# Galvanic corrosion **Galvanic corrosion**, also called bimetallic or dissimilar-metal corrosion, is the accelerated [[Corrosion|corrosion]] of the less noble of two metals that are electrically connected and share an [[Electrolyte|electrolyte]]. The pair is a short-circuited [[Galvanic_cell|galvanic cell]]: the metal with the more negative [[Standard_electrode_potential|electrode potential]] becomes the anode and dissolves, while the more noble metal becomes the cathode and is protected, usually by reducing dissolved [[Oxygen|oxygen]] on its surface. Three conditions must hold together — two different metals, a metallic path between them, and a continuous electrolyte bridging them — and removing any one of the three stops the process. In the microsim below the reader chooses a couple from the [[Galvanic_series|galvanic series]] and sets the single control, the area ratio `A_c/A_a` between cathode and anode. The cell voltage follows from the two electrode potentials, but the damage follows from the geometry: the whole cathodic current has to leave through the anode, so the anode's current density is `i_a = i_c·(A_c/A_a)` and the penetration rate follows by [[Faraday's_laws_of_electrolysis|Faraday's law]]. A small steel rivet in a large [[Copper|copper]] plate is destroyed; a small copper rivet in a large steel plate is harmless. A [[Zinc|zinc]] preset shows the same arithmetic used deliberately, as a sacrificial anode protecting [[Steel|steel]]. On the [[Materials_science|Materials science]] flagship this article serves the *Galvanic corrosion* section of Part VIII, Industry, as the zinc-placed sibling of the corrosion sim; the passivity that keeps some couples safe is the subject of the [[Pourbaix_diagram|Pourbaix diagram]]. ## Overview Every metal in contact with an electrolyte sits at some potential relative to a reference, and the ordering of those potentials is what makes one metal attack another. A tabulation of standard reduction potentials gives −0.44 V for Fe²⁺/Fe, +0.34 V for Cu²⁺/Cu, −0.76 V for Zn²⁺/Zn, −1.66 V for Al³⁺/Al and −2.37 V for Mg²⁺/Mg, all against the standard hydrogen electrode.[^os-appl] Coupling iron to copper therefore offers a driving force of 0.78 V, and coupling zinc to iron offers 0.32 V in the opposite sense (derived).[^os-appl] In a corroding couple the anodic [[Redox|half-reaction]] is the dissolution of the less noble metal, `Fe -> Fe²⁺ + 2e⁻`, and the cathode reaction in aerated near-neutral water is oxygen reduction, `O₂ + 2H₂O + 4e⁻ -> 4OH⁻`.[^os-166] What the potentials do not give is the rate. Charge conservation does: every electron released at the anode is consumed at the cathode, so the total anodic current equals the total cathodic current. If the cathode reaction is limited by how fast oxygen reaches the surface — which it usually is in quiet, aerated water — then the total current is proportional to the cathode *area*, and concentrating it onto a small anode raises that anode's current density in proportion. This is the area-ratio effect, and it is why the sim's control is a ratio rather than a metal. Faraday's law then converts current density into loss of metal. For iron, with a molar mass of 55.85 g/mol, two electrons per atom, a density of 7,870 kg/m³ and the Faraday constant `F = 96,485 C/mol`,[^haverkort-f] a current density of 1 A/m² corresponds to a uniform penetration of 1.16 mm per year (derived). Taking an ILLUSTRATIVE oxygen-limited cathodic current density of 0.1 A/m², the sim reads out 0.12 mm/yr at an area ratio of 1, 11.6 mm/yr at a ratio of 100 — a 2 mm steel rivet gone within a year — and 0.0012 mm/yr at a ratio of 0.01 (derived). The rule of thumb falls out of the arithmetic: never put a small anode with a large cathode. ## Examples The same three ingredients recur in ships, monuments, buildings and kitchens, and the historical cases are worth reading as controlled experiments that nobody meant to run. ### Statue of Liberty The statue is a copper skin hung on an iron armature designed by Gustave Eiffel, with the two separated by shellac-impregnated asbestos to keep them apart electrically. Over a century in a salt-laden harbour atmosphere the insulation absorbed water and failed in places, and the small-area iron bars, coupled to an enormous copper cathode, corroded and swelled — [[Iron|iron]] corrosion products occupy more volume than the metal, so the bars jacked against their copper saddles and tore the skin. The 1980s restoration replaced the iron armature with [[Stainless_steel|stainless steel]] and separated the two metals with an inert polymer tape, treating the problem as an insulation problem rather than a metal-selection one.