ESOH Technical Planning Integrated Product Team

ESOH TPIPT Need Assessment Summary

Need: #953

A Nonchromated Conversion Coating for Ion Vapor Deposited Aluminum Coating Which Replaces Hazardous Cadmium Plating

POC: Capt. Frank Wilson (513) 255-6388



1.0 Statement of Need

Ion Vapor Deposited (IVD) aluminum is a soft ductile coating similar to pure aluminum. The coating is applied by a dry process performed in a vacuum chamber that is environmentally clean. The coating has been widely accepted by the customers as a replacement for cadmium plating. Cadmium plating has been used as a corrosion resistant finish on steel. Substitution of IVD aluminum for cadmium has eliminated the hazardous waste produced by the cadmium plating.

The postcoat process steps for IVD aluminum and cadmium plating are essentially the same. Both use a chromate conversion coating (CCC) to provide additional corrosion protection and an improved base for paint adhesion, per MIL-C-83488. CCCs have come under intense scrutiny by regulatory agencies because they contain hexavalent chrome, a known carcinogen. Chrome is on the EPA 17 list of the worst industrial toxins. An effective conversion coating is essential to the functional performance of IVD aluminum in many applications. The primary need is to identify and qualify a conversion coating that does not contain EPA 17 substances. A secondary need is to identify technologies that reduce the hazard incurred by the use of existing CCCs.
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2.0 Objective

The objective of this action plan is to suggest existing technologies able to reduce the emission of chromium from use of CCCs, and to report on current research efforts that have identified effective nonchromate conversion coatings for use with IVD aluminum coatings.
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3.0 Technical Approach

At present there are drop-in alternative conversion coatings that pass the minimum standards for MIL-C-83488, but in practice, CCCs perform far in excess of the standards. Examples of adequate nonchromate conversion coatings are discussed. If an application demands the highest performance, CCCs may still be necessary. In these cases, technologies exist that reduce the hazard from use of CCCs. Finally, an experimental conversion coating is described that may one day match the performance of CCCs.

Existing Technologies to Reduce Emissions of Chrome

No-Rinse Chromate CCC Process - As a waste minimization measure, NADEP, Cherry Point established a no-rinse procedure for chem film treatment of aluminum surfaces. The procedure requires application of the CCC onto the aluminum surface with scotch brite or cheesecloth. After sufficient reaction time is allowed, residual CCC is wiped off the surface with a damp cloth. Northrop Grumman is currently evaluating this process for use in their aircraft rework facility and in production operations (touch-up). Preliminary results indicate that acceptable coating weights can be obtained using this procedure (40 to 60 mg/ft2). From a visual perspective, the aluminum panels coated in this manner are of comparable quality to an immersion or spray application. Adhesion characteristics of the CCC are currently under evaluation.

No-Rinse Chromate CCC (Touch-Up) - Several years ago Hughes Aircraft developed a chem film pen for use in touch-up operations. The pens that were originally developed by Hughes were not compatible with production operations from a number of different perspectives. In 1995, Parker Amchem purchased the technology concept from Hughes and improved the design of the touch-up pen. The Parker Amchem system, Touch-N-Prepä , uses a unique no rinse conversion coating, Alodineâ 1132. The no-rinse conversion coating can be used for surface preparation of aluminum, steel or galvanized metal. Testing performed by Parker Amchem indicates that the CCC coating will pass 336 hour salt spray resistance. The Touch-N-Prep system has been submitted to NAWCAD, Pax River for qualification under MIL-C-81706.

Nonchromated Conversion Coatings

Industry has made significant efforts to develop nonchromated conversion coatings that meet the requirements of MIL-C-83488. To meet this specification a conversion coating must pass salt spray corrosion, electrical resistance, and primer adhesion tests. Typically, the salt spray corrosion test is the most demanding to meet. Table 1 presents the corrosion resistance standard for IVD aluminum. For the test, an AISI 4130 steel panel, conforming to MIL-S-18729 is coated with IVD aluminum in conformance with MIL-C-83488 (0.5 mil thick minimum for Class 2). In accordance with standard practice, the IVD aluminum coating is glass bead peened prior to application of the conversion coating.

