steel interfaces

Author’s Accepted Manuscript Ageing mechanisms of polyurethane adhesive/steel interfaces J. Weiss, M. Voigt, C. Kunze, J.E. Huacuja Sánchez, W. Possart, G. Grundmeier www.elsevier.com/locate/ijadhadh

PII: DOI: Reference:

S0143-7496(16)30132-4 http://dx.doi.org/10.1016/j.ijadhadh.2016.06.009 JAAD1863

To appear in: International Journal of Adhesion and Adhesives Received date: 14 December 2015 Accepted date: 25 June 2016 Cite this article as: J. Weiss, M. Voigt, C. Kunze, J.E. Huacuja Sánchez, W. Possart and G. Grundmeier, Ageing mechanisms of polyurethane adhesive/steel inte r f a c e s , International Journal of Adhesion and Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2016.06.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Ageing mechanisms of polyurethane adhesive/steel interfaces J. Weissa, M. Voigta, C. Kunzea, J. E. Huacuja Sánchezb, W. Possartb, G. Grundmeiera a: University of Paderborn, Technical and Macromolecular Chemistry, Warburgerstr. 100, D33102, Germany b: Saarland University, Chair for Adhesion and Interphases in Polymers, Campus C 6.3–6, D-66123 Saarbruecken, Germany

Corresponding author: Guido Grundmeier (e-mail: [email protected]) Abstract: For the achievement of a complete understanding of the ageing and corrosion of adhesively bonded steel substrates, both the ageing of a polyurethane reference adhesive and the simultaneous corrosion of a mild steel substrate were studied by combined spectroscopic, electrochemical and microscopic techniques. A corundum-blasted mild steel was used as metal substrate in combination with a cross-linked reference polyurethane adhesive free from additional additives or fillers. The ageing was performed under full immersion in a chloride-free aerated electrolyte at elevated temperature. A strong influence of the water saturated polymer phase on the change in the interfacial oxide film structure of the metal substrate was observed. The thickening and hydroxylation of the metal oxide film during the corrosive delamination of the polyurethane film indicated that the de-bonding mechanism of the polyurethane adhesive is mainly based on interfacial water enrichment and a thereby induced growth of the interfacial oxide. The measured electrochemical properties of this oxide indicate that it is highly conductive and allows for oxygen reduction as the counter reaction to metal oxidation. Keywords: polyurethane, adhesive, steel, corrosion, interface

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1. Introduction The corrosive degradation of adhesively bonded steel joints is of significant importance for many technical applications. In most cases such adhesively joined structures are exposed to corrosive environments and the functionality of the component relies on the durability of the joint over long times of exposure. However, it has been observed that the bonded joint experiences a decrease in shear strength after industrial ageing tests or in the field [1-5]. As the durability of such a joint is based on the ageing of the steel/adhesive composite system, the overall observation of loss in performance is not sufficient to clarify the underlying critical mechanisms of degradation. The fundamental mechanisms of corrosive delamination of organic coatings and adhesives so far have been mainly studied in their initial states neglecting the ageing process of the polymer phase itself [6, 7]. Mainly, free standing adhesive films, not adhesive joints, were studied to predict critical mechanisms of corrosive delamination [8-11]. Wapner and Posner combined spectroscopic, microscopic and electrochemical techniques such as the Scanning Kelvin Probe to reveal the corrosive delamination process of epoxy adhesive films on steel and galvanized steel [12, 13]. They could show that the cathodic oxygen reduction reaction dominates the delamination for epoxy adhesive coated mild steel substrates [13]. Wapner et al. could moreover reveal by means of FTIR-spectroscopy the kinetics of water transport into these adhesive joints. Their studies showed the crucial role of interfacial water activity in combination with corrosive anions which attack the ultra-thin surface oxides of the metal alloys [12]. However in such studies, the state of ageing of the adhesive/metal interface prior to the analysis of the corrosive de-adhesion process was neglected. Moreover, the analytical approaches which considered free-standing adhesive films, led to an acceleration of the access of water and oxygen through the open surface of the adhesive film. Meiser et al. considered mainly the chemical ageing of several epoxy networks both at the free contact to the environment (moist air or water at different temperatures) and at the buried contact between the adhesive and metals (Al, Cu, Au) [14]. The authors showed that the chemical degradation of the epoxy network responds to the ageing temperature. For given temperature, it follows similar basic steps of oxidation in both regions but the kinetics and the hierarchy of these reactions are different in both regions. Water significantly accelerates these oxidations, especially for moderate ageing temperatures. Copper migrates deeply into the epoxy and forms amine complexes [15]. Moreover, the ageing regimes change the network morphology and the mechanical properties to some extent, in addition to the plasticization and the shift of the glass transition due to the diffusive uptake of water [16]. Due to the wide range of chemical and structural variations that can be achieved with polyurethanes, their ageing behaviour cannot be discussed in general terms. Concerning the poly(ether urethanes) utilised in this work, the literature attests very good stability to such polymers, at least under mild conditions in the dark. For example, thermoplastic poly(ether urethane) improved compatibility between hard and soft segments during aging at temperatures from 40 - 70 °C for up to 300 days but degradation did not occur [17]. Another technical poly(ether urethane) for bio-applications resisted chemically to water at 95°C and dry air at 90 °C for up to 56 days [18]. In crosslinked systems the stability against chemical degradation is similar. Hints of hydrolytic aging are observed in a poly(ether urethane) coating after 100 days of ageing in salt water above a threshold temperature of 70 °C [19]. Lower temperatures do not start a chemical degradation in this material even if this is aged up to 18 months. A crosslinked poly(ether urethane) used as a shape memory polymer does not show 2

