The galvanic series: the full table and how to actually use it

What is the galvanic series?

The galvanic series ranks metals and alloys by the electrochemical potential they exhibit in a given electrolyte — conventionally seawater. When two metals from different positions in the series are electrically connected in that electrolyte, the more active (anodic) metal corrodes preferentially and the more noble (cathodic) metal is protected. The table below is the working version; the three rules after it are what make it usable.

One caveat before the table, because it is the most common misuse: the series is measured in seawater near ambient temperature. In another electrolyte — fresh water, process chemistry, soil — the order can shift and even invert for some pairs. The series tells you where to look; it does not replace checking the actual service environment.

The galvanic series in seawater

From most noble (protected) to most active (sacrificed):

Three entries deserve a second look. Stainless steel appears twice — passive and active — because its position depends entirely on the state of its oxide film; a crevice or a deposit can locally turn a "noble" stainless surface into an active one. Graphite sits at the very top, which is why graphite gaskets against steel flanges, or graphitically corroded cast iron, create aggressive couples. Cast iron that has corroded graphitically becomes, in effect, a noble graphite electrode — a repaired or new steel part bolted to it becomes the anode.

The three rules that make the table useful

Rule 1 — Distance in the series sets the driving force. The further apart two connected metals sit, the larger the potential difference driving the galvanic current. Couples close together (copper and bronze) are usually tolerable; couples far apart (stainless bolted to aluminum, steel piping into a copper-nickel system) demand attention.

Rule 2 — Area ratio decides the severity. The galvanic current concentrates on the anode. A large anode feeding a small cathode is often acceptable; a small anode serving a large cathode is the disaster case — carbon steel bolts in a stainless flange corrode fast, while stainless bolts in a carbon steel flange are generally fine. When in doubt, make the fastener (the small part) the more noble metal.

Rule 3 — No electrolyte, no couple. Galvanic corrosion needs an ionic path. Two dissimilar metals in dry indoor service can be coupled indefinitely; the same pair under insulation that traps moisture, in a splash zone, or in buried service becomes an active cell. This is also why the two engineering remedies work: electrical isolation (isolating gaskets, sleeves and washers breaking the metallic path) and barrier coatings — with the counter-intuitive but firm rule that if only one metal can be coated, you coat the cathode, never only the anode: a coating holiday on a coated anode concentrates the whole galvanic current on a pinpoint.

Where this shows up in real equipment

The couples we are most often called about: carbon steel piping bolted to stainless vessels; steel tube sheets carrying copper-alloy or titanium tubes in heat exchangers; repaired steel patches on graphitically corroded cast iron pump casings; aluminum instrument supports on steel structures in coastal plants; and new steel spool pieces inserted into old, noble-scaled systems. In every case the diagnosis follows the same three rules — position gap, area ratio, electrolyte — and the fix is isolation, coating the cathode, deliberate sacrificial anodes, or materials substitution, chosen against the corrosion protection requirements of the service.

FAQ

Is the galvanic series the same in every environment? No. The conventional series is measured in seawater. Relative positions can shift in other electrolytes, with temperature, and with flow. Use the series as a screening tool and confirm against the actual service environment.

How far apart in the series is "too far"? There is no universal cutoff — severity depends on the potential difference, the area ratio and the electrolyte conductivity together. A large gap with a favorable area ratio in a poor electrolyte may outlast a small gap in the disaster configuration.

Why did my stainless steel corrode next to carbon steel — is the table wrong? Most likely the stainless went active locally (crevice, deposit, oxygen depletion) or another mechanism entirely is at work. The series describes coupled behavior of each metal in its stated condition — passive and active stainless are different entries for that reason.

Should I coat the anode to protect it? Coat the cathode first, or both. Coating only the anode is the classic error: any defect in that coating becomes a tiny anode carrying the full galvanic current of the large cathode, and perforates quickly.

Are sacrificial anodes just applied galvanic corrosion? Exactly. Zinc, aluminum and magnesium anodes deliberately occupy the active end of the series to corrode in place of the structure — the same table, used on purpose.

Dealing with a galvanic problem in your plant

Induscoat Canada diagnoses dissimilar-metal corrosion and delivers the fix — isolation hardware, barrier coating systems selected for the service, and repair of equipment already damaged by galvanic attack. Send photos of the couple and the service conditions through our request a quote page; we respond within 24 business hours.