WO2018169397A1

WO2018169397A1 - Diatoms as natural carriers for controlled release for metal protection and coatings

Info

Publication number: WO2018169397A1
Application number: PCT/NL2018/050161
Authority: WO
Inventors: Santiago Juan Garcia ESPALLARGAS; Paul Johan DENISSEN
Original assignee: Technische Universiteit Delft
Current assignee: Technische Universiteit Delft
Priority date: 2017-03-15
Filing date: 2018-03-15
Publication date: 2018-09-20
Anticipated expiration: 2019-09-15
Also published as: NL2018517B1

Abstract

The present invention relates to a coating comprising a natural carrier for controlled release of a compound, such as for metal protection, a product comprising said coating or coating application, a method of modifying a natural carrier for controlled release, and methods of forming said coating or coating application, wherein the natural carrier is selected from exoskeletons of a Heterokontophyta species.

Description

Diatoms as natural carriers for controlled release for metal protection and coatings

FIELD OF THE INVENTION

The present invention relates to a coating and coating application comprising a natural carrier for controlled release of a compound, such as for metal protection, a product comprising said coating or coating application, a method of modifying a natural carrier for controlled release, and methods of forming said coating or coating application, wherein the natural carrier is selected from exoskeletons of a Heterokontophyta species.

BACKGROUND OF THE INVENTION

The present invention is in the field of said coating or coating application.

Some major challenges faced when replacing toxic and carcinogenic Cr (VI) -based corrosion inhibitors by

environmentally friendly ones are (i) reduction of negative inhibitor-coating matrix interactions that limit a novel inhibitors efficiency, and (ii) control over release of the inhibitor in time. For this reason several encapsulation methods have been proposed in the last decade. The most common and successful concepts use 2D inorganic nanoparticles (e.g. montmorillonites , bentonites and hydrotalcites and more recently 3D inorganic nanocarriers (e.g. zeolites and

halloysites) . Such carriers are considered to allow

controlling inhibitor release by different mechanisms (e.g. diffusion, pH, redox, ion exchange) while at the same time prevent unwanted inhibitor reactions with e.g. a surrounding polymer matrix and too fast inhibitor release leading to blistering. Despite significant progress and reported evidence for nanocarriers yielding protection of small damages (<100 μιη width scratches) for short periods of immersion time, their long-term protection of relatively large damages is still under question. Together with their limited versatility, often synthesis complexity, and insufficient local release capacity motivates the constant search for alternatives.

Diatoms are a major group of unicellular algae with the unique feature of forming highly ordered hollow nanoporous silica exoskeletons {named frustules) . Each of the estimated 100.000 extant species as well as the species found as mineral (diatomaceous earth) has a distinctive frustule (typically two symmetric sides hold together) which varies in size (from 2 pm to 4 mm) , shape and nanopore distribution and size. For their characteristics the diatom exoskeletons may be described as forming "pill-box" structures. The availability, morphological characteristics and potential application of fragmented bio- based diatom exoskeletons as carriers has recently attracted significant attention in the biomedical field where their use as drug delivery systems in fluid media has been studied.

Some prior art documents recite applications of

microencapsulated biocides. WO 2010/148158 Al recites a process for preparation of microencapsulated release biocidal actives and aqueous or solvent compositions thereof. Said document does not relate to a coating. EP 1 024 181 Al recites an anti-corrosive coating an teaches to use large amounts of hollow filler, with therein inhibitors and anti-oxidants . Both documents relate to adsorption, i.e. only a surface is

provided with a biocide or inhibitor, respectively.

The present invention therefore relates to and further aspects thereof, which overcomes one or more of the above disadvantages, without compromising functionality and

advantages .

SUMMARY OF THE INVENTION

It is an object of the invention to overcome one or more limitations of the devices of the prior art and at the very least to provide an alternative thereto. The present invention relates to a coating, and likewise a coating application, comprising a natural carrier for controlled release of a compound, such as for metal protection, comprising 1-20 wt.%, preferably 2-19 wt.%, more preferably 3-18 wt.%, even more preferably 4-15 wt.%, such as 5-10 wt.%, of hollow structures, which may also be a halve structure, each structure enclosing an internal space thereof, wherein the hollow structures are selected from exoskeletons of a Heterokontophyta species, wherein the walls of the structure are mainly (e.g. 50-99.5 wt.%, such as 75-99 wt.%) of natural porous silica, wherein both the internal space and the surface of the hollow structure is provided with 0.1-100 vol.%, preferably 0.2-80 vol.%, more preferably 0.5-50 vol.%, even more preferably 1-40 vol.%, such as 5-30 vol.%, of at least one of an organic and inorganic active compound, wherein wt . % and vol.% are based on a total weight of the coating and volume of the hollow

