1. Galvanic (Dissimilar‑Metal) Corrosion
- Why it happens: Stainless steel is nobler than aluminum on the galvanic series. If the two metals are in electrical contact and there’s an electrolyte present (water, especially salt water), aluminum will corrode preferentially where it contacts the stainless screw.
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How to prevent it:
- Barrier coatings – apply a thin film of anti‑seize compound or zinc chromate primer to the screw shank or into the tapped hole before assembly.
- Insulating washers or bushings – nylon or fiber washers isolate the two metals.
- Sealants – a bead of silicon or polyurethane around the head and thread entry keeps moisture out.
⚠️ If your assembly will sit outdoors, or in a humid/marine environment, you must use both a barrier coating and a sealant/washer to stop moisture migration.
2. Thread Engagement & Strip‑Out Strength
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Aluminum is soft compared to steel; over‑torquing can strip threads.
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Best practice:
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Pre‑tap the hole to the correct size (e.g. for a ¼‑20 screw, use a #7 drill for 18‑8 stainless).
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Engage at least 1.5× to 2× the screw’s diameter in aluminum:
- ¼″ screw → .375″ to .5″ thread depth.
- Torque to the recommended value for steel‑in‑aluminum, which is roughly 20–30% below that for steel‑in‑steel.
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Example: a ¼‑20 A2 stainless cap‑screw in 6061‑T6 aluminum typically torques to about 60–70 in‑lb rather than the ~90 in‑lb you’d use in steel.
3. Choosing the Right Stainless Grade
- A2 (304) Stainless is common and cost‑effective for indoor use.
- A4 (316) Stainless contains molybdenum for better corrosion resistance in marine or chemical environments.
- If you’re really concerned about head galling when you install, consider screws with a PTFE (Teflon) dry‑film lubricant.
4. Alternatives & Add‑Ons
- Aluminum screws or fasteners: completely eliminates dissimilar‑metal issues, but you lose the strength and corrosion resistance of stainless.
- Thread inserts (e.g., Heli‑Coil®): install a stainless‑steel insert into the aluminum; now you have a steel‑steel interface inside the hole.
- Self‑clinching nuts or rivet nuts: embed a steel nut in the aluminum sheet to receive the stainless screw.
