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Basic Comparisons Of Chrome & Non Chromate Alternative Anodizing

September - 2008
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By Steve Anzelc


What are the differences between chromic acid anodize (CAA), sulfuric acid anodize (SAA) and boric sulfuric acid anodize (BSAA) for aluminum in the aerospace industry? When is chromic acid anodizing required versus sulfuric acid anodizing? Since Boeing has come up with a non-chrome replacement with Boric Sulfuric, why would you still use CAA? Additionally, the type, number, and class are sometimes confusing to those not using them regularly, so I have summarized the basics for each.

Types & Classes

Chromic Acid Anodizing (CAA) is Type I or IB; Sulfuric Acid Anodize (BSAA) is Type IC; and Hard Chrome is Type III. Suffixes are added to these types and are called "classes" to indicate dyes and seal combinations. Military (Mil) and Boeing Airplane Company (BAC) Class 1 is non-dyed, and Mil Class 2 is dyed prior to seal. BAC Class 3 is dilute chromate seal only, and BAC Class 5 is unsealed. A summary is shown in Table 1 (please refer to September issue PDF).

Since this paper is primarily discussing conventional chrome versus sulfuric coatings, items not discussed here but relate include Type III hardcoat anodizing (HCA) for high wear performance and electrical insulation, and phosphoric acid anodizing (PAA) used for metal bonding (structural adhesive) preparation. This paper will focus only on Chromic Acid Anodize and the non-chrome alternatives Sulfuric Acid Anodize and Boric Sulfuric Acid Anodize processes only.

Type I and IB Chromic Acid Anodize [BAC 5019, Mil-A-8625F]
This process utilizes chromic acid in an electrified bath (40 Volt for Type I and 22 Volt for Type IB) for building a thin, dense anodic coating on aluminum providing excellent corrosion resistance and fair paint pretreatment. Half of the coating penetrates into the substrate and the other half of the coating is a dimensional increase. It generates a .02 to.05 and up to a max of about 0.1 mil film thickness on parts and has a light grey color. The film is usually harder than the sulfuric acid anodizing film for the same thickness. CAA is the most heavily regulated process because of the hexavalent chrome issues in an agitated, electrified bath.

Environmental permitting, testing, monitoring, reporting and compliance issues will be a priority. This falls under the Aerospace National Emissions Standards for Hazardous Air Pollutants (ANESHAP). Mist eliminators at the tank and downstream hexavalent chrome air scrubbers are utilized on the process tank exhaust. Compliance is expensive and can be time consuming.
Because the chromic acid is inhibitive to the aluminum, it is not an issue like it is with sulfuric acid. CAA is specified for certain military applications and aerospace parts that have complex geometries, weldments, crevices, lapped or recessed surfaces that may trap electrolyte. It is the only one typically allowed to be used for assemblies for similar reasons. Assembles with dissimilar materials besides aluminum are typically prohibited without special masking or other steps. On assemblies with mixed aluminum alloys, CAA allows for an even anodic coating, where SAA would not. It is typically a good process for castings. CAA also preserves the metals fatigue strength with minimal dimensional changes. CAA will provide 336 hours in a 5% salts pray test.

At one time, CAA used to be inexpensive, but now you should use Sulfuric Acid Anodize unless you absolutely have to have Type I CAA because of environmental compliance. In many regards, the SAA is actually better and unfortunately it is getting harder to find shops that will even do Type I processing when you need it.

Type II and IIB Sulfuric Acid Anodize [BAC 5022, Mil-A-8625]
This process utilizes sulfuric acid in in an electrified bath for building an anodic coating on aluminum. It has many advantages, including the elimination of the use of hazardous chromium based coatings. The SAA conventional coatings are used for corrosion protection, decorative purposes, fatigue resistance and paint adhesion, while the hard coatings are for engineered purposes such as hardness and abrasion. The SAA baths will produce from 0.1 mil to 1 mil thickness for conventional coatings and around 4 mils for hard coatings.