structure, respectively. With the present method also hollow structures from a single (one) typically living species may be obtained, or of a limited number of such species. The term "active" may refer to "biologically active", "chemically active", "physically active", and combinations thereof, i.e. capable of acting or reacting as such or providing such activity. The amount of active compound may also be calculated on a weight/weight basis, and is typically in a range of 0.1- 1000 g active compound/kg carrier, preferably 1/100 g/kg, such as 2-50 g/kg, typically also depending on a molar weight of the active compound. Contrary to the prior art both the internal space and the surface of the hollow structure may be provided with active compound. The natural carrier is formed by diatom species. The carrier is a hollow structure which is formed by the exoskeleton of said diatom species. The walls of the exoskeleton, and likewise if applicable the top and bottom (caps) are mainly of natural porous silica, and may contain further oxides, such as titanate. The hollow structures have an internal space, which internal space is provided (such as filled, doped) with an active compound, such as a corrosion inhibitor. The internal space may be largely or fully filled with said active compound (up to about 100 vol.%) or may be partly filled or even only slightly filled, depending on an application and/or intended use. The active compound may be organic and inorganic. Also combinations of active compounds are envisaged. The active compound may be present as such, or in a suitable solvent. The present coating or application therewith overcomes prior art and provides a controlled release of the active compound, especially in engineering applications, corrosion inhibition of underlying surfaces, such as in pipelines, aerospace coatings, coatings for bridge structures, in concrete applications, etc. The present diatoms can be used as carriers for the release of single or multiple chemical species to be released from the particles themselves or for the particles embedded in bigger matrices such as coatings or concrete. The chemical species to be used can be broad in nature and can have different uses such as corrosion inhibition, self-healing, hydrophobicity, anti-biofouling, fire-retardant, anti-bacteria, anti-insects, colour

restoration, lubricants, etc. The present coatings give fast and adequate protection that can be sustained long; in

addition relatively large (mm-scale width/length) damages in coatings can be overcome or the effects thereof can at least be mitigated. A lower inhibitor-matrix interaction, a high local inhibitor storage, and a time-based release leading to sustained protection at damaged coated metals e.g. under immersion in salt solution is provided. The present

exoskeletons comprising an active compound can be produced with ease, can comprise high amounts of active compound, can have complex architectures at a microscale, and relates to naturally formed products (including those grown in a

bioreactor) being inherently environmentally friendly. The present coatings offer sufficient and constant supply of active compound, if applicable, and good release kinetics. The present nanoporous diatom algae exoskeletons allow for local inhibitor loading. In an example Cerium loaded exoskeletons show a fast diffusion controlled release. The Cerium loaded exoskeletons show long-term corrosion protection at damaged coatings. The Cerium loaded exoskeletons have comparable protection to the chromium based primer. Although the present invention has been proven for a cerium salt-epoxy-aluminium alloy system and a lithium salt-epoxy-aluminium alloy it is applicable to other inhibitor-coating-metal systems. It is possible to follow degradation processes of a damaged coating any electrochemical or optical technique designed to monitor corrosion processes such as scanning vibrating electrode technique or electrochemical impedance spectroscopy as well as other tools more common in industrial settings such as salt- fog spray.

In the present coating the active compound is at least one of a corrosion inhibitor, a self-healing compound, a compound for modifying surface tension (e.g. hydrophobicity or

hydrophilicity) , a precursor for a coating, an anti-bio fouling compound, a fire-retardant, a bactericide, an

insecticide, a colour restoration compound, an anti-icing agent, a de-icing agent, an anti-oxidant, a UV-protector, a lubricant, and an electrical conductor.

In the description the term "coating" is intended also to include "coating application", in so far as applicable.

In a second aspect the present invention relates to a product comprising the present coating or coating application, such as a pipeline, a platform, such as an off-shore platform, an oil platform, a rocket/shuttle platform, an aerospace carrier, a vehicle, an automotive, an airplane, a train, a bridge, a bridge structure, and concrete.

In a third aspect the present invention relates to a method of modifying a natural carrier for controlled release, such as for metal protection, comprising providing natural porous silica exoskeletons structures of a Heterokontophyta species, wherein the structures have an average height of 1 μπι-5000 urn, an empty inner space with a cross section of 80 nm-49 urn, and pores in the exoskeleton with an average size of 5-100 nm, wherein the inner space and pores of the structures is provided with 0.1-100 vol.%, preferably 0.2-80 vol.%, more preferably 0.5-50 vol.%, even more preferably 1-40 vol.%, such as 5-30 vol.%, of at least one of an organic or inorganic active compound, such as an inhibitor selected from rare earth salts, Li salts, etc., wherein the active compound is

preferably also provided by precipitation on the surface.

In a fourth aspect the present invention relates to a method of forming a coating or coating application according to the invention. The coating may comprise a thermoset and/or a thermoplast.

The present invention provides a solution to one or more of the above mentioned problems and overcomes drawbacks of the prior art.

The present invention is also topic of a scientific article by S.J. Garcia et al . , entitled "Cerium-loaded algae exoskeletons for active corrosion protection of coated AA2024- T3", which is submitted for publication, and which paper and contents and details thereof are incorporated by reference. Some of the paragraphs below relate closely to said article. The paper provides various experimental results and characterizations of the present invention.

Advantages of the present description are detailed throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

In an exemplary embodiment of the present coating the Heterokontophyta species may be an autotroph species, in particular a Bacillariophyceae, and more particular a

diatomophyceae . The species can be readily grown in a

bioreactor.

In an exemplary embodiment of the present coating the exoskeleton may be naturally grown, obtained from diatomaceous earth, or produced in a bioreactor with extant diatom species. When obtained from diatomaceous earth smaller particles typically need to be separated from the intact or largely intact exoskeletons; such may also be the case for naturally grown exoskeletons or those produced in a bioreactor, though to a lesser extent typically.

