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Galvanic corrosion

Pick the two metals in contact, their wetted areas and the environment. See which metal becomes the anode, whether the potential difference exceeds the usual limit and how to isolate the joint.

Pre-sizing and estimating tool: results do not replace design, calculations or reports signed by a licensed professional, nor verification against the applicable codes and standards.

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Metals in contact

Area exposed to the electrolyte (water, condensation, salt spray) near the joint. For bolts, use the head and nut area.

Environment
—Couple potential difference
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What to do

Measures in order of effectiveness for this couple and environment.

    Galvanic series in seawater

    Potential range of each metal, volts versus saturated calomel electrode. The two metals chosen are highlighted.

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    Couple difference and usual limits

    Difference between mean potentials, compared with the limits for harsh, normal and controlled environments.

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    Method, assumptions and references

    Equations used

    ΔE = |E1 − E2|, with E the mean corrosion potential in seawater (V vs SCE, ASTM G82); anode = metal with the more negative E
    Risk level: ΔE ≤ 0.05 V negligible; ΔE ÷ limit ≤ 1 low; ≤ 2 moderate; > 2 high; limit = 0.15 V (harsh), 0.25 V (normal), 0.50 V (controlled)
    Areas: Ac ÷ Aa ≥ 10 raises one level; ≤ 0.1 lowers one level; in immersion, Ac ÷ Aa > 1 raises one more level. Anode current density ≈ i_cathode × Ac ÷ Aa

    Assumptions and limits

    • Approximate potentials from the ASTM G82 galvanic series (seawater flowing at 8 to 13 ft/s, 50 to 80 °F). In fresh water, soil or chemicals the order can change.
    • The rating is qualitative: the actual rate depends on electrolyte conductivity, metal polarization, time of wetness and temperature. For critical parts, test per ASTM G71.
    • Without electrical contact or a continuous electrolyte between the metals there is no galvanic corrosion.

    Standards and technical documents

    • ASTM G82: Standard Guide for Development and Use of a Galvanic Series for Predicting Galvanic Corrosion Performance
    • ASTM G71: Standard Guide for Conducting and Evaluating Galvanic Corrosion Tests in Electrolytes
    • MIL-STD-889: Dissimilar Metals (anodic index and compatible couples)
    • Revie, R. W.; Uhlig, H. H.: Corrosion and Corrosion Control, 4th ed., Wiley
    • ISO 9223: Corrosion of metals and alloys — Corrosivity of atmospheres — Classification, determination and estimation

    U.S. regulations

    • MIL-STD-889 (Department of Defense): selection and protection of dissimilar metal couples
    • 49 CFR Part 192 Subpart I: requirements for corrosion control of gas pipelines
    • AMPP SP0169: control of external corrosion on underground or submerged metallic piping systems

    Frequently asked questions

    Can I use stainless bolts in a carbon steel structure?

    Usually yes, because the cathode (stainless) is small and the anode (steel) is large. The difference is about 0.58 V, so in marine exposure isolate with washers and sleeves and paint the assembly.

    What about carbon steel bolts in stainless sheet?

    That is the worst case: the small anode takes all the current from the large cathode. With an area ratio of 50 the bolt corrodes dozens of times faster than on its own.

    Can aluminum touch stainless steel?

    The difference is about 0.8 V. Indoors with climate control the risk is low; outdoors or near the sea, isolate the joint and seal it.

    What potential difference is acceptable?

    Common practice is up to 0.15 V in harsh environments, 0.25 V in normal environments and 0.50 V in climate-controlled spaces.