The objective of this project was to evaluate and demonstrate novel magnesium (Mg)-rich primers that are environmentally friendly and potentially have higher performance compared to legacy non-hexavalent chromium primers.
Several hexavalent chromium-free primers have been investigated, but unfortunately, none have possessed the technical performance of the chromate coatings. One approach is to employ a sacrificial metal-rich primer in the overall protection scheme, like the use of zinc-rich coatings for steel substrates to provide galvanic corrosion protection. In galvanic protection systems, the metal in the coating acts as an anode (more negative electrical potential) and oxidizes preferentially to the substrate. The substrate acts as a cathode (more positive electrical potential), and is protected from corrosion at the sacrifice of the anodic metal in the coating.
Mg is more anodic than aluminum and its alloys in the galvanic series, thereby giving it the ability to cathodically protect aluminum substrates. When Mg-rich primers are formulated, small particles of Mg metal are added at, or even beyond, the critical pigment volume concentration (CPVC). The high loading of the Mg particles ensures that nearly all of the metal particles are in electrical contact with each other and with the substrate. The electrical contact of metal particles is a key requirement in this corrosion protection mechanism, establishing the anode/cathode relationship. If successful, the Mg-rich primer could serve as a drop-in replacement for current chromate primers.
Extensive testing prior to this project, and throughout its execution, have led to several iterative improvements to the formulation and marked increase in the corrosion performance. While these improvements have increased the performance of the Mg-rich primer such that they perform better than other non-hexavalent chromium primers and hexavalent chromium primers in certain situations, they are not equivalent across the board. Accordingly, since the performance of the Mg-rich primer was not shown to be equivalent to currently-qualified hexavalent chromium primers, the demonstration and validation portion of this project was canceled.
Therefore, a key performance objective of this project – product testing – was not met. The performance objectives of “commercial-off-the-shelf procurement” and “ease of use” were met successfully. Since no field demonstrations were performed, the “hazardous material reduction” objective was not tested.
One key step in the broad implementation of alternatives to hexavalent chromium is the development of specifications to govern the new coatings. As many companies and government agencies are bound by military and commercial specifications, a coordinated approach to updating the related specifications is essential. Individual organizations or programs may implement alternatives based on mechanisms such as approval letters, local process specification changes, or contract modifications. These methods will need to be considered by the user on a case-by-case basis depending on the success of future field-testing.
An additional consideration may be which pretreatment(s) are available to the user. The relative performance of the Mg-rich primer, as compared to control coatings, is highly variable as the pretreatment changes. In some instances, such as with PreKote and BoeGel, the Mg-rich primer out-performs the non-chromate and the chromate controls. However, over pretreatments such as chromate conversion coating (CCC), trivalent chromium pretreatment (TCP) and anodized, the Mg-rich is not as good as the control coatings. Therefore, while the best non-chromate alternative is the qualified Class N primers over TCP, if a user was only authorized to use PreKote, the Mg-rich primers are a viable alternative.
While the Mg-rich primer has promise as a potential chromate primer alternative, additional work needs to be conducted to improve the performance over qualified pretreatments and to address the self-corrosion issue seen in primer-only test panels.