Primers for Corrosion Protection

When a corrosive medium contacts the edge of a bonded joint and finds an extremely active surface, such as that produced by a fresh acid treatment of the metal substrate to improve adhesion, corrosion at the metal-adhesives interface can occur. This initial corrosion and the subsequent penetration can take several forms. 

Representative data are shown in Fig. 10.5 for aluminum joints bonded with an epoxy film adhesive and a standard chromate-containing primer. Up until recently, standard corrosion-resistant primers contained high levels of solvent, contributing to high levels of volatile organic compounds (VOCs) and chromium compounds, which are considered to be carcinogens. As a result, development programs have been conducted on waterborne adhesive primers that contain low VOC levels and little or no chrome. Data are presented on several of these primers in Tables 10.8 and 10.9. 

Note To increase test s edges everity, 1 /2-in shear coup ut, were exposed to salt ons, I in wide with both spray.  

FIGURE 10.5 Effect of primer on lap shear strength of aluminum joints exposed to 5% salt spray.31 

TABLE 10.8 Examples of Commercial Primers for Structural Adhesives32 

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Pyrrole-based silane primer for corrosion protection of... 

The complete elimination of heavy metals and other hazardous compounds from the coating systems was the main objective of SAIE. Chromated spray primers with hazardous chromate components do not fit such an objective. Therefore, two kinds of nonchromated and water-borne spray primers, Spraylat EWAE118 (D) and Dexter lO-PW-22-2 (X), were selected to produce chromate-free plasma coating systems for corrosion protection of A1 alloys. 

The epoxy resin primers used for corrosion protection proved distinctly brittle and exhibited considerable rust formation beneath the layers which peeled off. 

Painting for corrosion protection usually involves a thorough cleaning of the metal surface to remove any corrosion products followed by application of a primer containing a corrosion inhibitor such as zinc chromate or calcium plumbate next follows a heavily pigmented undercoat for the purpose of reducing the permeation of water to the metal surface elimination of the internal circuit— the electrolyte), finally a third decorative coat that is resistant to the atmosphere. 

A complete coating system, recommended for highly demanding situations, consists of several layers - the primer, the intermediate coats and the topcoat. The primer is an essential component for corrosion protection, because it is responsible for the adhesion of the coating to the substrate and also it provides most of the anticorrosive properties, through the use of inhibitors. Its requirements are a good adhesion to the substrate and to the intermediate coat, flexibility and cohesion and good chemical resistance. 

The cabinets for cellular telephones are made of steel or aluminum. A double system of galvanizing and painting steel cabinets is applied for corrosion protection. Surface preparation by grinding and the application of zinc chromate primer is done prior to galvanizing. There are nearly 4000 cabinets for cellular telephones in use, and the estimated cost for corrosion is approximately 4 million. 

Uses. Electrodeposition coatings are used primarily for corrosion protection of steel, galvanized steel, and aluminum (e.g., in automobiles, agricultural machinery, steel furniture, and appliances). New areas of application include coil coating primers [3.127], beverage can coatings [3.128], and photocurable electrodeposition paints as photoresists [3.129]. 

Uses Binder, antisettling agent for ready-to-use one-pack paints, zinc-rich paints used as primers or on their own (for corrosion protection on steel)... 

Dominis, A.J., G.M. Spinks, and G.G. Wallace. 2003. Comparison of polyaniline primers prepared with different dopants for corrosion protection of steel. Prog Org Coat 48 (1) 43. 

At an electrolyte pH of 8, the passivation region extended for over 0.5 V. Electrochemical tests were performed on acid etched, bare A1 2024-T3 panels with artificial pits as a control experimental to determine if the increased passivation with increasing electrolyte pH was a result of self-passivation of the bare aluminum alloy surface (Fig. 6.12(b)). The polarization curves for the bare A12024-T3 showed no appreciable passivation and no significant difference with pH, indicating that the passivation observed in the primers was not a result of self-passivation of the substrates, but activity from the primer. This behavior provides important clues not only to the inherent electrochemical properties of the initial coatings, but also the mechanisms responsible for corrosion protection. 

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