Towards anti-corrosion coatings from surfactant-free latexes based on maleic anhydride containing polymers

Progress in Organic Coatings 61 (2008) 224–232

Towards anti-corrosion coatings from surfactant-free latexes based on maleic anhydride containing polymers Willem Jan Soer a,b , Weihua Ming a,∗ , Cor E. Koning b,∗∗ , Rolf A.T.M. van Benthem a a

Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands b Laboratory of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600MB Eindhoven, The Netherlands Received 20 August 2007; accepted 20 September 2007

Abstract We report on the film formation of surfactant-free, artificial latexes based on copolymers containing maleic anhydride. Different metallic substrates, such as aluminum, steel and magnesium alloys, were coated with three different latexes. A commercial polyester based coating was used as a comparative sample. Two of the latexes were based on polymer with a high Tg (resp. ∼100 and 130 ◦ C), and one on a polymer with a Tg of −70 ◦ C. The wetting of the substrates could be optimized by etching the metal substrate, acidic or alkaline, leading to homogenous film formation and improved adhesion. For aluminum substrates an alkaline pretreatment improved the adhesion with the polymer films, whereas for magnesium the acidic pretreatment improved the adhesion. Furthermore, acid pretreatment increased the corrosion resistance of the coated magnesium substrate, when compared to an alkaline pretreatment. The films formed from latex displayed comparable or better anti-corrosive properties when compared to the commercial polyester. © 2007 Elsevier B.V. All rights reserved. Keywords: Surfactant-free latex; Coating properties; Anti-corrosion

1. Introduction Waterborne coatings, such as latex coatings, are a very important class of coatings, due to legal restriction of the use of volatile organic contents. In this work, surfactant-free latexes consisting of maleic anhydride containing polymers [1,2] are studied, to serve as a potential anti-corrosion coating for metallic substrates, such as aluminum and magnesium alloys. The importance of surfactant-free systems becomes apparent if the water sensitivity of the final films is considered. Although the amount of surfactant that is used in most latexes is small, the water sensitive surfactants may lead to defects in the coating as soon as the films are inserted into water, which in turn affects the anti-corrosion properties of the coating. Magnesium is one of the most common elements found on this planet, representing about 2% of the earth’s mass [3]. Being one third lighter than aluminum, magnesium is the lightest of all structural metals. This makes this metal an interesting ∗

Corresponding author. Tel.: +31 40 247 3066; fax: +31 40 244 5619. Corresponding author. Tel.: +31 40 247 5353; fax: +31 40 246 3966. E-mail addresses: [email protected] (W. Ming), [email protected] (C.E. Koning). ∗∗

0300-9440/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.porgcoat.2007.09.032

material for use in light-weight applications such as automotive, aircraft or portable devices. Alloys of magnesium, such as AZ31 or AZ51 (Mg, containing 3 and 5 wt.% Al, respectively, and 1 wt.% Zn), are very sensitive to corrosion. This drawback limits their use in a number of potential applications. To overcome the corrosion of magnesium alloys, a large number of different techniques is used to protect the substrate [3]. Most of the treatments however, are accompanied by either high costs, or by the use of environmentally unfriendly methods. Polymers that have anhydrides incorporated into the backbone or in side groups, are known to give good adhesion to metallic substrates [4–6]. The dicarboxylic acid, the hydrolyzed product of the cyclic anhydride, for instance is known to have strong interaction with metal oxides, such as Al2 O3 or MgO [7], or hydroxides [6], and forms back cyclic anhydrides upon heating. Adhesion, in combination with good barrier properties of the coating, is known to increase the corrosion resistance of the substrates, such as aluminium or magnesium alloys [6,8–13]. Furthermore, anhydride can be used to react with amine functional reactants to irreversibly form hydrophobic cyclic imides upon heating [1,14], giving it advantage over regular acids, for which these reactions are not possible. This makes the final

