For more than 20 years the finishing industry has known that pigments containing chromium, while unrivalled in their performance and flexibility in inhibiting corrosion, are carcinogenic and hazardous to human health if they are inhaled as dust or particulate. However, maintenance systems for the corrosion protection of metal structures such as aircraft or bridges usually require thoroughly removing old coatings by sanding or blasting, which generates a considerable amount of chromate dust. This dust affects both the workers and the immediate environment.
International legislative, political and ethical pressures have been growing to limit the use of chromates and find viable commercial alternatives. New regulations for the control of chromate dust have lowered the safe exposure levels to five parts per million, a level that can easily be exceeded in the production of paints and the removal of coatings during refurbishment. Exposure levels must be closely monitored, and individuals who handle chromium products have to create multiple systems to prevent these new limits from being reached.
Ongoing development work has come up with an increasingly sophisticated and effective range of chromate-free alternatives to hexavalent chromium compounds, such as strontium chromate. However, even the best systems cannot completely replace the use of chromates. They still require chromated pretreatments or metal preparation to control corrosion, and they generally do not offer the same flexibility in use and application as chromated products. Consequently, these products have not been widely used or commercially successful. Lacking a suitable alternative, many companies have simply minimized their use of chromates wherever possible and resigned themselves to the significant challenges and expense of dealing with the associated regulations.
A "Universal" Solution
Chromated pigments are “universal” in the sense that they can be used in multiple resin systems with excellent results. In contrast, non-chromate pigments typically are specific to the resin system and are affected by factors such as pretreatments and substrate and environmental conditions. This limitation has delayed technological improvements in chromate-free coatings.Recently, a new technology based on magnesium has been developed that offers a promising solution. Magnesium has many distinct advantages over previous chromate replacement technologies. It is a common, safe, naturally occurring element (one of the most abundant elements on earth), and no known health, safety or environmental issues are associated with its use. Magnesium-based coatings also offer high performance. Magnesium pigmentation works under the same conditions as chromate, in that it is not specific to the resin system or the metal it is intended to protect. For coatings manufacturers, this characteristic means that a single magnesium pigment can be used in multiple formulations rather than having to use different pigments for each formula or end use. For the aerospace sector, where many airplane parts are composed of different alloys of aluminium and other metals, such as titanium or even steel, a single magnesium formulation will protect all of these substrates. Few of the other non-chromate corrosion inhibiting pigments have this capability.
There are cost-saving benefits for magnesium-based paints, as well. To begin with, totally chromate-free systems mean that applicators will not have to bear the financial burden of air supply and inhalation monitoring to meet their new legal obligations. Additionally, testing has provided early indications that magnesium primers will deliver savings in coatings weight over current primers. This weight savings can affect both fuel costs and emissions levels, which are both particularly sensitive issues for operators in 2007. Weight reduction is high on the list of needs by the OEMs and airlines; a coating that not only is chromate-free but also reduces aircraft weight is a win-win.
A Commercial Effort
The evidence for magnesium-based coatings has been building, thanks to pioneering technology-based research at North Dakota State University (NDSU). Dr. Gordon Bierwagon, a professor in the Department of Coatings and Polymeric Materials at NDSU who has been developing technology for metal substrates common to the international aerospace industries, has been looking at the use of magnesium metal as a corrosion inhibiting pigment for more than five years.Through the efforts of Bierwagon and other researchers, NDSU now has multiple patents on the use of magnesium. To commercialize this technology, NDSU has entered into a licensing agreement with Akzo Nobel Aerospace Coatings (ANAC) that will allow ANAC to exercise the patents and to continue research and development in magnesium coatings and related areas.
The focus of ANAC’s research, which is being carried out with various industry partners under nondisclosure agreements, is to bring to the aerospace market a corrosion control coating system that is 100% chromate-free and that also meets or exceeds the protective capabilities of traditional chromate systems. The company has already developed a promising magnesium-rich primer (MgRP) that can be applied to metal substrates after following current commercial cleaning processes. No conversion/intermediate coatings are necessary. This feature equates to large savings in time and labor costs.
Extensive testing in ANAC laboratories, at numerous military corrosion laboratories, and by commercial aircraft producers has proven the success of the new system. The MgRP consistently performs at levels equal to or higher than conventional chromate systems in preventing corrosion on aluminium and other metals. Based on these test results, documents for the first qualifications are expected to be submitted within the next three months so that the coating can become a qualified product at key OEMs and government approval authorities. It is likely that magnesium-based coatings will receive the necessary approvals/Qualified Product List (QPL) listings during 2007 and will be on the market before the end of the year.
