Improvement of the Durability of Zinc-Coated Steel/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and hot-dipped galvanized (G2F) or electroplated-phosphated (EZ2) steel have been investigated. The degradation mechanisms have been studied after three accelerated ageing tests: the “cataplasme humide” (“C.H.T.”), immersion (“I.T.”), and salt spray (“S.S.T.”) tests. X-ray photoelectron spectroscopy (XPS) analysis of fracture surfaces after ageing have shown that anodic dissolution of the zinc-coating is responsible for debonding in all cases and that intergranular corrosion phenomena account for poorer performances of the hot-dipped galvanized substrate during “C.H.T.” and “I.T.” Silane coupling agents were successfully used as primers on both substrates to increase the hydrolytic stability of the metal/adhesive interface. XPS results indicate that both the interfacial dissolution of the phosphate coating of EZ2 and intergranular corrosion of G2F are delayed for silane-primed specimens. The observed improvements do not appear to depend on the nature of the silane coupling agents. Alkylsilanes have been found to perform as well as silanes having a group capable of reacting with the epoxy/dicyandiamide system.

Additional tests were carried out in view of the possible application of organosilane reagents as additives in corrosion-protective oils. Good durability properties have been obtained by priming the metal coupons with a standard oil/silane mixture prior to bonding.

When corrosion was the controlling degradation mechanism as is the case during the salt spray test, silane treated specimens did not generally perform better than control specimens.     

Water-Resistant Oxide Layers at the Interface of Zinc/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and zinc metal coupons have been investigated. The metal coupons were anodized in sodium hydroxide solutions before bonding. The influence of the anodizing conditions on the morphology and composition of the oxide layers has been studied using SEM and TEM imaging analyses as well as X-ray photoelectron spectroscopy. The hydrolytic stability of the bonded joints has been assessed by storing the joints in water at 70 °C or 90 °C for periods of time up to 5 weeks. Polypropylene has been used as a model adhesive to study the influence of mechanical interlocking effects on the performance of the bonded joints. Depending on the anodizing conditions, the improved durability properties have been attributed either to „mechanical interlocking effects“ or to the higher hydrolytic stability of the oxide layers generated during the anodizing treatment.

Some of the results gained from the anodization of zinc have been extrapolated to hot-dipped galvanized steel. Bonded joints made from hot-dipped galvanized coupons anodized under smooth conditions (2% NaOH) displayed residual shear strengths of up to 70% higher than specimens simply degreased after immersion test. The generation of stable oxide layers as well as the suppression of intergranular corrosion phenomena at the metal/adhesive interface can explain the improved durability properties.     

Comparison of the Degradation Mechanisms of Zinc-Coated Steel, Cold-Rolled Steel, and Aluminium/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and zinc, zinc-coated steel, two different aluminium alloys or cold-rolled steel metal coupons have been investigated. The influence of the dicyandiamide content of the adhesive on the durability properties-has been assessed by salt spray testing or by storing the joints in water at 70°C or 90°C for periods of time up to five weeks. The degradation products formed during ageing of the epoxy adhesive in water have been investigated using high performance liquid chromatography (HPLC) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). The degradation mechanisms of aluminium/epoxy bonded joints have been thoroughly studied using X-ray photoelectron spectroscopy.

The performances of the bonded joints under a pure corrosive environment have been found to be little influenced by the quantity of dicyandiamide in the adhesive. When the bonded joints were aged in hot water, the stability of the interface toward an excess of dicyandiamide directly followed the sensitivity of the oxide layer at high pH values. Optimal durability properties without peel strength losses of the adhesive were aehieved both with zinc and aluminium-coated substrates by reducing the quantity of dicyandiamide in the epoxy adhesive by 20% (the initial dicyandiamide content in the commercial adhesive being ca. 9%, with respect to the epoxy resin).     