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films less sensitive to hydrolysis and therewith more suitable as protective coating for metallic substrates. To serve as a protective coating, the latexes need to form homogeneous and crosslinked films, to prevent the transport of ionic species that are the products of oxidation reactions in order to keep electrical neutrality [7]. Crosslinking of anhydride containing polymers can be done with multi-functional alcohols but reaction times between anhydrides and alcohols to form esters are usually long, even at high temperatures [15–17]. Besides, the esters that are formed are sensitive to hydrolysis, leading to loss of network properties. Another class of crosslinkers are bifunctional amines [18], but apart from toxicity and a strong smell, high temperatures for these reactions are required (>125 ◦ C) to form irreversible imide bonds [14,19]. The crosslinker used in this study is based on a difunctional hydrazide, i.e. adipic dihydrazide (ADH) [2], a water soluble compound that lacks the toxicity and strong smell of primary amines (such as the commonly used 1,6-diaminohexane (DAH)). In this paper we explore the possibilities of using these surfactant-free latexes, based on anhydride containing polymers, as anti-corrosion coatings. A high crosslink density combined with good adhesion to the substrate makes this a potentially very interesting system for this type of coatings for light-weight metals such as aluminum or magnesium alloys.

2.3. Film studies

2. Experimental

The testing of the properties of the coatings was performed on coatings applied on Al3003 H14 Q-panels. The coatings were applied with a 100 ␮m spiral roller applicator, resulting in coatings of 10–15 ␮m. In the case of K¨onig hardness measurements, a 250 ␮m doctor blade applicator was used, leading to dry film thicknesses of approximately 30 ␮m. The commercially available polyester was applied from NMP solution, and cured according to the manufacturer’s prescription.

2.1. Materials All materials were purchased from Aldrich and used as received, unless noted otherwise. Solvents were purchased from Biosolve and used without further purification. 1,6-Diaminohexane (DAH) was purchased from VWR. SMA1000F, poly(styrene-alt-maleic anhydride) (PSMA) and Ricon 131MA17, maleinized polybutadiene (17 wt.% maleic anhydride) (PBDMA), were kindly provided by Sartomer. Poly(octadecene-alt-maleic anhydride) (POMA) was purchased from Aldrich. Adipic dihydrazide (ADH) was kindly supplied by DSM NeoResins and used without purification. Aluminum and steel substrates were obtained from Q-panel Lab Products, and magnesium alloy (AZ31) substrates were obtained from Salzgitter Magnesium-Technologie GmbH. 2.2. Latex preparation Latexes of commercially available polymers, i.e. poly(styrene-alt-maleic anhydride) (PSMA), poly(octadecenealt-maleic anhydride) (POMA) and maleinized polybutadiene (PBDMA), were obtained by partial ammonolysis of the anhydride moieties in acetone, as described in more detail elsewhere [1,2]. The obtained latex particles were studied with a Malvern ZetaSizer Nano-ZS to obtain particle diameters as well as the ζ-potential. The interaction with crosslinker was studied by adding crosslinker dissolved in water to the latexes. The kinetics of the curing were studied with ATR-FTIR on a Bio-Rad Excalibur FTS3000MX infrared spectrometer, as described elsewhere [2].

Film formation was performed by applying the latexes, containing crosslinker, to either aluminum or glass substrates, immediately followed by a curing step in an oven at elevated temperatures, between 80 and 200 ◦ C. AFM experiments on the films were performed on a NT/MDT Solver P47HT in semi-contact mode under atmospheric conditions, using high resolution non-contact gold coated silicon cantilevers from the NSG11 series (NTI-Europe) with spring constant k = 2.5–10 N/m and a typical resonance frequency of 150 kHz. Scanning of the surfaces was done at 0.2 Hz. 2.4. Surface pretreatments The panels were cleaned from grease and other organic material by washing with acetone and ethanol. The AZ31 magnesium panels were sanded to remove the top-layer, by using sand paper with decreasing grid sizes, i.e. P360, P600 and P1200. Possible etching was performed by either immersing the substrate in a 0.1 NaOH (aq) solution for 1 or 2 min, or in a 5% HNO3 (aq) solution for 30 s, followed by rinsing with H2 O for 1 min. 2.5. Film application

