Southern Metal Finishing

Re: Alternative to Traditional Yellow Hex Films (Part 2)

This article was published in the March 06 issue of Southern Metal Finishing. If you would like register to receive our free newsletter and review our online archives please visit

Alternative to Traditional Yellow Hex Films (Part 2)
The operational parameters of the product are monitored with readily available laboratory equipment.  The concentration can be checked titrimetrically by oxidizing the trivalent chromium to the hexavalent state making it readily measured by wet analytical methodology. When instrumentation is available, concentration can also be determined using atomic absorption spectroscopy or color spectrophotometry to measure chromium levels or by ion-selective electrode to measure fluoride concentration.  pH should be measured routinely using a standard laboratory pH meter.

The reaction of dissolved aluminum with available fluoride slowly leads to the formation of a white aluminum fluoride precipitate which may cloud the working solution.  While it appears not to interfere with the conversion of the aluminum, it can be easily filtered away to keep the working solution clear and the aluminum substrates free from precipitate roughness.

Tanks used to hold the working solution must be resistant to acidic solutions containing fluoride.  Polyethylene, polypropylene, PVC and Koroseal lined steel tanks are all acceptable construction materials.  Air sparges, heaters, or spray systems when desired, should also be of suitable construction materials.  Appliances, used to hold aluminum to be processed, constructed of dissimilar metals may cause galvanic reactions which interfere with film formation.  Pumps or filtration units should have acid and fluoride resistant wetted materials.

Performance Results Various aluminum alloys have been processed and then tested to ensure universality of the invention.  The alloys tested by the Navy include 2024, 2219, 5083, 6061, and 7075.  Luster-On in-house testing focused on 2024, 3105, 6061, 7075 and A380.  All in-house testing was done using a 1.5 oz/gal solution at room temperature (65-85F) within a pH range of 3.5-4.0.  Immersion times were typically held at three minutes; five minutes for castings.  The NAVAIR testing centered on an equivalent of a 1.0 oz/gal solution at room temperature with a pH of 3.7-3.8 and typically two minute immersion times.   In both cases, pre-treatment deoxidizers were determined by the test alloy. 

The function of the coating as a pre-treatment chem film was examined in-house by applying paint and then testing paint adhesion according to ASTM D-1654 after 3000 hours of exposure to salt spray per ASTM B-117.  After exposure, the test panels achieved a rating of 9 for minimal creepage from the scribe and a 10 rating for no failure in unscribed areas.  The Navy compared the TCP process with hexavalent counterparts and determined that the trivalent process was capable of achieving similar results under conditions of cyclic testing, long term weather exposures, scribed salt spray studies, filliform testing and various adhesion tests.

Bare metal corrosion studies were performed in conjunction with Mil-DTL-81706 and ASTM B-117.  The Navy tested alloys 2024, 2219, 5083, and 7075 with all but the 2219 passing 336 hours of exposure without first sign of corrosion.  The 2219 alloy showed initial signs of corrosion but not enough damage to cause failure under the stipulations of the military specification.  Luster-On processed and tested alloys 2024, 3105, 6061, 7075 and A380 with all but the A380 surpassing 336 hours of exposure without first sign of corrosion.  The A380 castings showed initial signs of corrosion between 240-288 hours of exposure.  The salt spray test on alloys 3105, 6061 and 7075 was halted at 504 hours of exposure; no signs of corrosion were visible.

The NAVAIR group tested the electrical resistance of the coating in conjunction with Mil-DTL-81706, Type 3.  The films were checked as required before and after exposure to a salt fog for 168 hours.  Prior to salt spray exposure, the average contact resistance of 5 test panels averaged 1.57 milliohms/in2 (0.24 milliohms/cm2).  After 168 hours of exposure, the contact resistance was determined to average 3.19 milliohms/in2 (0.49 milliohms/cm2) on five test panels.

The invention shows promise as a hexchrome replacement in the fields of zinc and zinc alloy passivation, a sealant for anodized aluminum and zinc and iron phosphate conversion coatings, generating protective coatings on metallic substrates and pre-treatment chem films for magnesium alloys.  The U.S. Navy and Luster-On are continuing their work on the performance of the process on various aspects of the metal finishing industry. 

The Luster-On process compares favorably with its hexavalent counterparts in providing a conversion coating on aluminum which meets or exceeds the requirements of Mil-C-5541, Type 1A and Type 3.  The product is a drop-in replacement for its hexavalent counterparts.  The conversion coating which simultaneously fulfills the corrosion and electrical resistance requirements can be generated using the same operating parameters; something that most hexavalent systems cannot do.  The chem film is an excellent paint base prior to e-coat, powder coat or traditional primer and paint systems.  The finished product is compliant with all European Directives by being completely free of hexavalent chromium, cadmium, lead or mercury.   All of the present needs for aluminum chem film technology have been met and the new term in the industry is Aluminescent.

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