Guide
Galvanic corrosion between aluminum and stainless steel: how do you read a galvanic corrosion chart and prevent it?
Galvanic corrosion is the accelerated attack on one metal that happens when it is electrically connected to a more noble metal and both are wetted by the same electrolyte. In an aluminum housing held together with stainless steel screws, the aluminum is the metal that pays: it becomes the anode, and it corrodes faster than it would on its own, mostly right around the fastener.
By Saola Manufacturing engineering team · Published · Updated
A galvanic corrosion chart (the galvanic series) tells you which metal in a pair will be the anode. It does not tell you how fast that anode will corrode. The speed comes from three other things you control on the drawing: the ratio of exposed cathode area to anode area, how wet and salty the joint gets, and whether anything breaks the electrical or electrolyte path. This guide covers all four, with aluminum and stainless steel as the main example and the other pairs that show up on industrial parts.
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How do you read a galvanic corrosion chart?
Find both metals on the series: the one nearer the active (negative) end becomes the anode and corrodes faster, and the one nearer the noble (positive) end is protected. The wider the gap between them, the stronger the push, but the chart alone cannot predict a corrosion rate.
ASTM G82 is the guide for building and using such a series: metals are ranked by the corrosion potential they actually show in a stated electrolyte, and upon coupling, the more active metal tends to corrode more while the more noble one tends to corrode less. The series most charts reproduce was measured in flowing seawater against a saturated calomel reference electrode. Each metal is shown as a range rather than a single value, because its surface condition, temperature, aeration and flow all move the reading.
Two habits keep the chart honest. First, use a series measured in an electrolyte that resembles your service: seawater is a reasonable stand-in for coastal splash and road salt, much less so for a dry indoor enclosure. Second, read stainless steels twice. A passive stainless surface sits near the noble end, but the same alloy inside a crevice or in stagnant, poorly aerated water can lose its film and behave as a far more active metal, as the table note shows.
| Metal or alloy (active end first) | Potential range, V vs SCE | Coupled to aluminum, the aluminum is |
|---|---|---|
| Magnesium | −1.60 to −1.63 | The cathode (magnesium corrodes) |
| Zinc | −0.98 to −1.03 | The cathode (zinc corrodes, mildly) |
| Aluminum alloys | −0.70 to −0.90 | Same family; small gaps between alloys |
| Cadmium | −0.70 to −0.76 | Close; low drive |
| Carbon steel | −0.60 to −0.70 | The anode, moderate drive |
| Yellow and naval brass | −0.30 to −0.40 | The anode, strong drive |
| Copper | −0.28 to −0.36 | The anode, strong drive |
| 400 series stainless steel | −0.20 to −0.35 | The anode, strong drive |
| 17-4 PH stainless steel | −0.10 to −0.20 | The anode, strong drive |
| 300 series stainless steel, passive | 0.00 to −0.15 | The anode, strong drive |
| 300 series stainless steel, in crevices or stagnant water | near −0.5 | The anode, smaller drive, but the stainless may pit too |
| Titanium and its alloys | +0.06 to −0.05 | The anode, strong drive |
| Graphite (and carbon-fiber composites) | +0.20 to +0.30 | The anode, strongest drive |
Why does aluminum corrode next to stainless steel?
Aluminum sits far toward the active end of the series and passive 300 series stainless sits near the noble end, so whenever water bridges the two, current flows and the aluminum dissolves. Remove the water, the metal-to-metal contact or the potential difference, and the cell stops.
The University of Delaware Sea Grant note works this exact pair: an aluminum alloy averaging about −0.8 V coupled to 300 series stainless averaging about −0.07 V gives a driving difference of roughly 0.73 V, with aluminum as the anode (Dexter, Galvanic Corrosion). The U.S. Naval Academy course notes list five conditions for a corrosion cell (an anode, a cathode, an electrolyte, an electrical connection and a potential difference) and note that eliminating any one of them stops galvanic corrosion (USNA EN380, chapter 2). That list is the whole design toolbox: every fix later in this guide removes one of those conditions or weakens it.
Aluminum alloy matters at the margin. Copper-bearing alloys such as 2011, 2024 and 7075, and die-cast alloys such as ADC12 or A380, are less corrosion resistant than 6061 to begin with, so the same stainless fastener does visibly more damage in them. ADC12 typically carries 1.5 to 3.5% copper per JIS H 5302, which is part of why cast housings are usually painted or powder coated rather than left bare. Stainless grade matters less than designers expect: 304 and 316 are both far more noble than aluminum, and choosing 316 improves the fastener’s own crevice resistance in chlorides, not the aluminum’s.
