Corrosion, adhesive joints affected

A surface is that part of an object which is in direct contact with its environment and hence, is most affected by it. The surface properties of solid organic polymers have a strong impact on many, if not most, of their apphcations. The properties and structure of these surfaces are, therefore, of utmost importance. The chemical stmcture and thermodynamic state of polymer surfaces are important factors that determine many of their practical characteristics. Examples of properties affected by polymer surface stmcture include adhesion, wettability, friction, coatability, permeability, dyeabil-ity, gloss, corrosion, surface electrostatic charging, cellular recognition, and biocompatibility. Interfacial characteristics of polymer systems control the domain size and the stability of polymer-polymer dispersions, adhesive strength of laminates and composites, cohesive strength of polymer blends, mechanical properties of adhesive joints, etc. 

Chemical Surface Modification. In considering the interface, one must contemplate not only the possibility of moisture disrupting the bond but also the possibility of corrosion of the substrate. Corrosion can quickly deteriorate the bond by providing a weak boundary layer before the adhesive or sealant is applied. Corrosion can also occur after the joint is made and, thereby, affect its durability. Mechanical abrasion or solvent cleaning can provide adhesive joints that are strong in the dry condition. However, this is not always the case when joints are exposed to water or water vapor. Resistance to water is much improved if metal surfaces can be treated with a protective coating before being bonded. 

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. 

This method is the simplest of all the methods and is capable of detecting surface flaws such as corrosion, contamination, surface finish and surface discontinuities on joints.47 The discontinuities on joints such as welds, seals, solder connections and adhesive bonds can be detected. General corrosion, qualitative pitting corrosion, stress-corrosion cracking, weld-heat-affected zone attack, erosion corrosion and other type of degradation can be observed by visual examination aided by microscopes with sufficient magnification. Degradation of plastics can also be detected by visual examination. Visual examination is also used in conjunction with other techniques, such as powerful microscopes. 

What is the use of an optimum surface treatment if creep corrosion affects the adhesive layer from outside the glueline, which results in the destruction of the bonded joint This process is shown in Figure 7.8. 

The physical and chemical properties of both the solidified adhesive and the polymer affect the quality of the bonded joint. Major elements of concern are the corrosion resistance of the adhesive, its thermal expansion coefficient, and the glass transition temperature of the polymer relative to the adhesive. 

Stressed-bond joint durability is markedly affected by the adherend prebond surface treatment and the adhesive/primer system in contact with it. This is evidenced by the poor performance of FM 123-L/BR 123 (non-CIAP) adhesive/primer system on FPL-etched and chromic acid-anodized 2024-T3 aluminum alloy, clad and bare, and the superior performance of the same systems when BR 127 (corrosion-inhibiting adhesive primer (CIAP)) is substituted for BR 123 (non-CIAP). 

However, other approaches have also successfully been adopted. For example, Dickie and Ward [53] have studied single-lap joints exposed to a high humidity at moderately elevated temperatures but maintained a constant stress on the joints. Also, periodically the joints were removed from the high-humidity environment and exposed to a salt solution for a short time period. Using this accelerated-ageing test they were able to rank the durability performance of various adhesive systems in a comparatively short timescale. Further, they reported that not only were the kinetics of mechanisms of environmental attack accelerated, but also the exact details of the mechanisms were affected by the levels of the applied load. For example, for joints which consisted of bonded galvanised steel substrates, the effect of relatively high applied loads was to prevent the formation of an effective barrier of corrosion products, i.e. passivation of the substrate surface was prevented. This allowed the electrochemical corrosion process to proceed unimpeded, and hence at a faster rate than for similar, but unstressed, joints. Thus,... 

Examination of fractured surfaces after tensile testing revealed evidence of FFC on the alkaline-etched, phosphatemermanganate coated, and FPL-etched specimens. Corrosion filaments provide paths for ingress of moisture to the substrate/adhesive interface and corrosion products may also have contributed to a loss of adhesion by creating transverse stresses. To what extent the rate of degradation of adhesive bonded joints is affected by FFC requires further investigation. 

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