WO2013169130A1

WO2013169130A1 - Hybrid coatings for improved corrosion protection of magnesium alloys

Info

Publication number: WO2013169130A1
Application number: PCT/PT2013/000026
Authority: WO
Inventors: Sviatlana V. LAMAKA; Fabiola BRUSCIOTTI; Darya V. SNIHIROVA; Maria de Fátima MONTEMOR
Original assignee: Instituto Superior Tecnico
Priority date: 2012-05-09
Filing date: 2013-05-03
Publication date: 2013-11-14
Also published as: PT106302A; WO2013169130A8

Abstract

The proposed invention concerns anti-corrosion coatings for magnesium alloys, used in automotive and aeronautic industry, and their preparation method. In particular, it relates to the coatings composition, the films formed over the substrate and the anti-corrosion, properties of the overall system when immersed in an aggressive electrolyte. More specifically, the coatings consist of an epoxy component (e.g. Poly (bisphenol A-co-epichlorohydrin), glycidyl end-capped), a silane (e.g. AminoPropylTriEthoxySilane, APTES and an amine (e.g. Diethylenetriamine, DETA), in organic solvents. The coating solution can be applied to the magnesium alloys substrates by dip-coating or spraying, followed by thermal curing in a specific range of conditions. The thickness of the coating varies from 5 to 20 micrometers. The magnesium alloys coated with such films have an excellent resistance to corrosion and after one-month immersion in sodium chloride their performance is remarkably higher compared to conventional coatings, as presented in the state of the art.

Description

DESCRIPTION

Hybrid coatings for improved corrosion protection of

magnesium alloys

Field of the invention

The proposed invention concerns coatings for magnesium alloys used in automotive and aeronautic industry in order to improve their corrosion resistance. The three main components of the coatings are: a silane, an epoxy and an amine. It was proved that the coating can resist the immersion into a solution of 0.05 M sodium chloride (NaCl) during a month without any damage. The coating performance was evaluated through electrochemical impedance spectroscopy.

Background of the invention

Magnesium alloys are characterized by a unique set of properties (lightweight, high thermal conductivity, dimensional stability and damping characteristics, recyclability, ...) [1-5], which make them valuable materials for many industrial applications, such as automotive and aerospace components, sporting goods, electronics and biocompatible implants.

In aeronautics the use of magnesium alloys is particularly growing, mostly driven by the increasing importance of fuel economy and reduction of carbon dioxide (C0₂) emissions, hence the need to reduce the weight of the aircraft [6,7]. In this perspective, Mg alloys represent an excellent candidate because of their high strength-to-weight ratio.

However, a major drawback in the use of magnesium alloys is their high susceptibility to corrosion, caused by internal galvanic couples due to second phases or impurities and the nature of the hydroxide film on the surface, which is porous and poorly protective [5] .

Literature on anti-corrosion coatings for Mg alloys is still poor if compared to aluminium and steel. Anodizing has proved to improve the corrosion resistance of the Mg alloys under aggressive environments, as anodic oxidation produces and oxide film with good corrosion resistance and reasonable adhesion properties [8-13] . However, layers obtained by anodization are porous and require an extra sealing layer, such as a sol-gel silane-based coating. Besides this, anodizing has elevated costs, as it requires high current intensity and also a very efficient cooling system in order to avoid over-heating of the system. This translates into high energy consumption. In addition, the anodized film often presents defects, especially when the substrate material (magnesium alloys) has a complex geometry.

Commercial conversion treatments as MagPass [14] and Gardobond [15] provide a corrosion protection weaker than the anodized layers and still require the application of an additional protection system, sealants, based on sol-gel films or silanes. Again, the anodizing process makes the corrosion protection process more expensive.

Chromates-based coatings are also efficient anti- corrosion methods [16] . However, chromates-based treatments and coatings were banned and they are in the process of elimination from production because of their toxic effects on health and environment.

In this perspective, sol-gel based organically modified silanes have attracted significant interest as versatile, easily processed coating materials with potential to replace chromate-based corrosion protection surface treatments for Mg alloys [1-3,17-24].

The performance of such surface treatments is mostly dependent on their ability to form a dense barrier against the penetration of water and corrosion initiators, and on their adhesive bond strength to the substrate, which can be achieved through introduction of various organo-functional groups into the silane matrix, thus tailoring their chemical composition. Hence sol-gel coatings represent an effective and environmentally friendly route to prepare films on metallic substrates at low cost.

