Southern Metal Finishing

Re: Alternative to Traditional Yellow Hex Films (Part 1)

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 www.southernmetalfinishing.com

Alternative to Traditional Yellow Hex Films (Part 1)
Individuals long associated with the metal finishing industry immediately recognize the terms “Iriditing” and “Alodining” as nicknames for a process protecting aluminum with a yellow chromate conversion coating.  The terms stem from the two most prominent proprietary products, Iridite and Alodine, of aluminum finishing since its inception in the early 1950’s.   However, recent European Union Directives such as End-of-Life Vehicles (ELV) approved in 2000, Restriction of Hazardous Substances (RoHS) and Waste Electrical and Electronic Equipment (WEEE) both approved in 2003, now place restrictions on traditional hexavalent chromium containing finishes. The proposed OSHA limitation on hexavalent chromium exposure of 0.1 mg/m3, presently under review, has the American metal finishing and electroplating industries deeply concerned over the future of hexavalent bearing chemistries of any kind.  The combination of these new governmental policies creates a need for a replacement of hexavalent chromium based finishes with technology that rivals the performance but remains hexchrome-free.  This need has created the new name in chemical conversion coating technology, “Aluminescent”.

Luster-On Products’ Aluminescent is based on the Trivalent Chromium Process, “TCP”, developed by United States Navy researchers at their NAVAIR research center in Patuxent River, MD.  The patented process generates both pre-treatment and post-treatment chem films on wrought and cast grades of aluminum.  The conversion coating itself meets or exceeds the requirements of Mil-DTL-81706, Type II for both Class 1A, base metal corrosion protection, and Class 3, minimum electrical resistivity.

The United States government recently offered licenses to market the invention to interested parties and Luster-On Products was granted a license to blend and market the material.  Aluminescent generates a hex-free chemical conversion coating on aluminum fulfilling the requirements of Mil-C-5541 and compliant with the demands of the European Directives, “ELV”, “RoHS”, and “WEEE”. 

General Chemical Description  Aluminescent is a blended powder containing a unique mixture of trivalent chromium salts and metallic fluoride compounds.  The working solution is simply prepared by dissolution of the material in water.  The product has a limited solubility in water of about 20 g/L.  The novel technology can be described as a steady-state system, one in which the active complexes do not change state or valence at any time throughout the process; constantly providing a uniform consistent finish.  Only a change in the operational parameters can change the physical nature of the conversion coating itself.

Aluminescent contains trivalent chromium and although still considered a hazardous material, the product is far more user friendly and “greener” than its hexavalent counterparts. The U.S. Navy tested the toxicology of the invention following EPA guidelines in the categories of acute oral, acute dermal and acute inhalation sensitivity.  The LD50 for both the oral and dermal testing was greater than 2000 mg/kg and the LC50 for inhalation is greater than 2.25 mg/L.  The dermal classification for the invention is non-sensitizer.   By comparison, chromic acid has an acute oral LD50 of 52 mg/kg and an acute dermal LD50 of 57 mg/kg.  The acute inhalation LC50 is set at 0.217 mg/L.

Aluminescent is a drop-in replacement for similar aluminum chromates based on the hexavalent chromium ion and may be used as an effective paint base or independently providing excellent corrosion protection with low electrical resistance.  The pre-treatment chemical requirements presently used prior to any hexavalent process suitably meet, in most applications, those of the trivalent process.  The invention can be applied by immersion, spray or brush application without the need for current or cooling.  Similar to hexavalent technologies, the process is effective on both cast and wrought alloys but usually requires specific pre-treatment chemistries determined by the alloy to be treated.

The process generates a colorless conversion coating on the metal surface with only a slight level of iridescence of bluish or golden yellow hues.  Coating weights for the film generally fall in the range of 20-25 mg/ft2. 
Operational Parameters/ Maintenance and Control

Luster-On Aluminescent, used at 1-2 oz/gal (7-15 g/L) with a working pH range of 3.5 – 4.0, best meets the parameters of the Navy’s original invention.  The process most satisfactorily operates between room temperature and 120F (18-50C). Immersion times of 1 – 10 minutes are acceptable with 3-5 minutes suggested as optimum.

The pH of the working solution was found to be critical to the success of the film.  The pH will slowly rise as the solution forms the conversion coating.  Adjustments can be made with a mild (10% v/v) solution of sulfuric acid to lower the pH and a mild (10% wt) solution of potassium hydroxide to raise the pH into acceptable range.  As in most chromating situations, agitation is recommended, but not required, to assist in bringing fresh solution to the interface.  Constant movement about the surface of the substrate also aids in keeping the pH equilibrium at its optimum.

Please see the following post for part 2 of this article or visit http://www.southernmetalfinishing.com/n … tails.html to read the full text.