However, this is not a suitable replacement for CAA on aircraft parts subjected to stress as they cannot have the corrosive nature of any residual sulfuric acid left on them. SAA parts do have a greater durability than Type I, but they should not be specified when the part geometries may trap electrolyte. The SAA also produces a much thicker oxide layer which has a reduced fatigue life than the thinner oxides specified in BAC 5019 (CAA) or 5632 (BSAA). The film growth rate occurs rapidly so the process must be controlled tightly to produce the desired film thickness by varying the current or amp density and time in the bath, but it can be successfully accomplished.

The Type IIB thing coat alternative can be specified as a non-chrome version of Type I CAA, if any of the above concerns are ruled out by the aerospace customer and specifically specified.
SAA Class 2 can have the pores of the anodic coating trap dye in them prior to final seal; for instance, for architectural coatings such as anodized window frames for buildings. Aerospace manufacturers may also specify class 2 to use the dye for identification or other purposes.

BSAA, Boric Sulfuric Acid Anodize (BAC 5632)2
This is a Boeing specified process developed as a chrome free replacement for CAA in the early 90s for their non critical fatigue sensitive parts that are almost always painted afterwards. Typically, it can be used in place of BAC 5019 (CAA) but Boeing drawings for various aircraft may have specifics called out for specific parts requiring only BAC 5019. If parts are to be painted they may allow BAC 5632 Class 5 (unsealed) as the paint adhesion is superior to CAA parts with the unsealed BSAA coating. BSAA is approved by Boeing in the place of CAA for all models of commercial airplanes where electrolyte entrapment is not a concern with over 15 years of experience with their commercial fleet.

The BSAA process is more energy efficient than chrome based processes (lower temperature, lower voltage (15 V), 20 minutes, 30 to 60% less time that CAA). A dilute chromate seal must be used in lieu of a hot DI seal and materials of construction will require 316L stainless steel for process materials but you will not need the hexavalent chrome scrubber on the anodize bath. You still have some chrome to deal with for the seal tank, but it is less than 75ppm hexchrome Cr(VI). Hot DI (Deionized) water seal works, but is not as robust. In dusty environments, sodium benzoate or benzoic acid can be used to help prevent the growth of fungus.

Summary (see Table 2 in September PDF)
Alternatives to chrome based anodic coatings are preferred due to the environmental hazards and costs associated with environmental regulatory compliance.
Boric Sulfuric Acid Anodize (BSAA) is utilized for Boeing Commercial Airplanes (BCA) as a non-chrome substitution for Chromic Acid Anodize or CAA. It is used for corrosion protection, paint adhesion and fatigue reduction. It cannot be used where there is concern with electrolyte being trapped in the parts or assemblies.

Sulfuric Acid Anodize (SAA) is utilized as a replacement for Chromic Acid Anodize as well, and are also utilized for corrosion protection, paint adhesion and fatigue reduction. It is used for dying of product for identification in the aerospace industry and for decorative purposes in architectural and other industries. It cannot be used where there is a concern with electrolyte being trapped in other parts or assemblies.

Chromic Acid Anodizing (CAA) is under heavy environmental regulation so there are subsequent compliance costs and activities required by the anodizing shop but it is still a required coating for certain military applications as well as on commercial airplane parts that may trap electrolyte (crevices, complex geometries, weldments, lapped or recessed parts). It is also used for multiple alloy assemblies. Suitable replacements have not been found for those types of applications.

1 Mil-A-8625F Military Specification Anodic Coatings for Aluminum and Aluminum Alloys
2 Nonchromate Conversion Coatings in use at Boeing. Osborne, Joseph H., Boeing Phantom Works - Seattle. Presenter at Hazmat Alternatives - Metal Finishing Workshop May 16 & 17, 2007. JHOB.pdf

About the Author:
Steve Anzelc, PE LEED AP, is a Senior Project Manager with Burns & McDonnell. He has 20 years of business experience, including design, project and proposal management on some of the largest anodizing, plating and paint finishing lines in the world. Burns & McDonnell is a Global Engineer-Architect-Environmental Consultant & Contractor with projects in 70 countries. You can reach him at or (816) 823-7083, USA CST.

Steve is the Winner of our 2008 article contest! Congratulations, Steve!