2.6. Substrate surface studies XPS (X-ray photoelectron spectroscopy) measurements were performed with a VG-Escalab 200 spectrometer using an aluminum anode (Al K␣ = 1486.3 eV) operating at 510 W with a background pressure of 2 × 10−9 mbar. Spectra were recorded using the VGX900 data system. The data was analyzed using CasaXPS software. The Mg 2p, O 1s and Al 2p signals were collected and the peak areas were calculated. In order to compare ratios between different atoms, sensitivity factors were used (Mg: 0.252, O: 0.711, Al: 0.234). They were determined for this specific instrument. 2.7. Coating properties Contact angle measurements were performed on a Dataphysics OCA 30 instrument. The advancing contact angles were measured by adding water at 0.5 ␮l/s. The reported contact angle is the average value from three individual measurements on different spots on the film. The K¨onig hardness was measured by a pendulum test with an Erichsen Pendulum Hardness meter, model 299/300. All values are averages of three measurements on three different places, and errors are within 5 s. Reversed

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Table 1 Polymers used for the preparation of latexes Polymer

Mn (g/mol)

Tg (◦ C)

PSMA POMA PBDMA

5,000 30,000–50,000 5,500

130 90–100 −70

impact testing was performed on a BYK Gardner impact tester with a 1 kg weight, having a diameter of 20 mm, falling from 100 or 50 cm. Adhesion testing was performed by a crosshatch tape test, using an Erichsen Scratch Stylus 463 to apply the cuts, and scotch tape to test the adhesion of the coating to the substrate. The wet adhesion was tested by applying a droplet on the coating for 4 h, followed by the crosshatch tape test. Chemical resistance was measured by 100 acetone double rubs with a soaked piece of paper. 2.8. Corrosion studies Anti-corrosion properties were studied by immersing coated steel or Al 2024 substrates into a 5 wt.% NaCl bath at room temperature. Magnesium alloy substrates were studied by applying a 5 wt.% NaCl solution drop on top of the coating, covered by a 10 ml sample bottle with its contact side to the coating sealed with vacuum grease to prevent evaporation of water. 3. Results and discussion 3.1. Coatings from latexes based on maleic anhydride containing polymers Surfactant-free latexes of maleic anhydride containing polymers are obtained by partial ammonolysis of the anhydride functionality [1,2]. In this study three different polymers were used (Table 1) to obtain these latexes. The PSMA was partly imidized by n-heptylamine to prevent complete dissolution of the polymer in water [1]. With these polymer, stable latexes with particle sizes in the range of 100–200 nm are obtained [2]. The latexes of these three polymers are stabilized by electrostatic interactions, as is indicated by a ζ-potential of about −70 mV for all samples. For the PSMA latex it is shown that both particle size and ζ-potential do not change during a period of over 2 years. Furthermore, the particles are stable in a wide pH range; from pH 2 to 7 there are no significant differences in particle size [2]. Only the absolute value of the ζ-potential decreases when

