# Phosphoric acid anodizing

Aluminum surface preparation for structural adhesive bonding, with oxide, primer, handling, geometry, and qualification boundaries.

Reviewed: 2026-11-01

Tags: processes, finishing, anodizing, adhesive-bonding

A hypothetical **5,000 N** lap-joint failure over **25.0 mm width × 12.5 mm overlap** gives **16.0 MPa apparent average shear**. Doubling the overlap at the same load gives **8.0 MPa**. Compare matched specimens, including peel, failure mode and exposure. Qualify the complete bond system for service.

ASTM D3933-98(2025) addresses aluminum preparation for structural adhesive bonding. Qualify the anodic surface as part of the complete bond system and assess durability after exposure. [ASTM D3933-98(2025)](https://store.astm.org/standards/d3933)

## Process identity and specification

Specify the substrate alloy, temper, clad/bare condition, part revision, bonding surfaces, and complete pretreatment/primer/adhesive system. “Anodize before bonding” leaves important choices unresolved: different anodizing chemistries, sealing routes, and downstream materials are not automatically interchangeable.

ASTM identifies D3933-98(2025) as active, with a catalog update of **February 27, 2025**. The designation records the 1998 base issue and 2025 reapproval; it is not a claim that every processing instruction was newly developed in 2025. Its scope says that preparation used for adhesive qualification and quality-control testing should be agreed between manufacturer and user. [ASTM, current guide and scope](https://store.astm.org/standards/d3933)

SAE **ARP1524A**, reaffirmed **July 19, 2013**, describes a processing system for surface preparation and priming of aluminum alloy parts for durable structural bonding. SAE separately identifies **ARP1575C**, stabilized **September 20, 2017**, as a hand-applied, non-tank PAA process. Those are distinct implementation scopes; a local repair procedure should not inherit a tank-line instruction without the applicable approval. Use the contractual specification issue and process documentation. [SAE ARP1524A](https://saemobilus.sae.org/standards/arp1524a-surface-preparation-priming-aluminum-alloy-parts-high-durability-structural-adhesive-bonding), [SAE ARP1575C](https://saemobilus.sae.org/standards/arp1575c-surface-preparation-priming-aluminum-alloy-parts-high-durability-structural-adhesive-bonding-hand-applied-phosphoric-acid-anodizing)

## Oxide morphology, sealing, and primer

The Aluminum Anodizers Council's **Technical Bulletin 1-08**, issued **February 2008** and carrying a 2013 copyright, describes PAA as a thin, unsealed oxide used for adhesive bonding. This distinguishes the intended bonding surface from a routine sealed decorative finish. Do not add a conventional hot-water seal to a specified PAA bond preparation or substitute a sealed sulfuric coating simply because both surfaces are anodized. Any alternative post-treatment needs qualification as part of the actual bonding system. [AAC, guide to adhesives for anodizing applications](https://cdn.ymaws.com/anodizing.site-ym.com/resource/resmgr/Docs/Technical_Bulletin_1-08_Prop.pdf)

A **1997 University of Washington/Boeing microscopy study** specifically investigated how PAA parameter changes affect oxide morphology and adhesive-primer penetration. Its scope links the porous oxide and primer interaction to bond performance; it does not establish a universal pore dimension, film thickness, or recipe. A visually similar surface cannot by itself demonstrate the same microscopic condition or primer access. [Cole and colleagues, original microscopy study](https://doi.org/10.1017/S1431927600010771)

Primer is a separate controlled material and application stage, not another name for the anodic oxide. Establish primer identity, compatibility with the adhesive, application coverage, film requirement, cure/dry state, storage, and permitted time before bonding. Do not treat a primer change as cosmetic or assume it compensates for contamination or an unqualified oxide.