Damage shows up as white, powdery aluminum oxide around the fastener head, pitting in the bore threads and, in the worst cases, a seized screw whose thread in the aluminum has turned to corrosion product. Because the attack concentrates where the two metals meet, it often weakens a joint long before the rest of the part looks corroded.
Why does the area ratio matter as much as the voltage gap?
The anode has to supply all the current the cathode can consume, so a small anode wired to a large cathode corrodes fast and deep. A small stainless screw in a large aluminum plate is usually tolerable; an aluminum rivet in a stainless sheet is not.
The Delaware note illustrates this with two couples of the same metals: an aluminum rivet in a stainless plate corrodes severely, while a stainless rivet in an aluminum plate does much less harm, “even though the potential difference is the same in each case” (Dexter). The NIH Office of Research Facilities makes the same point in its design guidance: a small anode coupled to a large cathode significantly increases corrosion current density (NIH ORF Technical News Bulletin, June 2026).
Three practical consequences follow. Make the small, critical part from the more noble metal, so stainless fasteners in an aluminum housing are the right way round and aluminum fasteners in a stainless frame are the wrong way round. Count the exposed area, not the part size: a stainless bracket bolted under an aluminum cover counts as a large cathode even though the fasteners are small. And never paint only the anode. A scratch in paint on the aluminum leaves a tiny bare anode facing the whole uncoated stainless part, which concentrates the attack in the scratch. If only one member can be coated, coat the cathode (the stainless) or coat both.
How much does the environment change the risk?
Without an electrolyte there is no galvanic cell, so a dry indoor enclosure with stainless screws in aluminum rarely has a problem. Condensation, salt and trapped water turn the same joint into a working cell, and chlorides make it worse.
Think about where water collects rather than where the part is installed. Outdoor equipment in a coastal or road-salt environment, washdown areas in food and medical equipment, rooftop and mechanical-room hardware, and anything that cycles through condensation all keep a film of conductive moisture on the joint. The NIH bulletin lists five factors that drive the probability of failure: potential difference, anode-to-cathode area ratio, electrolyte conductivity, coating condition and environmental aggressivity (NIH ORF).
Geometry decides how long water stays. A blind tapped hole that faces up, a washer clamped against a painted face and a lap joint without sealant all hold water in a crevice, where oxygen is depleted and the electrolyte grows more aggressive. The same crevice is where passive stainless can turn active, so a joint that traps water can hurt both metals. Drain holes, fasteners entering from below and sealant that fills the joint instead of bridging it are cheap design changes that remove the electrolyte from the equation.
Which design and finish fixes actually work for aluminum and stainless?
In order of reliability: use compatible metals, break the electrical path, keep water out of the joint, then coat. The NIH mitigation hierarchy also puts compatible materials first, electrical isolation second and barrier coatings after them, and notes that coatings need ongoing inspection and maintenance to stay effective.
- Choose a closer fastener metal. Aluminum fasteners in aluminum remove the couple entirely. Zinc-plated steel sits close to aluminum on the series, and the zinc corrodes before either base metal, which is why plated steel is a common choice for aluminum indoors; it trades galvanic risk for the plating’s own life in a wet environment.
- Isolate. Nylon or other non-absorbent plastic washers under the head and a sleeve through the clearance hole break metal contact. The Delaware note warns against fiber or paper washers that absorb water. Isolation only works if the fastener cannot touch the aluminum anywhere, including the threads, so it suits through-bolted joints far better than screws tapped straight into aluminum.
- Install fasteners wet. A corrosion-inhibiting sealing compound (MIL-PRF-81733 is one common military specification) or a barrier primer applied to the threads and under the head both insulates and keeps water out of the crevice.
- Anodize the aluminum. An anodic film is electrically insulating and slows the cell, but threads tapped after anodizing, scratches from the screwdriver and the bearing face under the head cut through it. Treat anodizing as a contributor, not as isolation. ADC12 is not a good anodizing alloy, so ADC12 die-cast housings are normally powder coated instead.
- Coat the cathode. Painting the stainless (or both parts) shrinks the exposed cathode area, which is the safe direction for a coating defect.
- Use thread inserts with care. A stainless helical-coil insert in aluminum is a strong, common design, but it is a stainless-aluminum couple in a crevice; in wet service, specify inserts installed with sealant or with a protective coating.
Passivating the stainless part cleans free iron off its surface and improves its own corrosion resistance, but it does not make stainless less noble to aluminum and does not reduce galvanic attack on the aluminum. The passivation guide covers what that treatment does do.
Which other metal pairs need the same attention?
The pairs that cause most field failures put a reactive metal (aluminum, zinc, zinc coatings or carbon steel) against copper alloys, stainless steel, titanium or carbon fiber. Pairs within one family, such as two aluminum alloys or two copper alloys, are rarely a design problem.