Table 1. Required corrosion resistance duration times for IVD aluminum coated steel subjected to a 5% neutral salt fog environment per MIL-C-83488

Class

Type I-No Conversion Coating (hours)

Type II-Chromate Conversion Coating (hours)

1 504 672
2 336 504
3 168 336

The performance standards for Type II (chromated) coatings and Type I (no conversion coat) coatings are similar. However, the actual performance times for Type II coatings typically far exceed the MIL-C-83488 requirements with averages of 8600, 2800, and 900 hours, respectively for Class 1, 2, and 3 coatings. On the other hand, the actual performance times for Type I coatings do no better than meet the minimum requirements. Some applications may require a nonchromated coating able to perform in excess of the minimum standard.

Under funding by Armstrong Laboratory, McDonnell Douglas Corp. reported on its evaluation of 6 nonchromated conversion coatings for potential use with IVD aluminum coatings (contract F08635-94-C-0004). The coatings were selected from a larger list based on its own internal evaluations. A list of the 6 coatings is provided in Table 2.

Table 2. List of Nonchromated Conversion Coatings Evaluated by McDonnell Douglas

Nonchromate Conversion Coating

Chemical Description

Manufacturer

Sanchem-CC (Steps I, II, and III) Permanganate based Sanchem, Inc.
Sanchem-CC(Steps I, and II) Permanganate based Sanchem, Inc.
Alodine 2000 Cobalt based Parker+Amchem
Sanchem-CC(Sealing step II only) Permanganate based Sanchem, Inc.
Permatreat 1001 Organometallic (Zirconia based) Betz Laboratories
Alodine NC 90/91 Organometallic Parker+Amchem

Under salt spray corrosion testing for 3000 hours, only Sanchem-CC (Sealing step II only) survived without failure. However, it failed the electrical resistance and primer adhesion tests. After exhaustive testing beyond MIL-C-83488, Sanchem-CC (Steps I, II, and III) was recommended as a satisfactory alternative to CCCs. The Sanchem process steps are as follows:

anchem Process:

For an immersion application, this process requires the use of several heated tanks. In general, it is felt that the energy and facility requirements create substantial difficulty from the standpoint of implementation into a production facility. For this reason, Sanchem, in conjunction with Electro-Steam Corporation has modified the process using a specially designed steam cabinet for application of the coating. This pilot scale coating operation, utilizing this technology is currently being installed by Concurrent Technologies Corporation at the National Defense Center for Environmental Excellence (NDCEE). This process does not readily lend itself to aircraft rework operations involving spray application of the CCC.

Alodine 2000 was also judged to be a satisfactory alternative to chromate conversion coating. It performed poorer than the Sanchem-CC process described above only with regard to a sulfur dioxide salt fog test for corrosion.

Current Research Efforts:

Sol-Gel Coatings: Thiokol has developed a nonchromated surface treatment process for aluminum which greatly enhances the adhesion of applied paints and coatings. The process consists of a metasilicate and organofunctional silane which provides a covalent chemical bond with the paint or coating. This process was evaluated for NASA (Space Shuttle) and found to be equivalent to unpainted chromate CCC from the standpoint of corrosion resistance. This process requires military qualification. The Thiokol pretreatment process is a two step process that is applicable to immersion applications and potentially, spray applications.

Sol-Gel coatings have been under development for a number of years. To date, the development effort has primarily focused on surface preparation of aluminum and titanium for adhesive bonding. Recently, Northrop Grumman Corporation has begun working with Chemat Technologies to optimize this coating technology to replace chromated CCC. Both spray and immersion applications are being considered. Similar projects are ongoing at Boeing, McDonnell Douglas and NAWCAD, Pax River. It should be noted that these coatings do not contain active corrosion inhibitors. Rather, they act primarily as thin barrier coatings that provide an adhesion base for epoxy films that is superior to CCC’s. When these types of coatings are used, the corrosion inhibitors contained in the primer impart the necessary corrosion resistance properties of the "coating system" and the surface preparation process is optimized for paint adhesion. The following serves as an example of a nonchromated surface prep for paint application:

Acceptance of a process of this type would require deviation from the typical product qualification test requirements. In essence, the surface prep and coating would be evaluated and qualified as a system for a specific coating application rather than individually. These coatings may have potential for treatment of IVD aluminum coatings on steel panels. Sol-gel coatings would not be useful for IVD aluminum coatings on fasteners.
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