Galvanic couples in industrial plants: common metal pairs and how to control them

A galvanic couple forms when two dissimilar metals are in electrical contact and wetted by the same electrolyte. The less noble metal becomes the anode and corrodes faster than it would on its own; the more noble metal becomes the cathode and is protected. Remove any one of the four conditions — anode, cathode, electrolyte, metallic contact — and the couple stops.

The galvanic series table tells you which metal will be the anode; galvanic corrosion in heat exchangers covers tube sheets and water boxes. This article is the field catalogue: the pairs found in plants, which metal pays, and the fix for each.

What is a galvanic couple?

A galvanic couple, or galvanic pair, is a short-circuited corrosion cell. One metal gives up electrons and dissolves; the other consumes them in a reduction reaction — in neutral water, usually the reduction of dissolved oxygen. The resulting attack on the anode is galvanic corrosion; "galvanic coupling" is the act of connecting the two metals, by design or by accident.

The cell needs four elements at the same time:

This is why the same pair can last for years in a dry electrical room and fail quickly on an outdoor platform: the metals did not change, the electrolyte did.

What sets the severity: potential gap, area ratio, electrolyte

Three factors decide whether a couple is a curiosity or a failure.

Potential gap. The further apart the two metals sit in the galvanic series, the larger the driving force. Copper against bronze is usually tolerable; stainless steel against aluminum is not, in wet service.

Area ratio. The current collected by the cathode is supplied by the anode. A small anode facing a large cathode corrodes fast — carbon steel bolts in a stainless flange are the textbook case. A large anode facing a small cathode spreads the attack, which is why stainless bolts in a carbon steel flange are generally fine.

Electrolyte. Conductivity and wetting time matter. Seawater, de-icing salt and chemical splash make a strong electrolyte; condensation and rain an intermittent one. In low-conductivity water the attack concentrates right at the junction.

What "galvanically compatible" means

Two metals are galvanically compatible when their coupling does not produce an unacceptable corrosion rate in the intended environment. For fasteners, platings and enclosures, designers check this with the anodic index: usual design-guide limits for the index difference are 0.15 V in harsh, 0.25 V in normal and 0.50 V in controlled environments. The index ignores the area ratio and immersion, so for plant equipment in water, soil or process service, the series and the three factors above remain the right tool.

Common galvanic couples in industrial plants

Notes on the pairs that cause the most trouble

Galvanic couple aluminum / stainless. The most common couple on outdoor equipment: aluminum covers, cable trays and instrument supports fastened with stainless hardware. The aluminum around the fastener corrodes into a white, powdery product and the threads seize. A large stainless panel riveted with aluminum is the severe version.

Carbon steel / copper. Found wherever steel piping meets copper or brass: process water, condenser connections, instrument lines. The steel next to the joint thins locally. Dissolved copper adds a second effect: copper ions can plate out on steel or aluminum downstream and create new local cathodes far from the joint.

Galvanized steel / stainless. The zinc layer is designed to be sacrificed; coupled to stainless in a wet environment, it is consumed faster around the contact, and once it is gone the bare steel becomes the anode. Stainless fasteners on galvanized structures are generally acceptable in atmospheric service with isolating washers; the reverse is not.

Cast iron / bronze. The classic case is a bronze-fitted pump or valve — bronze impeller or trim in a cast iron casing — put into seawater or aggressive cooling water. The cast iron suffers graphitic corrosion: the iron dissolves and leaves a graphite network that looks intact but can be scraped with a knife. That surface is itself noble, so a new steel part bolted to it becomes the next anode.

Titanium / carbon steel. Titanium is very noble and often presents a large surface — tubes, plates, liners — so the steel it touches receives a strong galvanic current. Cathodic protection potentials must stay within the titanium supplier's limits to avoid hydrogen uptake by the titanium.

