Adhesives corrosion caused

Adhesive failure Is a problem In solar systems. In the past, polymers have been used to protect the mechanical Integrity of wood and metal structures In severe outdoor environments and to protect sensitive electronic components In relatively benign enclosed environments. Polymers used In solar equipment will have to protect the optical properties of reflectors, thln-fllm electrical conductors, and thln-fllm photovoltalcs from the effects of moisture and atmospheric pollutants In severe outdoor environments while simultaneously maintaining optical, mechanical, and chemical Integrity. In some systems, the prevention of mechanical failure Is Important frequently, adhesive failure at the metal/polymer Interface Is of particular concern because the ensuing corrosion causes optical failure. 

The potential for corrosion caused by adhesives requires evaluations in both cured and uncured states. In one test, the adhesive components or the cured adhesive are applied to aluminized-Mylar film and examined after 48 hours for etching or transmittance of light through the Mylar. A second test involves embedding copper wire with the adhesive and measuring corrosion by resistance changes in copper wire. Because of the wide variety of tests and the fact that they are closely related to adhesive reliability issues discussed in Chapter 6, the reader is referred to that chapter. 

The potential for corrosion caused by adhesives requires evaluations in both the cured and uncured state. In one test, developed for NASA MSFC-SPEC-5 92, the adhesive components orthe cured adhesive are applied to... 

Vapors emitted from the materials of closed storage and exhibit cases have been a frequent source of pollution problems. Oak wood, which in the past was often used for the constmction of such cases, emits a significant amount of organic acid vapors, including formic and acetic acids, which have caused corrosion of metal objects, as well as shell and mineral specimens in natural history collections. Plywood and particle board, especially those with a urea—formaldehyde adhesive, similarly often emit appreciable amounts of corrosive vapors. Sealing of these materials has proven to be not sufficiently rehable to prevent the problem, and generally thek use for these purposes is not considered acceptable practice. 

It is worthwhile drawing attention to health hazards associated with film infected water systems which also cause corrosion. Two of the most common are Legionnaires disease and so called humidifier fever . Because of strong adhesion of biofilms and diffusion rates through the film treatment based on cleaners and chemical sterilisers such as chlorine often fail similar considerations apply to other systems in industry, e.g. food, paint, oil and gas are examples where biofilm activities have given massive problems. 

It will be seen that the design of articles to be electroplated can have a considerable effect on the corrosion resistance of the electrodeposited coating. The chief effects are the result of variations in deposit thickness, but also important are features which can influence the adhesion, porosity and physical properties of the deposit. Good design will also avoid features of the plated article capable of trapping liquids or solid contaminants which might cause more rapid corrosion. 

To check the efficacy of grease removal, the alkali solution is rinsed away or neutralised by dipping in dilute acid. If, after removal from the acid, the draining metal surface remains wetted evenly all over for 30-60 s (or until it dries by evaporation), hydrophobic soils have been removed. Traces of grease cause the surface to de-wet, and surface tension draws the water into separate droplets. This is the water-break test. Traces of grease which remain when the work is plated do not prevent electrodeposition, but are detrimental to adhesion and corrosion resistance. 

When corrosion develops on painted steel the question is often raised as to whether corrosion was a result of paint failure or the paint failure was caused by corrosion. Several studies have shown that adhesion forces are reduced greatly after water soaking or even at very high humidity -and it has been argued that film detachment by water usually precedes underfilm corrosion . Against this view others have claimed that those paints known to have reduced wet adhesion, e.g. those based on alkyd resins, are not uniquely, or even especially, subject to underfilm corrosion Several factors should be considered in this discussion ... 

Abrasive wear is caused by sharp asperities cutting the plastic fatigue wear is caused by particles of plastic being detached as a result of dynamic stressing on a localised scale adhesive wear is the transfer of plastic to another surface as a result of adhesive forces between the two surfaces. There can also be corrosive wear due to the direct chemical attack on the surface and the term erosive wear is sometimes used for the action of particles in a liquid stream. 

The interfacial chemistry of corrosion-induced failure on galvanized steel has been investigated (2) adhesion of a polyurethane coating was not found to involve chemical transformations detectable by XPS, but exposure to Kesternich aging caused zinc diffusion into the coating. Similar results were obtained with an alkyd coating. Adhesion loss was proposed to be due to formation of a weak boundary layer of zinc soaps or water-soluble zinc corrosion products at the paint metal Interface. 

Major causes for coating failure are surface cracking and undetected pinholes or voids. These can be repaired and serious problems avoided. Coatings generally fail in different modes, these are chemical failure, abrasion failure, adhesive failure, cohesive failure and undercoat corrosion. For performance evaluation of coatings on experimental basis on these parameters various ASTM and BS specifications are presently being used. 

Some rubber compounds can cause corrosion of, and tend to stick to, metal surfaces with which they are in contact, and corrosion can even be caused to a metal in close proximity but not touching the rubber. Although not a very widespread problem, there has been sufficient concern, particularly for some military applications, for tests to be devised to assess the relative degree of corrosion and adhesion caused by different compounds. 

When a ferrous alloy is immersed in phosphoric acid, il initially forms a soluble phosphate. As the pH rises at the mclal/solutiun interface, the phosphate becomes insoluble and crystallizes epitaxially on Ihe substrate metal. The phosphate coating thus produced consists of a nonconduciivc layer nf crysinlx that insulates the metal from any subsequently applied film and provides a topography with enhanced tooth" for increased adhesion. The cry stals insulate microanode and microcathode centers caused by stress or imperfections in the metal surface. This greatly reduces Ihe severity of electrochemical corrosion. 

Water is sometimes used as a solvent for water-soluble resins. Certain epoxy adhesives are available as water-based emulsion or latex formulations. In the early 1970s, during the time of the petroleum crisis, water-based adhesives were thought of as a possible replacement for solvent-based adhesives systems. However, water-based adhesives never met the lofty expectations primarily because of the time and energy required to remove water from the bond line, the corrosion that the water causes in drying ovens, and the poor moisture resistance of cured water-based adhesives. 

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. 

Almost any epoxy adhesive can be used on magnesium provided that proper surface protection is maintained. In view of the corrosion potential, water-based adhesives or adhesives that allow water permeation may be expected to cause problems with magnesium substrates. 

Mechanism. Basically, fretting is a form of adhesive or abrasive wear, where the normal load causes adhesion between asperities and oscillatory movement causes ruptures, resulting in wear debris. Most commonly, fretting is combined with corrosion, in which case the wear mode is known as fretting corrosion. For example, in the case of steel particles, the freshly worn nascent surfaces oxidize (corrode) to FejO, and the... 

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