any chemical degradation of the urethane bond while being stored at 60 °C during 180 days [20]. Ageing of polyurethane adhesives is rarely studied. The few reported results indicate a good stability of the urethane groups, as shown in [21] for medical poly(ether urethane) adhesive in distilled water or physiological solution at 20 °C and 100 °C, respectively. In summary, the ageing behaviour of poly(ether urethane) deserves further attention. Particularly, this holds for the situation in a polyurethane adhesive joint. Based on these findings the study reported here is focused on the interplay between the water up-take and ageing of a polyurethane adhesive and the interfacial chemistry and electrochemistry of the mild steel substrate in an adhesive joint. Reference materials were employed in this study to reduce the complexity compared to technical adhesives while keeping the principal materials properties as close as possible to the application.

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2. Materials and Methods Materials and Chemicals Sheets (2 x 20 x 130 mm³) of S235 mild steel alloy were utilised as adherend. Just prior to bonding, the sheets were solvent cleaned in an ultrasonic bath with a sequence of solvents (tetrahydrofuran (purity >99.5 %), 2-propanol (97 %), ethanol (99 %)) for 10 min each step. Then, they were grit blasted (white corundum from EW Würth, Bad Friedrichshall, Germany, grain size 125-180 µm, blasting with dried air at 6 bar, angle 90°) at room temperature (RT). Loose corundum particles and dust was removed by acetone washing in an ultrasonic bath. The elemental composition of the corundum particles is shown in Fig. 1.

Figure 1: XPS survey spectrum of corundum on an indium foil Besides Al2O3 and the usual carbonaceous surface contamination, the corundum particles contain some sodium and calcium most probably as carbonates as sodium and calcium ions are not stable under atmospheric conditions. Except the area for adhesive bonding (100 x 20 mm²), all faces and edges of the steel sheets (130 x 20 mm², 2 mm thick) were protected with a 3-layer anti-corrosion coating (Sika Zink R, EG 5 and EG1, Zurich, Switzerland). Finally, for reproducible cleanliness, the bonding area was gently blasted with pressurized dried air. The reactive PU adhesive is formulated from three technical monomers (Bayer Material Science, Leverkusen, Germany): The diisocyanate Desmodur VP.PU1806 consisting of the isomers 2,4’-methylene diphenyl diisocyanate (2,4’-MDI, 53.7 mass-%), 4,4‘-MDI (46.2 mass%) and 2,2’-MDI (0.1 mass-%), the polyether diol Desmophen 3600Z, and the polyether triol Baygal K55 as the cross-linker. For their chemical formulae, see Fig. 2.