[^nps-stli] ### Royal Navy and HMS <i>Alarm</i> Copper sheathing was fitted to wooden hulls to stop shipworm and fouling, and HMS *Alarm* was among the first ships so sheathed, in the early 1760s. When the ship was docked and inspected, the copper was found in good order and the iron fastenings holding it were not: the iron nails and bolts had wasted away where they touched the copper, and in places the sheathing was held on only by the paper that had been laid under it. The lesson — that the copper had been protected at the iron's expense — took decades to be understood, and copper fastenings replaced iron ones long before there was a theory for it.[^alarm] Humphry Davy's investigation of the same copper sheathing in the 1820s, and his proposal to protect it with attached pieces of zinc or iron, is the origin of [[Cathodic_protection|cathodic protection]].[^davy1824] ### US Navy littoral combat ship <i>Independence</i> The *Independence* variant of the littoral combat ship has an [[Aluminium|aluminium]] hull and steel components in its waterjet propulsion system, an arrangement that puts a large, nobler steel cathode in permanent contact with an aluminium anode in seawater. Aggressive galvanic corrosion around the propulsion units was reported in 2011, and the remedy was the standard one: isolation, coatings and an impressed-current cathodic protection system.[^lcs2] ### Corroding lighting fixtures Outdoor and marine lighting is the commonest small-scale case. An aluminium housing fastened with stainless steel or [[Brass|brass]] screws presents exactly the unfavourable geometry — a large cathode is not needed when the anode is the thin thread of aluminium around a screw — and coastal fixtures fail at their fasteners long before the housing itself is spent. The same appears in curtain-wall construction wherever aluminium meets stainless fixings without an isolating washer or sleeve. ### Lasagna cell A salty, acidic food baked in a steel pan and covered with aluminium foil makes a working galvanic cell, and the foil in contact with the food develops holes. The food is the electrolyte, the foil the anode and the pan the cathode, and the geometry is unfavourable to the foil because it is thin. The cell is harmless but instructive: it shows that neither seawater nor an engineered structure is required, only the three ingredients.[^os-166] ### Electrolytic cleaning The same electrochemistry is run backwards on purpose in conservation. An artifact is made the cathode of a cell — by connecting it to a sacrificial anode or to the negative terminal of a power supply in a suitable electrolyte — so that corrosion products on it are reduced rather than grown, and chloride ions trapped in them are driven out. The treatment is deliberate galvanic action with the sign chosen by the conservator, and it is the bench-scale relative of cathodic protection. ## Prevention The three necessary conditions give three families of remedy. Break the electrolyte path by designing so that water drains and does not pool, by sealing joints, and by keeping insulation continuous; break the metallic path with dielectric unions, non-conducting washers, sleeves and gaskets; or remove the potential difference by choosing metals close together in the galvanic series, which makes this a problem of [[Material_selection|material selection]] as much as of [[Electrochemistry|electrochemistry]]. Where a mixed couple is unavoidable, geometry is the next lever, and it is the sim's point. Make the anode large and the cathode small, and never the reverse: steel fasteners in an aluminium plate are a poor choice, aluminium fasteners in a steel plate are worse still, and the standard resolution is to use a fastener of the nobler metal so that any attack is spread over the larger, less noble member. Coating follows the same logic with a twist that surprises people — coat the cathode, not the anode. A pinhole in a coating on the anode concentrates the entire current into that pinhole; a pinhole in a coating on the cathode merely exposes a small cathode. The last family is to supply the current from elsewhere. A sacrificial anode of zinc, aluminium or [[Magnesium|magnesium]], bolted to the structure, is deliberately the most active metal present, so that it corrodes and everything attached to it is cathodic and safe; the magnesium–iron couple offers 1.93 V of driving force and the aluminium–iron couple 1.22 V (derived), which is why those two are used where zinc's 0.32 V is not enough, for instance in fresh water. [[Galvanization|Galvanizing]] applies the same idea as a coating: a zinc layer on steel is both a barrier and a sacrificial anode, so a scratch through it does not undercut, because the exposed steel is cathodic to the zinc around it. A zinc anode carrying 0.1 A/m² wastes at 0.15 mm/yr (derived), which is how anode life is estimated. Impressed-current systems replace the sacrificial metal with a rectifier and an inert anode, and are used where the current demand is too large to supply by consuming metal. ## Galvanic series A galvanic series is an ordered list of metals and alloys as they actually behave in a stated electrolyte, most often seawater, from most noble at one end to most active at the other. It differs from a table of standard electrode potentials in three ways that matter in practice: it ranks alloys rather than pure elements, it is specific to one environment, and it reports the mixed potential a freely corroding surface adopts rather than a thermodynamic half-cell value. The standard-potential table is still the right starting point, and the sim uses it for the cell voltage. Selected values against the standard hydrogen electrode, from the appendix of *Chemistry: Atoms First*:[^os-appl] | couple | E° (V) | note | |---|---|---| | Au³⁺ + 3e⁻ → Au | +1.50 | most noble of the entries here | | Ag⁺ + e⁻ → Ag | +0.80 | | | Cu²⁺ + 2e⁻ → Cu | +0.34 | the cathode in the sim's copper preset | | 2H⁺ + 2e⁻ → H₂ | 0.000 | reference | | Pb²⁺ + 2e⁻ → Pb | −0.13 | | | Ni²⁺ + 2e⁻ → Ni | −0.26 | | | Fe²⁺ + 2e⁻ → Fe | −0.44 | the anode in the sim's steel preset | | Zn²⁺ + 2e⁻ → Zn | −0.76 | the sacrificial preset | | Al³⁺ + 3e⁻ → Al | −1.66 | | | Mg²⁺ + 2e⁻ → Mg | −2.37 | most active of the entries here | Two cautions travel with the table. Passive alloys occupy two places in a real galvanic series — [[Stainless_steel|stainless steel]] ranks close to copper while its oxide film holds and close to plain iron once the film is broken, so a crevice can move it several volts' worth of position without any change of composition. And aluminium and [[Titanium|titanium]] behave far more nobly than their standard potentials suggest, for the same reason. ## Anodic index For design rather than analysis, military and aerospace practice compresses the series into a single number per metal, the anodic index, expressed in volts, and specifies the largest difference allowed between two metals in contact. The commonly quoted limits tighten with the severity of the service: about 0.15 V for harsh, wet, salt-laden or outdoor environments, about 0.25 V for normal indoor conditions, and about 0.50 V for controlled environments that stay dry and temperature-regulated.[^milstd889] The virtue of the index is that it turns metal selection into arithmetic that a draughtsman can do — look up two numbers, subtract, compare with a limit — without any electrochemistry. Its weakness is that it ignores everything the overview section says about rate. Two metals 0.10 V apart with a thousand-to-one area ratio against the anode can do more damage than two metals 0.40 V apart in a favourable geometry, and no index can see that. [[Corrosion_engineering|Corrosion-engineering]] standards therefore pair the index with a rule on relative areas and with a coating requirement, and the sim is best read as the argument for why that pairing is necessary. ## Electrolytic corrosion Galvanic corrosion supplies its own driving voltage from the metals themselves. Electrolytic corrosion, also called stray-current corrosion, is driven by a current from an external source that finds an unintended path through a structure and an electrolyte. Direct-current traction systems, [[Welding|welding]] return currents and poorly grounded cathodic-protection installations are the classic sources: an [[Electric_current|electric current]] enters a buried pipe at one place and leaves it at another, and at the point where it leaves, the pipe is an anode and dissolves at a rate set by that current, regardless of what the pipe is made of. The distinction matters because the remedies differ. A galvanic couple is fixed by separating the metals or improving the area ratio; a stray-current problem is fixed by finding the source, by bonding the structure so that current has a metallic route back, or by installing drainage bonds that return the current before it can leave through the electrolyte. Both are the same electrochemistry, and Faraday's law converts current to metal loss in exactly the same way; only the origin of the current is different. The single-metal case also makes the point that the word "galvanic" refers to the couple, not to the corrosion: a structure with no dissimilar metal anywhere in it can still be eaten by a current it did not generate. ## See also - [[Galvanization]] - [[Cathodic_protection]] - [[Galvanic_series]] - [[Zinc]] - [[Corrosion]] - [[Pourbaix_diagram]] - [[Electrochemical_cell]] - [[Passivation_(chemistry)]] ## References [^os-appl]: Flowers, Paul; Neth, Edward; Robinson, William; et al. (2019). *Chemistry: