Galvanic Corrosion: How Dissimilar Metals Drive Attack: and How to Prevent It

Key Takeaways on Galvanic Corrosion

• Galvanic corrosion needs three things at once: dissimilar metals, an electrolyte, and metallic contact between them. Take away any one of those and the attack stops cold.

• Which metal corrodes, the anode, comes down to its position in the galvanic series. How fast it corrodes is a different question, governed by the ratio of cathode area to anode area.

• Watch for a small anode paired with a large cathode. That's the setup that does real damage fast, as with carbon-steel bolts fastened into a big stainless plate.

• Stainless grades like 304, 316, and 904L, along with nickel alloys such as Incoloy 825, sit on the cathodic, noble end of the scale. They hold up well on their own, but couple them with carbon steel or aluminum and they'll accelerate the other metal's corrosion.
• A high PREN does not help here, PREN ranks pitting and crevice resistance, not galvanic polarity.

What Is Galvanic Corrosion?

Galvanic corrosion, sometimes called dissimilar-metal or bimetallic corrosion, is the accelerated loss of metal that occurs when two different metals form an electrical couple in the presence of an electrolyte. One metal becomes the anode and corrodes faster than it would on its own. The other becomes the cathode and gets partly protected. This is a coupling problem, not a flaw in either grade. A stainless steel that holds up fine by itself can still push a neighboring carbon-steel part toward failure once the two are wetted and wired together.

Because the mechanism is electrochemical, it falls under the localized attacks covered in our types of corrosion overview. Still, it stands apart from single-metal mechanisms like pitting and crevice corrosion.

Galvanic Corrosion vs Electrolysis

Galvanic corrosion comes from the natural difference in corrosion potential between two metals that are coupled together, no outside power involved. Electrolysis, or stray-current corrosion, works differently: an external current, maybe a leaking DC source, bad grounding, or impressed current, pushes the metal into solution. People mix these two up constantly. The galvanic series can predict the first kind. It has nothing to say about the second.

How Galvanic Corrosion Works: The Mechanism

A galvanic couple works like a short-circuited battery. At the anode, metal atoms oxidize and dissolve into solution as ions, which is the visible metal loss. At the cathode, a reduction reaction eats up the electrons flowing through the metallic path, usually oxygen reduction or hydrogen evolution, depending on the medium. The electrolyte completes the circuit, carrying ions between the two electrodes. Current flows. The anode dissolves faster than it would under free corrosion, and the cathode gets cathodically protected.

Attack direction tracks the galvanic series every time: whichever metal sits at the more active, less-noble end of that series is the anode. Attack size is a different story. That's governed by the area ratio and the electrolyte's conductivity, and those two factors set the rate of corrosion, not which metal gets eaten.

The Three Conditions Required

All three have to be there at once. Break even one link, and galvanic corrosion simply can't happen:

1. Two dissimilar metals with a measurable corrosion-potential difference (ΔE).
2. An electrolyte bridging them, seawater, condensation, process fluid, even a damp salt film.
3. Electrical (metallic) continuity, direct contact or a conductive fastener/path between them.

Electrical isolation and keeping joints dry work so well for one simple reason: each one knocks out a condition the failure needs to happen.

The Galvanic Series: Which Metal Is the Anode?

The galvanic series ranks metals and alloys from active (anodic) to noble (cathodic) in a given electrolyte, seawater by convention. Couple any two of them, and the one higher on the active side corrodes while the one nearer the noble side stays protected. One detail is easy to miss: stainless steel appears twice on the list, once in its passive state (noble) and once in its active state (much less noble), a state it can enter inside an oxygen-starved crevice. The table below covers the grades Nandini Steel stocks, so the ranking applies directly to real dissimilar-metal coupling decisions.

Factors That Control the Rate

Potential Difference (Series Separation)

The farther apart two metals sit in the galvanic series, the bigger the driving voltage, and the more aggressive the couple becomes. Metals that sit close together, say 304 and 316 in their passive state, couple safely because the voltage gap stays small.

Area Ratio: The Single Most Important Design Factor

Rate depends on the cathode-to-anode area ratio. A large cathode feeding a small anode dumps the whole couple's current onto a tiny spot, and the anode corrodes catastrophically. That's why carbon-steel bolts in a large stainless plate fail fast. Stainless bolts in a large carbon-steel plate, though, spread the minor anodic attack over a wide area and are usually fine. Design for a large anode and a small cathode. Never the reverse.

Electrolyte Conductivity

Seawater's the worst case for carrying current; brackish water and salt-laden industrial condensate come next. Clean, dry conditions barely support any galvanic current at all.

Distance, Temperature and Time

Attack concentrates nearest the contact point and fades out with distance in low-conductivity media. Higher temperature generally speeds up the reactions, and cumulative metal loss builds as exposure time grows.

Passive vs Active State

A passive stainless film is what keeps the grade noble and cathodic. Break that film down locally, say inside a crevice or a pitting site, and the exposed area turns active. It can then flip into the anode of a micro-couple, which ties galvanic behavior straight back to crevice and pitting resistance.

Which Grades It Affects: Stainless & Nickel Alloys in a Galvanic Couple

In almost every practical couple, stainless steels and nickel alloys end up as the cathode, they survive, but they accelerate whatever less-noble metal they're joined to.

Stainless / Nickel ↔ Carbon Steel (A516)

When stainless steel or a nickel alloy gets bolted or welded to carbon steel in wet service, the carbon steel becomes the anode and corrodes. That fact shapes flange-to-bolt, vessel-to-internal and pipe-support details across the industry. Specify matched components and area ratio with care. See A516 pressure-vessel carbon steel and the grades it's commonly paired with: 316 stainless steel, 904L super-austenitic stainless and Incoloy 825.