Cathodic Delamination of an Epoxy/Polyamide Coating from Steel

X-ray photoelectron spectroscopy (XPS) has been used to determine the mechanism responsible for debonding of an epoxy/polyamide coating from steel during cathodic delamination in 3.5% aqueous NaCl solutions. Coating failure always occurred near the interface between the coating and the oxide. The nitrogen content of the free surface of the prepared coatings was about 10%. However, the nitrogen content of the free surface dropped to only 5% after exposure to 1 N NaOH for four weeks and that of the coating failure surface after cathodic delamination was only about 2%, implying that the failure involved degradation of the polyamide curing agent by hydroxide ions formed at the steel surface by reduction of oxygen. That conclusion was supported by results obtained from curve fitting of C(1s) and O(1s) spectra. The intensity of components in the C(1s) spectra due to C—N and C≡O bonds in amide functional groups decreased significantly after coatings were exposed to NaOH or subjected to cathodic delamination. Small amounts of organic materials characteristic of the coating were observed on the substrate failure surface, perhaps indicating that the failure was cohesive within the coating but very close to the interface or that some products from degradation of the curing agent precipitated on the substrate. Use of silane coupling agents to retard cathodic delamination was also investigated. Coupling agents were added directly to the coating or applied to the substrate as a primer before application of the coating. Significant reduction in the rate of cathodic delamination was seen only when the silane coupling agent was applied to the substrate and cured at elevated temperatures before the epoxy/polyamide coating was applied.     

Investigation of Steel/Epoxy Adhesion Durability Using Polymeric Coupling Agents III. Influence of Coupling Agent Layer Thickness

Steel/epoxy peel specimens were prepared using ethylene-mercaptoester (EME) copolymer coupling agents (90 wt% mercaptoester units) applied in thickness ranging from 25 to 350 Å. An optimum thickness of approximately 140 Å, which corresponded to an over 200% increase in peel strength when compared to 50 Å thick samples, was determined from ellipsometry and 90° peel strength measurements. The corrosion protection obtained was essentially independent of coupling agent thickness.     

Cathodic Delamination of an Epoxy/Polyamide Coating from Steel

X-ray photoelectron spectroscopy (XPS) has been used to determine the mechanism responsible for debonding of an epoxy/polyamide coating from steel during cathodic delamination in 3.5% aqueous NaCl solutions. Coating failure always occurred near the interface between the coating and the oxide. The nitrogen content of the free surface of the prepared coatings was about 10%. However, the nitrogen content of the free surface dropped to only 5% after exposure to 1 N NaOH for four weeks and that of the coating failure surface after cathodic delamination was only about 2%, implying that the failure involved degradation of the polyamide curing agent by hydroxide ions formed at the steel surface by reduction of oxygen. That conclusion was supported by results obtained from curve fitting of C(1s) and O(1s) spectra. The intensity of components in the C(1s) spectra due to C—N and C≡O bonds in amide functional groups decreased significantly after coatings were exposed to NaOH or subjected to cathodic delamination. Small amounts of organic materials characteristic of the coating were observed on the substrate failure surface, perhaps indicating that the failure was cohesive within the coating but very close to the interface or that some products from degradation of the curing agent precipitated on the substrate. Use of silane coupling agents to retard cathodic delamination was also investigated. Coupling agents were added directly to the coating or applied to the substrate as a primer before application of the coating. Significant reduction in the rate of cathodic delamination was seen only when the silane coupling agent was applied to the substrate and cured at elevated temperatures before the epoxy/polyamide coating was applied.     

X-Ray Photoelectron Spectroscopy of the Fracture Surfaces of Iron/ Epoxy and Titanium/Epoxy Adhesive Joints

The failure path of iron/epoxy and titanium/epoxy lap joints prepared from adherends pretreated with dilute aqueous solutions of ω-aminopropyltriethoxysilane (ω-APS) and then exposed to water at 60°C for varying times was determined using X-ray photoelectron spectroscopy (XPS). Iron/epoxy lap joints prepared using T-APS primers applied at pH 8.0 and tested without exposure to water failed near the oxide/primer interface with only a small amount of adhesive left on the substrate failure surface. Iron/epoxy joints that were primed with T-APS at pH 10.4 and tested without immersion in water failed partly within the adhesive and partly near the interface. All of the iron/epoxy joints failed in the primer near the primer/oxide interface when tested after exposure to water for seven days but very little corrosion of the substrates was observed. When titanium/epoxy lap joints were tested without exposure to water, failure was partly within the adhesive and partly near the primer/oxide interface regardless of the pH at which the primer was applied. However, the failure path shifted to near the primer/oxide interface after the joints were immersed in water for seven days. It was concluded that failure of the joints after exposure to water was associated with hydrolysis within the primer rather than with extensive corrosion of the substrate.     