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Figure 2: Chemical structure of the polyethertriol Baygal K55, polyetherdiol Desmophen 3600Z, 2,4’-, 4,4’- and 2,2’-isomers of methylene diphenyl diisocyanate The triol and diol monomers were mixed (OH group ratio OHtriol : OHdiol = 90 : 10 mole-%). The MDI isomer mixture was added slowly up to the stoichiometric ratio of hydroxyl and isocyanate groups. All components were handled and stirred at RT in dried air (dew point below -50 °C). For removal of dissolved gas, the monomer mixture was evacuated at 0.01 mbar for 15 minutes. Curing of bulk samples at RT (ca. 23 °C) for 7 days plus 7 days at 60 °C in dried air results in a fully cured polyurethane network (the caloric glass transition extends from ca. -18 °C to +59 °C with Tg = 27.3 °C). Preparation of adhesive joints Right after the final step of steel surface treatment, adhesive joints (bonding area 20 x 100 mm²) were prepared at RT by filling the space between two steel sheets with the liquid reactive adhesive (average thickness of 500, 300 and 100 µm) by a syringe. Bonding was carried out immediately after the adhesive was mixed (cf. previous section) and in the same dried atmosphere. The bond line thickness was set by two polytetrafluoroethylene separators which were placed between the two steel substrates but outside the bond line. The adherends were fastened upright in a metal jig keeping the bond line at the desired thickness. Except for the upper slit, all sides of the bond line were sealed by adhesive tape in order to avoid liquid reactive spill out. The joints were then cured in the same two-step regime as bulk samples for full crosslinking and stable mechanical properties. Ageing regime The fully cured samples (bulk and adhesive joints) were immersed in bi-distilled water at 60 °C as sketched in Figure 3. They were taken from the bath for analysis at different times from 7 to 180 days. The water was not exchanged during the course of ageing. 5

Figure 3: Storage of PU-steel adhesive joints immersed in water for up to 180 days

Analytical Techniques

Raman Spectroscopy Raman spectroscopy measurements were conducted with an InVia Renishaw R40 Raman microscope system (Renishaw Gloucestershire, UK). The Raman spectra were collected with an excitation wavelength of 633 nm (He-Ne laser) and 8.8 mW output through a 50x objective lens. X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) experiments were performed on an Omicron ESCA+ System (Omicron NanoTechnology GmbH, Taunusstein, Germany) with a base pressure of < 3·10-7 Pa. The system is equipped with a hemispherical energy analyser, the element spectra were recorded at 20 eV pass energy. For photoelectron excitation a monochromatic Al-Kα (1486.7 eV) X-ray source with a spot diameter of 800 µm was used. The take-off angle of the detected photoelectrons was 60° with respect to the surface normal. For data evaluation the Casa XPS software (version 2.3.15) was used. All quantification of the XPS data was performed by integration of the peaks with regard to the relative sensitivity factors of the elements. Scanning Electron Microscopy (FE-SEM) The morphology of the samples was characterised by means of scanning electron microscopy using a NEON 40 FE‐SEM (Carl Zeiss SMT AG, Oberkochen, Germany) at an accelerating voltage of 2.0 kV with an aperture size of 30 µm and a working distance of approximately 4.5 mm. Prior to the measurements, the polymer samples were sputtered with a 4 nm thick metal layer consisting of 80% Au with 20% Pd. Electrochemistry The linear sweep voltammetry and cyclic voltammetry measurements were performed in a three electrode setup with a Reference 600 potentiostat (Gamry, Germany). Measurements were done in chloride free borate buffer (pH 8.4, 19.1 g/L sodium tetraborate decahydrate, 12.4 g/L boric acid und 7.1 g/L sodium sulfate) as electrolyte at 25 °C. A gold wire and a saturated Ag/AgCl electrode (Radiometer analytical, France) were used as counter and reference electrodes, respectively. For the linear sweep voltammetry measurements, the potential was scanned with a rate of 2 mV/s from -0.3 V to 0.3 V vs. open circuit potential. For cyclic voltammetry measurements six cycles were collected from -0.8 V to 0.4 V and back to 0.8 V 6

with a sweep rate of 100 mV/s. After the second sweep the sample behavior was stationary, so the third sweep is presented as the representative sweep.