the pH is decreased to values <4, but the electrostatic interactions over this pH range remain sufficient to stabilize the particles. When a crosslinker is added to the PSMA based latex it has been shown that the chemical nature of the crosslinker plays a crucial role in the properties of the final latex. Adding a strongly basic diamine, 1,6-diaminohexane (DAH), shows an increase in the initial pH of the system of 7 to almost 9 [2]. Furthermore, a drop in absolute ζ-potential of −70 to −45 mV is observed. Lowering the pH to 5 leads to flocculation of the latex due to the lack of sufficient electrostatic stabilization. When a dihydrazide based crosslinker is used, i.e. adipic acid dihydrazide (ADH), the initial pH of the system hardly changes, from 7 to 6.5. Furthermore, no difference in ζ-potential was observed. Lowering the pH of the system does affect the ζ-potential in a similar way as for the system without crosslinker. This indicates that there is no significant interaction between ADH and polymer particles as long as the two are in the water phase, whereas DAH appears to interact with the particles. This has been further confirmed with 1 H NMR [2]. When no water is present, crosslinking between anhydride units (of the model compound cis-1,2-cyclohexyldicarboxylic anhydride (CDA)) and ADH or DAH takes place at room temperature, as demonstrated with both 1 H NMR and ATR–FTIR [2]. This reaction yields the ring-opened amic acid structure (1). To obtain hydrophobic, non-hydrolysable bonds, the amic acid moieties need to form imides by ring closure (Scheme 1), which are obtained by heating [14]. This reaction can be followed by monitoring the carbonyl peak originating from the imide moieties in the system. The imide formation of the amic acid with the ADH proceeds at temperatures of 90 ◦ C and higher, whereas imide formation with DAH takes place only at temperatures of 125 ◦ C and higher [2]. Film formation of these latexes depends on the Tg of the original polymer. When PSMA-based latexes are used, either with ADH or DAH as crosslinker, film formation temperatures of 160 ◦ C are needed to obtain homogeneously crosslinked films, as AFM studies revealed [2] (Fig. 1). When POMA-based latexes are used the homogeneous film formation temperature is decreased to 100 ◦ C. With PBDMA based latexes homogeneous films can be obtained at room temperature, however, these films have very poor chemical resistance due to the absence of imide formation. To prepare crosslinked coatings from these formulations, a minimum temperature of 90 ◦ C should therefore always be used. In practice temperatures of 120 ◦ C and higher were used to make sure full crosslinking took place, as well as to reduce the time of curing.

Scheme 1. Schematic representation of the reaction between model compound CDA and ADH. The first step takes place at room temperature to yield the hydrophilic amic acid (1), while upon heating the ring-closed imide is formed (2).

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Fig. 1. AFM height images of films of (a) PSMA film formed at 160 ◦ C, (b) POMA film formed at 100 ◦ C and (c) PBDMA formed at 120 ◦ C, all crosslinked by ADH.

3.2. Coating properties 3.2.1. Chemical resistance The chemical resistance of the films formed from a PSMA based latex was studied as a function of curing temperature. The temperature, at which the PSMA latexes form homogeneously crosslinked networks, is shown to be 160 ◦ C, so results obtained for curing temperatures of 120 and 140 ◦ C were for non-homogeneous films. Although not perfectly film formed, films cured with ADH (30 mol% to initial anhydride) at a temperature of 140 ◦ C for 20 min, have very good chemical resistance. One hundred double rubs with an acetone soaked tissue did not lead to any visible damage. At 120 ◦ C curing times of 10 min did not result in good chemical resistance. For longer curing times, some increase in chemical resistance was shown, but damage to the film was observed after 100 double rubs. When films from this latex were formed with DAH as crosslinker at 120 ◦ C for 20 min, no chemical resistance was observed, but for the higher curing temperatures no differences with ADH were found (Table 2). This correlates to the results obtained by the kinetic experiments in previous work [2]. In view of film formation of a PSMA based latex; there is no obvious advantage of using ADH over DAH. Both crosslinkers lead to comparable films, and the quality is mainly determined by the MFFT of the PSMA based latex. But due to the better stability of the latex containing crosslinker, the lower reaction temperature to form imides [2], as well as the better environmental aspects, ADH is used for the remainder of this paper.