In addition, they have a simple application procedure, easily adaptable within industry. From the point of view of synthesis, the sol-gel route offers versatile ways to synthesize effective coatings with specific properties. Functionality is optimized by variation of experimental parameters such as chemical structure, composition and ratio of precursors and complexing agents, rate and conditions of hydrolysis, synthesis media, embedding of additional active species, aging and curing conditions, deposition procedure, etc... [3] .

Sol-gel processed organic-inorganic hybrid coatings exhibit increased flexibility and thickness as compared to their inorganic counterparts. In general, these sol-gel derived coatings have been found to provide good corrosion resistance to metal substrates due to their barrier properties, strong adhesion, chemical inertness, versatility in coating formulation, and ease of application under ambient temperature conditions [4,25]. Often, corrosion inhibitors are added to the system in order to provide added functionalities for improved performance and durability, with self-repair ability for damage recovery. The presence of additives should further decrease the corrosion rate. The concept is based on damage recovery from corrosion attack via controlled release of corrosion inhibitors stored in the organic coating with a specially designed assembly or simply dispersed in the polymeric matrix [2-4,18-21,26,27].

US Patents by Ostrovsky [28,29] disclose a treatment for improved surface corrosion resistance of Mg and its alloys. The patents describe the acid pickling solutions for surface pretreatment and the compositions of water/organic solutions of hydrolyzed silanes. The coated magnesium alloys reveal the presence of corrosion pits after a period of 8-24 hours, because silane-only treatments (without additives) do not provide efficient protection of magnesium alloys. The treatments described in the patents [28-29] are mainly designed for the adhesion of paint or sealing of anodization, and they are not efficient for corrosion protection by themselves.

Khramov et al. developed sol-gel processed organic- inorganic hybrid coatings with phosphonate functionalities [18,19]. Due to the chemical interaction between the phosphonate groups and the surface of magnesium substrate, these specially designed organo-silicate barrier coatings are expected to generate protective coatings with improved adhesion and corrosion resistance for magnesium materials. The organo-silicate sols were prepared via acid-catalyzed hydrolysis and condensation of a mixture of TetraEthOxySilane (TEOS) and DiethylPHosphonatoethyl-triethoxySilane (PHS) in ethanol/water solution at low water-to-silane ratio. Zhang et al. elaborated a novel sol-gel process, where appropriate additives were used to stabilize and disperse uniformly the inorganic salts precursors (such as cerium) ; the sols with appropriate pH could be applied directly on Mg alloys [17]. Montemor et al. [24] investigated the protective behavior of Bis- [TriEthoxySilysPropyl] Tetrasulfide (BTESPT) modified with the addition of cerium nitrate or lanthanum nitrate in order to introduce active corrosion protection in the silane film.

Lamaka et al. [3] developed hybrid organic-inorganic sol- gel coatings synthesized by copolymerization of epoxy- siloxane and titanium or zirconium alkoxides. Addition of Tris ( trimethylsilyl ) phosphate was also studied to improve the corrosion protection of the Mg alloy.

Even though all these newly-developed coatings show some improvement in terms of more environmentally friendly corrosion protection for Mg alloys, they still present some limitations as they provide low durability and poor resistance to corrosion even in presence of active additives. Some of these coating systems were only tested in diluted solutions - 0.005 M NaCl and/or for a short time (30-min immersion) [17-19]. Montemor [24] and Lamaka [3] reached good results in terms of corrosion resistance, but the performance was evaluated during only one- and two- week immersion time, respectively. In addition the electrolyte used was 0.005 M NaCl, which is still considered diluted, if compared to the actual trend of industry and to what was used for the purpose of the present invention.

Very good results were recently reported by Wang et al.

[22]. The hybrid sol-gel/polyaniline coating applied to AZ31 alloy withstood immersion in 3.5% NaCl with the value of the low frequency impedance above 10⁶ Ω cm² after 27 days of immersion. The thickness of the coating was around 53 micrometers. Surface preparation of the substrate also plays a very important role in the overall performance of the coating assembly, as the presence of impurities on g alloys has a crucial effect on their corrosion behavior [27,30,31]. Hence coating treatments are always preceded by a cleaning procedure that might consist of a simple grinding and polishing process, or of a chemical process such as acid pickling .