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3 Results and Discussion: 3.1 Substrate surface state prior to bonding The topography of the corundum blasted steel surface in comparison with the polished and etched state is shown in Figure 4. The etched surface was prepared in a sequence of steps: grinding with silicon carbide paper (up to 4000 grade), polishing with diamond paste (1µm), rinsing with ethanol (99%), ultrasonicating for 15 minutes in ethanol to remove any residues from the process, etching for 2 minutes in a mixture of ethanol and nitric acid (9:1 HNO3 (65%): ethanol (99%), rinsing with ultrapure water (Milli-Q), and finally, drying in a stream of dry air. While the etched state reveals the grains in the size range from 20 to 70 µm and the corresponding grain boundaries, the blasted substrate is characterised by a highly deformed surface near region with a roughness on the micrometre scale.

a)

Figure 4 a, b): FE-SEM images of the polished and chemically etched surface (a) in comparison to the corundum blasted surface (b) of the steel substrate

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Thereby, the corundum blasting could in principle lead even to cavities enabling mechanical interlocking. The XPS analysis of the two surfaces is shown in Figure 5 a-c. The corresponding surface chemical composition as well as the peak evaluation is presented in Tables 1 and 2. The corundum blasting process leads to chemical modification of the surface as Al-oxide and in addition Ca and Na ions are deposited onto the surface and most probably lead to the formation of the corresponding hydroxides and carbonates under atmospheric conditions.

a)

b)

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c) Figure 5 a-c): a) Fe2p spectra of the chemically etched and the corundum blasted surface prior to adhesive application, b) O1s spectra of the chemically etched and the corundum blasted surface prior to adhesive application, c) C1s of the chemically etched and the corundum blasted surface prior to application of the adhesive.

Table 1: Analysis of the surface compositions of the corundum, chemically etched and the corundum blasted steel surface (binding energy in eV (BE) and compositions in at.-%) steel substrate – corundum blasted

corundum particles

steel substrate – etched

BE (eV)

at.-%

BE (eV)

at.-%

BE (eV)

at.-%

Fe2p

-

-

712

9

710

22

Na1s

1071

5

1072

4

-

-

C1s

285

28

285

39

284

36

O1s

531

45

531

41

529

42

Al 2s

119

22

119

7

-

-

Table 2 a-c): Decomposition of O 1s, C1s and Fe 2p peaks for chemically etched and corundum blasted samples (binding energy in eV (BE); compositions in at.-%) 2 a): Decomposition of the O1s peak O 1s

BE (eV) chemically etched

OH-, CO32-

H2O, -COOH

at.-% BE (eV)

O2at.-%

BE (eV)

at.-%

532.6

16

531.5

29

530.2

55

533.0

10

531.8

60

530.4

30

corundum blasted

10

2 b): Decomposition of the C1s peak C 1s

CO32-

-C=O, -COOH

-C-OH, -C-HO, -C-O-C

-C-C, -C-H

BE (eV) at.-%

BE (eV) at.-%

BE (eV)

at.-%

BE (eV)

at.-%

chemically etched corundum blasted

289.6

-

288.5

10

285.8

34

284.9

56

10

288.2

4

285.9

32

284.9

54

2 c): Decomposition of the Fe 2p peak Fe0

Fe 2p

FeII, FeIII oxides and hydroxides

BE (eV)

at.-%

BE (eV)

at.-%

chemically etched

707.0

31

710.9

69

corundum blasted

706.7

39

710.9

61

The chemical compositions as estimated from the XPS data are shown in Tables 1 and 2. The blasting process leads to a mixture of iron and aluminium oxides on the surface. Both oxide layers show a passive film thickness formed under atmospheric conditions below 3 nm as indicated by the significant contribution of the Fe0(2p) peak in Figure 5a). The O1s data (see Figure 5b) indicate that the surface after corundum blasting is rich of hydroxyls. Moreover, the formation of metal carbonates on the surface is confirmed. In agreement with the O1s peak, the C1s peak (See Figure 5c) shows an increased carbonate/carboxylate contribution after the blasting process suggesting that the surface contains metal oxides, hydroxides but moreover metal carbonates prior to adhesive bonding. The dominating contribution however is still the aliphatic component based on atmospheric contaminations. Based on the surface analytical findings the polyurethane adhesive was applied to a rough surface containing a density of hydroxyl and carbonate groups. Moreover, the average thickness of the surface inorganic non-metallic layer is below 3 nm. Hence, an electronic barrier is not formed which is proven by the electrochemical studies shown below.

