What Is Galvanic Corrosion?
Quick Answer: Galvanic corrosion occurs when two dissimilar metals are electrically connected in the presence of an electrolyte. The more active (anodic) metal corrodes preferentially, while the more noble (cathodic) metal is protected.
Three Conditions for Galvanic Corrosion
Galvanic corrosion requires all three conditions simultaneously:
- Two metals with different electrochemical potentials (position in the galvanic series)
- An electrically conductive path between the metals (direct contact or a metallic bridge)
- An electrolyte bridging both metals (seawater, process fluid, condensation)
Remove any one condition and galvanic corrosion cannot occur. This principle drives all prevention strategies.
Galvanic Series in Seawater
| Position | Metal/Alloy | Behavior |
|---|---|---|
| Most Noble (Cathodic) | Platinum, Gold | Protected; corrodes least |
| Titanium | Protected | |
| Alloy C-276, Inconel 625 | Protected | |
| 316L Stainless Steel (passive) | Protected | |
| 304L Stainless Steel (passive) | Protected | |
| Monel 400 | Intermediate | |
| Copper-Nickel (90/10) | Intermediate | |
| Bronze, Brass | Intermediate | |
| Carbon Steel, Cast Iron | Active | |
| Aluminum Alloys | Active | |
| Most Active (Anodic) | Zinc, Magnesium | Corrodes first |
The farther apart two metals sit in this series, the greater the driving voltage and the faster the anodic metal corrodes. A carbon steel pipe connected to a 316L stainless steel flange in a wet environment will corrode aggressively at the junction.
Common Piping Scenarios
- Carbon steel pipe with stainless steel fittings: the carbon steel corrodes at the weld or bolted connection
- Brass valves in galvanized steel piping: zinc coating on the steel corrodes preferentially (de-zincification of brass can also occur)
- Stainless steel stud bolts on carbon steel flanges: bolt area is small (cathode) vs. large flange (anode), reducing the effect; but reverse the ratio and damage accelerates
- Copper tubing connected to carbon steel headers: the steel header corrodes at the transition
Prevention Methods
| Method | Application |
|---|---|
| Isolating gaskets/kits | Flange isolation kits with dielectric sleeves break the electrical path |
| Coatings | Paint or epoxy on the cathodic metal (or both metals) |
| Material selection | Pair metals close together in the galvanic series |
| Cathodic protection | Sacrificial anodes (zinc, magnesium) protect the structure |
| Insulating washers | Nylon or phenolic washers under bolt heads |
For offshore and seawater piping, NACE SP0169 and DNV-RP-B401 provide cathodic protection design criteria. In process piping, the most effective approach is to use isolation kits at every dissimilar-metal flanged joint and coat exposed carbon steel surfaces.
Leave a Comment
Have a question or feedback? Send us a message.