Salt spray exposure, wet tape adhesion tests, and measurement of electrical contact resistance were performed according to the military specifications that ...
CORROSION ENGINEERING SECTION
Corrosion Performance of Field-Applied Chromate Conversion Coatings R.B. Leggat, S.R. Taylor,* W. Zhang, and R.G. Buchheit**
ABSTRACT Alternative conversion coatings designed to replace chromate conversion coatings (CCC) are typically compared to laboratory-prepared CCC with regard to stand-alone corrosion performance. This study seeks to determine the stand-alone corrosion resistance of field-applied CCC to establish a more realistic benchmark for new nonchromate conversion coatings. Salt spray exposure, wet tape adhesion tests, and measurement of electrical contact resistance were performed according to the military specifications that govern what conversion coatings are accepted for use by the U.S. Department of Defense (DOD) facilities. In addition, other analytical techniques such as electrochemical impedance spectroscopy and Auger electron spectroscopy depth profiling were used. In an initial study, all of the field-applied coatings on AA2024 (UNS A92024), AA6061 (UNS A96061), and AA7075 (UNS A97075) displayed significant pitting after 168 h salt spray exposure. Additionally, attention to pre-cleaning the surface prior to coating was found to increase CCC corrosion resistance. Based on the results of the field-applied coatings, a second phase of investigation was initiated to examine the effects of coating time and application method (spray vs immersion) on the corrosion resistance and paint adhesion. This study was conducted using facilities designed to simulate conditions in an aircraft maintenance depot. It was found that spray and immersion application produced coatings with equivalent performance. Regardless of application method, 5 min of continuous exposure to solution was required to obtain adequate coating weight. The results of this study suggest that a reSubmitted for publication March 2001; in revised form, August 2001. * Center for Electrochemical Science and Engineering, University of Virginia, Charlottesville, VA 22903. ** Fontana Corrosion Center, The Ohio State University, Columbus, OH 43210.
CORROSION—Vol. 58, No. 3
view of the relevant military specifications is merited so that emerging, environmentally benign conversion coatings can be evaluated against an appropriate metric. KEY WORDS: adhesion, aluminum aerospace alloys, chromate conversion coating, corrosion resistance, electrical contact resistance, military specifications, salt spray
INTRODUCTION Conversion coatings are applied to metals in order to increase corrosion resistance and increase adhesion of subsequently applied organic coatings. A conversion coating is the result of two chemical reactions: dissolution of the native surface oxide and formation of a more corrosion-resistant, inorganic compound of the original metal.1 Chromate conversion coatings (CCC) are the most common conversion coatings applied to aluminum aerospace alloys. Although CCC provide excellent corrosion resistance and adhesion, many of the chemicals used in the processing are under strict regulation because of their toxicity. Epidemiological studies have established the carcinogenicity of hexavalent chromium.2-3 The worker’s health and safety risks of using chromate-based conversion coatings are compounded by effluent disposal costs.4 Thus, the health risks and cost of producing CCC has driven the development of nontoxic, environmentally benign alternatives, which have recently been reviewed by Nylund.5 Presently, CCC remain an essential component in effective corrosion protection systems on aluminum alloys for military applications. In order for
0010-9312/02/000061/$5.00+$0.50/0 © 2002, NACE International
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CORROSION ENGINEERING SECTION
TABLE 1 Corrosion-Resistance Requirements of MIL-C-81706 Corrosion-Resistance Properties For qualification For quality conformance inspection (A) (B)
Treatment Class
Aluminum Alloys Treated with Film-Forming Materials
Exposure to 5% Salt Spray (h)
1A(A) 3(B) 1A 3
2024-T3, 7075-T6 6061-T6 2024-T3 6061-T6
336 168 168 168
Class 1A coatings are used for maximum protection against corrosion. Class 3 coatings are used for protection against corrosion where low electrical (contact) resistance is needed.
candidate nonchromate conversion coatings to gain attention as viable candidates for use by the U.S. Department of Defense, they must meet the requirements of two military specifications: MIL-C-81706, “Chemical Conversion Coatings for Coating Aluminum and Aluminum Alloys,” and MIL-C-5541, “Chemical Conversion Coatings on Aluminum and Aluminum Alloys.” The two aforementioned specifications both govern the same systems (i.e., chromate conversion coatings on aluminum) but have important differences with regard to whether it is the process or the end product that is monitored. MIL-C-81706 is a “materials” specification and defines the characteristics of the conversion coating chemistry including appearance and properties of the applied coating. This specification differentiates two classes of coatings, 1A and 3, depending on the intended purpose of the coating. Class 1A coatings are used when maximum corrosion protection is needed, while Class 3 coatings are used when corrosion protection and low electrical contact resistance are needed. In addition, MIL-C-81706 specifies two levels of performance for qualification and quality conformance. To be placed on the Qualified Products List (QPL) associated with MIL-C-81706 (QPL-81706), a product must perform at the qualification level. The quality conformance requirements are used mainly for quality control and lot inspection. On the other hand, MIL-C-5541 is a “process” specification and defines the characteristics of a coating applied in a production environment. This specification defines the requirements for monitoring the processes used in coating production and is independent of the materials that are used. A “process” specification is typically used when the properties of the end product are not easily measured. The process is controlled assuming that the product will be within specification as long as the process is within its parameters. Under the military specifications, corrosion performance of conversion-coated aluminum is assessed by exposure to 5% salt spray according to ASTM B 117.6 The corrosion-resistance requirements are more rigorous for MIL-C-81706 than MIL-C-5541. To meet MIL-C-81706 requirements, no evidence of corrosion with the exception of 1/4 in. of the edge is
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allowed after exposure of five panels for a specified time. The required exposure time varies depending on the alloy and coating class as shown in Table 1. Under MIL-C-5541, a single panel can have no more than five pits and no more than a total of 15 pits for five panels to meet the requirement. In addition to corrosion resistance, these two military specifications also mandate adhesion performance, electrical contact resistance, and coating weight. Adhesion of an epoxy primer and topcoat to the conversion coating is satisfactory for both military specifications when no intercoat separation is observed after wet tape testing per Method 6301 of Federal Test Method Standard no. 141.7 Electrical contact resistance of
