Corrosion, Dissimilar Metals and a Blend-Out Limit on a Coastal Operator
An agricultural aeroplane operating from a coastal strip is in for a 100-hour inspection. Findings: (1) white powdery deposits around cadmium-plated steel screws securing a stainless steel access panel to a 2024-T3 wing upper skin; (2) a 7075-T6 aluminium alloy flap hinge bracket with blistering and lifting of the surface on the short-transverse face; (3) graphite-based anti-seize compound found on the stainless fasteners. The structural repair manual permits local blending of corrosion on the bracket to a maximum of 10 percent of nominal section thickness. The bracket is 4.00 mm nominal and the deepest blend measured is 0.45 mm. The operator proposes to clean the area, refit the same fasteners with fresh anti-seize, and release the aircraft.
1. Using the galvanic series, explain which metal is being consumed at the access panel and why the graphite compound makes the situation worse. State the correct installation practice.
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Galvanic series (simplified, anodic to cathodic):
Magnesium - Zinc - Aluminium alloys - Cadmium - Mild steel - Cast iron - Stainless steel (passive) - Lead - Tin - Brass - Copper - Bronze - Graphite - Gold
Which metal is consumed
The aluminium alloy is the anodic member of every couple present, so the aluminium is the one that corrodes. The white powder is aluminium corrosion product in the region around the fastener. Two factors make it aggressive:
- Unfavourable area ratio. The cathode (panel and screw head) is large and the anode (the small area of skin at the hole) is small, so the anodic current density is high and pitting at the hole is rapid.
- Cadmium versus stainless. Cadmium is close to aluminium in the series and is a relatively benign partner; the damaging couple here is stainless steel (and graphite) against aluminium.
Why graphite is the worst choice
Graphite sits at the extreme cathodic end, near gold. Applied in the joint it becomes a large-area cathode in intimate contact with the aluminium, so it accelerates attack and drives deep pitting. A graphite-loaded grease on an aluminium structure is a design error, not a convenience.
Correct practice: remove the fasteners, clean and inspect the holes for corrosion, elongation and cracking; treat the bare aluminium with the specified surface treatment and primer; use an approved non-graphite corrosion-inhibiting compound; wet-install the fasteners with sealant to exclude electrolyte; where the manual permits, use a compatible fastener material. Re-inspect at a shortened interval on a coastal operation.
2. Check the blend-out against the SRM limit and decide whether the bracket can be released under the SRM.
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Calculation
Allowable blend = 10 percent of nominal thickness = 0.10 x 4.00 mm = 0.40 mm
Measured deepest blend = 0.45 mm
Percentage = 0.45 / 4.00 x 100 = 11.25 percent
Verdict
0.45 mm exceeds the 0.40 mm permitted by 0.05 mm, so the blend is outside the SRM. There is no discretion here: 11.25 percent is not 10 percent, and a blend of this size on a hinge bracket, which is a fatigue-critical attachment, cannot be signed off as a standard repair.
Action
Stop work, raise an engineering query to the type certificate holder or an approved design organisation for a repair scheme or replacement, and record the blend depth. If the part is replaced, no disposition is needed. Note also that any blend must be re-protected with the specified treatment and blended out with a smooth radius so no stress raiser is left.
3. Explain why the 7075-T6 bracket blistered on the short-transverse face, and how that affects reliance on the measured blend depth.
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Why this alloy and this face
7075-T6 is a copper- and zinc-bearing high-strength alloy with poor resistance to exfoliation and intergranular corrosion. Extruded or forged sections have elongated grain structure: the short-transverse direction is across the grains.
Corrosion attacking a grain boundary runs fast along the boundary and is stopped at the next boundary, so the attack spreads laterally as thin planes and lifts the surface in layers. That is exactly what blistering on the short-transverse face indicates. It is not cosmetic surface corrosion.
Consequence for the blend
Intergranular attack follows grain boundaries ahead of what is visible at the surface. Blending until the pit looks clean can therefore leave a tail of attacked grain boundary below the blended surface, and the material removed is not a reliable measure of the depth of damage. Removing 0.45 mm may look like a repair while the section is still degraded, and the blend itself thins a fatigue-critical part.
Judgement
The correct route is replacement or an approved repair with the true extent of attack established (strip and inspect, or penetrant and eddy current as directed). The operator's proposal to clean, refit the same fasteners and release must be rejected on both grounds: the blend is outside the SRM, and the corrosion type makes the measured depth an unreliable indicator.