Chromate-containing epoxy primer

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. 

Structural adhesives are not usually applied directly to the prepared metal. Rather, a primer is applied first. The primers are usually of the same generic chemical composition as the adhesive being used over them (i.e., epoxy primer to epoxy adhesive). The primers may or may not contain a corrosion inhibiting pigment and may or may not have to be cured before application of the adhesive. EC-1593 is a 350 F (121 C) curing nitrile-phenolic primer while EC-3917 is a chromated epoxy-phenolic primer curing... 

A. V. Pocius (3M Corporation) The primer EC-1945BIA is a 2-part solvent diluted amine-epoxy primer. The primer also contains chromates for corrosion inhibition. 

The selection of primers and subsequent coats is critical. Fujisawa et al. (1992) report unusual 10-50 mm diameter corrosion patches under snow on profiled roofs in Japan after 5 years. Attack on the polyvinyl difluoride (25 p.m) over epoxy-primed (7 p.m) galvanized steel was worse when the primer contained potassium chromate than when the less soluble strontium chromate was present. Excess dissolved chromate at the primer-phosphate interface produced excessive acidity, anodic dissolution of zinc, and evolution of hydrogen. It was also found that when the top coat was too thick and rigid, hydrogen could not diffuse out. 

Corrosion inhibitors in paints protect metallic surfaces from oxidation. Coating primers are used when there is continuous exposure to corrosive elements, e.g., in marine applications. Examples are coal-tar derivatives, epoxy resins and coal-tar modified epoxies. Primers that inhibit corrosion by anodic or cathodic polarization contain inorganic metallic pigments such as chromates or leads or both. Composite pigments containing calcium oxide, zinc, silica, and oxides of phosphorus and boron can also be used (Mathias 1984). Nowadays, powder paints such as polyester and epoxy powder paints can also be used for corrosion inhibition (Rose and Vance 1997). 

The films contain soluble chromates that act as corrosion inhibitors that provide a modest improvement in corrosion resistance. As with phosphatizing, the film produced serves as an excellent coating base. Indeed, it can be difficult to make most polymeric coatings adhere to the nonferrous alloys without such a treatment. Epoxy primer, for example, which does not adhere well to bare aluminum, adheres very well to chemical conversion coatings. Recent concerns with the toxicity of the hexavalent chromium used in chromating have, however, limited its usage. 

This yellow anticorrosive pigment is among the most expensive chromate-based pigments. It is mainly used to protect aluminum from corrosion. Epoxy primers containing strontium chromate are well-accepted in the aircraft industry. They are effective at low pigment levels and are also used in some coil coating applications. 

Other forms of coating systems which contain corrosion inhibitors include in-situ phosphatising coatings such as those described by Neuder et al. [126] where functionalised aryl-phosphonic and arylphosphoric acids were added to an epoxy primer. The inhibitors were shown to deposit uniform metal-phosphate products on the substrate surface and provide a similar level of performance to chromate-inhibited control samples when tested over 3000 h in a neutral salt spray test. 

Another type of primer which has already been mentioned are those especially formulated to be applied to chemically treated aluminium alloys in particular, with the main aim of further improving the durability of joints prepared from these pretreated substrates [e.g. 205-209]. They are commonly referred to as corrosion inhibiting primers and are proprietary formulations which typically contain phenolic, epoxy and chromate species. They are used extensively in modern aircraft structures. They will be discussed in greater... 

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