In an exemplary embodiment of the present coating the naturally grown exoskeleton or the exoskeleton obtained from diatomaceous earth may be obtained by removing non-intact skeletons, such as by filtering, by mass-separation, or by settling, whereby a part or most, or even all, non-intact skeletons are removed, or wherein the exoskeleton produced in the bioreactor may be obtained by removing organic matter from the diatom species, such as by heating. Therewith good control over and selection of e.g. a size distribution of exoskeletons is obtained.

In an exemplary embodiment of the present coating the active compound may be at least one of phosphates, benzoates, silicates, vanadates, tungstates, zirconates, borates, molybdates, carbonic acids, amines, ketones, aldehydes, and heterocyclic compounds. Hence a large variety of active compounds may be applied, alone and in combination, amongst others showing the versatility of the present coating.

In an exemplary embodiment of the present coating the corrosion inhibitor comprises one or more of a salt, such as an organic or inorganic salt, such as a rare earth salt, such as wherein the cation is Ce, Nd, La, Sc, or Dy, a Li-salt, wherein the anion is one or more of N03-, alkyl phosphate, such as dibutyl phosphate, a thiol, a C(SH)=S comprising compound, and a carboxylate, such as diethyl dithiocarbamate, carbonate, etc.

In an exemplary embodiment of the present coating structures may have at least one of a cross section selected from

circular, triangular, hexangular, square, rectangular, starlike, oval, and multiangular, such as octangular, an average height of 1 μιη-5000 μιη, preferably 10-1000 μιη, such as 20-500 μπι, an inner space with a cross section of 80 nm-49 μιη, preferably 0.1-30 μπι, such as 1-20 μιη, and pores in the exoskeleton with an average size of 5-100 nm, such as 10-50 nm.

In an exemplary embodiment of the present coating

structures may be partly or fully capped, preferably fully capped. Such is a clear advantage as the inner space can then be filled fully or almost fully. Having partly or fully capped hollow structures typically implies a careful selection and/or growth method of exoskeletons .

In an exemplary embodiment of the present coating

structures may be partly or fully provided with a partly or fully chemically modified surface. The present hollow

structures can also comprise a modified surface. The surface may be modified before providing the active compound, after providing the active compound, or during provision of the active compound. The surface may be fully modified or partly modified, such as providing an adeguate amount of surface modifier. The surface may be chemically modified in a variety of ways providing various characteristics thereto.

In an exemplary embodiment of the present coating the active compound may be provided by precipitation on the surface .

In an exemplary embodiment the present coating may comprise 5- 99 wt.%, preferably 10-95 wt . % , more preferably 15-90 wt.%, even more preferably 20-80 wt.%, such as 50-75 wt.%, of at least one of a polymer of an epoxy resin, a phenolic resin, a polyurethane, a polyester, a polyamide, a polyimide, a silicone, an alkyd resin, an amino resin, and combinations thereof. It is an important advantage that the present hollow structures can be provided in small amounts, thereby leaving the characteristics of coatings largely unaltered. In fact the addition of the present hollow structures may in this respect be regarded as a provision of a small amount of additive to the coating.

In an exemplary embodiment of the present coating the exoskeleton particle size distribution may be a Gaussian- shaped size distribution. In view of using the present

particles such offers advantages in controllable

characteristics of the coating.

In an exemplary embodiment of the present coating the standard deviation in particle size may be very limited, such as wherein 3σ< ( 3*average exoskeleton particle size),

preferably 3σ< (2*average exoskeleton particle size), more preferably 3σ< ( l*average exoskeleton particle size), such as preferably 3σ< (0.5*average exoskeleton particle size) or (for larger particle sizes (>20pm) preferably 3σ< (0. l*average exoskeleton particle size) .

In an exemplary embodiment of the present coating an average exoskeleton particle may be from 1-lOOum, preferably

5-50 m, more preferably 10-20μπι, such as 12-15um. Such uniform and small particle sizes are found to be beneficial to the present coatings.

In an exemplary embodiment the present coating may comprise 2-20 Heterokontophyta species, i.e. a limited number of species with well-defined characteristics, preferably 3-12 species, such as 5-10 species, each species having a different average size. For instance a first species may have an average size of 5μπι, a second species of 15μπι, and a third species of 50pm. It is preferred to have size distributions that are not overlapping, for instance in view of final characteristics of a coating, i.e. that within e.g. 3σ two adjacent size distributions do not overlap. Such is very well achievable with the present structures as very well defined structures may be obtained, especially when obtained from species grown in a bioreactor.

In an exemplary embodiment the present coating may comprise

>99% intact hollow structures, preferably > 99.5%, such as > 99.9%, based on a total number of species; i.e. practically no broken structures are present. Such is especially achievable with the living species grown in a bioreactor. For comparison, structures obtained in mining typically may have up to 50% broken structures, which percentage could be lowered with big efforts to 20%, and with even more effort to 5% or slightly lower.

The present coating can be applied to a large number and variety of products.