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3.2 Water in the polyurethane adhesive In accordance with the chosen ageing conditions, the water diffusion into the cured PU bulk was monitored gravimetrically. Briefly, PU sheets (l x w = 90 mm x 35 mm, t = 3 – 4 mm) were immersed in bi-distilled water at 60 °C as for the ageing of the adhesive joints. After appropriate periods of immersion, samples were taken from the bath, superficially dried with a wipe, and weighed on lab scales. Figure 6 shows that the water uptake saturates at csat(H2O) = 2.6 %-weight within ca. 160 h.

a)

b) Figure 6 a, b): Gravimetric data for the water uptake of PU bulk during immersion at 60 °C. The PU thickness is 3 – 4 mm. a): Measured water content m(t) normalized to the dry state mdry. The dashed line indicates sample saturation at 2.6 %-weight. b): Data with rescaled axes (msat, mass of water-saturated sample) for the determination of Fick’s diffusion coefficient as the slope of the straight line. That behaviour justifies data treatment along the lines of Fickian diffusion (cf. [22] for details) which results in a water diffusion constant DH2O(60 °C) = (1.9  0.14)10-11 m2s-1. Then,, the water diffusion profile can be calculated for a homogeneous PU bulk layer (l  w = 100 mm x 20 mm) between two steel sheets as a function of immersion time, tim. Figure 7 illustrates the situation for 7 and 150 days, respectively. We note in passing that the PUmetal interphase cannot be taken into account for this simulation. Nevertheless, that model 12

may serve as a reasonable first approximation for the water distribution in the bondline of the real adhesive joint after 150 days of water immersion.

a)

b) Figure 7 a, b): Simulated water concentration profiles mH2O(x) / msat in a 100 mm  20 mm homogeneous PU bulk at 60 °C for (a) 7 and (b) 150 days between two semi-infinite walls (not shown) that seal the lower and upper PU faces tightly against water penetration. Right: 1D profile along the width, x. Left: 2D profile for the cross-section. In particular, Figure 7 reveals that the water arrives quickly at the centre of the sample and, therefore, the contact to the steel substrate faces the presence of water very early. The combination of penetrating water, dissolved oxygen, the raised temperature and, last not least, the presence of the substrate surface could provoke chemical ageing processes in the PU inside the adhesive joint. µ-ATR IR spectroscopy was carried out on the PU side of the fracture faces produced by a mechanical shear test of the aged adhesive joints. Within the limits of sensitivity of the method, detailed IR spectra analysis revealed no clear indication for chemical ageing in the PU for up to 180 days of ageing in water. The GC-MS analysis of products leached from the aged PU bulk by dichloromethane (HCCl2)) reveals amine or urea molecules based on the diphenyl methane units from MDI as well as fragments from the polypropylene glycol diol. This points to the degradation of urethane links in the polymer network (cf. [23] for more details). In general, the PU itself resists the chosen ageing procedure 13

very well and hence the very contact between the PU adhesive and the steel surface moves into the focus. 3.3 Fracture face studies after ageing The FE-SEM images of the metal and polymer fracture faces after the ageing process (shown in Figure 8 for an adhesive joint after 180 days of water immersion at 60 °C) show that an additional inorganic layer has been formed on the steel side. The surface presents a rougher structure in terms of small grains and crystallites. In comparison, the polymer side replicates the topography of the initial state of blasted steel substrate (compare to Figure 4b). The interfacial layer growth on the metal side which can be deduced by the change in the microscopic roughness could be proven by XPS data as shown below.

a)

b) Figure 8 a, b): FE-SEM data of the metal substrate a) and the adhesive side b) after fracture of the aged joint under shear load. 14

The corresponding Raman spectroscopy measurements (see Figure 9) on the metal fracture face of the delaminated adhesive joint show the formation of mixed iron oxides and hydroxide species during the exposure. The steel surface after corundum blasting did not show any significant peaks due to the low oxide film thickness after this process.