Aging Process of Metal/Epoxy Laminates Investigated with X-ray Photoelectron Microscopy and Spectroscopy

In this article we describe the application of X-ray photoelectron spectroscopy to epoxy/dicyandiamide laminates on zinc galvanized steel which were aged under different environmental conditions involving high humidity and temperatures.

X-ray photoelectron microscopy allows us to identify the distribution of chemical elements with a lateral resolution of 10μm. Areas selected in the microscopy mode were then analyzed in the spectroscopy mode in order to get information on the local chemical composition.

We compared the spectroscopic features of the aged but freshly delaminated surfaces of samples stored under ambient conditions at room temperature with samples exposed to the “Kataplasmann” and the “KWT” test, respectively. Furthermore, a comparison was made with a model sample which was prepared in vacuum and on which the curing process was investigated.

Though there is no substantial loss in the lap-shear strength of the samples, we find drastic spectroscopic changes in the Kataplasma and KWT treated samples compared with the sample kept at room temperature. We conclude that the chemical changes induced by these tests cause an internal interphase boundary between the epoxy/metal interface and the bulk adhesive along which delamination occurs. Comparison with the behavior of the water-vapor-treated model sample gives evidence that hydrolysis is the main reaction in these tests.

The results described here complement our former study.¹     

Aging Process of Metal/Epoxy Laminates Investigated with X-ray Photoelectron Microscopy and Spectroscopy

In this article we describe the application of X-ray photoelectron spectroscopy to epoxy/dicyandiamide laminates on zinc galvanized steel which were aged under different environmental conditions involving high humidity and temperatures.

X-ray photoelectron microscopy allows us to identify the distribution of chemical elements with a lateral resolution of 10μm. Areas selected in the microscopy mode were then analyzed in the spectroscopy mode in order to get information on the local chemical composition.

We compared the spectroscopic features of the aged but freshly delaminated surfaces of samples stored under ambient conditions at room temperature with samples exposed to the “Kataplasmann” and the “KWT” test, respectively. Furthermore, a comparison was made with a model sample which was prepared in vacuum and on which the curing process was investigated.

Though there is no substantial loss in the lap-shear strength of the samples, we find drastic spectroscopic changes in the Kataplasma and KWT treated samples compared with the sample kept at room temperature. We conclude that the chemical changes induced by these tests cause an internal interphase boundary between the epoxy/metal interface and the bulk adhesive along which delamination occurs. Comparison with the behavior of the water-vapor-treated model sample gives evidence that hydrolysis is the main reaction in these tests.

The results described here complement our former study.¹     

Investigation of Steel/Epoxy Adhesion Durability Using Polymeric Coupling Agents. II. Factors Affecting Adhesion Durability

Ethylene mercaptoester (EME) copolymers containing 23-90 wt% mercaptoester units were employed separately as coupling agents in steel/epoxy peel adhesion systems. As coupling agent functionality is increased the initial adhesion strength of, the steel/EME/epoxy peel systems also increased, approaching a ten-fold improvement over controls for the EME 90 coupling agent. However, the degree of corrosion protection decreased significantly with increasing concentration of mercaptoester units. The best corrosion protection was obtained with the EME 23/epoxy resin system which protected the steel adherend for an average of 48 hours in 57°C water baths. Regardless of the coupling agent used, all of the peel specimens exhibited poor adhesion after 5-11 hours exposure to hot water. Information from hydrolysis stability, water absorption, locus of failure and internal stress analyses when pooled show that the observed adhesion loss is attributable to the effect of water on the epoxy resin used; failure occurred within the epoxy.     