Table 2 Chemical resistance of films from PSMA based latexes crosslinked with ADH (30 mol% relative to initial anhydride) and DAH, tested with 100 acetone double rubs Curing temperature (◦ C)

200 180 160 140 120

ADH

Table 3 Compositions of the polymers used for the different latexes and curing/filmforming conditions of the applied latexes PSMA

POMA

PBDMA

Reactions with anhydride Imidizeda (%) Ammonolyzed (%) ADH (%)

30 30 40

– 60 40

– 60 40

Film formation conditions T (◦ C) Time (min)

160 15

120 15

120 15

All percentages are relative to initial anhydride. a mol% of anhydrides that reacted with n-heptylamine [1].

The latexes based on POMA and PBDMA are studied in a similar way. The compositions of the latexes were optimized and curing conditions above MFFT were used to get the best possible film for each sample, provided that all initial anhydride was used in different reactions, to prevent the possible formation of hydrophilic channels through which ion transport is possible. The reactions of the anhydrides initially present in the polymer with different compounds and application conditions are listed in Table 3. All films were applied from a 10 wt.% latex at 100 ␮m by a roller bar applicator. 3.2.2. Mechanical properties If the films are formed at temperatures that allow for homogeneous film formation as well as complete crosslinking, excellent chemical resistance is obtained for the different latexes, as indicated by no visible damage after 100 acetone double rubs. Furthermore, it is shown that the surface of the polymers is hydrophobic (Table 4). The POMA sample is the most hydropho-

DAH

Table 4 Properties of polymer films obtained from different latexes

10 min

20 min

30 min

20 min

Polymer

MFFT (◦ C)

+ + + + ±

+ + + + ±

+ + + + −

K¨onig hardness (s)

Impacta

+ + + ± −

Adv. water contact angle (◦ )

PSMA POMA PBDMA

160 100
∼95 ∼105 ∼95

205 ± 5 135 ± 5 45 ± 5

Failed Failed No damage

(+) No damage visible; (±) some damage; (−) film removed completely.

a b

1 kg from 50 to 100 cm. Film forming at RT does not lead to a crosslinked network.

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Table 5 Composition of the used metal substrates

Aluminum Chromium Copper Iron Manganese Silicon Titanium Zinc Others (total) a

Al3003 H14a

AA1050 [22]

Al2024 T3a

AZ31B-O [23]

Remainder

Remainder <0.01 <0.01 0.3 <0.01 <0.1 0.01 <0.01

Remainder 0.1 3.8–4.9 0.5 1.2–1.8 0.5 0.15 0.25 0.15

2.5–3.5

0.05–0.20 0.7 0.6 0.10 0.15

CRSa

<0.05 <0.01 ∼0.5 <0.1

Remainder 0.25–0.6

0.5–1.5 Remainder (magnesium)

<0.13 (carbon) <0.1 (other)

Data submitted by supplier.

bic sample of these polymers, due to its long aliphatic tails, which, together with the dense network and a relatively high Tg , is expected to act as good barrier against the transport of ionic corrosion products in water [12,20,21]. As can also be seen in Table 4, the K¨onig hardness for the polymer films depends very strongly on the initial Tg of the used polymer. The polymer system with the highest Tg , PSMA, has a K¨onig hardness of ∼200 s. This however leads to a very brittle film resulting in very poor impact resistance, for both crosslinked and non-crosslinked films. The film cracks and delaminates from the substrate when a 1 kg weight with a 20 mm diameter is dropped from 100 or 50 cm on the back of a coated 0.6 mm thick aluminum panel. Although the POMA based films are softer (as shown with pendulum testing), these films are not resistant against this reversed impact either. PBDMA does not crack or delaminate after reversed impact testing, due to its more flexible nature, indicated by the lower Tg of the initial polymer. This increased impact resistance however is combined with a very low hardness, giving the film a tacky character. 3.2.3. Adhesion of coatings to different metallic substrates One of the key parameters in corrosion protection is the adhesion of the polymer to the substrate. In this paper we study the adhesion of the coatings on different metallic substrates, i.e. steel (CRS), different grades of aluminum (Al3003, AA1050 and Al2024), and a magnesium alloy (AZ31). The compositions of the aluminum, steel and magnesium alloy panels that were investigated are given in Table 5. All substrates were cleaned with ethanol and acetone. The Al3003 substrate was also given an alkalic pretreatment, by