Figure 9: Raman spectra of the unaged and aged substrate side

The spectra of the aged samples present a peak with the highest intensity at 668 cm-1, which has a small shoulder at around 720 cm-1. A very small peak at 298 cm-1and two small features at around 400 cm-1 and 520 cm-1 over the overall high background could be determined. A broader peak arises at around 1400 cm-1. The peak positions as well as their correlation with relevant iron oxides and hydroxide species according to different references are listed in Table 3. According to Table 3, the strong peak at 668 cm-1 observed in the spectra could be associated with different iron oxides or hydroxide species. The Raman spectra of the oxide layer formed during ageing can be assigned to an iron oxide mixture of maghemite (-Fe2O3) and magnetite (Fe3O4) as found found by Bellot-Gurlet et al [24] and a hydrated iron (III) oxy-hydroxide [25]. The small peak at 298 cm-1 could be due to an oxide or hydroxide contribution, e.g. magnetite and/or a slightly amorphous FeOOH.

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Table 3: Summary of the wavenumbers found in this work in comparison with eight relevant iron oxide and hydroxide species according to different references

wavenumber ṽ/cm-1

Compound name

References

observed

literature

298

301.6

[28]

Goethite α-FeOOH

298, 300

[29]

Feroxyhite δ-FeOOH

297

[29]

510

[24]

513.0, 533.6

[28]

670

[24,25,28,29]

Wustit FeO

662.7

[28]

Ferrihydrites

680-700 (broad)

[24,25]

Akaganeite (β-FeOOH)

670

[25,29]

Feroxyhite δ-FeOOH

666, 677

[25,29]

Magnetite Fe3O4

667, 676

[28,29]

Magnetite Fe3O4,

Maghemite -Fe2O3

520

Magnetite Fe3O4 Maghemite -Fe2O3

668

Maghemite -Fe2O3

720

700, 720, 718

[24,25,28]

Maghemite -Fe2O3

1400

~1400 (broad)

[24,25]

~1400 (broad)

[24,25]

Akaganeite (β-FeOOH)

The observed metal oxide layers in the joint are formed at high water activities but at reduced oxygen partial pressure in the adhesive joint of the two steel substrates. Moreover, the exclusion of chlorides from the ageing medium simulating a purely water and oxygen induced ageing process at increased temperatures led to a de-adhesion process where chloride induced attack of the oxide film can be neglected. Corresponding XPS data of the non-aged steel surface and the steel side of the fracture face after ageing were measured. The Fe2p spectra of the metal and the adhesive side as well as the O1s spectrum of the metal side are shown in Figure 10 and the peak evaluations for all elements are presented in Table 4.

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a)

b)

c) Figure 10 a-c): XPS-analysis of the interface chemical state of the fracture faces after the ageing test based on the Fe2p peak with a) metal side and b) adhesive side, as well as c) the O1s peak of the metal side. 17

Table 4: Analysis of the surface compositions of the aged and unaged adhesive and sample surfaces (binding energy in eV (BE); compositions in atomic percent (at.-%)) adhesive side – unaged

adhesive side aged

substrate side – unaged

substrate side aged

BE (eV)

at.-%

BE (eV)

at.-%

BE (eV)

at.-%

BE (eV)