The Effect of Immersion on the Breaking Force and Failure Locus in an Epoxy/Mild Steel System

This paper presents the effects of immersion on the adhesion behavior in a polyamide-cured epoxy system immersed in sodium chloride electrolyte adjusted to three different pH values. The strength of lap shear joints was measured before and after exposure and after redrying. The failure locus was determined on a macroscopic and microscopic level. It was found that a large adhesion loss occurred upon immersion. Most of that loss was recovered upon redrying. All of the breaking force was recovered when the immersion fluid was distilled water. The locus of failure was primarily through the bulk of the adhesive before immersion. After immersion the failure was interfacial with a thin residue of polymer remaining on the metal surface. These results are discussed with respect to earlier work on the water absorption properties of the system.     

The Effect of Immersion on the Breaking Force and Failure Locus in an Epoxy/Mild Steel System

This paper presents the effects of immersion on the adhesion behavior in a polyamide-cured epoxy system immersed in sodium chloride electrolyte adjusted to three different pH values. The strength of lap shear joints was measured before and after exposure and after redrying. The failure locus was determined on a macroscopic and microscopic level. It was found that a large adhesion loss occurred upon immersion. Most of that loss was recovered upon redrying. All of the breaking force was recovered when the immersion fluid was distilled water. The locus of failure was primarily through the bulk of the adhesive before immersion. After immersion the failure was interfacial with a thin residue of polymer remaining on the metal surface. These results are discussed with respect to earlier work on the water absorption properties of the system.     

Interphase Composition in Aluminum/Epoxy Adhesive Joints

Model epoxy/aluminum adhesive joints were constructed with a geometry that allowed cracks to be propagated extremely close to the adhesive/adherend interface. The joints were fractured in air and the fracture surfaces analyzed using angle resolved X-ray photoelectron spectroscopy. Fracture occurred in a manner that left a significant amount of aluminum oxide on the epoxy side of the fracture surface and very little epoxy on the aluminum side. Aliphatic amine curing agent found associated with the aluminum oxide on both the adhesive and the adherend sides of the fracture surface was protonated by the acidic hydroxyls present in the aluminum hydroxide. Moreover, catalysis of the curing reaction by these hydroxyls resulted in an increased degree of crosslinking in the regions of the adhesive very close to the oxide surface. Thus, the aluminum oxide surface modified the structure of the adhesive in the near surface regions and resulted in the formation of a distinct interphase region with a composition different from that of the bulk adhesive.     

Interphase Composition in Aluminum/Epoxy Adhesive Joints

Model epoxy/aluminum adhesive joints were constructed with a geometry that allowed cracks to be propagated extremely close to the adhesive/adherend interface. The joints were fractured in air and the fracture surfaces analyzed using angle resolved X-ray photoelectron spectroscopy. Fracture occurred in a manner that left a significant amount of aluminum oxide on the epoxy side of the fracture surface and very little epoxy on the aluminum side. Aliphatic amine curing agent found associated with the aluminum oxide on both the adhesive and the adherend sides of the fracture surface was protonated by the acidic hydroxyls present in the aluminum hydroxide. Moreover, catalysis of the curing reaction by these hydroxyls resulted in an increased degree of crosslinking in the regions of the adhesive very close to the oxide surface. Thus, the aluminum oxide surface modified the structure of the adhesive in the near surface regions and resulted in the formation of a distinct interphase region with a composition different from that of the bulk adhesive.     

Adhesive Bonding of Clean and Oil-Contaminated Electrogalvanized Steel Substrates

The performance of two-part, amidoamine-cured epoxy adhesives on clean and oil-contaminated electrogalvanized steel (EGS) was studied using screening and lap shear tests. On exposure to boiling water, the cured epoxy adhesives with amidoamines having higher amine value delaminated from the clean and oil-contaminated EGS surfaces before those cured with amidoamines having low amine value. The results of X-ray photoelectron spectroscopy (XPS) showed that the adhesives cured with amidoamines having high amine value were unable to displace the oil from the EGS substrate. However, the durability and the strength of the adhesive bonds on the oiled EGS could be improved by adding proper amounts of silane or wetting agent to the adhesive. The preferential adsorption of amino curing agents occurred on the clean EGS surface, confirmed by XPS and reflection absorption infrared spectroscopy, and this decreased the durability of the bonds in boiling water. In addition, from XPS analyses of various specimens, different amounts of cured resins were detected in the adhesive/EGS interfacial regions which affecting the durability of the adhesive bonds. In addition, the amidoamine curing agents may form complexes on the EGS surface.     

