NCMS Project #170604, Recent Alternatives for Chromate Conversion Coatings

Statement of Work

Purpose

Chromate conversion coatings are applied to aluminum as a corrosion inhibitor and to provide an adhesion base for paints. Particularly for the high strength copper-bearing aluminum alloys used extensively in airframes, there is currently no generally accepted alternative. All conversion coatings which currently qualify as Class 1A or as Class 3 coatings under MIL-C-81706 contain chromate.

As a known human carcinogen, chromate is subject to strict health and environmental regulation, and its use in manufacturing involves an increasing burden of expense and potential liability. The need to find alternatives to chromate has been recognized as an important priority, both in defense maintenance facilities and in the private sector.

In 1995, NCMS released a widely circulated report of a study which provided, for the first time, a side-by-side comparison of 29 chromate free alternative coatings, together with four standard chromate coatings. The coatings were applied by their manufacturers on test coupons of five representative aluminum alloys and were submitted to NCMS project participants for testing. Salt fog corrosion, electrical conductivity, and wet and dry paint adhesion testing was carried out on all samples. The detailed test results were included in the report, together with an environmental impact study of the alternatives, compared to the standard coatings.

The report indicated that, while suitable alternatives were available for a large number of applications, it was still difficult to find substitutes for the most difficult alloys under the most demanding conditions. This was particularly problematic for aerospace applications.

Since the appearance of the report, several manufacturers have released products which may offer significantly improved performance compared to those available in 1994, when the original testing was carried out.

This project has been organized under the NCMS Commercial Technologies for Maintenance Activities (CTMA) program to conduct a test series for recently developed chromate-free alternatives for conversion coating of aluminum to determine the extent to which the new alternatives can substitute for existing applications.

Project overview

Project participants will decide which substrate materials and which test methods are of greatest interest.  Suppliers will be asked to provide samples of their coatings on standard test panels of the selected substrates supplied by the project.   To encourage suppliers to participate, they may choose to submit samples on only some of the alloys, and may indicate that the coatings are only intended for applications relevant to some tests and not others.  At the option of the any supplier, NCMS will provide non-disclosure assurances stipulating that no further analysis will be carried out beyond the announced tests without the supplier's permission, and will return the samples after testing upon request.  Suppliers will be asked to provide as much detail as possible about process conditions and coatings composition, to allow project participants to determine the suitability of those coatings for their ultimate intended applications.

Project participants will select laboratories to conduct the tests. One project participant will assume responsibility for distributing uncoated test panels to suppliers, for receiving the coated samples back and redistributing the samples to the test laboratories, and for archiving the samples after the tests are completed. Coated samples provided to the test laboratories will be designated by a code which does not indicate the source of the coating.

The test laboratories will report their results to the project participant] which has assumed the responsibility of data collection.  When the testing has been completed, NCMS will distribute the data to project participants for review, together with a report describing the test procedures and summarizing the results.   NCMS will draft the report, edit the final version, and distribute the report to the general public.

Scope of work

The number of coatings which can be tested depends on the number of substrates submitted and the number of test methods chosen.  The resources available to the project can be specified in advance, but the precise mix of samples and tests will depend on what the coatings suppliers make available, and can only be determined as the project proceeds.

The following calculations are intended to provide a guideline estimate for planning purposes, and will be refined during the initial phases of the project as information on coating availability develops.

To provide a common unit of measure, the calculation will be expressed in terms of a "coating-test" (CT).  One CT will denote one complete test carried out on one specimen.  (Generally, one "specimen" will correspond to one panel, but it may be possible to carry out some tests on portions of panels.  Here, "specimen" means one unit used as a sample in a given test.)

Some tests will require several panels.  Salt fog testing, for example, typically requires five panels of a given coating type on a given alloy.   Therefore, one salt fog test of one coating type on one alloy will require five CTs.  Note that, for example, in the salt fog test procedure used in the first NCMS study, each panel was inspected numerous times.  In the contact resistivity test, a single panel was measured eight times.  However, by definition the CT unit refers to the complete test, not an individual measurement.  Thus, a complete salt fog test takes five CTs, a complete contact resistivity test will require one CT, etc.

Thus, 100 CTs are required to run 5 tests on 20 coating-substrate combinations, or 20 tests on 5 combinations, etc., with some test methods (e. g. salt fog) requiring multiple tests.

It is assumed for the purposes of this calculation that it is meaningful to assign an average cost to carry out one CT.  While the actual costs per CT will clearly vary from test to test, taking those cost differences into account will be postponed as a refinement.

This calculation assumes that an average of one hour is assigned to each CT.  The hour includes preparation, measurement, and cleanup time on the actual specimen.  It also takes into account overall setup time, time required for data formatting, and other costs which cannot be assigned to each individual specimen, by dividing the total of those costs by the number of CTs in the particular test series.   For each test series, the total number of labor hours required to run the test series is thus assumed to equal one hour times the number of specimens tested.  It is also assumed that cost of supplies and other non-labor expenses is minimal compared to the labor costs of conducting the test series, so that non-labor expenses are not included.

A fully burdened cost per hour of $75 will be used in this calculation.  (For testing that is carried out on a cost-shared basis, participants will be asked to use that figure when estimating their cost-share contribution to the project.  In many cases, participants will have federally-approved rates in excess of $75/hour.  When reporting time spent for cost-sharing purposes, participants will of course use actual hours and rates.  This may result in the project's accruing a larger dollar amount of cost-sharing than reflected in this calculation.  But the bottom line for this calculation is the work which can get done, rather than the dollar amount of value assigned to it.  So for the purposes of this estimate, the $75/hour figure will be used regardless of each participant's actual rates.  For testing that is carried out under contract and paid for by the project, the same cost, $75 per CT, is assumed.)

The project's resources include $148,551 in cash and cost-sharing.  As an example, if 35 coatings are submitted on an average of 5 substrates apiece, and 10 tests are run on them, the work would require 35 x 5x10 =  1750 CTs, and the total cost would be 1750 x $75/CT = $131,250, leaving $17,301 for expenses (purchase of uncoated test panels, etc.).  This provides a baseline estimate for the extent of testing which can be anticipated under this project, given the available resources.  (The dollar amounts represent actual resources available to the project.  Costs incurred by NCMS for project management and infrastructure expenses are funded separately and do not come out of this total.)

Work plan

The testing will be carried out in two phases.

• The first phase will be broadly inclusive, with minimal prescreening of alternative material and process types as proposed by coatings suppliers, but will comprise salt fog and paint adhesion testing only. The object of the first phase is to develop state-of-the-art information and to identify potential candidates for targeted applications of particular interest to participants.
• The second phase will be directed toward developing more detailed information required to support the substitution of the most promising chromate-free coatings, as identified in the first phase, in the targeted applications.

Significant tasks are listed below, together with the project participant assuming primary responsibility for execution of each task:

1. Contact suppliers and obtain commitments for participation. Prepare list of coating types and alloys to be coated from each supplier. (NCMS, with assistance from other participants)
2. Purchase test coupons. (NCMS)
3. Distribute test coupons to suppliers; receive coated coupons and distribute to test laboratories. (UTRC)
4. Perform salt fog testing. (Raytheon)
5. Perform paint adhesion testing. (UTRC, through Pratt & Whitney Canada)
6. Collate data from phase 1. (UTRC)
7. Select candidates and prepare detailed test plan for phase 2. (All)
8. Execute phase 2. (To be determined)
9. Prepare and distribute final project report. (NCMS, with assistance from other participants)

Target Schedule

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National Center for Manufacturing Sciences