at.-%

Fe2p

-

-

711.2

<1

709.5

9

711.7

5

N1s

400.3

3

399.9

3

-

-

400.4

<1

C1s

284.9

74

284.6

70

284.0

39

285.2

40

O1s

532.8

20

532.5

25

529.0

41

531.8

46

Al 2s

-

-

119.2

1

118.5

7

119.4

9

Si 2p

102.1

3

101.8

<1

-

-

-

Na 2s

-

-

-

1071.5

4

-

-

Ca 2p

-

-

-

347.0

<1

-

-

-

Based on the XPS data as summarized in Table 4 only minor amounts of Fe and Al are transferred to the adhesive fracture face with resulting concentrations of not more than one atom percent. However, the oxide layer on the steel side is thickened and hydroxyl-rich. Alcompounds remain on the metal side of the interface even after ageing. However, the Na and Ca-carbonates could not be detected after ageing which might be due to dissolution of such compounds in the formed interfacial aqueous layer during the exposure. The curve fitting of the Fe2p 3/2 spectrum was based on the multiplet peak model proposed by Biesinger et al. [26]. According to this model, it was found that the Fe2p 3/2 peak is composed of a large fraction of oxy-hydroxide (87% FeOOH) and a small fraction of different Fe(II) and Fe(III) oxide species (10% of FeO, 3% of Fe2O3). No contribution of Fe(0) was found anymore on the metal fracture face of the aged sample. The minor transfer of Al and Fe-ions to the adhesive side could be a result of an adsorption of corrosion products on the adhesive interface as it offers Lewis base sites. Interestingly, the shape of the very weak Fe2p signal on the polyurethane fracture face replicates the Fe2p line shape on the metal side very well. That shows that some material of the aged oxide adheres well enough on the polyurethane interface to resist fracture. However, it is clear that the major part of fracture did not occur within the oxide film. The small contribution of nitrogen (less than 1%) on the metal side after fracture indicates the adsorption of few macromolecular components of the adhesive since the polyurethane is the only N-containing material in the joint. However, no significant part of fracture occurred in the adhesive phase. In conclusion, the delamination was mainly a result of the interfacial water accumulation and corrosion reaction which destroyed the adhesive interactions. No significant chemical ageing of the polyurethane at the delaminated interface could be detected by means of XPS in agreement with the infrared studies on bulk ageing (cf. sect. 3.2). It can be assumed that an alkaline pH establishes underneath the adhesive which is based on the interfacial oxygen reduction as the main interfacial electrode reaction. The interface side of the adhesive indicates a small but measurable transfer of oxides / hydroxides from 18

the substrate to the adhesive. It can be concluded that the interfacial oxide thickening and hydroxylation mainly lowers the strength of the steel/polyurethane interface.

3.4 Potentiodynamic studies of the oxide formed at the aged PU-steel contact To analyse the changes in the electrochemical properties of the steel substrate aged in the adhesive joint, cyclic voltammetry and linear sweep voltammetry measurements were performed with the substrate part after shear fracture (Fig. 11 a, b). In the utilised chloride-free electrolyte both surfaces show a similar behaviour in the cathodic and anodic trees of the Tafel plot (Figure 11a). However, a decrease of the anodic current density and an increase in the cathodic current density is observed after the ageing process as indicated by the shift in the trees of the Tafel plot and the resulting slight anodic shift of the free corrosion potential by about 50 mV. The difference in the redox chemistry of the surface oxides becomes however more obvious in the cyclic voltammograms (Figure 11b). The cyclic voltammogram of the surface after ageing shows a high capacity of the semiconducting oxide and again the strong contribution of oxygen reduction resulting in a broad, featureless but tilted cyclic voltammogram.

a)

19

b) Figure 11 a, b): LSV curves and CV data of the corundum blasted surface before and after ageing.

The explanation for this observation is that after ageing, the kinetics of the oxygen reduction are probably even slightly accelerated as the formed Fe2+ doped oxide film is nsemiconducting and the oxygen reduction shows rather low over-potentials on nsemiconducting iron oxides. However, as also the microscopic and sub-microscopic roughness increases in comparison to the initial state as shown by the FE-SEM images in Figure 8, no quantitative analysis is possible. As shown by Bockris et al. [27], the oxygen reduction on iron oxides occurs via a two-electron pathway with the formation of peroxy radicals as intermediate species. Therefore, we can predict that during the delamination of the adhesive the interfacial oxygen reduction process leads to oxidative intermediates which could promote the de-adhesion process in combination with the wet-de-adhesion.

