Can You Use Stainless Steel Screws in Aluminum? (Galvanic Corrosion)

Yes. Stainless steel screws are used in aluminum every day, on gutters, signage, HVAC ducting, boat hardware and sheet-metal enclosures, and in a dry or indoor joint there is usually nothing to worry about. The Australian Stainless Steel Development Association (ASSDA) says it plainly in its technical FAQ on galvanic corrosion: a stainless screw in aluminum is frequently used. The same FAQ adds the catch: corrosion of the aluminum immediately around the stainless is quite possible.

That is the whole risk in one sentence. When a conductive liquid (rain, condensation, salt spray, wash-down water) wets the joint, the two metals form a battery, and the aluminum is the side that gives up metal. The screw stays bright. The aluminum immediately around it corrodes. The fix is to break one of the three things the battery needs: keep the joint dry, put an insulator between the two metals, or seal the stainless so the wet area is small. The rest of this article is how to do that with the 18-8 and 316 stainless screws and rivets Fastenio sells.

Why does aluminum corrode next to a stainless screw?

Galvanic (dissimilar-metal) corrosion needs three things at once. ASSDA lists them: the joint must be wet with a conductive liquid, the metals must be in metal-to-metal contact, and the two metals must have sufficiently different potentials. Take away any one and the galvanic attack stops.

Stainless steel and aluminum are far enough apart for the third condition to be met. In the galvanic series (ASSDA's chart follows ASTM G82; MIL-STD-889 is the US military standard on galvanic compatibility), aluminum is the active metal of this pair and stainless steel the noble one. In a wet joint the active metal is the anode and corrodes, and the noble metal is the cathode and is protected. So the aluminum corrodes, and the stainless screw does not.

That is also why the reverse pairing is the bad one. ASSDA states that aluminum fasteners in stainless steel are not acceptable: the aluminum part is small, the stainless part is large, and the aluminum fastener corrodes quickly.

What makes it worse?

Three things, and they stack.

Salt and standing water. The liquid has to conduct. Rain water conducts a little; salt spray, de-icing salt and pool water conduct much more, so coastal and marine aluminum is the case to take seriously. Water that pools and sits (a horizontal joint, a cupped washer, a seam that never dries) keeps the circuit closed for longer than a surface that drains and dries.

Area ratio. This is the one most people get backwards. ASSDA's FAQ explains that a large wetted area of stainless steel in contact with a small wetted area of aluminum generates a large corrosion current concentrated on that small aluminum area. The favourable case is the opposite: a small cathode (the stainless screw) on a large anode (the aluminum sheet). ASSDA gives a rule of thumb that galvanic effects are not serious if the wetted area of the corroding metal is about ten times the wetted area of the noble metal, and the effect lessens as the ratio grows. A stainless screw in an aluminum panel is on the right side of that ratio. A stainless bracket, rail or plate bolted to a thin aluminum tab is on the wrong side.

Crevices. ASSDA notes that corrosion in salt water is worse where there is a crevice, and names a nut and bolt as the example. Under a screw head or washer, the water that gets in doesn't get out, so the joint stays wet after the surface has dried. That is why the damage shows immediately around the screw.

How do you isolate a stainless screw from aluminum?

Break the electrical contact, keep the water out, or both. ASSDA's methods:

1. Insulate the contact. Separate the metals with plastic strips, washers or sleeves. A nylon or similar isolating washer under the head and a sleeve through the hole keep the screw from touching the aluminum anywhere. Fastenio's flat washers are stainless steel and silicon bronze, so a metal washer spreads the load but doesn't isolate anything; the isolating washer itself comes from a plastics or electrical supplier.
2. Seal the joint. ASSDA says water can be kept out by design or by using adhesive sealants. A bead of sealant under the head and around the hole, applied so it stays bonded, keeps the electrolyte away from the contact.
3. Paint the right metal. This is the one people get wrong. ASSDA is specific that paint is not a reliable electrical insulator, especially under bolt heads, nuts or washers, and that painting the active metal (the aluminum) can cause deep holes at coating defects. If you coat one side, coat the stainless, and ASSDA recommends extending that coating 30 to 50 mm (about 1-1/4 to 2 in) beyond the joint.
4. Design the water out. Drain holes, joints that shed water, and no cupped pockets where a screw head sits in a puddle.