In an exemplary embodiment the present method of modifying a natural carrier for controlled release, such as for metal protection, may comprise providing natural porous silica exoskeletons structures of a Heterokontophyta species, wherein the structures have an average height of 1 μιη-5000 um, an empty inner space with a cross section of 80 nm-49 μτα, and pores in the exoskeleton with an average size of 5-100 nm, wherein the inner space and pores of the structures is

provided with 0.1-100 vol.% of at least one of an organic or inorganic active compound. The modifying method may comprise further steps. For instance the present carrier may be

separated, such that largely intact exoskeletons are obtained. Separation may e.g. be performed by suspending exoskeletons in a solvent, treating the suspension, such as by sonification, such that larger and smaller particles are spatially

separated, removing the smaller typically not-intact

particles, such as by decanting, and repeating the steps as often as required to obtain a full separation, such as 2-10 times, typically 3-7 times. Thereafter an optional post- treatment on the obtained exoskeletons may be performed, or a combination of post-treatment steps. For instance an acid treatment may be performed, such as by providing a 1-5M acid, such as H2SO4, and mixing at room temperature at 50-500 rpm, such as 100-200 rpm, for a period of 1-24 hrs, such as 2-16 hrs, thereby removing impurities, such as metal species.

Thereafter the exoskeletons are typically washed with water, and dried, such as at 40-80 °C, typically at 50-60 °C, during 2-48 hrs, such as 12-24 hrs. Likewise an alkaline treatment may be performed using a 10^_1-10^"4 M, such as 10^"3 M alkaline solution, such as NaOH, during 1-3 hrs, such as 1.5 hrs, and further following the steps mentioned above. Also a combination of an acid and alkaline post-treatment may be performed.

In an exemplary embodiment the present method of modifying a natural carrier for controlled release the active compound may be provided by ion exchange, typically after structural modification of the exoskeleton.

In a further aspect the present invention may relate to a method of forming the present coating or coating application comprising providing 1-20 wt . % of a hollow structure enclosing an internal space thereof, wherein the walls of the hollow structure is at least one natural porous silica exoskeleton of a Heterokontophyta species, wherein the internal space and the surface of the hollow structure is provided with 0.1-100 vol.% of at least one of an organic and inorganic active compound, providing 5-99 wt . % of at least one of an uncured polymer, mixing the hollow structures and uncured polymer, applying the mixed polymer to a surface, and curing the polymer, wherein wt.%/vol.% are based on a total weight/volume of the

coating/hollow structure.

In a further aspect the present invention may relate to a method of forming the present coating or coating application comprising providing 1-20 wt.% of a hollow structure enclosing an internal space thereof, wherein the walls of the hollow structure is at least one natural porous silica exoskeleton of a Heterokontophyta species, wherein the internal space and the surface of the hollow structure is provided with 0.1-100 vol.% of at least one of an organic and inorganic active compound, providing 5-99 wt.% of at least one of a thermoplastic polymer in a solvent, mixing the hollow structures and polymer, such as by extrusion, applying the mixed polymer to a surface, and increasing the temperature, wherein wt.%/vol.% are based on a total weight/volume of the coating/hollow structure.

The method of forming the present coating may further comprises providing 1-40 wt.% of a hollow structure enclosing an internal space thereof, preferably 2-30 wt.%, more

preferably 5-20 wt.%, such as 10-15 wt.%, wherein the walls of the hollow structure is at least one natural porous silica exoskeleton of a Heterokontophyta species, wherein the exoskeleton is produced in a bioreactor, wherein both the internal space and the surface of the hollow structure is provided with 0.1-100 vol.% of at least one of an organic and inorganic active compound. It has been found that especially for exoskeletons produced in a bioreactor much high loadings of filler can be provided to coatings with a more controlled particle size and property distribution.

In view of changes in the coating the present use of exoskeletons and embedded inhibitors is found to lead to a clear decrease in inhibitor-polymer chemical interactions, such as can be seen in a strong decrease in yellowing in the case of Ce-loaded exoskeletons in an epoxy matrix. Figure 6 shows microscope images of (a-b) Cerium Nitrate and (c-d) Ce- DE mixed with Ancamine®2500 just after mixing (a, c) and after 1 hour (b, d) . Images illustrate the yellowing process in the case of the salt-amine couple and its absence in the case of the Ce-DE amine one. Image confirms the strong reduction of the yellowing when the Ce-salt is included in an exoskeleton nanoporous micro particle.

The invention will hereafter be further elucidated through the following examples which are exemplary and explanatory of nature and are not intended to be considered limiting of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims .

FIGURES

Figure 1 shows a set of SEM images and EDX spectra of the diatom exoskeletons before (la and lc) and after (lb and lc) the purification process as well as the particle size

distribution (Id) .

Figure 2. (a) SEM image of a single fractured diatom exoskeleton after corrosion inhibitor, (b) Comparative XRD spectra of DE, Ce (NO3) 3 · 6H2O corrosion inhibitor salt, and Ce- DE.

Figure 3. Cerium release kinetics of cerium nitrate from the salt powder (top-line) and from the Ce-DE particles

(bottom-line) obtained with a real-time UV/VIS spectroscopy from the 252nm wavelength. Figure 4a-d. SEM (a) and EDS micrographs for C (b) , Ce (c) and Si (d) of the fractured plane of an epoxy coating loaded with Ce-DE.

Figure 5. Microscope images of polished AA2024-T3 before, during, and after 7 days of immersion in 0.05M NaCl (a) without corrosion inhibitor (b) 0.05mM Ce( 03)3 and (c) Ce- doped DE containing 0.05 m Ce(N03) 3. Note, images during immersion do not represent real sizes due to the distortion by the solution.

Figure 6. Microscope images of (a-b) Cerium Nitrate and

(c-d) Ce-DE mixed with Ancamine®2500 just after mixing (a, c) and after 1 hour (b, d) .

