Galvanic Corrosion at Dissimilar Metal Joints: What Pipe Fitting Engineers Must Know
When two dissimilar metals are electrically connected in an electrolyte, the less noble metal (the anode) corrodes preferentially. In piping systems, this galvanic mechanism most commonly attacks carbon steel fittings connected to stainless steel, carbon steel bolts on stainless flanges, or austenitic stainless fittings connected to titanium in seawater — often in locations invisible during normal inspection.
The Galvanic Series in Seawater
The galvanic potential of a metal in seawater determines whether it acts as an anode (corrodes) or cathode (protected) when coupled to another metal. The greater the potential difference, the faster the anodic metal corrodes. Approximate nobility ranking from most active (anodic, corrodes) to most noble (cathodic, protected):
| Rank | Material | Behaviour when coupled to more noble grade |
|---|---|---|
| Most active | Zinc, Aluminium | Sacrificial anode — used deliberately for cathodic protection |
| — | Carbon Steel / Cast Iron | Corrodes when coupled to stainless or copper alloys |
| — | Copper alloys (brass, bronze) | Corrodes when coupled to stainless or nickel alloys |
| — | Austenitic SS (316L, 304L) | Cathodic to CS; anodic to duplex and nickel alloys |
| — | Duplex / Super Duplex | More noble than austenitic — accelerates 316L corrosion when coupled |
| — | Inconel 625, Hastelloy C-276 | Noble — will accelerate corrosion of all grades below |
| Most noble | Platinum, Titanium | Cathodic to all common engineering alloys |
The Three Conditions Required for Galvanic Corrosion
- Dissimilar metals — different electrochemical potential (different alloys, or same alloy in different heat treatment states)
- Electrical contact — the metals must be electrically connected (direct metal-to-metal contact, or conductive path through bolts/pipe wall)
- Electrolyte — a conductive liquid bridging both metals (seawater, process water, rain, condensate)
Removing any one condition prevents galvanic corrosion. In practice, the most practical control measure is electrical isolation (insulating gaskets and sleeves at flange joints) or designing with compatible metals throughout the piping system.
Critical Piping Joints Where Galvanic Corrosion Occurs
Carbon steel pipe to stainless steel fitting: The large cathode (SS) to small anode (CS) ratio drives rapid pitting of the carbon steel adjacent to the weld. Common in mixed-material offshore topside piping where CS structural connections meet SS process piping.
Carbon steel bolts on stainless flanges: The large stainless flange acts as cathode; the small carbon steel bolt acts as anode — bolt corrosion can be rapid in marine environments. Always use matching or more noble bolt material (e.g. B8M / 316 SS studs on 316L flanges, or duplex studs on duplex flanges).
Stainless fitting to titanium heat exchanger: Titanium is very noble — it will drive pitting of the 316L fitting in seawater service if both are wetted. Inconel 625 or super duplex are better matches for titanium proximity.
Practical Mitigation for Piping Systems
- Match bolt material to the flange grade — or use a more noble bolt material (duplex bolts on 316L flanges are common and correct)
- Use insulating gaskets (PTFE envelope or mica sheet) and bolt sleeve kits at any flange joint between dissimilar grades in aqueous service
- For CS-to-SS transitions: use a short spool piece of a compatible intermediate grade, or apply fusion bonded epoxy (FBE) coating on the CS side
- Keep dissimilar metal joints above the waterline where possible — galvanic corrosion requires electrolyte bridging; in dry gas service it does not occur
- Cathodic protection (zinc anodes or ICCP) on submerged carbon steel piping near stainless fittings — the anodes provide the sacrificial metal rather than the carbon steel pipe