Hidden couples nobody drew on the drawings

• Mill scale is cathodic to bare steel; breaks in the scale corrode preferentially, one reason coating specifications require its removal.
• New steel spliced into an old line becomes anodic to the old, scale-covered pipe around it.
• Weld metal that is less noble than the base metal corrodes preferentially; filler metal for wet service is chosen to be at least as noble as the base metal.
• Passive and active stainless behave as two different metals: a crevice or a deposit turns part of the surface active against the passive rest — see crevice corrosion and pitting corrosion.
• Graphite gaskets and packings act as cathodes against steel flange faces.

How to control a galvanic couple

Each fix breaks one condition; durable solutions combine two or three.

1. Choose metals close together in the series. For new equipment and replacement parts, the most durable measure: match the fastener to the part, the spool to the system, the filler to the base metal. When mixing is unavoidable, make the small part the noble one.

2. Isolate electrically. An isolating flange kit — insulating gasket, a sleeve over each bolt, insulating washers under the nuts — breaks the metallic path between two piping systems; on structures, isolating washers, sleeves and pads do the same. The isolation must be checked electrically after installation, it is easily bridged by a pipe support, a grounding conductor or instrument tubing, and it has to respect the plant's electrical bonding requirements.

3. Coat the cathode, or both — never only the anode. If only one metal can be coated, coat the noble one: it reduces the cathodic area feeding the attack. A coating on the anode alone turns every holiday into a small anode facing the whole cathode, and it perforates faster than the bare metal would have corroded. Coating both, with a holiday-tested system, is the robust option.

4. Add a deliberate anode. Cathodic protection uses the same mechanism on purpose: zinc, aluminum or magnesium sacrificial anodes, or an impressed-current system, make the structure the cathode. Anodes are consumed by design and must be inspected and replaced.

5. Remove the electrolyte. Drain holes in channels and supports, sealant on faying surfaces, no pockets where water stands against a joint. Wet insulation is an electrolyte reservoir: a mixed-metal joint under it deserves the attention given to a splash zone.

6. Inhibit closed loops. In closed water and glycol circuits with several metals, the inhibitor package must cover all of them — copper alloys typically need an azole-type inhibitor. Attack appears when the inhibitor is depleted, so fluid analysis belongs in the maintenance plan.

These measures are part of the broader toolbox described in our guide to corrosion protection methods for industrial metals.

Fasteners: stainless screws in aluminum, steel bolts on copper

Fasteners create most galvanic couples, and the fastener choice sets the area ratio.

Stainless screws in aluminum. The screw is a small cathode in a large anode: indoors and dry, it is usually acceptable. Outdoors, or wherever salt and water reach the joint, the aluminum around the thread corrodes and the screw seizes. Use isolating washers and sleeves, apply a thread sealant or jointing compound so water cannot fill the thread, and avoid placing the joint where water collects.

Steel bolts on copper or bronze. The worst configuration: a small steel anode against a large copper cathode, with the attack on the shank and under the head, where strength matters. Use bronze or stainless bolting.

The same logic rules out carbon steel bolts in stainless flanges and galvanized bolts on stainless or copper parts: when in doubt, make the fastener the more noble metal.

FAQ

Does a galvanic couple corrode if the joint is dry? No. Without an electrolyte there is no ionic path and no galvanic current. The risk returns once condensation, rain or wet insulation reaches the joint.

Which metal corrodes in an aluminum and stainless steel couple? The aluminum. Stainless steel in its passive state is far more noble, so the aluminum becomes the anode and corrodes at the contact.

Is galvanic coupling always harmful? No. Galvanized steel and sacrificial anodes are galvanic couples used on purpose: the zinc or the anode corrodes so the steel does not.

How do I check that an isolating flange kit works? Measure the electrical isolation across the flange after installation, then check that no support, grounding conductor or tubing bridges it.

In practice

A galvanic problem is solved by breaking one or two of the four conditions at the right place: an isolating kit, a coating on the cathode, an anode, a drain hole. Induscoat diagnoses dissimilar-metal corrosion, repairs equipment already damaged by galvanic attack and applies barrier coating systems selected for the service through its protective coatings service, as part of its corrosion protection work for plants in Quebec and Ontario. Photos of the joint, the materials of each part and a description of the service environment are enough to start: request a quote. Technical data sheets for the Induscoat range are published on the brand site, induscoat.com.