Chromium, in certain forms, is on its way off of the proverbial production map, and with this comes the swan song of a staple of engineering-centric coatings such as hard chrome chromate conversion coatings and chromic acid anodize.
Chromic acid anodize, aka Type I per MIL-DTL-8625, has traditionally been used in the following applications:
• Airframe components, due to its increased corrosion resistance
• Tight-tolerance aluminum parts requiring corrosion protection
• Aluminum components adhesively bonded to one another
These attributes have been ideal for aircraft coatings.
As its name denotes, chromic acid anodize (CAA) is an aluminum oxide coating created in a chromic acid solution. EH&S and EU legislations (including RoHS, REACH and WEEE) have called for the ban of hexavalent chromium (aka chromates) not only in coatings but also in the manufacturing process. In response to this trend, Boeing in 1990 developed and patented boric-sulfuric acid anodize (BSAA). The company initially licensed the patented process to qualified vendors, however, that patent has since expired.
The BSAA oxide coating exhibits many of the same properties as the aluminum oxide coating that is derived from the chromic acid process. The accompanying table compares boric-sulfuric and chromic acid anodize.
Design Considerations with BSAA
In considering whether or not BSAA would be a functional alternative to a specified chromic acid anodize, there is very little to consider. If parts require hardcoat anodize and chromic anodize on the same part, and the process has previously included using chromic acid anodize as a stop-off for the hardcoat, BSAA will not be appropriate.
Also, because the electrolyte used for developing this coating contains sulfuric acid, there’s a possibility of entrapping solution in welds, crimps, and seams, which can lead to corrosion and part degradation. Parts with these features would not be good candidates for BSAA. Furthermore, while modifying electrolyte conditions for chromic anodize allows those anodic oxide films to be dyed (black, in particular), BSAA doesn’t allow opportunity for dyeing. Aesthetic design considerations aside, it is strongly recommended that users prototype the performance of any coating/part system in the application before bringing it to market.
One of the largest benefits of thin anodic films used in the aerospace industry—be it chromic anodize or boric-sulfuric—is their ability to provide corrosion protection without greatly diminishing the fatigue strength of the aluminum base material. It’s been well demonstrated that with increasing oxide film thickness, the fatigue strength of aluminum can degrade by as much as 50 percent for the thickest, Type III, hardcoat anodize films. Type II sulfuric anodize coatings typically fall in the 25 percent fatigue reduction realm while the thin Type I films are generally around 5 percent or less.
For years, aerospace primes such as Sikorsky have been permitting the switch to BSAA where Type I chromic anodize was specified without actually mandating it. This may be attributed to lack of adoption in the metal finishing industry, particularly since hexavalent chromium was still required for sealing the anodic film allowing it to meet stringent 336-hour salt spray requirements—the industry standard for anodize films. Sikorsky has taken BSAA to the next level when it comes to environmental impact, specifying the use of a trivalent chromium seal (for instance, Metalast TCP-HF) rather than the traditional dilute dichromate. Anoplate is approved by Sikorsky to process to Type IC (BSAA) with this seal and has already seen an influx of incoming work to this specification.
Although BSAA is suitable as a primer for subsequent adhesive bonding, many primes are looking to other chromic anodize alternatives due to their ability to provide even greater bonding strength. For instance, Sikorsky and Bell Helicopter both favor phosphoric acid anodize as optimum for adhesive bonding applications, whereas Airbus is turning toward tartaric acid anodize. While BSAA will exceed the standard corrosion performance threshold exhibited by chromic anodize, neither phosphoric nor tartaric anodize can. Thus, these chromic alternatives are relegated to applications where only adhesive bonding is required.
Boric-Sulfuric Acid Anodize represents a leap forward in providing an environmentally-responsible replacement to chromic acid anodize for straightforward corrosion resistance applications.
Sean Novak is a process/application engineer at Anoplate (Syracuse, N.Y.). For more information, please contact him at email@example.com, or call 315-471-6143.