Galvanic Corrosion: The Electrochemical Series

## Electrochemistry of Galvanic Corrosion Every metal immersed in an electrolyte develops a characteristic equilibrium electrode potential determined by the metal/metal-ion equilibrium and the ion activity in solution. The standard electrode potential (E°) measured against the standard hydrogen electrode (SHE) at 25 °C and unit activity places metals and ions on an absolute scale: | Half-Reaction | E° (V vs. SHE) | |--------------|----------------| | Au³⁺ + 3e⁻ → Au | +1.50 | | Pt²⁺ + 2e⁻ → Pt | +1.19 | | Ag⁺ + e⁻ → Ag | +0.80 | | Cu²⁺ + 2e⁻ → Cu | +0.34 | | 2H⁺ + 2e⁻ → H₂ | 0.00 (reference) | | Ni²⁺ + 2e⁻ → Ni | -0.25 | | Fe²⁺ + 2e⁻ → Fe | -0.44 | | Zn²⁺ + 2e⁻ → Zn | -0.76 | | Al³⁺ + 3e⁻ → Al | -1.66 | | Mg²⁺ + 2e⁻ → Mg | -2.37 | The more negative the standard potential, the more thermodynamically active (less noble) the metal. When two metals are coupled in an electrolyte, the metal with the more negative potential acts as the anode (corrodes) and the metal with the more positive potential acts as the cathode (protected or undergoes a cathodic reaction such as oxygen reduction or hydrogen evolution). ## The Practical Galvanic Series The standard electrode potentials are measured under idealized conditions that rarely match engineering reality. In practice, the galvanic series in flowing seawater (the most common aggressive engineering environment) is more relevant. Passive films on stainless steels, titanium, and nickel alloys shift their practical potential significantly more noble than their standard electrode potential would suggest. A simplified galvanic series in seawater, from most noble (cathodic) to most active (anodic): 1. Platinum, graphite 2. Titanium alloys (Ti-6Al-4V, commercially pure Ti) 3. Hastelloy C, Inconel 625 (passive) 4. Stainless steel 316 (passive) 5. Stainless steel 304 (passive) 6. Silver 7. Nickel (passive) 8. Copper alloys (Cu-Ni, bronzes, brasses) 9. Lead 10. Carbon and alloy steels 11. Cast iron 12. Aluminum alloys (2xxx series most active, 5xxx less so) 13. Cadmium 14. Zinc 15. Magnesium alloys Metals close together on this list will exhibit little galvanic effect when coupled. Metals far apart will show significant galvanic attack on the more active metal. ## Factors Controlling Attack Severity ### Area Ratio The area ratio of cathode to anode is the most critical practical factor. A small anode (active metal) coupled to a large cathode (noble metal) concentrates all the corrosion current on a small area, causing rapid penetration. A large anode coupled to a small cathode spreads the corrosion over a large area, reducing the rate at any point. **Practical consequences**: Carbon steel fasteners in a stainless steel assembly corrode rapidly (small anode, large cathode). Stainless steel fasteners in a carbon steel assembly are acceptable because the large carbon steel anode area spreads any galvanic current to negligible levels. Always ensure the more active metal is the larger area member. ### Electrolyte Conductivity Galvanic current can only flow through a conducting electrolyte. In freshwater with low dissolved salt (100–500 μS/cm conductivity), galvanic effects are confined to within a few millimeters of the contact point. In seawater (50,000 μS/cm), current flows freely over large distances, and galvanic attack can extend far from the bimetallic contact. ### Distance Between Metals Because electrolyte resistance increases with distance, galvanic attack is most severe immediately adjacent to the bimetallic junction and diminishes with distance. In low-conductivity environments this gradient is steep; in seawater it is shallow. ## Design Rules to Prevent Galvanic Corrosion 1. **Select metals close on the galvanic series**: In seawater, any pair within 50 mV of open-circuit potential difference is generally considered compatible. 2. **Insulate dissimilar metals**: Electrical isolation via non-conducting gaskets, sleeves, and washers breaks the circuit. Insulating flange kits for pipelines carrying dissimilar metal sections are a standard solution. 3. **Apply a coating to the cathode, not the anodes**: If the noble metal (cathode) is coated, any holiday (coating defect) creates a small cathodic area connected to a large anodic area, which is the favorable area ratio. Coating the active metal (anode) while leaving the cathode bare creates the dangerous small-anode/large-cathode situation if the anode coating is damaged. 4. **Use sacrificial anodes or impressed current cathodic protection**: Attach zinc or aluminum sacrificial anodes to the structure to be protected. The sacrificial anode corrodes preferentially, protecting the structure. Ship hulls, offshore platforms, and buried pipelines rely on this approach. 5. **Avoid crevices at bimetallic junctions**: Crevice corrosion and galvanic corrosion combine to produce especially aggressive attack in trapped electrolyte at bimetallic joints.