Phosphoric Acid Anodizing (PAA) was developed in the late 1960s and early 1970s to improve the performance of bonded primary structures. Bonds formed with PAA-treated adherends exhibit superior durability during exposure to humid environments compared to those formed with FPL-treated adherends, especially when epoxy adhesives are used. The PAA procedure has consequently become the treatment of choice in the United States for critical aerospace applications. ASTM D3933-98(2017): Standard Guide For Preparation Of Aluminum Surfaces For Structural Adhesives Bonding (Phosphoric Acid Anodizing) provides requirements for phosphoric acid anodizing of aluminum and its alloys for structural adhesive bonding.
What Is Phosphoric Acid Anodizing (PAA) of Aluminum?
Phosphoric Acid Anodizing (PAA) is an electrolytic passivation process that changes the microscopic texture of the surface of aluminum. By providing a superior porous surface for bonding to composites, PAA serves as an excellent pretreatment for adhesive bonding or painting of aluminum parts, especially in high-humidity environments.
What Is ASTM D3933-98(2017)?
ASTM D3933-98(2017) describes the requirements for phosphoric acid anodizing of aluminum and its alloys for structural adhesive bonding. This standard describes one method on how to properly prepare aluminum surfaces can be obtained. The procedure included in ASTM D3933-98(2017) is based on the commercial practice of numerous agencies and organizations. The surface preparation of metal systems used for qualification and quality-control testing of the adhesive should be agreed upon between the manufacturer and the user.
What Adhesive Is Used in Aerospace Industry?
Anaerobics, structural acrylics, epoxies, cyanoacrylates are all common adhesives used in the aerospace industry as well as adhesive tapes and films. The properties of adhesives are tailored to the needs of the aerospace industry.
- Anaerobic adhesives: are used for threadlocking, retaining, thread sealing, and form in place gasketing. Most of these applications are in the engine compartment, however threadlockers are important throughout the aircraft to prevent loosening of components from vibration.
- Structural acrylics: are used throughout in diverse applications such as bonding magnets into the electric motors and to attach harness clips. An adhesive is considered structural if it can withstand 1000 psi of shear force.
- Epoxies: are the dominant aerospace adhesives as they are widely used for their strength and low density—a huge factor driving the widespread use of adhesives in the aerospace industry. Epoxies are used as honeycomb edge fillers for bonding composites. Their reduced weight helps keep the aircraft as light as possible.
- Cyanoacrylates: are not only used to permanently bond various items, but they are also used as a processing aid to instantly hold parts that are being bonded with slower curing adhesives. They are useful as quick fixers for damaged interior coverings.
To preserve the structural integrity of critical aircraft components, aircraft parts require high quality adhesives. As a result, manufacturers need adhesives that ensure strong bonds between composite sub-components with excellent chemical resistance and extraordinary mechanical performance. Compliance to specifications like ASTM D3933-98(2017) helps provide product reliability.
ASTM D3933-98(2017): Standard Guide For Preparation Of Aluminum Surfaces For Structural Adhesives Bonding (Phosphoric Acid Anodizing) is available on the ANSI Webstore.