The galvanic series ranks metals and alloys by their electrochemical potential in seawater at 25°C (77°F). Materials at the top are more cathodic (noble), while those at the bottom are more anodic (active). When two metals are coupled in seawater, the anodic material will corrode preferentially.

Electrochemical Potential Ranges

The potential difference ( $\Delta E$ ) between two materials predicts galvanic corrosion severity:

$\Delta E = E_{\text{cathode}} - E_{\text{anode}}$

Where:

$E_{\text{cathode}}$ = potential of the more noble metal (V vs SCE)
$E_{\text{anode}}$ = potential of the more active metal (V vs SCE)

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Galvanic series of metals and alloys in flowing seawater at 25°C (77°F). Potentials shown vs. saturated calomel electrode (SCE).
Material	Potential Range(V vs SCE)	Classification
Platinum	+0.20 to +0.30	Noble
Gold	+0.20 to +0.30	Noble
Graphite	+0.20 to +0.30	Noble
Titanium	-0.05 to +0.10	Noble
Silver	+0.08 to +0.15	Noble
HASTELLOY® C	-0.05 to +0.10	Noble
INCONEL® 625	-0.05 to +0.10	Noble
316 SS (passive)	-0.05 to +0.05	Noble
304 SS (passive)	-0.10 to 0.00	Noble
Monel 400	-0.15 to -0.05	Intermediate
Copper 90/10 Ni	-0.20 to -0.10	Intermediate
Red Brass (85% Cu)	-0.25 to -0.15	Intermediate
Yellow Brass (65% Cu)	-0.30 to -0.20	Intermediate
Naval Brass	-0.35 to -0.25	Intermediate
Tin	-0.35 to -0.25	Intermediate
Lead	-0.50 to -0.40	Intermediate
316 SS (active)	-0.50 to -0.35	Active
304 SS (active)	-0.55 to -0.40	Active
410 SS (active)	-0.55 to -0.40	Active
Cast Iron	-0.65 to -0.50	Active
Wrought Iron	-0.65 to -0.50	Active
Low Carbon Steel	-0.65 to -0.50	Active
Aluminum 2024	-0.75 to -0.60	Active
Cadmium	-0.80 to -0.70	Active
Aluminum 1100	-0.80 to -0.65	Active
Galvanized Steel	-1.05 to -0.90	Very Active
Zinc	-1.05 to -0.90	Very Active
Magnesium Alloys	-1.70 to -1.50	Very Active

Source: engineeringtoolbox.com

Galvanic Corrosion Risk Guidelines

Potential Difference $\Delta E$	Corrosion Risk
< 0.15 V	Low
0.15 – 0.25 V	Moderate
0.25 – 0.50 V	High
> 0.50 V	Severe

Practical Considerations

Passive vs. Active States: Stainless steels exhibit dramatically different potentials depending on their surface condition. In the passive state (protective oxide layer intact), they behave as noble materials. In the active state (oxide layer compromised), they shift significantly anodic and may corrode rapidly.

Area Ratio Effect: The galvanic corrosion rate is amplified when a small anode is coupled to a large cathode. The current density on the anode increases, accelerating localized attack:

$\frac{i_{\text{anode}}}{i_{\text{cathode}}} = \frac{A_{\text{cathode}}}{A_{\text{anode}}}$

Where $i$ is current density and $A$ is exposed surface area.

Environmental Factors: Temperature, salinity, flow velocity, and dissolved oxygen content can shift potentials significantly from laboratory values. Always consider site-specific conditions when selecting materials.

References

Original Source

Source: engineeringtoolbox.com