Among the acids used for cleaning, hydrofluoric acid (HF) has received particular attention, due to the formation of a protective layer on the metal surface, which improves its corrosion resistance [32,33]. Coatings provide a barrier against corrosion attack; however, once the water reaches the metal/coating interface, delamination of the film might occur, together with formation of a defect due to gas evolution, which considerably decreases the protective properties. Hence, a pretreatment that removes the impurities, increases the hydrophobicity of the metal surface and enhances adhesion is necessary. HF is a good candidate for this purpose, as it provides these characteristics.

Summary of the invention

The proposed invention concerns anti-corrosion coatings for magnesium alloys and their preparation method. In particular, it relates to the coatings composition, the films formed over the substrate and the anti-corrosion properties of the overall system when immersed in an aggressive electrolyte .

The coating composition comprises a silane (e.g. AminoPropylTriEthoxySilane, APTES or

AminoPropylTriMethoxySilane, APTMS) , an epoxy component (e.g. Poly (bisphenol A-co-epichlorohydrin) , glycidyl end- capped) and an amine (e.g. Diethylenetriamine, DETA) , in organic solvents.

A wide range of magnesium alloys used in the aeronautic and automotive industries can be protected efficiently with the coating proposed in this invention. The magnesium alloys that can be protected include, but are not limited to, alloys containing aluminium such as AZ31, AZ91 and AM60, alloys containing zinc such as ZK30, ZK60 and also alloys modified with rare earth metals such as WE43, WE45, Elektron 21 and ZE41.

These coating solutions can be applied to the magnesium alloys substrates by dip-coating or spraying, followed by thermal curing in a specific range of conditions. The thickness of the coating varies from 5 to 20 micrometers.

The anti-corrosion performance of this set of coating is evaluated by immersing the final product in an aggressive environment such as NaCl for a long period of time and assessing the outcome through optical evaluation and electrochemical methods. In particular, electrochemical impedance spectroscopy (EIS) is used for analysis, as it gives the possibility to estimate the corrosion protection efficiency of coatings.

The proposed invention can be applied to automotive and aerospace components, sporting material, electronic material and bio-compatible implants. Detailed description of the invention

The present invention refers to a corrosion protection coating for magnesium alloys and its preparation method. In particular it refers to the coatings composition, the films formed on the substrate and the corrosion properties of the full system when immersed in an aggressive electrolyte. The coating solution object of this invention comprises three main ingredients: a silane (AminoPropylTriEthoxySilane , APTES or AminoPropylTriMethoxySilane, APTMS) , an epoxy component (Poly (bisphenol A-co-epichlorohydrin) , glycidyl end-capped) and an amine ( Diethylenetriamine, DETA) as cross linker. The silane and epoxy components are first prepared separately by diluting them in ethanol and acetone, and let stirring for 1 hour. Then the two solutions are mixed and the amine is added. The final concentration of the main components is between 1 to 10 wt% silane, preferably between 3 and 5%, between 5 and 50 wt% epoxy, preferably between 30 and 40 wt%, and between 0,5 and 10 wt% amine, preferably between 2 and 4 %. The solution is stirred for 1 to 6 hours before deposition on the substrate, which can be done by spraying or dipping.

The preparation of the magnesium alloys herein designated by way of example as substrates consists of mechanical polishing with silicon/carbon paper, followed by chemical etching in 12% hydrofluoric acid (HF) for 15 minutes. This etching treatment is very efficient as it helps removing impurities on the alloy surface and forming Mg hydroxides, oxides and fluorides at the metal surface [25] . It is crucial for the formation of an efficient anti-corrosion coating. The coating is deposited on the magnesium alloys by dipping the substrate into the coating solution (dip- coating) . The substrate is immersed 3 times for 5 seconds each. After dipping, the coated Mq alloys, herein designated by way of example as samples, are cured in an oven at various temperatures between 120°C and 180°C, during a period of time between 0,5 and 5 hours, preferably between 1 and 2 hours. The thickness of the coatings obtained with the described procedure varies from 5 to 20 micrometers, depending on the specific conditions.

