Coating failure

Surface preparation, always important in obtaining optimal coatings performance, is critical for marine coatings (see Metal surface treatments). Surface preparation usually comprises about half of the total coating costs, and if inadequate may be responsible for early coating failure. Proper surface preparation includes cleaning to remove contaminants and roughening the surface to faciUtate adhesion. 

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. 

Kinetic Monte Carlo and hyperdynamics methods have yet to be applied to processes involved in thermal barrier coating failure or even simpler model metal-ceramic or ceramic-ceramic interface degradation as a function of time. A hindrance to their application is lack of a clear consensus on how to describe the interatomic interactions by an analytic potential function. If instead, for lack of an analytic potential, one must resort to full-blown density functional theory to calculate the interatomic forces, this will become the bottleneck that will limit the size and complexity of systems one may examine, even with multiscale methods. 

Understanding the behavior of the interfaces and bulk materials involved in thermal barrier coating failure due to the extreme environment created in aircraft engines is still in its infancy. This is primarily because the system involves complex interfacial chemistry and the materials issues span large length and time scales. In this review, we have focused on the atomic level characterization. Once that is specified, it will be imperative to draw links between the atomic and the microstructural scales in order to understand the materials failure mechanisms completely. 

As to the concept of mild steel liners, the main problems have been due to coating failures. When you consider the service conditions within a tall steel liner, it s not hard to see why certain surface applied systems have not held up. As opposed to the normal ductwork for which most coating systems have been developed, steel chimney linings exhibit the following characteristics. 

Coatings failure occurs by delamination (separating by layers) or by peeling (separating from the concrete). The latter happens most frequently when the adhesive bond with the concrete is lost. This type of failure occurs when coatings are applied without first removing the surface contaminants and laitance. Laitance, a film which can form on the surface of the concrete, is unreacted cement or cement which has risen to the surface of the concrete surface due to over-working fresh concrete. 

Figure 9. Population growth curve. No physical phenomena, except perhaps the growth of the universe, follow the exponential law shown by the dashed line. The vertical line denotes the half-life x of the growth. Coatings failure is manifested as a curve that peaks below the saturation limit and levels off at a lower value.

The hydrophilic thermosetting materials do not usually satisfy well these two requirements. Water concentrates at their coating/substrate interface, causes corrosion, delamination, and coatings failure. As a result of these problems in the hydrophilic thermosets, there is a strong need for a hydrophobic material. 

In order to size the mitigation bond, the point of maximum current flow between the two affected structures must be located. Typically, this point is situated near the point of pipeline crossing, where the circuit resistances are the lowest, but it can be situated some distance away, particularly with well[Pg.421]

For aluminum and magnesium, FFC and blistering are the predominant coating failure mechanisms. For iron, FFC is observed only under special conditions and cathodic delamination is the primary failure mechanism see Ref. [168] and references therein. 

Subsurface defects, such as cracks, within the TBC layer are also of concern as they can also lead to coating failure [4]. 

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