Stainless ↔ Aluminum, Zinc or Copper

Against aluminum (aluminium), the aluminum takes the anode role and corrodes, you'll see this a lot at stainless fasteners sitting in aluminum structures that face salt exposure. With zinc (galvanizing), it's the zinc that sacrifices itself, protecting the couple until it's used up. That's often exactly what the design intends. Copper is trickier: the outcome hinges on the medium, so treat both metals as candidates for isolation or testing.

When Stainless Is the Anode

Stainless surface in an active, crevice state beneath a nobler metal, graphite gaskets, copper alloys, titanium, and it can turn into the anode itself. Stainless isn't always safe. The fix: design out the crevice and keep the passive film intact.

How to Prevent Galvanic Corrosion

The controls run from most to least preferred. In demanding service, combine them.

1. Material matching, couple metals that sit close together in the galvanic series. Matched stainless-to-stainless or nickel-to-nickel joints have too little ΔE to drive significant attack.
2. Favorable area ratio, make the anode large and the cathode small. Stainless fasteners on a carbon-steel structure are acceptable; carbon-steel fasteners on a stainless structure are not.
3. Electrical isolation, break the metallic path with insulating gaskets, sleeves and washers (isolation kits) so no current can flow.
4. Coatings, coat the cathode, or both metals, to reduce the effective cathode area. Never coat the anode alone: a holiday in that coating creates a tiny exposed anode under a huge cathode, the worst possible area ratio.
5. Cathodic protection, add sacrificial anodes (zinc, aluminum or magnesium) or impressed current to make the whole assembly cathodic.
6. Design out the electrolyte, drain stagnant water, eliminate crevices and traps, and keep joints dry so the couple is rarely wetted.

The Fastener Rule of Thumb

Use noble fasteners (stainless) to join less-noble plate (carbon steel). Small cathode, large anode: tolerable. Reverse it, though, and those small anodic fasteners corrode fast. When you're unsure, match the fastener to the plate, or isolate the two.

Galvanic Corrosion vs the Other Mechanisms

Galvanic corrosion is a coupling mechanism. The items below work on a single metal, and keeping that distinction straight matters: blur it, and you'll end up misdiagnosing a failure or specifying more than the job needs.

PREN Does Not Rank Galvanic Behavior

A common mistake in specs is thinking a high PREN grade will protect whatever partner it's coupled to. PREN only ranks resistance to pitting and crevice attack, nothing more. Take 904L, with a PREN around 34: it's still firmly cathodic. It won't protect a carbon-steel partner, and it can actually push corrosion on that partner harder. Keep these two ideas apart. For more, see PREN (pitting resistance equivalent).

Real-World Examples

• Marine flange and bolt couples, carbon-steel bolting into a stainless flange in splash or marine / seawater service; the small bolts become the anode and fail first.
• Heat-exchanger tube-to-tubesheet, a nobler tube in a less-noble tubesheet (or the reverse) in cooling water concentrates attack at the joint.
• Carbon-steel vessel with stainless internals, a large carbon-steel surface is the anode; here the unfavorable ratio runs in the operator's favor, but weld and support details still need isolation.
• Aluminum-to-stainless in oil & gas and chemical processing structures exposed to salt-laden condensate, the aluminum is consumed unless isolated.

How Grade Selection Controls Galvanic Risk

The first check on galvanic compatibility happens at the purchase order, long before anyone touches a gasket. Grade selection matters here, and so does traceability. Pick grades that sit close together in the galvanic series. Detail fasteners for a favorable area ratio. Know exactly what you've coupled. Use the grade selector tool and the how to select a stainless grade guide to narrow your candidates, then confirm the delivered heat against its certificate.

From the Nandini Steel Engineering Team: the dissimilar-metal error we see most at RFQ stage is carbon-steel bolting specified into a stainless or nickel-alloy flange in wet duty. That's a small anode sitting under a large cathode. Specify matched or isolated fasteners up front, and the couple problem never makes it past the drawing board. Getting the designation right matters just as much. Incoloy 825 (UNS N08825) still gets written as "Inconel 825" far too often, and putting the correct designation on the PO and the MTC heads off receiving-inspection disputes about what actually showed up.

Every heat ships with an EN 10204 3.1/3.2 mill test certificate provided with every order, so you can check grade, composition and standard for both sides of a couple. That's the foundation of material traceability and the EN 10204 3.1 mill test certificate. For how the delivered material gets verified, see testing, MTC, NDT & PMI.

Sourcing the Matched Grades

Nandini Steel holds ISO 9001:2015 certification and operates as a stockist and supplier (Cert No. 1283438, Unitas Certification Services UCS-UK, valid to 11 January 2028). Founded in 2003, the company brings more than 20 years of experience in corrosion-resistant steel and is a certified specialist in 904L, Alloy 28, Incoloy 800 and Hastelloy plate. Every stainless, nickel-alloy and carbon-steel product sits ex-stock, ready for immediate dispatch. Each piece comes from genuine mill plate sourced from Aperam, Outokumpu, Jindal, POSCO, Nippon Steel and JFE. Pricing runs by RFQ, so quotes reflect live rates. Need a dissimilar-metal joint specified? Reach the metallurgist on WhatsApp at +91 99202 09184, or email reachus@nandinisteel.com.

Frequently Asked Questions