immersing it in a 0.1 M NaOH (aq) solution for 60 s to study the effects of the chemical composition on the wetting and adhesion of the polymer. The magnesium substrate (AZ31) was sanded to remove the dirty top-layer, followed by etching in a 5% HNO3 (aq) solution. The coatings were applied from 10 wt.% in water by a roller applicator (100 ␮m), immediately followed by curing at an elevated temperature. The films prepared from the different latexes are compared with a commercially available acid functional polyester cured with TGIC. The adhesion of the films to the substrates was studied by crosshatch scotch tape adhesion testing. As can be seen in Table 6 the adhesion of most of the polymers on the different aluminum substrates is good, both dry and after 4 h covered by water. Only the non-etched Al3003 substrates show poor adhesion for a number of polymers, as well as moderate dewetting of most latexes. Films were obtained, but upon applying the latex, some dewetting was observed at the edges of the film, leading to an inhomogeneous film thickness. These problems were merely overcome by giving the substrates an alkalic etching pretreatment. Increasing the etching time did not further improve the adhesion. Only for the POMA samples, poor adhesion and some dewetting was still observed. This may be due to the hydrophobic character of the alkyl chains of the polymer, causing dewetting of the substrate upon applying the film, and shielding off the acid groups of the polymer backbone, preventing sufficient interaction with the metal surface. Adding a small amount of NMP (∼10 vol.%) did improve the wetting of the substrate, but still resulted in the formation of some spots that were not perfectly wetted, and therefore caused defects in the coating.

Table 6 Adhesion of different coatings on the different substrates

Dry (PSMA) Wet (PSMA) Dry (PBDMA) Wet (PBDMA) Dry (POMA) Wet (POMA) Dry (Polyester) Wet (Polyester) a

Al2024

AA1050

Al3003

Al3003 alkalic etching

Steel

AZ31 acid etching

+ + + +

+ + + + + − + +

− − + − − − + +

+ + + − − − + +

+ − + − − − + −

+ + + − + + + +

a a

+ +

No film could be formed due to severe dewetting problems. (+) Indicates good adhesion; (−) indicates complete loss of adhesion.

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The wet adhesion to the steel substrates was found to be very poor in all cases. This may be due to the pretreatment of the substrate, but the influence of the pretreatment is not studied in more detail in this paper. Both the dry and wet adhesion of the polymer to the acid pretreated magnesium alloys is good for all of the polymers, except for the PBDMA wet adhesion. When the substrates were given an alkali pretreatment, the adhesion was poor for both the dry and wet conditions. 3.3. Corrosion protection 3.3.1. Aluminum alloy substrates To study the anti-corrosion properties of the films, aluminum substrates of the Al2024, due to its tendency to corrode relatively easily (unlike the Al3003 and AA1050), were coated with three different coatings (PBDMA, PSMA and polyester) and immersed in a 5 wt.% NaCl solution. The POMA based coatings were not investigated, since the films could not be formed properly due to dewetting problems, which would not lead to reliable results. It can be seen in Fig. 2a that after 1 week immersion, as expected, corrosion starts at the unprotected sides of the samples.

