Epoxy corrosive degradation

Thus, the transport of hydrated ions and chemical debonding processes can be studied by means of the SKP. Fig. 31.6 shows the potential distribution measured with the SKP when a thin electrolyte layer enters the interface between an adhesive and an iron surface covered by a thin (about 6 nm) nonconducting SiOx layer precipitated by a plasma-polymerization process [51, 52]. The SiO layer inhibits the electron-transfer reaction. Consequently, no corrosive degradation of the interface takes place (see Section 31.3.2.1). However, as the adhesion of the epoxy adhesive to the siUca-Uke layer is weak, the polymer is replaced by... 

Although the above experiments involved exposure to the environment of unbonded surfaees, the same proeess oeeurs for buried interfaces within an adhesive bond. This was first demonstrated by using electrochemical impedance spectroscopy (EIS) on an adhesive-covered FPL aluminum adherend immersed in hot water for several months [46]. EIS, which is commonly used to study paint degradation and substrate corrosion [47,48], showed absorption of moisture by the epoxy adhesive and subsequent hydration of the underlying aluminum oxide after 100 days (Fig. 10). After 175 days, aluminum hydroxide had erupted through the adhesive. 

Different behaviors and mechanisms were clearly recognized between these resins. Epoxy resin cured with amine showed no degradation during immersion because of its stable crosslinks. Epoxy resin cured with anhydride showed the uniform corrosion with the softening and dissolution of the surface and also behaved similar to the oxidation corrosion of the metal at high temperature obeying linear law. 

For example, elevated-temperature exposure could cause oxidation or pyrolysis and change the rheological characteristics of the adhesive. Thus, not only is the cohesive strength of the adhesive weakened, but also its ability to absorb stresses due to thermal expansion or impact is degraded. Chemical environments may affect the physical properties of the adhesive and also cause corrosion at the interface however, the adhesive may actually become more flexible and be better able to withstand cyclic stress. Exposure to a chemical environment may also result in unexpected elements from the environment replacing the adhesive at the interface and creating a weak boundary layer. These effects are dependent not only on the type and degree of environment but also on the specific epoxy adhesive formulation. 

The above analysis applies to degradation processes that relate to the bulk adhesive. Interfacial degradation processes such as corrosion can be similarly determined. Thermal and oxidative stability, as well as corrosion and water resistance, depends on the adherend surface as well as on the adhesive itself. Epoxy-based adhesives degrade less rapidly at elevated temperatures when in contact with glass or aluminum than when in contact with copper, nickel, magnesium, or zinc. The divalent metals have a more basic oxide surface than the higher-valence metal oxides and hence serve to promote dehydrogenation reactions, which lead to anion formation and chain scission.7... 

Salt-spray data (for nos. 1 and 2 in Table V) indicated that, on zinc-phosphated steel, cathodic ED indeed led to considerably better corrosion resistance. On bare steel, however, both cathodic and anodic coatings failed completely. Further, we prepared a number of different, amine-modified epoxy resin esters containing between 30 and 50 %w of drying fatty acids (e.g. no. 3 in Table V). These could all be deposited cathodically and again attained excellent salt-spray ratings on phosphated steel but performed poorly on bare steel. We concluded that cathodic ED prevents phosphate layer degradation (a well-known phenomenon with anodic ED) and thus leads to superior corrosion resistance on pretreated steel. On bare steel the binders performed too poorly to allow comparison between cathodic and anodic ED. 

Of finishes used outside, the polymers that can offer the best protection against acid deposition are those not containing acid-sensitive groups such as esters. Inclusion of acid resistant paint binders such as vinyls, urethane, and epoxies would produce acid resistance only if the other components are also acid resistant. Saponification of esters catalyzed by hydroxyls formed during corrosion of steel substrates may also degrade polyesters (7). 

Fibre-reinforced polymers (FRP) rebars, usually made of an epoxy matrix reinforced with carbon or aramide fibres, have also been proposed both as prestressing wires and reinforcement. Nevertheless, they are not discussed here, because these applications are still in the experimental phase and there is a lack of experience on their durability. In fact, while they are not affected by electrochemical corrosion typical of metals, they are not immune to other types of degradation. FRP are also used in the form of laminates or sheets as externally bonded reinforcement in the rehabilitation of damaged structures this application will be addressed in Chapter 19. 

Honeycomb structures can be susceptible to water intrusion, which may affect, for e. am-ple. the weight and balance of an aircraft, and in the long term induce corrosion. During winter, or at high altitude in the case of aircraft, trapped water in such structures may freeze, and the subsequent expansion of ice causes cracks, breakage of the honeyeomb cells, and disbonds. Moisture absorption in unidirectional carbon fiber epoxy-matrix composites causes swelling and degradation of epoxy film adhesive joints. 

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