Aluminum against copper or brass is as bad as aluminum against stainless and adds a twist: copper ions washed off the brass can plate onto aluminum downstream and start pits away from the joint. Aluminum against carbon fiber composite or graphite-filled gaskets has the widest gap on the chart. Zinc die castings and zinc-plated steel against brass or stainless lose their zinc first; the NIH bulletin describes a potential failure in which galvanized steel water piping fails early because of yellow brass ball valves, with the zinc coating acting as the anode (NIH ORF). Carbon steel framing fastened with stainless hardware is listed there as an amplified risk too; with small stainless fasteners in large steel members the area ratio is favorable, but the reverse arrangement is not.
When two metals are close on the chart, their ranges overlap and either one can become the anode depending on conditions, so the chart cannot settle it. For a couple that matters, test the actual joint: ASTM G71 is the guide for conducting and evaluating galvanic corrosion tests of two dissimilar metals in electrical contact in an electrolyte. Military programs classify compatibility and protection requirements for dissimilar couples under MIL-STD-889, which is worth naming on the drawing if your customer already works to it.
What should the drawing and RFQ say for a dissimilar-metal joint?
Name both metals by alloy, the service environment, the finish on each part and the isolation or sealing method at the joint. A supplier can only protect a joint the drawing describes; “stainless hardware” in a parts list is not a corrosion design.
- Materials by grade. For example, “HOUSING: ADC12 DIE CASTING” and “SCREWS: 316 STAINLESS, ISO 3506 A4-70,” never just “aluminum” and “SS.”
- Environment. One line such as “OUTDOOR, COASTAL, CONDENSING” tells everyone downstream which joints need protection.
- Joint note. For example, “INSTALL FASTENERS WET WITH CORROSION-INHIBITING SEALANT; NYLON WASHER UNDER EACH HEAD.”
- Finish per part, with masking. State which faces stay bare for grounding or sealing, since a bare contact face is also a galvanic contact face.
- Verification. If corrosion performance matters, ask for a salt-spray test on the assembled joint rather than on loose coupons of each finish, and agree the standard, duration and pass rule in the quote.
Saola manages surface finishing through partner plants from a fixed menu: anodizing on aluminum, zinc plating, RoHS-compliant blue trivalent chromate, powder coating, electroplating, gold plating, pad printing and oil-dip rust protection. Specialty conversion coatings and any finish outside that menu are not offered today, so chem film on aluminum and passivation of stainless are not on the list. Salt-spray results are supplied on request, with the test standard and duration confirmed in the quote. ADC12 aluminum housings are die cast by partner foundries (see die casting), with tapped holes and datum faces added by CNC milling at our lead CNC plant; the quote names the production site and origin. To have a dissimilar-metal assembly quoted, send the drawing with the joint note and environment through the quote form; the RFQ guide lists what else the package should carry.
Frequently asked
Is it safe to use stainless steel screws in aluminum?
Indoors and dry, usually yes, because there is no electrolyte and the area ratio (small stainless screw, large aluminum part) is the favorable way round. Outdoors, in washdown or in salt, install the screws with sealant, isolate them where the joint allows, and keep water from pooling at the head.
Does anodizing stop galvanic corrosion?
It helps, because the anodic film is a poor conductor, but it is not reliable isolation. Threads tapped after anodizing, fastener bearing faces and scratches expose bare aluminum right where the stainless touches it. Combine anodizing with sealant or washers in wet service.
Is 316 stainless better than 304 against aluminum?
Not for the aluminum. Both are far toward the noble end of the chart, so the aluminum sees much the same drive. 316 resists pitting and crevice corrosion of the fastener itself better in chloride environments, which is a reason to choose it, but not a galvanic fix.
Why do different galvanic corrosion charts disagree?
They were measured in different electrolytes and against different reference electrodes, and some are anodic-index tables built for specific industries rather than measured series. Use the ranking, not the third decimal place, and check that the chart’s electrolyte resembles your service before trusting a close call.
Sources
- ASTM G82-98(2021)e1, Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance (scope and significance), ASTM International, 2021
- ASTM G71-81(2024), Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes (scope), ASTM International, 2024
- Galvanic Corrosion, Marine Advisory Service Note, S. C. Dexter, University of Delaware Sea Grant College Program, 1999
- EN380 Naval Materials Science and Engineering Course Notes, Chapter 2: Corrosion and Electromotive Potential, U.S. Naval Academy, accessed October 2026
- Galvanic Corrosion in Design and Construction, Technical News Bulletin Issue 161, National Institutes of Health, Office of Research Facilities, June 2026