On a screw with a built-in washer face, such as a hex washer head sheet metal screw, the washer face is stainless too. It doesn't isolate; it only spreads load. Some hex washer head self-drilling screws in the catalog come with a bonded sealing washer; otherwise put sealant under the head if the joint will get wet.

Does 316 stainless steel help more than 18-8?

Not for the galvanic couple. Both 18-8 and 316 are austenitic stainless steels, and ASSDA's own example in the 316 vs. 18-8 hex nut article is that 304 bolts in a 316 panel show no galvanic effect at all, which tells you how close the two grades sit to each other. Moving from an 18-8 screw to a 316 screw doesn't move the screw meaningfully away from aluminum in the galvanic series, so it doesn't protect the aluminum.

What 316 does buy is resistance for the screw itself. Per the 18-8 vs. other grades guide, 18-8 is the default for general outdoor use and 316 is the grade for salt water, coastal and chemical exposure where 18-8 can pit. If the aluminum assembly lives near the coast, 316 is the grade the guide names for the screw. It is a corrosion choice for the screw, not a cure for the aluminum around it. Isolation and sealing do that job whichever grade you use.

One grade to think twice about is 410. It is the hardened, magnetic grade used for self-drilling screws into steel, and the catalog notes it has less corrosion resistance than 18-8 or 316. Fastenio's 18-8, 305 and 316 self-drilling screw pages say those grades are best for aluminum and thin sheet, with 410 for steel; the self-drilling screw point size and material post explains the split.

Will a stainless screw seize in tapped aluminum?

It can, and that is a separate problem from corrosion. Galling is adhesive wear between sliding threads, and aluminum is prone to it as well as stainless steel. A stainless machine screw run fast and dry into a tapped aluminum hole can bind before it seats. The prevention is the same as for stainless-on-stainless: slow down, keep the threads clean, and use a lubricant or anti-seize. The stainless steel galling article covers the warning signs and the fixes.

Anti-seize is not an electrical insulator, so don't count it as isolation.

When should you use an aluminum rivet instead?

When the two parts are both aluminum and the joint will be wet, the cleanest answer is to remove the third condition altogether: fasten aluminum with aluminum. Fastenio's rivet catalog describes its aluminum blind rivets as the choice for aluminum structures where a steel part would corrode the base metal. The all-aluminum dome head blind rivet has a 5052 aluminum body with an aluminum mandrel, so nothing dissimilar is left in the joint after setting.

Read the mandrel material before you buy. The catalog also carries an aluminum-body rivet with a steel mandrel, which the product page says gives a higher setting force and a stronger joint than an all-aluminum rivet. If the joint will be wet and you want nothing but aluminum in it, choose the all-aluminum rivet; each product page lists the body and mandrel material. For stainless sheet, the 18-8 and 316 blind rivets in the dome head range keep the whole joint stainless.

The sheet metal screw catalog lists silicon bronze, 18-8, 316, 410 and case-hardened steel, not aluminum, so for an aluminum-on-aluminum joint the same-metal fastener in the catalog is the rivet.

The bottom line

A stainless steel screw in aluminum is normal practice and is safe when the joint stays dry. When it will be wet, especially with salt, the aluminum around the screw is what corrodes, so keep water out of the contact, isolate the screw with a plastic washer and sleeve, seal under the head, and if you coat anything, coat the stainless side. Keep the stainless part small relative to the aluminum, never the other way round. Choosing 316 over 18-8 protects the screw in salt air but does nothing for the aluminum.

For the screws, start with sheet metal screws in 18-8 or 316, and for aluminum-to-aluminum joints that will get wet, use the all-aluminum blind rivets in the rivet catalog. A stainless flat washer spreads the load under the head; the isolating washer goes between it and the aluminum.