For the last year or so we have been talking about the July 2006 RoHS deadline that puts a ban on the use of three common metals used in the surface engineering industry. We have touched on both lead and cadmium recently and the new emphasis will be on educating our customers on the use (or non-use) of hexavalent chrome. Its use as a chromate film on zinc and cadmium has been discussed in previous issues of Anoplate News, but little discussion has taken place about the ban with regard to chemical conversion films on aluminum. This is that iridescent, rainbow film used for both corrosion resistant purposes as well as paint adhesion on aluminum. For those of you that live and die by specifications, these are the films covered by MIL-C-5541. Now there has been lots of activity among our suppliers to provide an acceptable substitute, one that has all of the characteristics of the hex chrome film (Alodine or Irridite, for example). This has not been an easy task, however. Some of the requirements involve salt spray exposure, low electric contact resistance, paint adhesion properties and (not a requirement but hopefully) that rainbow color.  
  
Well, we have good news and bad news. First, the good news. After 18 months of testing the Navy has approved the material which is used in our AnoChem TCP process. It has met the qualification requirements for MIL-DTL-81706B, Type 2 Chemical Conversion Materials for Coating Aluminum and Aluminum Alloys. This material meets all the technical requirements, but it does not have that iridescent film that we are used to. That is the bad news. It is also important to note that very stringent cleaning requirements have to be adhered to when processing certain aluminum alloys. At the present time MIL-C-5541 only pertains to hex chrome conversion coatings. 
  
Let's get a few coating descriptions listed here:

• 100% hexavalent chromium free conversion coating for aluminum and RoHS compliant.
• Harder, denser and more abrasion resistant finish than the chromate version it replaces.
• Highly corrosion resistant, exceeding salt spray requirements of MIL-C-5541, MIL-DTL-81706B and AMS 2473.
• Outperforms conventional chromate coatings in corrosion resistance testing on high copper bearing aluminum alloys.
• Unlike chromate conversion coatings, corrosion resistant properties do not degrade upon exposure to temperatures above 140 degrees F.
• Blue-bright finish, not iridescent rainbow color.
• Compliant with IBM 41-218A and Lucent WL-2156 (#571).

We suggest the possible use of this wording on your finishing requests; “apply a non-hexavalent chromium conversion coating using a material per MIL-DTL-81706, Type II, whose resulting coating meets the process performance requirements of MIL-C-5541, Class 3 or 1A (as appropriate)”. As mentioned above, just using a material qualified under MIL-DTL specification does not mean that the resulting coating will meet MIL-C-5541. Certain modifications in the cleaning cycle are necessary for certain alloys so be careful of which metal finisher you choose. Sorry about losing that nice rainbow, but you will have some explaining to do with your customer. That is the bottom line why we wrote this article:  to better educate you and your end user and give you options.