4. Conclusions The presented data address both the response of the adhesive and the metal substrate on the environmental conditions during an artificial ageing process at high water activity. Prior to adhesive application the initial corundum blasted steel surface consists of an ultra-thin iron oxide film and aluminium oxyhydroxides. Moreover, the surface contains water-soluble surface sodium carbonates which are deposited by this mechanical pre-treatment. The measurement and calculation of water transport in the adhesive joint show that the adhesive layer sandwiched between the two steel substrates is fully saturated within the exposure time. The saturation of the bulk phase and the interphase with water led to an increase of the oxide film thickness of the steel substrate. In contradiction to dry adhesive joints, which typically fail mechanically by crack propagation through the polyurethane layer, water accumulation in 20

combination with the presence of oxygen at the polyurethane/steel interface leads to a thickening of the Fe-oxide film, to surface hydroxylation and, as the consequence, to an almost complete de-adhesion of the adhesive film at the oxide interface. While sodium ions are dissolved in the interfacial electrolyte, the initially observed Al-compounds remain at the interface even after the ageing test. The formed interfacial iron oxides and oxyhydroxides do not show insulating properties but allow for an accelerated oxygen reduction process in comparison to the non-aged initial state. In conclusion, both interfacial water enrichment and a corrosion process of the steel surface leading to iron oxyhydroxide scale formation, lead to the macroscopic de-adhesion process of the joint at the given ageing conditions.

Acknowledgements: The authors gratefully acknowledge the financial support by the German Science Foundation (AiF-DFG joint project grants No. GR 1709/14-1 and Po 577/25-1). J. Weiss also acknowledges the financial support by the state of North-Rhine Westphalia (“NRW Fortschrittskolleg Leicht-Effizient-Mobil”).

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References:

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Figure Captions Figure 1: XPS survey spectrum of corundum on an indium foil Figure 2: Chemical structure of the polyethertriol Baygal K55, polyetherdiol Desmophen 3600Z, 2,4’-, 4,4’- and 2,2’-isomers of methylene diphenyl diisocyanate Figure 3: Storage of PU-steel adhesive joints immersed in water for up to 180 days Figure 4 a, b): FE-SEM images of the polished and chemically etched surface (a) in comparison to the corundum blasted surface (b) of the steel substrate Figure 5 a-c): a) Fe2p spectra of the chemically etched and the corundum blasted surface prior to adhesive application, b) O1s spectra of the chemically etched and the corundum blasted surface prior to adhesive application, c) C1s of the chemically etched and the corundum blasted surface prior to application of the adhesive. Figure 6 a, b): Gravimetric data for the water uptake of PU bulk during immersion at 60 °C. The PU thickness is 3 – 4 mm. a): Measured water content m(t) normalized to the dry state mdry. The dashed line indicates sample saturation at 2.6 %-weight. b): Data with rescaled axes (msat, mass of water-saturated sample) for the determination of Fick’s diffusion coefficient as the slope of the straight line. Figure 7: Simulated water concentration profiles mH2O(x) / msat in a 100 mm  20 mm homogeneous PU bulk at 60 °C for 7 and 150 days between two semi-infinite walls (not shown) that seal the lower and upper PU faces tightly against water penetration. Right: 1D profile along the width, x. Left: 2D profile for the cross-section. Figure 8: FE-SEM data of the metal substrate a) and the adhesive side b) after fracture under shear load Figure 9: Raman spectra of the unaged and aged substrate side. Figure 10 a-c): XPS analysis of the interface chemical state of the fracture faces after the ageing test based on the Fe2p peak with a) metal side and b) adhesive side as well as the O1s peak of the metal side. Figure 11 a, b): LSV curves and CV data of the corundum blasted surface before and after ageing.

Table captions: Table 1: Analysis of the surface compositions of the corundum, chemically etched and the corundum blasted steel surface (binding energy in eV (BE) and compositions in atomic percent at.-%) Table 2 a-c): Decomposition of O 1s, C1s and Fe 2p peaks for chemically etched and corundum blasted samples (binding energy in eV (BE); compositions in atomic percent (at.-%)) Table 3: Summary of the wavenumbers found in this work in comparison with eight relevant iron oxide and hydroxide species according to different references Table 4: Analysis of the surface compositions of the aged and unaged adhesive and sample surfaces (binding energy in eV (BE); compositions in atomic percent (at.-%))

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