BSAA as a Chromium-Free Alternative for Bonded Structures

Chromic acid anodizing (Type I, per MIL-A-8625) has long been the aerospace default for adhesive bonding surface preparation — its thin, fine-pored oxide layer gives adhesives an excellent mechanical key, and decades of bond durability data back it up. It also relies on hexavalent chromium, and environmental and worker-safety pressure has been pushing the industry toward a chromium-free alternative for years. That alternative is boric sulfuric acid anodizing — BSAA.

What BSAA is

BSAA replaces the chromic acid electrolyte with a boric acid and sulfuric acid solution, originally developed to give aerospace manufacturers a Cr6+-free bonding surface preparation without giving up bond performance. Like Type I, it produces a thin oxide layer with a pore structure suited to adhesive bonding rather than a thick, corrosion-resistant coating in its own right.

How it compares on bond performance

BSAA has been validated against chromic acid anodizing in wedge crack and fracture testing (methods consistent with ASTM D3762 and equivalent aerospace test protocols) and shown comparable bond durability in qualification testing across multiple aerospace programmes. It is not a drop-in replacement without validation, though — as with any bonding surface preparation, primer compatibility and bond performance need to be qualified for the specific adhesive system and application, the same discipline that would apply to a chromic anodize changeover.

Where this matters for your programme

• Aerospace primes and their supply chains are increasingly specifying BSAA over chromic anodizing on new programmes and requalification efforts, driven by REACH and similar regulatory pressure on hexavalent chromium.
• BSAA and Type I share the same low fatigue debit that makes them suitable for structural, fatigue-critical bonded assemblies — unlike Type II or Type III.
• A changeover from chromic to BSAA on an existing bonded assembly should go through the same bond qualification process as any other surface-prep change, not be treated as a like-for-like swap.

We run BSAA alongside chromic, Type II and hard anodizing, so a chromium-free bonding surface prep is available without needing a separate vendor relationship for it.