Sacrificial primers

Paint 29 Zinc Dust Sacrificial Primer, Performance-Based... 

Finally, mention should be made of the concept of a sacrificial primer which enables metals to be readily bonded underwater [210,211]. From the comments in Section 2.6.2 it is obvious that when trying to apply an adhesive to a substrate underwater it will be difficult for the adhesive to completely displace the water adsorbed on the surface and establish interfacial contact with the substrate a layer of water would remain adsorbed onto the substrate and act as a weak boundary layer. The idea of the sacrificial primer is that the substrate is firstly cleaned, typically by an abrasion pretreatment, but simultaneously a primer is applied which is formulated so that it displaces any water and is more readily adsorbed onto the substrate surface. This primer, although water repellant, is also formulated so that it is compatible with the adhesive to the extent that it, in turn, is displaced and dissolved by the adhesive when it is applied. Thus, the concept of a sacrificial primer enables the adhesive to wet the substrate completely without a weak boundary layer of water being retained at the interface. The adhesive is also especially formulated to aid these processes and contains silanes which will diffuse to the interface and establish strong and stable interfacial bonds to ensure a long service life for the joint in its underwater environment [212]. 

Primers containing 93-95% zinc dust by weight in non-saponifiable media provide sacrificial protection to clean steel (see Section 14.3). 

Sacrificial coatings consist of (i) organic zinc-rich primers (ii) inorganic zinc-rich primers. Corrosion control mechanism by zinc-rich primers may be visualized to occur as shown in Figure 1.65. 

Lead powder. The oil-suspension of metalhc lead powder is used in the preparation of protective primers for steelwork where it is normally present as the sole pigment. Lead pigments act as a barrier and provide a degree of sacrificial protection similar to that of zinc dust. 

Cationic electrodeposition coatings. A sacrificial metal (yttrium) is used as the primer for automobile and truck bodies to reduce corrosion. 

Coatings such as a zinc silicate primer covered with a layer of an epoxy-based polymer are routinely applied to steel structures to protect them against corrosion. However, cracks or flaws in the coating expose Fe which then undergoes oxidation in an anodic process. To prevent this, a second protection system is put in place cathodic protection. By placing a block of a more electropositive metal on the surface, this second metal is preferentially oxidized. This is the same principle as the use of zinc in galvanized steel (see Section 6.7). From Table 8.1, you can see why Zn, A1 and Mg (or alloys of these metals) are typically chosen as sacrificial anodes. The most electropositive metals (Li, Na, K and Ca) are unsuitable because they react with cold water. The relevant half-equations (at pH 7) are now ... 

For sacrificial coatings, such as zinc-rich primers, the high chloride content together with the constant high humidity means that the zinc is not likely to form a passive film as it does in the field. 

Zinc epoxy primer applied to the rebar, with sacrificial corrosion protection for the rebar in the repair zone... 

Primers containing high levels of zinc dust are used to protect steel by a sacrificial cathodic mechanism. The less noble zinc acts as the electrochemical anode and corrodes instead of the steel substrate. The concentration of zinc powder in the dry primer film must be sufficiently high to allow the particles to make contact and allow an electrical current to be estabhshed between these particles and the steel. 

As a result, barrier pigments can be incorporated into primer, intermediate coat, or top-coat since they are chemically inert and do not react with the metal, unlike inhibitive or sacrificial... 

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