DETAILED DESCRIPTION OF FIGURES

The figures are further detailed throughout the description.

EXPERIMENTS

The present invention is demonstrated for an epoxy-amine coating on an aerospace aluminium alloy AA2024-T3 system.

Cerium nitrate was used as the corrosion inhibitor giving an excellent corrosion inhibition in the copper-rich aluminium alloy. Cerium nitrate was here stored into refined

diatomaceous earth (DE) and its loading, release kinetics and corrosion inhibition efficiency of bare AA2024-T3 in salt solution. These systems were studied by real-time UV/VIS spectroscopy, SEM/EDS and Raman spectroscopy. The active corrosion protection of the cerium-loaded DE particles (Ce-DE) was then evaluated in a particle-loaded epoxy coating by an in-situ hyphenated opto-electrochemical device after creating highly controlled scratches of 130 μπι width at the bottom of the scratch. The results obtained are compared to those obtained for an unloaded epoxy coating and two epoxy coatings directly loaded with cerium nitrate and potassium dichromate respectively. The in-situ hyphenated opto-electrochemical approach allowed for obtaining real-time optical and

electrochemical information on the corrosion/protection processes. The analysis of the results showed a clear delay of the on-set and kinetics of the degradation process when the Ce-DE were used reaching protective values similar to those of the chromium-based system for the studied conditions. The involvement of the cerium ions on the corrosion inhibition was further confirmed by a post-mortem analysis of the damaged site by SEM-EDS and Raman spectroscopy. While the protection offered by the inhibiting species was detected with both techniques, the higher spatial resolution of the Raman signal at the scratch gained additional information on the

interaction between cerium species and copper-rich

intermetallic phases. The results here presented prove active corrosion protection of coated metal structures.

Materials and preparation

Diatomaceous earth Diafil 525 mainly consisting of

cylinder shaped diatom exoskeletons was supplied by Profiltra Customized Solutions (NL) . The as-received diatomaceous earth consists of 89.0 wt. % amorphous silica (S1O2) , a tapped powder bulk density of 0.42 g/cm³ and a mean particle size of 12 μπι. Cerium nitrate hexahydrate (Ce (NO3) 3.6H20) and Potassium

dichromate (K^C^O?) with >99% purity were purchased from

Sigma-Aldrich. Commercial 2 mm thick bare AA2024-T3 sheet obtained from Kaizer Aluminium was used as metallic substrate. Commercially available bisphenol-A based epoxy resin (Epikote™ 828) and amine cross linker (Ancamine®2500 ) were supplied by AkzoNobel (NL) and used as received to form the coating binder. Xylene with a purity of 99% was used as epoxy solvent. All aqueous solutions for the particle doping, corrosion and release studies were prepared using Millipore® Elix 3 UV filtered water.

Refining of the as-received diatomaceous earth

In order maximize the amount of intact diatom exoskeletons and reduce the impurities content (i.e. non-silica) a refining process was applied. Therein 8.0 g of as-received diatomaceous earth was suspended in 120 ml demineralized water and

sonicated for 30 min. This was left in unstirred condition for 30 min to allow the silica intact exoskeletons to settle down. The supernatant (containing the impurities and small diatom parts) was discarded with the help of a glass pipette. This settling process was repeated 5 times (without sonication) . Finally the settled particles were filtered using a Whatman® grade 595 paper filter and dried in a vacuum oven at 60 °C for 24 h. The refined diatomaceous earth, referred to as "DE", was used in the rest of the study as the corrosion inhibitor carrier.

Post-treatment of DE

DE may be post-treated in order to remove impurities. Almost all impurities can thereby be removed. The treated DE shows a somewhat better behaviour, e.g. in terms of release of active compound over time and in amount.

For acid treatment, 200 mg refined DE was dispersed in 20ml solution of 3.0M Sulfuric acid (H2S04) using a 50ml flask fitted with a condenser and controlled at 100 in an oil bath. The flask was fitted with magnetic stirrer at around 200 rpm. After 16 hours the content was filtered using a Whatman® grade 595 paper filter. The acid treated product was repeatedly washed with water until the filtrate reached a pH of 7. The residue was removed from the filter and dried in a vacuum oven at 60°C for 24 hours to completely dry.

For alkaline treatment, two batches were used. Namely, 200mg refined DE for the first batch and 200 mg acid treated DE for the second batch. This was mixed with 5.0 ml of sodium hydroxide solution (NaOH) having a pH of 11. The content was magnetically stirred for 1.5 hours at 200 rpm in a 50 ml flask. Finally the diatoms were filtered and water cleaned with the use of a Whatman® grade 595 paper filter until the filtrate reached a pH of 7. The residue was removed from the filter and dried in a vacuum oven at 60 °C for 24 hours to completely dry.

DE doping with cerium nitrate

For the doping procedure a powder mixture of 0.85 g refined DE and 0.15 g cerium nitrate hexahydrate was added to 2.0 ml demineralized water. The mixture was then placed on a shaking table at 320 rpm for 24 h followed by complete drying in an oven at 80 °C under ambient atmosphere for another 24 h. As a result of the process a DE powder containing 15 wt. % of cerium nitrate was obtained. The loaded powder was then screened through a stainless steel sieve of 50 μπι aperture to reduce agglomerates and conform the here on called cerium- doped DE (Ce-DE) .

