprising sodium gluconate loaded halloysite or LDH nanocontainers were provided with two micro-defects to probe the active corrosion protective properties of the coatings. The coated substrates were then immersed in a sodium chloride solution (0.05 M) and impedance spectra were obtained after 3, 30, 50, 75 and 100 hours of immersion (Fig 1). The impedance measurements (R_p) show that enhanced corrosion protection is obtained when steel substrates are provided with a polyetherimide coating comprising sodium gluconate loaded halloysite or sodium gluconate loaded LDH nanocontainers relative to polyetherimide coated substrates in which the nanocontainers and corrosion inhibitors are absent.

The impedance measurements also show the effect of aging the nanocontainer/corrosion inhibitor mixture. In this respect polyetherimide coatings comprising LDH and sodium gluconate that were aged in sodium chloride solution for 72 hours were characterised by an increase in polarisation resistance relative to those coatings which comprised LDH and sodium gluconate that were aged in sodium chloride only for 3 hours. Polarisation resistance is used to determine the rate of corrosion.

Experiment 2 - Barrier and active corrosion protection ,

Barrier and active corrosion properties were also assessed as a function of loaded nanocontainer concentration. Polyetherimide coatings were doped with 2.5 wt%, 5 wt% or 10 wt% of sodium gluconate loaded halloysites or sodium gluconate loaded LDH (Table 1 and Table 2 respectively). Impedance measurements were obtained after 1 day and 14 days of immersion in sodium chloride solution (0.05 M).

The impedance measurements show that polyetherimide coatings doped with sodium gluconate loaded LDH exhibit improved barrier properties relative to polyetherimide coatings that were doped with sodium gluconate loaded halloysites when self healing polyetherimide coated steel substrates were immersed in saline solution for 1 day. This indicates that sodium gluconate loaded LDH nanocontainers are particularly effective at reducing the detrimental effects of corrosion when the coating is initially damaged.

On the other hand sodium gluconate loaded halloysites afford better barrier properties relative to their LDH counterparts following the immersion of the coated substrates in saline solution for 14 days. This suggests that corrosion inhibitors are released from halloysites in a more controlled manner relative to corrosion inhibitors released from LDH. Thus, the advantageous effects of active corrosion protection are prolonged when halloysite nanocontainers are used. The measurements also indicate that increasing the content of sodium gluconate loaded halloysites in the polyetherimide coating results in an improvement in the barrier properties of the coating. In contrast the best barrier properties were obtained when polyetherimide coatings comprise 2.5 wt% sodium gluconate loaded LDH.

Table 1 : Assessment of barrier and active corrosion protection properties of doped polyetherimide coatings containing sodium gluconate loaded halloysites.

Table 2: Assessment of barrier and active corrosion protection properties of doped polyetherimide coatings containing sodium gluconate loaded LDH doped polyetherimide coatings.

Claims

1. A method of manufacturing a coated steel substrate for use in construction or automotive applications which comprises the steps of:

(i) providing a steel substrate;

(iv) combining the first coating mixture and the second coating mixture;

(v) applying the combined mixture on the steel substrate;

2. Method of manufacturing a coated steel substrate according to claim 1 wherein the nanocontainers comprise layered double hydroxides, halloysites, CaC0₃, polymeric capsules or mixtures thereof.

3. Method of manufacturing a coated steel substrate according to claim 1 or claim 2 wherein the nanocontainers comprise halloysites having an internal diameter between 10 and 150 nm.

4. Method of manufacturing a coated steel substrate according to any one of the preceding claims wherein the first coating mixture comprises 1-50 wt%, preferably 2.5-20 wt%, more preferably 2.5-10 wt% of the nanocontainers with nanoscale corrosion inhibitors contained therein.

5. Method of manufacturing a coated steel substrate according to any one of the preceding claims wherein the nanocontainer is surface modified with a surface modifying component, preferably an organofunctional silane or a polyelectrolyte.

6. Method of manufacturing a coated steel substrate according to claim 5 wherein the organofunctional silane comprises alkylsilanes, arylsilanes, alkoxysilanes, aminosilanes or mixtures thereof.

7. Method of manufacturing a coated steel substrate according to claim 5 wherein the polyelectrolyte comprises polystyrenesulfonate, polyallylamine hydrochloride, polyanetholesulphonic acid sodium salt, poly(4-vinylpyridene), polyethyleneimine, polyacrylic acid.

8. Method of manufacturing a coated steel substrate according to any one of the preceding claims wherein the nanoscale corrosion inhibitors comprise anionic corrosion inhibitors and cationic corrosion inhibitors, preferably one or more of Sodium molybdate Na₂Mo0₄, Cerium molybdate Ce2(Mo0₄)₃, Cerium nitrate Ce(N0₃)₃, Calcium nitrate Ca(N0₃)₂, Zinc sulfate ZnS0₄, Sodium tungstate NaW0₃, Sodium phosphomolybdate hydrate

Na₃Mo₁₂O₄₀P, Sodium phosphate Na₃P0₄, Sodium hydrophosphate Na₂HP0₄, Sodium dihydrophosphate NaH₂P0₄, Sodium carbonate Na₂C0₃, sodium polyphosphate NaP0₃x, Sodium Gluconate, 2-Mercaptobenzothiazole, Benzimidazole, Quinaldic acid, Sodium Citrate, Glycine, 8-hydroxyquinoline, Sodium Salycilate, Sodium benzoate, 1- Hydroxyethylidenediphosphonic acid (Etidronic acid) , Nitrilo-tris-phosphonic acid , N,N dimethylamine , Di-azo compounds , Cu-thalocyanine, dyes tartrazine (TZ)).

9. Method of manufacturing a coated steel substrate according to any one of the preceding claims wherein the curable organic component is polyamic acid or a derivative thereof.

10. Method of manufacturing a coated steel substrate according to any one of the preceding claims wherein the content of organic curable component in the second coating mixture is between 10 and 25 wt%.

11. Method of a coated steel substrate according to any one of the preceding claims wherein the applied combined coating mixture is water based.

12. Method of manufacturing a coated steel substrate according to any one of the preceding claims wherein the combined mixture is cured at a temperature between 80 and 250^°C.

13. Method of manufacturing a coated substrate according to any one of the preceding claims wherein the steel substrate is provided with a corrosion protective coating prior to applying the combined mixture, preferably the corrosion protective coating is a zinc or zinc alloy.

14. Coated steel substrate produced according to the method of any one of the preceding claims.

15. Use of the coated steel substrate according to claim 14 in an organic coated strip product for building and construction applications.