This study found that an etch-primer formulation that incorporated Nubirox 301 showed comparable results in adhesion and corrosion inhibition when compared to a traditional, commercial etch primer that contained zinc tetraoxy chromate. Nubirox 301, a calcium and strontium phosphosilicate corrosion inhibitor, gives an effective and cost-efficient pathway to making an environmentally friendly self-etching primer. The formulations were applied to three different substrates (cold rolled steel, galvanized steel and aluminum) and were put through a series of tests (EIS, SEM/EDX Analysis, ASTM B 117, ASTM D 5894 and ASTM D 4585). Each of the tests concluded that, overall, the addition of Nubirox 301 showed the most advantageous results. The formulations were in compliance with low-VOC requirements; however, since the commercial etch primer contained the zinc tetraoxy chromate, it still required the reporting of that environmentally hazardous substance. Nubirox 301, however, is not only chrome-free, it is zinc-free as well – therefore, it is not required to report this inhibitor for environmental compliance.
In the automobile refinishing and industrial manufacturing industries, and where untreated or bare metal is painted, an etch primer is typically used to increase adhesion of the paint to the metal surface and to improve the anticorrosive quality of the coating. Historically, etch primers have utilized zinc tetraoxy chromate as the primary corrosion inhibitor. However due to the toxicity and carcinogenic concerns associated with chromates, as well as the dawning of the new era of “green regulations”, an environmentally friendly alternative is needed for this market.
Zinc phosphate corrosion inhibitors have been studied as a replacement for zinc tetraoxy chromate in this type of application, and they have seen limited success. Historically, zinc phosphate has exhibited similar performance to chromates in real-world outside exposure. However, in harsher conditions such as marine environments and accelerated salt spray or prohesion testing, the results are less favorable; however, it is important to note that under these more harsh conditions, zinc phosphate still helps in the preservation of the metal substrate, just not to the degree the chromate presents. Even though zinc is not considered a heavy metal, it is still a requirement to report its use.
Etch primers are traditionally formulated with phosphoric acid and zinc tetraoxy chromate in an alcohol medium, using polyvinyl butyral as the binder, which acts as a ligand of the reaction products. As new global environmental standards are put in place, stricter formulation guidelines will become prevalent. In the near future a push to lower VOCs and HAPS-free systems, to eliminate chrome, to limit reportable compounds, and to develop water-based hybrids are in store for the etch-primer market.
The objective of the work presented herein was to develop a chrome-free self-etching primer that meets current and future global environmental demands, as well as offer improved performance over an existing chrome-free formula utilizing zinc phosphate and comparable to improved results to an industry-standard zinc tetraoxy chromate-containing etch primer. A range of Nubirox corrosion inhibitors were evaluated in this study in order to determine which chrome-free inhibitor showed the best results.