Coatings preparation

AA2024-T3 metal sheets were cut into pieces of 25x50 mm prior to surface modification and coating application. The metal surface preparation consisted of the following sequential steps: (i) removal of native oxide layer and

surface chemistry homogenization using SiC sandpapers down to grit 320; (ii) surface roughness formation by Scotch Brite 3M "Clean N Finish grade AVFN"; (iii) degreasing with acetone; and (iv) immersion in a 2M NaOH aqueous solution for 10

seconds followed by rinsing with distilled water and air drying in order to increase the surface OH fraction and

therefore adhesion with the subsequent organic coating.

The organic coatings were prepared using a mixture of

Epikote™ 828, Ancamine®2500 and Xylene (2.70:1.57:1 weight ratio) . Five coating systems were formulated as summarized in Table 1. In all cases the epoxy-amine-xylene mixture was first high-shear mixed for 5 minutes at 2500 rpm in a high-speed mixer. In order to reduce possible side reactions with the epoxy/amine matrix the mixtures were then let pre-cure at ambient conditions for 30 min before the corrosion inhibiting components (cerium nitrate powder, DE, Ce-DE or potassium dichromate) were added. The mixtures were then manually

stirred to form a homogeneous mixture, applied on the AA2024- T3 coupons by a ΙΟΟμιη spiral bar coater and cured at 60 °C for 24h as reported elsewhere to achieve complete crosslinking. After curing the coated panels were stored in a desiccator until 30 min before testing. The final pigment volume

concentration (PVC) in the dry coatings could be calculated.

Table 1. Overview of the coatings compositions, sample coding and relevant coating parameters

Active

Coating

inhibitor (epoxy- Particle content Thickness PVC

content amine <wt. % over binder) (μπι) ⁽%⁾

{moles/kg based)

binder)

E None 110+20 0 0

DE Refined DE: 12 80±20 25 0

Ce As received Ce( 03)3: 2 80±20 0.5 0.04

Ce-DE Ce loaded DE: 14 110120 25 0.04 Cr As received K2Cr₂0₇: 2 100+20 0.8 0.06

Coatings damage formation for corrosion inhibition evaluation

Reproducible and controlled 5 mm long and 130 pm wide scratches (at the bottom of the scratch) were created on the coated panels with a CSM Microscratch tester using a 100 μιτι Rockwell C diamond tip in multi-pass mode. For this, the tip was programmed to give 5 passes at each load of 5N, 10N and 15N at the same location until the AA2024-T3 substrate was reached.

Details of testing methods and equipment used are given in the above mentioned publication.

Cerium interaction with the exposed AA.2024-T3 metal surf ce

In order to confirm the involvement of cerium in the active corrosion protection at damage sites both SEM-EDS and Raman spectroscopy analysis were performed at the scribe bottom (metal) of the damaged coated samples used in the opto- electrochemical study. For the Raman analysis a Renishaw inVia reflex microscope equipped with a research-grade Leica

microscope objective at 50 * magnification and numerical aperture of 0.55 was used. A 532-nm laser light with an effective laser power of 32mW in 1 second excitation

measurements was employed. Control tests were performed on a copper block, cerium oxide precipitates, and on bare AA2024-T3 exposed to inhibited and non-inhibiting solutions.

Particle and coating characterization

distribution (Id) . From the SEM images it becomes clear that the purification process significantly reduced the amount of broken diatoms and other impurities of the as received

diatomaceous earth as intended. The SEM inserts in Figures la and lb further confirm the removal of very small particles blocking the nano-pores in the as-received exoskeletons. The particle size analysis showed a near Gaussian-shaped size distribution with an increased peak centred at 12μπι (Figure Id) as well as a drop of the small fraction particles after the purifying process. Here four clearly different diatom species were present with mainly sp. Aulacoseira species having a cylindrical pill-box structure with nanopores of around 500nm evenly distributed around the exoskeleton wall.

EDX analysis was used to determine the effect of the purification process on the removal of the chemical species different than the silica of the diatom shell. Figure lc shows the predominance of silicon (Si) and oxygen (0) constituents as expected for diatom silica (Si02) exoskeletons. Small traces of aluminium (Al) , Iron (Fe) , Calcium (Ca) and

Magnesium (Mg) were also detected and assigned to impurity oxides (A1203, Fe203, CaC03, CaO, and MgO) . The purifying process used was not capable of fully removing oxide

impurities. The presence of the impurities in the clean DE did not have an effect on the ulterior cerium doping when the doping procedure proposed in this work was employed. As the impurities did not have a significant effect in the doping it was decided to skip the acid and alkali post-treatments to simplify the process.

Diatom exoskeleton loading with corrosion inhibitors was confirmed by SEM and XRD analysis (Figure 2) . The XRD spectra in Figure 2b show that the DE (refined diatomaceous earth) primarily consisted of amorphous silica with some minor diffraction peaks at 22.0° and 26.6° corresponding to

crystalline structures of quartz and cristobalite . The XRD spectra for the Ce-DE (cerium doped DE) shows the amorphous silica baseline combined with crystalline peaks corresponding to Ce(N03)₃. The increased intensity of the peaks compared to pure Ce (N0₃) 3.6H₂0 is presumably caused by the decrease of water in the crystal lattice due to drying and localized deposition on the diatom silica surface. The results confirm that, during the doping process, the cerium inhibitor did not change its crystalline structure and remained as an inorganic salt primarily inside the diatom exoskeletons body space and nanopores, thereby confirming the success of the developed doping procedure.