The anti-corrosion properties of the final product coated magnesium alloys are evaluated through electrochemical impedance spectroscopy (EIS) measurements. These are performed at the open circuit potential (OCP) of the system under study, in the 10⁵ Hz to 10 mHz frequency range, by means of a potentiostat in potentiostatic mode. The amplitude of the perturbation is lOmV rms .

The electrochemical cell consists of a three-electrode setup: a saturated calomel electrode (SCE) as a reference, a platinum coil as a counter electrode, and the working electrode, which is the coated magnesium. The measurements are performed in the electrolyte 0,05 M sodium chloride. Visual and optical inspections of the coated sample prove that after a month immersion the coated sample is intact and without any corrosion damage.

On the other side, the uncoated samples reveal strong signs of corrosion and accumulation of corrosion products on their surface. This superior corrosion performance is confirmed by the results of the electrochemical impedance spectroscopy measurements shown in Figure 1. Samples of magnesium alloy AZ31 with a coating of 12 + 2μπι, present very high impedance values, stable for immersion time up to 31 days. For these samples the corrosion resistance did not show any significant decrease. The fact that the impedance does not change during the entire immersion period, reveals the absence of pits or other corrosion events. The impedance values are above 1 Gohm cm² after 31 days of immersion, which is much higher than what is reported in literature [1-3, 17- 24, 26-33] . These results reveal that the proposed coatings have a performance considerably higher than those described in literature. The proposed invention can be applied to automotive and aerospace components, sporting material, electronic material and bio-compatible implants.

Description of the drawings

Figure 1 represents the graphical evolution of electrochemical impedance as a function of time for the coated sample immersed in 0.05 M sodium chloride.

In Figure 1(A) the y-axis, identified as \Z\ , refers to the electrochemical impedance, expressed in Qcm² and the x- axis refers to frequency, expressed in Hz.

In Figure 1(B) the y-axis, identified as phase angle, refers to the phase, expressed in degrees, and the x-axis refers to frequency, expressed in Hz.

References

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Date: May 2^nd, 2013

Claims

1. Hybrid coatings for improved corrosion protection of magnesium alloys, characterized in that their composition comprises a silane component, an epoxy and an amine.

2. Hybrid coatings according to claim 1, characterized in that their silane component is AminoPropylTriEthoxySilane or AminoPropylTriMethoxySilane .

3. Hybrid coatings according to claim 1, characterized in that their epoxy component is Poly (bisphenol A-co- epichlorohydrin) , glycidyl end-capped.

4. Hybrid coatings according to claim 1, characterized in that their amine component is Diethylenetriamine .

5. Hybrid coatings according to claims 1 and 2, characterized in that the concentration of AminoPropylTriEthoxySilane or AminoPropylTriMethoxySilane is between 1 and 10 wt%.

6. Hybrid coatings according to claim 5, characterized in that the concentration of AminoPropylTriEthoxySilane or AminoPropylTriMethoxySilane is between 3 and 5 wt%.

7. Hybrid coatings according to claims 1 and 3, characterized in that the concentration of Poly (bisphenol A-co-epichlorohydrin ) , glycidyl end-capped is between 5 and 50wt% .

8. Hybrid coatings according to claim 7, characterized in that the concentration of Poly (bisphenol A-co- epichlorohydrin) , glycidyl end-capped is between 30 and 40wt% .

9. Hybrid coatings according to claims 1 and 4, characterized in that the concentration of Diethylenetriamine is between 0.5 and 10wt%.

10. Hybrid coatings according to claim 9, characterized in that the concentration of Diethylenetriamine is between 2 and 4 wt%.

11. Method of preparation of the hybrid coatings, as defined in claims 1 to 10, is characterized in that: a) the components of the hybrid coating are dissolved in organic solvents; b) the hybrid coatings are deposited on a magnesium alloy, by spraying or dipping; c) the hybrid coatings are cured at a temperature between 120°C and 180°C; d) the hybrid coatings are cured for a period of time between 0,5 and 5 hours.

12. Method of preparation according to claim 11 characterized in that the organic solvents are ethanol and acetone.

13. Method of preparation according to claim 11 characterized in that the hybrid coatings are cured for a period of time between 1 and 2 hours.

14. Use of the hybrid coatings, as defined in claims 1 to 10, characterized in that it is used in: a) automotive and aerospace components; b) sporting material and electronic material;

c) bio-compatible implants. Date: May 2^nd, 2013