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Especially for the polyester coated sample corrosion is clearly observed. The sample with the PSMA based coating also shows some corrosion spots, but the overall film still is intact. The PBDMA gives the best protection against corrosion after 1 week of immersion, no corrosion spots are observed. When these samples are left for 7 weeks, the sample coated with the polyester coating has almost completely been corroded, having only a small part of the original coating intact (Fig. 2b). The sample coated with PSMA based latex is also partly corroded, but the majority of the original coating is still intact. Better results are observed for the PBDMA based coating. Some corrosion occurs on the sides of the panel, due to incomplete coverage of the surface, but no severe damage is observed underneath the coating. Furthermore, hardly any filiform corrosion is observed, indicating good barrier properties of the coating [8,13,24]. Therefore, the corrosion that is observed is most probably due to defects in the coating, caused by either dewetting during application, leading to pores through which water transport can take place, or degradation of the polymer in time. The first cause is more probable, since the oxidation takes place from the sides of the coating, where dewetting problems are most apparent. Furthermore, when the non-damaged parts of these coatings were submitted to a crosshatch test, the adhesion to the substrate was still good. The key parameter in the preparation of these systems appears to be excellent film formation, as well as the pretreatment of the substrate. 3.3.2. Steel substrates The washed steel coated samples were immersed in a 5% NaCl solution, but none of the coatings provided a good resistance against corrosion. Within 2 days, very severe corrosion was observed for all coated samples. For the PSMA coated substrate, strong filiform corrosion was observed next to pit-corrosion, indicating poor barrier properties (Fig. 3b). The reason for the filiform corrosion of PSMA coated substrates is not clear, since the coated Al2024 sample showed good barrier properties, which should not lead to filiform corrosion. Apart for PSMA based coatings, the relatively small amount of filiform corrosion indicates that the barrier properties of the coatings are good, and the observed corrosion is caused by poor adhesion to the substrate. The pretreatment of the steel substrates and its influence on the adhesion of a polymer coating is not studied in more detail in this paper, but it is expected that better corrosion-resistant coatings can be obtained once the adhesion of the polymer films is improved, for instance by using a conversion layer.

Fig. 2. Three different coatings on Al2024 after 1 week (a) and 7 weeks (b) immersion in 5 wt.% NaCl solution. The left sample is coated with PBDMA based latex, the middle with PSMA based latex and the right with polyester.

3.3.3. Magnesium alloy substrates 3.3.3.1. Substrate pretreatment. Two different pretreatments of the AZ31 alloys were compared to study the influence of the pretreatment on the adhesion and corrosion protection. Both acidic and alkaline etching steps were performed. It is expected that the acid pretreated substrate contains more aluminum oxide, since the magnesium phase is partly etched away. As can be seen from the XPS data in Fig. 4, the initial Al/Mg atomic ratio of ∼0.06 increased to 0.6 after etching in a 5% HNO3 solution for 30 s.

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Fig. 3. Corrosion of steel substrates after 1 week of immersion in 5 wt.% NaCl (aq) solution, coated with a PBDMA based coating (a) and filiform corrosion underneath a PSMA based coating (b).

Furthermore, it can be seen that after acid treatment the amount of oxygen increases strongly. This indicates that the substrate contains a lot of aluminum- and magnesium-oxides and possibly hydroxyl groups, which can interact or react with the carboxylic acid groups in the polymer film [6,11]. The aluminum oxide also increases the anti-corrosion properties of the substrate, since it is stable under atmospheric conditions. The composition of the acid pretreated substrate does not change significantly when the sample is stored for 1 week under atmospheric conditions. Others [25] showed that the adhesion of succinic acid to (untreated) AZ31 substrates deteriorates when the samples are left under atmospheric conditions due to the contamination of the surface with water. This loosely adsorbed water will not be detected in the XPS measurement due to evaporation, and therefore all substrates were etched prior to applying the coatings to make sure maximum adhesion is obtained. 3.3.3.2. Anti-corrosive properties. The magnesium substrates cannot be tested in a reliable way by immersion in a NaCl solution, since the substrates could not be fully covered, especially

Fig. 4. Relative amounts of the three main elements (Mg, Al, O) at the surface of the AZ31 sanded and acid pretreated substrate as determined by XPS, with a probing depth of 10 nm.

at the edges and sides of the substrate. This leads to contact between the uncovered substrate and the aqueous NaCl environment, leading to oxidation of the magnesium and the formation of hydrogen gas [26], indicated by bubbling. Mg + 2H2 O → Mg2+ + 2OH− + H2