Figure 3 shows a difference in release behaviour

(dissolution and diffusion) of the Ce(N03) 3 salt directly placed in a paper filter of an UV-Vis system and that of the cerium salt contained in the Ce-DE particles. By analysing the release plots it is possible to realize that the filter paper influenced the dissolution of the inhibitor by delaying its release. The release response for both systems is comparable whereby the release at the beginning increases exponentially due to the high solubility of Ce(N03) 3 and ends asymptotically. Interestingly, the release curve for the Ce-DE particles is slower than that of the cerium salt over the entire time domain. The increasing separation in time between the two release curves plotted against t^1/2 shows that there may not be a single ''trapping' time for the cerium but an entire spectrum delayed between 10 and 10³ seconds with respect to the cerium salt in the filter. These results confirmed that the cerium inhibitor loaded in the DE structure was able to come out by a time diffusion controlled process in aqueous solution as intended.

The active corrosion protection by the release of cerium from the Ce-DE particles in solution was studied by a detailed SEM/EDS and Raman study on bare AA2024-T3 immersed in 0.05M NaCl solutions. Despite the delay with respect to the salt, the results confirmed a sufficiently fast cerium release from the DE particles enough to prevent local corrosion by the formation of protective cerium precipitates at copper-rich phases .

Figure 4 shows the SEM-EDS micrograph of a fractured epoxy coating containing Ce-DE particles. The fractured plane shows a DE cylindrical particle (from the top) embedded in the epoxy matrix. The EDS analysis further confirmed the presence of high cerium concentrations inside the exoskeleton with a low carbon signal of the polymeric matrix thereby confirming the cerium inhibitor remained in the inner volume of the DE when the Ce-DE particles were mixed with the epoxy coating. This is a critical requirement of the localized long term corrosion protection.

Corrosion inhibition

Formation of corrosion products and local corrosion sites are found at the scribe of the non-inhibited coatings (Epoxy and DE) . On the other hand, both the Ce-DE coating and the Chromium containing coating (Cr) do not show significant variations with the immersion time at the scribe, indicating active corrosion protection of the damaged site.

Figure 5 (a) shows that pitting corrosion on AA2024 occurs within the first 3h of immersion in 0.05 M NaCl, indicating the susceptibility to localized corrosion in salt water for this alloy. The initiation of pitting did not occur when

0.05mM Ce(N03) 3 or Ce-doped DE was added, as shown in Figure M(b-c), confirming that the Ce-doped DE is actively protecting the substrate. DE particles were visible at the aluminium surface during the immersion-test for the Ce-doped DE solution after 3h. Furthermore, several secondary phases became visible at the surface after 7 days of immersion and rinsing with water .

For the quantitative analysis three main characteristic parameters were used, (1) Relative variation of the total impedance in time, (2) Open circuit potential (OCP} , and

(3) The degraded area around the scratch with immersion time. Upon studying these parameters results were found to be satisfactory.

Conclusions

A new biobased carrier for corrosion inhibition is introduced. The use of diatom algae silica exoskeletons , provides protection of AA2024-T3 structures by cerium nitrate corrosion inhibitor. Corrosion protection levels comparable to those given by a coating containing potassium dichromate were obtained. High protection levels are achieved, a reduction of unwanted reactions, a high inhibitor storage in the silica cages and the fast and sustained release of the cerium

inhibitor at the damaged site. Also protective systems based on fast release and inhibition at damaged sites followed by a time-sustained or on-demand release of corrosion inhibitors supplied at a sufficient concentration to ensure the long term protection are envisaged. The use of inhibitor loaded algae exoskeleton particles for sustained corrosion inhibition here presented is not restricted to cerium and epoxy coatings on aluminium substrates but should be regarded as being generic with a high versatility and potential for developing

environmentally friendly active corrosion protection in coated metals .

Claims

1. Coating comprising a natural carrier for controlled release of a compound, such as for metal protection,

comprising

1-20 wt.% of hollow structures, each structure enclosing an internal space thereof, wherein the hollow structures are selected from exoskeletons of at least one Heterokontophyta species, wherein the walls of the structure are mainly of natural porous silica,

wherein both the internal space and the surface of the hollow structure is provided with 0.1-100 vol . % of at least one of an organic and inorganic active compound, wherein the active compound is at least one of a corrosion inhibitor, a self- healing compound, a compound for modifying surface tension, a precursor for a coating, an anti-bio fouling compound, a fire- retardant, a bactericide, an insecticide, a colour restoration compound, an anti-icing agent, a de-icing agent, an antioxidant, a UV-protector, a lubricant, and an electrical conductor,

wherein wt.% and vol.% are based on a total weight of the coating and volume of the hollow structure, respectively.

2. Coating according to claim 1, comprising 2-19 wt.% hollow structures.

3. Coating according to any of the preceding claims, wherein the Heterokontophyta species is an autotroph species, in particular a Bacillariophyceae, and more particular a diatomophyceae .

4. Coating according to any of the preceding claims, wherein the exoskeleton is naturally grown, obtained from diatomaceous earth, or produced in a bioreactor with extant diatom species.

5. Coating according to claim 4, wherein the naturally grown exoskeleton or the exoskeleton obtained from

diatomaceous earth is obtained by removing non-intact

skeletons, or wherein the exoskeleton produced in the

bioreactor is obtained by removing organic matter from diatom species, such as by heating.

