Anticorrosive coatings barrier effects

Flake zinc pigments have a barrier effect and also act by a cathodic anticorrosive mechanism. Compared with zinc dust coatings, flake zinc pigments are formulated with lower pigment volume concentrations [5.178]. 

In order to reduce the intrinsic porosity and to increase the barrier effect of anticorrosion CP-based coatings, composite systems have been considered where one component is a CP and other component can be organic or inorganic. In literature, several recent papers have been published on this topic. Two classes of composites based on CPs could be individuated (Table 10.4). In the first class, the matrix is a CP, usually PANI and its... 

PANI/P-PVA (partially phosphorylated poly(vinyl alcohol)) nanoparticles were successfully dispersed in epoxy resin applied to steel [82]. P-PVA is fundamental to obtain a uniform dispersion of PANI nanoparticles, and this fact is responsible for the uniform formation of Fe Oj passive layer at the interface between coating and substrate and therefore for its effectiveness in corrosion protection. PANI was also used in combination with DBSA to be added to epoxy-ester (EPE) system to form a smart anticorrosion coating [73]. DBSA is used as both surfactant and doping agent. By EIS measurements it is deduced that the better anticorrosion performance of PANI (DBSA)/EPE coatings with respect to simple EPE is due to the formation of a second barrier layer by reaction between released DBSA anions and Fe cations at the defective locations of the coating. 

In an electroactive polymer system, Huang et al. (2011) attempted to prepare advanced PCN anticorrosive coating materials with the synergistic effects of redox catalytic capabihty and gas barrier properties by incorporation of well-dispersed organophilic clay platelets into amine-capped aniline trimer (ACAT)-based electroactive polyimide (EPl) matrix through chemical imidization (see Fig. 13.13). 

From a practical point of view, the attainment of the specified dry film thickness is important. In many cases, a minimum thickness must be achieved or protection will fail in a short period. This aspect demands skill in application and close inspection and control. For metallic zinc paints, the thickness would normally be of the order of 60-100/im, for anticorrosive pigmented paints, together with top coats, 100-200 //m, and for inert barrier types 150-300/im. For special environments or projects, these thicknesses may be varied but it should not be presumed that protection will always be improved or be more effective over a longer period of time by increasing the film thickness. 

An important aspect of the efficiency of inhibited plastics is the stock of Cl in the polymer matrix. If the anticorrosion system is designated for longterm operation, a moment when the Cl stock appears to be fully exhausted inevitably arrives. Naturally, this substantially lowers the effectiveness of the anticorrosion system, although does not necessarily lead to failure. Types of rust-inhibiting plastics such as coatings and preserving films retain their barrier properties even after volatilization or wash-out of the inhibitors and continue to insulate metal goods from the hostile environment. 

In addition, it should be noted that alkyd resins generally exhibit poor barrier properties against moisture vapor. Choosing an effective anticorrosion pigment is therefore important for this class of coating [1]. 

For many decades, chromate compounds have been successfully used as anticorrosive inhibitors in the surface treatment of aluminium and others alloys. The use of chromates is however restricted worldwide, as they are considered highly toxic and carcinogenic [1], This has stimulated research aimed at the development of effective and environmentally acceptable alternatives to chromates. To obtain the beneficial properties of chromatation, two approaches can be utilised passive [2] and active [3,4] corrosion protection. Passive protection is normally provided by a barrier film that prevents contact between the corrosive species and the metal surface and therefore hinders a corrosion process. However, when a defect is formed in the barrier layer, the coating cannot stop corrosion in that place. The second approach is active corrosion protection, which employs inhibitive species that can decrease corrosion activity. An important point is that both strategies must be used together to protect the metallic substrate adequately. 

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