Factors Affecting the Durability of Ti-6Al-4V/Epoxy Bonds

Factors influencing the durability of Ti-6Al-4V/epoxy interphases were studied by determining chemical and physical properties of Ti-6Al-4V adherend surfaces and by characterizing the strength and durability of Ti-6Al-4V/epoxy bonds.

Ti-6Al-4V adherend surfaces were oxidized either by chemical etch or anodization. Four principal pretreatments were studied: chromic acid anodization (CAA), sodium hydroxide anodization (SHA), phosphate fluoride acid etch (P/F) and TURCO basic etch (TURCO). The oxides were characterized by SEM, STEM, profilometry, contact angles and XPS.

All adhesive bonding was carried out using a structural epoxy, FM-300U. Both lap shear and wedge test samples were tested in hot, wet environments. The results lead to the conclusion that the interfacial area between the adhesive and adherend is the primary factor affecting bond durability.     

氨丙基甲基二甲氧基硅烷防腐保护镀锌钢板的研究

本次实验采用氨丙基甲基二甲氧基硅烷在镀锌钢板表面制备抗腐蚀保护膜,采用正交实验得出硅烷钝化最优工艺条件：硅烷浓度V（SCA）=2mL,钝化时间t1=10s,固化温度T=100℃,固化时间t2=5h。采用醋酸铅点滴和交流阻抗研究试样的耐腐蚀性能,实验结果表明试样经硅烷钝化后耐腐蚀性得到大幅度提高,耐黑变时间达到35s,阻抗值达到1600n。初步讨论了硅烷的成膜机理和各因素对成膜的影响。     

Adhesive Bonding to Galvanized Steel: I. Lap Shear Strengths and Environmental Durability

Initial (i.e., unaged) adhesion, as well as adhesion after seven day, 60°C water immersion and six week scab corrosion accelerated environmental exposures, has been assessed for five different one and two-part epoxy adhesives, bonded to three different types of galvanized steel substrates. We have shown that adhesion, as measured by lap shear strength, is specific to the galvanized substrate type. In general, for a given adhesive, adhesion to “hot-dipped” galvanized substrates is harder to achieve and maintain under accelerated environmental exposure than is adhesion to “electroplated” galvanized. Also, for a given type of galvanized steel, the one-part epoxies evaluated generally showed higher initial strengths, as well as better strength retention under environmental exposure than did the two-part epoxies.     

Adhesive Bonding to Galvanized Steel: I. Lap Shear Strengths and Environmental Durability

Initial (i.e., unaged) adhesion, as well as adhesion after seven day, 60°C water immersion and six week scab corrosion accelerated environmental exposures, has been assessed for five different one and two-part epoxy adhesives, bonded to three different types of galvanized steel substrates. We have shown that adhesion, as measured by lap shear strength, is specific to the galvanized substrate type. In general, for a given adhesive, adhesion to “hot-dipped” galvanized substrates is harder to achieve and maintain under accelerated environmental exposure than is adhesion to “electroplated” galvanized. Also, for a given type of galvanized steel, the one-part epoxies evaluated generally showed higher initial strengths, as well as better strength retention under environmental exposure than did the two-part epoxies.     

硅烷偶联剂和固化温度对LY12铝合金／环氧树脂黏接强度的影响

研究了含硫硅烷偶联剂和固化温度对LY12铝合金／环氧树脂黏接剪切强度、拉伸位移的影响规律，并用扫描电镜观察分析了黏接试样的拉伸剪切断面形貌特征。结果表明，铝合金表面经过硅烷偶联剂处理后，80～180℃固化时，黏接剪切强度介于19.4～22.2MPa之间，明显高于铝合金表面未用硅烷偶联剂处理时的最大黏接强度17.5MPa，断面呈现被平行裂纹一层层掀开的破坏现象，表明硅烷偶联剂起到了明显的界面改性作用。低温固化条件下黏接强度低，断面较平整；在220℃以上高温固化时，界面相脆化，黏接性能快速下降，断面呈现脆性断裂特征。