Atoms First*, 2nd ed. OpenStax (Portal Book 051). Appendix L, Standard Electrode (Half-Cell) Potentials, pp. 1125–1130. The individual E° entries quoted here are rounded to two decimals; the exact tabulated values are to pin against those pages. https://open.umn.edu/opentextbooks/textbooks/chemistry-atoms-first [^os-166]: Flowers, Paul; Neth, Edward; Robinson, William; et al. (2019). *Chemistry: Atoms First*, 2nd ed. OpenStax (Portal Book 051). Chapter 16, Electrochemistry, pp. 753–790; §16.6 Corrosion — the anodic dissolution and oxygen-reduction half-reactions, galvanic couples and sacrificial protection (section pages to pin within the chapter range). https://open.umn.edu/opentextbooks/textbooks/chemistry-atoms-first [^haverkort-f]: Haverkort, Willem (2024). *Electrolysers, Fuel Cells and Batteries: Analytical Modelling* (Portal Book 053). Chapter 1, Electrochemistry, pp. 20–41; the Faraday constant F ≈ 96,485 C per mole of electrons at p. 25, and Faraday's law in the form `j = n·F·N` at p. 28. https://open.umn.edu/opentextbooks/textbooks/electrolysers-fuel-cells-and-batteries-analytical-modelling [^nps-stli]: National Park Service. *Statue of Liberty National Monument*. https://www.nps.gov/stli/ (The specific restoration page documenting the armature replacement and the isolation detail is to pin; the site root is given here rather than an unverified deep link.) [^alarm]: Royal Navy Admiralty record of the coppering of HMS *Alarm* and the subsequent survey of its iron fastenings, early 1760s. (Report, exact dates and archival reference all to pin; no DOI or URL asserted.) [^davy1824]: Davy, Humphry (1824). On the corrosion of copper sheathing by sea water and on methods of preventing it. *Philosophical Transactions of the Royal Society of London* 114. (Article title as printed, pages and DOI to pin against the journal record.) [^lcs2]: United States Navy reporting from 2011 on galvanic corrosion in the waterjet propulsion system of the littoral combat ship USS *Independence* (LCS-2). (Specific report, issuing office and date to pin; no URL asserted. The mechanism described here — an aluminium hull coupled to steel propulsion components in seawater — follows from the arrangement itself.) [^milstd889]: United States Department of Defense. *MIL-STD-889, Dissimilar Metals*. The anodic-index table and the permissible-difference limits of about 0.15 V, 0.25 V and 0.50 V for harsh, normal and controlled environments. (Standard revision, date and table number to pin; no URL asserted.) ## Sources - Flowers, Paul; Neth, Edward; Robinson, William; et al. (2019). *Chemistry: Atoms First*, 2nd ed. OpenStax. Portal Book 051. Chapter 16 (pp. 753–790) and Appendix L (pp. 1125–1130). https://open.umn.edu/opentextbooks/textbooks/chemistry-atoms-first - Haverkort, Willem (2024). *Electrolysers, Fuel Cells and Batteries: Analytical Modelling*. Portal Book 053. Chapter 1 (pp. 20–41). https://open.umn.edu/opentextbooks/textbooks/electrolysers-fuel-cells-and-batteries-analytical-modelling - National Park Service, *Statue of Liberty National Monument*. https://www.nps.gov/stli/ - MIL-STD-889, *Dissimilar Metals*. United States Department of Defense. Not a Portal Book; revision to pin. ## External links - [*Chemistry: Atoms First*, 2nd ed.](https://open.umn.edu/opentextbooks/textbooks/chemistry-atoms-first), Open Textbook Library — Portal Book 051, open access; Chapter 16 is the electrochemistry this article rests on - [*Electrolysers, Fuel Cells and Batteries: Analytical Modelling*](https://open.umn.edu/opentextbooks/textbooks/electrolysers-fuel-cells-and-batteries-analytical-modelling), Open Textbook Library — Portal Book 053, open access - [Statue of Liberty National Monument](https://www.nps.gov/stli/), National Park Service - For standards and trade-association links, see the external links of the Wikipedia pair at the pinned revision below; none is reproduced here unverified. <!-- MATTERSIM:BEGIN g33 — Matter & Energy Cluster microsim (framework build, specs/sims/Galvanic_corrosion.json); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Galvanic corrosion* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/matter/Galvanic_corrosion.html" data-title="Galvanic corrosion"></div> *Built from `MICROSIM_GUIDE/specs/sims/Galvanic_corrosion.json`; part of the [[PORTAL_Matter|Matter portal]] spine (section sims and See-also variants).* <!-- MATTERSIM:END --> ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Galvanic_corrosion) : [Wikitube](https://en.wikitube.io/wiki/Galvanic_corrosion) · pinned revision [1367882661](https://en.wikipedia.org/w/index.php?oldid=1367882661) · 2026-09-11 ## Previous hub tags Hubs: `Life_Physics`. 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