Recent Alternatives to Chromate Conversion Coatings

DoD Participant:  U.S. Army – Corpus Christi Army Depot; Naval Air Systems Command (NAVAIR) Joint Group on Pollution Prevention (JG-PP)

This study compared the performance of eighteen conversion coatings, including 16 chromate-free alternatives and two controls, from seven suppliers, on six aluminum alloys. Data from side-by-side testing is provided for three typical tests:  salt spray corrosion resistance, contact electrical resistance, and paint adhesion.

These results extend and update a previous NCMS study, published in 1995, that provided corresponding data from the most advanced coatings then available. The current study was undertaken in order to gauge the extent to which the state of the art has advanced.

The 1995 study showed that, while there were several strong contenders on the horizon, the standard chromate coatings remained the unchallenged standard, particularly for the most demanding applications involving the high strength copper-bearing aluminum alloys used in aerospace applications (represented by aluminum alloy 2024).

In contrast, the present study indicates that the developers of chromate-free conversion coatings have made significant progress in the past few years.

For example, in the 1995 study, of the 33 coatings tested, only the chromate-containing control samples met the quality conformance standard in the salt spray test on alloy 2024. In the present study, four of the chromate-free test samples surpassed the 336-hour standard, in addition to the control. In addition, on other alloys evaluated on the salt spray test, many of the other chromate-free coatings also performed comparably to the controls.

It is not the purpose of this study to advocate the use, in a general sense, of any of the chromate-free coatings tested here. Any recommendation to use an alternative coating must be specific to an application, and can only be determined by an evaluation suited to that application. The intent of this report is to present a description of results, not a prescription for action.

Nevertheless, the results presented here indicate that it may be time to address some fundamental questions.

1.   When the standards for conversion coating of aluminum were being developed, and until quite recently, conversion coatings based on chromate were the only available choices that could do the job. That situation may be changing. We may therefore pose the following question:

On the basis of the results presented here, if the specifications for conversion coating were being written today, would chromate now be the only choice? Or would it be one option among several?

2.   The interest in developing a chromate-free conversion coating is driven by environmental and workplace health concerns. The availability of chromate-free alternatives would offer significant potential benefits, both directly in terms of pollution prevention opportunities, and indirectly as a competitive incentive to improve the process engineering and workplace practices for those areas where chromate remains in use. At the same time, changing any tried-and-true process, particularly in critical aerospace applications where lives may be at stake, involves a degree of risk.

While the development of chromate-free alternatives was in the research stage, the question of relative risk could be postponed. But if materials substitution becomes a viable option, it becomes time to pose another question:

At what point do the risks of environmental and employee liability begin to outweigh the risks associated with process change?

3.   Finally, the results bring up a somewhat more specialized question. One promising candidate identified in the study is a trivalent chromium process. While the environmental consequences of substituting a trivalent for a hexavalent chromium process are debatable, a more immediately persuasive case might be made for the workplace health and safety consequences of the substitution. Under the circumstances, it is reasonable to pose this question:

Is it time to conduct a thorough investigation of the workplace impact of substituting a trivalent process for a state-of-the-art hexavalent process?