6. Coating according to claim 5, wherein the exoskeleton is obtained by removing non-intact skeletons, or wherein the exoskeleton is obtained by removing organic matter from diatom species by heating.

7. Coating according to any of the preceding claims, wherein the active compound is at least one of a phosphate, a benzoate, a silicate, a vanadate, a tungstate, a zirconate, a borate, a molybdate, a carbonic acid, an amine, a ketone, an aldehyde, and a heterocyclic compound.

8. Coating according to any of the preceding claims, wherein the corrosion inhibitor comprises one or more of a salt .

9. Coating according to claim 8, wherein in the salt the cation is selected from Ce, Nd, La, Sc, Dy, and Li, and the anion is one or more of NO3^", alkyl phosphate, such as dibutyl phosphate, a thiol, a C(SH)=S comprising compound, and a carboxylate, such as diethyl dithiocarbamate .

10. Coating according to any of the preceding claims, wherein structures have at least one of

a cross section selected from circular, triangular,

hexangular, octangular, square, rectangular, star-like, oval, and multiangular, an average height of 1 μπι-5000 urn, an inner space with a cross section of 80 nm-49 pm, and pores in the exoskeleton with an average size of 5-100 nm.

11. Coating according to any of the preceding claims, wherein structures are partly or fully capped.

12. Coating according to any of the preceding claims, wherein structures are partly or fully provided with a partly or fully chemically modified surface.

13. Coating according to any of the preceding claims, wherein the active compound is provided by precipitation on the surface.

14. Coating according to any of the preceding claims, comprising 5-99 wt . % thermoset and/or thermoplast.

15. Coating according to any of the preceding claims, comprising 5-99 wt . % of at least one of a polymer of an epoxy resin, a phenolic resin, a polyurethane, a polyester, a polyamide, a polyimide, a silicone, an alkyd resin, an amino resin, and combinations thereof.

16. Coating according to any of the preceding claims, wherein an exoskeleton particle size distribution is a

Gaussian-shaped size distribution.

17. Coating according to claim 16, wherein 3σ (standard deviation) < (3*average exoskeleton particle size).

18. Coating according to claim 16 or 17, wherein an average exoskeleton particle is from 1-lOOum.

19. Coating according to any of the preceding claims, comprising 2-20 Heterokontophyta species, each species having a different average size.

20. Coating according to any of the preceding claims, comprising >99% intact hollow structures.

21. Product comprising a coating according to any of the preceding claims, wherein the product is selected from a pipeline, an aerospace carrier, a platform, an off-shore platform, an oil platform, a rocket/shuttle platform, a vehicle, an automotive, an airplane, a train, a bridge, a bridge structure, and concrete.

22. Method of modifying a natural carrier for controlled release, comprising

providing natural porous silica exoskeletons structures of a Heterokontophyta species,

wherein the structures have an average height of 1 μιτι-5000 μια, an empty inner space with a cross section of 80 niri-49 μιτι, and pores in the exoskeleton with an average size of 5-100 nm, wherein both the inner space and pores of the structures is provided with 0.1-100 vol . % of at least one of an organic or inorganic active compound, wherein the active compound is at least one of a corrosion inhibitor, a self-healing compound, a compound for modifying surface tension, a precursor for a coating, an anti-bio fouling compound, a fire-retardant, a bactericide, an insecticide, a colour restoration compound, an anti-icing agent, a de-icing agent, an anti-oxidant , a UV- protector, a lubricant, and an electrical conductor.

23. Method according to claim 22, wherein the active compound is provided by ion exchange.

24. Method according to claim 22 or 23, wherein the exoskeletons structures are pre-treated and post-treated, wherein treatment is selected from at least one of refining, washing, drying, purifying, filtering, fractioning, acid treatment, and alkaline treatment.

25. Method of forming a coating according to any of claims 1-20, comprising

providing 1-20 wt . % of a hollow structure enclosing an

internal space thereof, wherein the walls of the hollow

structure is at least one natural porous silica exoskeleton of a Heterokontophyta species,

wherein both the internal space and the surface of the hollow structure is provided with 0.1-100 vol.% of at least one of an organic and inorganic active compound, wherein the active compound is at least one of a corrosion inhibitor, a self- healing compound, a compound for modifying surface tension, a precursor for a coating, an anti-bio fouling compound, a fire- retardant, a bactericide, an insecticide, a colour restoration compound, an anti-icing agent, a de-icing agent, an antioxidant, a UV-protector, a lubricant, and an electrical conductor,

providing 5-99 wt.% of at least one of an uncured polymer or 5-99 wt.% of at least one of a thermoplastic polymer in a solvent,

mixing the hollow structures and uncured polymer or polymer, applying the mixed polymer to a surface, and

curing the polymer or increasing the temperature.

26. Method according to claim 25, wherein mixing the hollow structures and polymer is by extrusion.

27. Method according to claim 25 or 26, further comprising providing 1-40 wt.% of a hollow structure enclosing an

internal space thereof, wherein the walls of the hollow structure is at least one natural porous silica exoskeleton of a Heterokontophyta species, wherein the exoskeleton is

produced in a bioreactor, wherein both the internal space and the surface of the hollow structure is provided with 0.1-100 vol.% of at least one of an organic and inorganic active compound.