Boric-sulfuric acid anodizing
Specifying a qualified aluminum anodizing system, including pretreatment, sealing, downstream coating and dimensional acceptance.
Tags: processes, finishing, anodizing, aluminum, aerospace
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An assumed 0.002 mm growth per treated face changes an 8.000 mm pin to 8.004 mm and an 8.020 mm bore to 8.016 mm. The pair retains 0.012 mm clearance, down from 0.020 mm. Use the qualified BSAA route's measured growth and check the finished fit at assembly surfaces.
Process identity
Boric-sulfuric acid anodizing (BSAA) forms an anodic oxide using a boric/sulfuric-acid electrolyte. Specify the governing process and subsequent treatments.
Boeing's historical US4894127A, published 16 January 1990, describes boric/sulfuric anodizing developed as an alternative to chromic-acid processing. It discusses coating control, corrosion/paint performance and fatigue concerns. That disclosure establishes a documented process-development context; its experiments and claims are not current production acceptance requirements or a universal performance guarantee for every alloy. Boeing original technical disclosure
The DoD's SERDP/ESTCP technology overview identifies BSAA with BAC 5632 and gives Boeing structural applications as examples. It is an overview of alternatives, not the controlled BAC document or approval of a particular supplier, class or part. SERDP/ESTCP chromic-acid alternatives overview
Standard and authorization boundaries
The official ASSIST record checked here lists MIL-PRF-8625, active, with revision F, amendment 2, dated 23 November 2020. Its scope covers six types and two classes of electrolytically formed anodic coatings on aluminum and aluminum alloys for nonarchitectural use. The record identifies the publication; it does not establish that a named commercial process or specific component meets it. DLA ASSIST document record
The Aluminum Anodizers Council's legacy MIL-A-8625 overview describes Type IC as non-chromic-acid anodizing intended as an alternative to Types I/IB, and gives Boeing's boric-sulfuric process as an example. Use that as a terminology guide, with the actual drawing's required publication and revision controlling. Do not infer compliance with every Type IC requirement from “BSAA” on a certificate. AAC specification overview
Preserve the applicable OEM/customer process revision, type/class where required, alloy/temper, coating requirements and authorized processing route. If the controlled BAC text is unavailable, resolve it with the specification owner rather than constructing production limits from a patent or supplier summary. Chromic, boric-sulfuric, tartaric-sulfuric and conventional sulfuric processes must not be substituted solely because each is described as anodizing.
The complete treatment system matters
Anodizing, sealing and the organic coating or bonding operation answer different requirements. Boeing's technical disclosure discusses oxide treatment followed by sealing and/or organic coating, with different sequences for different purposes. Retain the specified sequence and condition instead of assuming every anodized surface must receive the same seal. Boeing disclosure, background and process discussion
A non-chromic anodizing bath does not establish that the entire finishing route is chromium-free. SERDP/ESTCP specifically notes that non-chromate anodized layers can receive chromate-containing sealing treatment. Check pretreatment, seal and downstream materials individually when a material restriction applies. SERDP/ESTCP treatment-system discussion
Do not equate a corrosion-test result with adhesive-bond durability, paint adhesion, fatigue performance or wear resistance. Each claim needs the applicable method, specimen/alloy, processing condition and acceptance criterion. A historical comparison on one test system is not authority to replace a qualified treatment on another part.
Geometry and production planning
Define significant surfaces, protected bores/threads, rack-contact locations and acceptable marks before processing. Establish dimensional effects through the approved process and representative geometry. A general anodizing growth estimate should not silently become a BSAA machining allowance for a precision fit.
The following is original dimensional arithmetic using deliberately assumed growth, not a measured BSAA result. Suppose an 8.000 mm pin and 8.020 mm bore initially have 0.020 mm diametral clearance. Assume outward growth into the available space of 0.002 mm per treated surface, on both parts:
finished pin diameter = 8.000 + 2 × 0.002 = 8.004 mm
finished bore diameter = 8.020 − 2 × 0.002 = 8.016 mm
finished clearance = 8.016 − 8.004 = 0.012 mm.
The clearance decreases by 0.008 mm, because two opposing surfaces on each component contribute. This example supplies no coating thickness, growth ratio, tolerance or processing recipe. Actual requirements may instead mask a feature, change a machining allowance or control the finished dimension; those choices need the product definition and qualified process.
Qualification and acceptance questions
| Definition | Question to resolve |
|---|---|
| Product and route | Which alloy/temper, part revision and controlled process revision apply? |
| Coating | What specified thickness/weight, appearance and coverage criteria apply? |
| Subsequent treatments | What seal, primer, paint or bonding condition is required? |
| Geometry | Which features are protected, where are contacts permitted, and what final dimensions apply? |
| Evidence | What lot controls, test specimens, methods and acceptance criteria demonstrate the actual requirement? |
These are proposed specification-review fields, not a normative test list. Keep process-control results separate from final-part inspection and identify which evidence represents the production lot. Retain the connection between coupons, alloy condition and processed parts rather than treating an unrelated certificate as proof of every part's performance.
Review changes to the alloy, preparation, racking, treatment sequence or downstream coating against the applicable qualification requirements. Reproducing the name of the electrolyte alone does not establish equivalence of the finished system.
Source scope and handoff
The Boeing disclosure is historical, with its publication date stated above; no patent legal-status conclusion is made here. ASSIST supplies the checked military-document status and revision. The AAC page is explicitly a legacy specification overview, and the SERDP/ESTCP page is a technology overview without a verified technical issue date here. Neither replaces the controlled OEM process specification.
A useful human or agent handoff records the full finishing callout, specification revisions, part/alloy state, protected features, treatment sequence, applicable qualification/lot evidence and final inspection. The dimensional example and review questions are original guidance.