For a named supplier example, **3M AF 163-2 technical datasheet, November 2009**, identifies PAA under 3M method C-2780 among preferred aluminum preparations for durable joints in moist environments. That sheet also contains other preparation routes and representative test results. It does not qualify every combination of PAA, primer, aluminum alloy, film variant, and cure schedule. Retain the exact tested/approved system and distinguish typical product data from acceptance values. [3M AF 163-2 datasheet, surface preparation](https://multimedia.3m.com/mws/media/282041O/3m-scotch-weld-structural-adhesive-film-af-163-2-af-163-3.pdf?fn=TDS_AF163-2_R1.pdf)

## Preparation, access, and geometry

Review incoming contamination, machining fluids, oxide/scale, previous coatings, and required cleaning/deoxidizing steps. Specify masking, electrical contact locations, significant surfaces, and which recesses or internal regions must receive treatment. Drainage, trapped air or solution, cathode arrangement, and local electrical access need review with the processor. An accessible exterior witness surface does not prove equivalent treatment in a deep pocket or narrow passage.

Maintain the qualified wet-processing sequence, rinsing, drying, and contamination controls. No acid concentration, voltage, temperature, or dwell schedule is prescribed here. A bath formulation alone does not define pretreatment condition, electrical arrangement, oxide acceptance, or subsequent bonding capability.

Confirm dimensional requirements after the complete route. Cleaning/etching, oxide formation, primer, and adhesive each affect the finished assembly differently. A thin PAA film does not justify ignoring cumulative effects on a close fit, a bondline gap, a mask boundary, or an edge. Resolve prefit before surface preparation where possible so later trial assembly does not damage or contaminate the bond surface.

## Handling and time windows are separate controls

Record preparation completion, primer application, primer cure, adhesive application, and adhesive cure as separate events. Apply the actual material/process limits and permitted storage environment at each stage. Moving a part into a bag does not establish that it remains acceptable indefinitely, and opening a new work order does not restart a process clock.

As a bounded example, **NASA/JSC PRC-1001 Rev. G**, whose cover states **December 2019**, applies to space-flight-related hardware fabricated under JSC authority. It references D3933 for aluminum PAA, prohibits solvent wiping of unprimed prepared surfaces before priming/bonding, and states a **12-hour maximum from surface priming to adhesive application** for metallic parts. Its primer section separately directs preparation-to-primer time, thickness, cure, and subsequent timing to the applicable material specification or manufacturer guidance. These are that document's requirements, not a universal 12-hour anodize-to-primer limit. [NASA PRC-1001 Rev. G, applicability and sections 6.6–6.7](https://www.nasa.gov/wp-content/uploads/2023/03/prc-1001-current.pdf)

If a delay, handling incident, or contamination occurs, follow the approved disposition/reprocessing route. Do not improvise a solvent wipe, abrasion, sealing step, or extra anodizing cycle that changes the qualified surface.

## Qualification and original calculation

Evaluate durability as well as initial bond strength using the approved material/process combination and exposure requirements. Include the actual alloy/condition, surface route, primer, adhesive, bondline, cure, specimen geometry, and failure mode in the evidence. A process certification does not by itself supply a structural joint allowable; nor does a high initial load prove moisture durability.

**Original illustrative calculation:** assume a lap-joint specimen has a **25.0 mm width**, **12.5 mm overlap**, and a hypothetical failure load of **5,000 N**.

`Nominal overlap area = 25.0 × 12.5 = 312.5 mm²`

`Apparent average shear stress = 5,000 / 312.5 = 16.0 N/mm² = 16.0 MPa`.

If another specimen has twice the overlap length at the same width and fails at the same load, its calculated average is **8.0 MPa**. This does not show that its PAA surface was half as effective: overlap geometry, peel and stress concentrations, substrate deformation, and failure mode affect the comparison. Neither number is an adhesive design allowable, a uniform local stress, or a qualification threshold. Use the specified test method and matched specimens for a valid comparison.

## Useful production handoff

Provide the part revision and substrate condition, bonded/masked/contact regions, governing process issue, tank or approved local route, exact primer/adhesive, final geometry, and approved timing/storage conditions. Include oxide/process acceptance, representative specimens, bonding/cure records, required environmental testing, and failure-mode reporting. Keep surface-preparation acceptance distinct from final bond acceptance and any required nondestructive inspection.

## Related reference

[Surface-finishing processes](index.md)