Coatings protection mechanisms

In some cases, the oxide-coating protects the surface from further oxide buildup. One example is that of aluminum where an oxide coating appears almost instantaneously once the pristine surface is exposed to air. Yet, there are many cases where the oxide layer continues to buildup until the metal is totally consumed (One example is that of iron and "rust"). How is this possible Wagner hypothesized that both metal and oxide ions difiosed through the metal oxide layer so as to build up the layer thickness from both sides. The following diagram is one representation of such a mechanism ... 

The electrochemical, physicochemical and adhesional aspects of corrosion protection by organic coatings are shortly discussed. Attention is drawn to some inoonsist-ancies in the interpretation of protective mechanisms and suggestions are given how protective principles may be optimally realized in practical systems. 

Ihe ocmibination of different protective mechanisms in one coat or one coating system is frequently reoonriended for optimal results in ocar-rosion protection. However, ways and measures to optimize protective mechanisms may be different and sometimes even exclude each other. 

Grundmeier G, Schmidt W and Stratmann M 2000 Corrosion protection by organic coatings electrochemical mechanism and novel methods of investigation Electrochim. Acta 45 2515... 

Formation and protective mechanisms of antimicrobial polymer coatings are in principal the same as in inhibited paint-and-varnish coats [97],... 

Conformal coatings protect electronic circuitry from moisture, dust, chemicals, solvents, and other harsh environments. Although usually only a few mils thick, they also dampen the effects of mechanical and thermal stresses, vibrations, and electrical noise. Conformal coatings also have been used to provide electrical insulation by inhibiting dendrite growth, a potential cause of short circuits. 

Zinc-nickel Zn-Ni alloys with 5 to 15 wt% Ni offer excellent corrosion resistance and are mainly used in the automotive, aerospace, and electronics industries. Above 15% Ni, the alloy coating becomes more noble than steel, and the corrosion-protection mechanism changes from a sacrificial to a pure physical one (comparable to pure Ni coatings, see Sect. 5.5.4.2.2). They can be electrode-posited from acid or alkaline baths. The acid baths are usually based on sulfate, chloride, sulfate-chloride, pyrophosphate, or acetate (Table 15). The system shows anomalous codeposition (see Sect. 5.5.1.2), which has been explained by a hydroxide suppression mechanism [47]. As in the case of Ni-Fe, the alkaline baths must contain complexing agents (see Sect. 5.5.4.6.2). The alloys electroplated from add haths contain approximately 10 to 14% Ni, whereas the alkaline Zn-Ni... 

These equations assume equilibrium adsorption exists at the passive film interface. Island-type adsorption instead of the uniform adsorption assumed in these equations has also been suggested. The standard coating based on chromate conversion layers is used to protect high-strength A1 alloys. The primer may contain chromate pigments that enhance protection. The chromium protection mechanism is described by Edeleanu and Evans [89]. 

Protection mechanisms (i) and (iii) are ineffective when a defect appears in a coating, and mechanism (ii) will fail for large defects, even though it may induce excellent protection for small ones. Furthermore, delamination of the coating upon reduction of the conductive polymer as well as the progressive incorporation of water and electrolyte within the coating can be responsible for failure. 

An advantage of incorporating ECPs into a binder such as epoxy or polyurethane is the good adhesion that such binders provide, and the actual performance of coatings based on formulation may be due to the fact that the adhesion is better than that obtained by electropolymerization. However, this strategy leads to less contact between metal and ECP and, in view of the protection mechanisms described in Section 16.2, this may be to the detriment of the anticorrosion properties. 

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