(1)

Within 1 day the coatings are completely delaminated and destroyed. This is due to the alkaline (pH ∼ 12) environment that is formed upon dissolution of the magnesium in an aqueous environment, as shown in reaction (1). The coatings are no longer stable under these conditions, leading to the aforementioned results. To test the PSMA based coatings on AZ31 substrates, a drop of NaCl solution was placed on top of a coated substrate, covered with a 10 ml sample bottle with vacuum grease at the interface to avoid evaporation of the water (Fig. 5a). After 1 week the underlying acid pretreated substrate was not affected by the exposure to the solution (Fig. 5b). The coating remained on the substrate when scotch tape adhesion testing was performed without damage, indicating a strong adhesion to the aluminum oxide bonds at the substrate surface, combined with good barrier properties. Due to the pretreatment good wetting is obtained, leading to homogeneous film formation and good adhesion of the film to the substrate. The polyester films that were applied from NMP solution did show similar results. No corrosion products were found, and the adhesion to the substrate was preserved. The POMA and PBDMA based coatings however did show delamination after 1 week exposure to the NaCl solution. Again, this may be due to defects in the films, since no filiform corrosion was observed. Samples that were pretreated with NaOH instead of HNO3 solution showed strong corrosion of the substrate and therewith damaging the coating after 1 week under NaCl (aq) solution exposure (Fig. 5c). Although the barrier properties of the coating might be good, there may be some transport of water via defects in the coating. The transport of a small amount of water reaching the interface, leads to delamination of the film, followed by oxidation of the substrate and an increase in pH, leading to failure of the coating. This indicates that the bonding to the substrate

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Fig. 5. Corrosion test set-up for PSMA coated AZ31 substrates: (a) for a pristine film on an acid pretreated substrate; (b) 1 week after applying the NaCl solution on a film on an acid pretreated substrate; (c) 1 week after applying the NaCl solution on a film on an alkaline pretreated substrate. The circular feature in (b) originates from the vacuum crease that was used to prevent evaporation of the solution.

was less effective for the alkali etched substrate when compared to the acidic etched substrate. Although the alkaline pretreatment gives a magnesium hydroxide layer which is expected to give very good adhesion to polymers, the magnesium hydroxide is not stable at pH values <12 [26], and will therefore dissolve upon applying the coating, leading to poor adhesion, as well as delamination of the coating. The penetrating water will eventually reach the magnesium phase, upon which dissolution of the magnesium hydroxide takes place, followed by an increase in pH, leading to failure of the film. The effects of the different pretreatment steps on adhesion to magnesium alloys was also described by others [25], who showed that on alkaline pretreated AZ31 adhesion of succinic acid was poor, whereas on acidic pretreated substrates good adhesion was obtained. Currently more detailed research, such as electrochemical impedance spectroscopy (EIS), about the barrier properties of the different coatings is in progress.

4. Conclusions Stable surfactant-free latexes from three different maleic anhydride containing polymers were applied to a metal substrate and cured above the Tg of the respective polymer, to form homogeneous, crosslinked films. Densely packed films and low mobility of the polymer chains are desirable for anti-corrosion coatings, but this makes the films also susceptible to cracking, as indicated by reversed impact testing. The polymer systems based on a maleic anhydride functional polymer do show good barrier properties as well as good adhesion to the different metal substrates. A comparative system, based on a polyester resin applied from NMP solution, showed comparable properties in both adhesion as well as barrier properties. For aluminum substrates a basic pretreatment improved the adhesion, whereas for magnesium the acidic pretreatment improved the adhesion. Furthermore, acid pretreatment increased the corrosion resistance of the coated magnesium substrate, when compared to a basic pretreatment. By mixing different latexes the final properties, such ass hardness and flexibility, of the film can be optimized to obtain good barrier type coatings that have good adhesion

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