Galley Floor Beams: Pitting at Stainless Brackets on a Coastal Aircraft

Three mechanisms overlap here.

• Galvanic corrosion. 2024-T3 is strongly anodic to stainless steel (and to cadmium-plated steel once the plating is breached). Wash water plus detergent residue is an excellent electrolyte. The cathode area (bracket and screw) is large compared with the anode area (the small ring of aluminium around each fastener), so the attack is intense and localised exactly where it is seen.
• Crevice / oxygen-concentration cell. The cracked, lifted sealant and the bracket-to-panel gap hold stagnant water while excluding oxygen. The shielded aluminium becomes anodic to the surrounding aerated metal, so corrosion starts at the crevice mouth and creeps under the bracket - often the hidden face is worse than what is visible.
• Cladding effects. The pure aluminium cladding is anodic to the 2024 core, so once the cladding is perforated the core corrodes quickly and the cladding lifts off, which is the blistering described.

The white-grey powder is aluminium hydroxide/oxide; the blue-green tinge is typical of copper-bearing corrosion products (2024 contains roughly 4% copper) and dissolved plating metals. Daily washing with detergent in a humid coastal environment supplies electrolyte, warmth and chloride continuously - a textbook aggressive combination.

Not acceptable as a mechanic's decision.

• 0.35 mm in a 1.6 mm sheet is about 22% of nominal thickness. The usual blend-out allowance (where approved data permits blending at all) is around 10%, i.e. about 0.16 mm here - and that allowance applies only where the approved data says it does.
• The clad layer is gone over a much wider area than the pit; the core now has no sacrificial protection, so any residual corrosion will keep running under the paint.
• The depth was measured after blending. Blending only deepens the excavation; the depth must be established before material is removed, using a pit gauge/profilometer, and the extent mapped by eddy current or ultrasonic inspection to find intergranular attack under sound-looking paint.
• Corrosion that has penetrated cladding into the core is normally a repair/replacement decision (doubler, splice or new section), not a blend-and-paint.

Correct action: stop the corrosion (remove products, neutralise, clean, apply temporary protective coating), then raise the finding for engineering disposition against the current SRM, and record it in the maintenance system. No release on 'blend and paint' without approved data.

Break the couple

• Replace cadmium-plated steel screws with compatible or properly protected hardware, and wet-install (sealant on the fastener and in the hole) so no electrolyte path exists.
• Use insulating washers/bushings at the bracket interface, or change the bracket material (titanium or a suitable aluminium alloy) so the couple disappears.
• Keep drain paths clear; never let the bracket form a water trap.

Restore the barrier

• Correct surface preparation, conversion coating and primer on bare metal, then the specified polysulphide/polyurethane sealant over a clean, dry surface with a fillet covering fastener heads and bracket edges.
• Add sealant condition to the scheduled inspection for this zone, so a cracked seal is found before the beam is attacked.

Control the electrolyte

• Rinse with potable water and dry; avoid strong alkaline cleaners and detergent residue on aluminium.
• Extend the inspection to the hidden faces and adjacent structure - crevice attack under a bracket is often far worse on the face you cannot see.