The Galvanic Series and Forms of Corrosion

Ranking the metals: the galvanic series

Guide 1 left you with a working corrosion cell: metal dissolving at an anode where it gives up electrons (oxidation), those electrons flowing through the metal to a cathode where something else is reduced, and an electrolyte closing the loop. That picture answered how metal corrodes. It left one sharp question hanging: when two different metals sit in the same puddle, which one plays the anode and pays with its own atoms? The answer is a ranking called the galvanic series — a list of metals and alloys ordered from the most active (eager to corrode) at one end to the most noble (content to stay put) at the other.

GALVANIC SERIES  (measured in seawater)

  ANODIC / ACTIVE   (gives up electrons -> becomes the ANODE, corrodes)
     Magnesium
     Zinc                 <- galvanizing; sacrificial anodes
     Aluminum alloys
     Carbon / mild steel, cast iron
     Lead, Tin
     Brass, Copper, Bronze
     Nickel
     Stainless steel (PASSIVE)
     Titanium (PASSIVE)
     Silver
     Graphite, Gold, Platinum
  CATHODIC / NOBLE  (takes electrons -> stays protected)

  Couple ANY two of them:
    the higher (more active) metal becomes the ANODE and corrodes;
    the lower (nobler) metal is the CATHODE and is protected.
  The wider the gap, the stronger the driving voltage.

Two honest cautions come with the list. First, this is the practical galvanic series, measured in one environment — usually seawater — and the order can shuffle in a different electrolyte; it is an experimental ranking, not the tidy standard EMF series of pure elements you may meet in chemistry. Second, the passivating metals — stainless steel, titanium, aluminum — sit high and noble only while their thin protective film holds; strip that film and they slump back toward their bare, active electrode potential, so one metal can occupy two places at once. And crucially, the series predicts the direction of attack, never the rate: how fast the anode actually corrodes depends on kinetics, conductivity, and area, which the rest of this guide unpacks.

When two metals touch: galvanic corrosion

Put a copper pipe in direct contact with a steel tank in wet ground, or drive a steel bolt into an aluminum boat hull, and you have built a battery you did not want. This is galvanic corrosion: two dissimilar metals, electrically connected and bathed in one electrolyte, form a couple in which the more active metal is forced to be the anode and corrodes faster than it ever would alone, while the nobler metal sits as a protected cathode. It is a tug-of-war the reactive metal always loses. The gap in the galvanic series sets the driving voltage — couple magnesium (very active) to copper (noble) and the pull is over a volt; couple two neighbours in the list and it is feeble.

That same one-sided outcome, deliberately arranged, is one of the oldest tricks in corrosion protection. Galvanized steel is steel dipped in zinc: zinc sits above iron in the series, so the zinc coating becomes the anode and corrodes first, feeding electrons to the steel and keeping it cathodic even where the coating is scratched clean through. The zinc is a sacrificial anode — a cheap metal that agrees to rust so the valuable part does not, exactly like the zinc or magnesium slabs bolted to a ship's hull and to buried pipelines. (Turning this into a full protection strategy — coatings, cathodic protection, inhibitors — is guide 5's job; here we only note that galvanic corrosion, aimed on purpose, becomes a shield.)

Uniform, and the treacherous localized kinds

Not all corrosion is treacherous. Uniform corrosion — the familiar even rusting of an unpainted steel gate, or the general tarnish of a copper roof — attacks the whole surface at nearly the same rate. It is, oddly, the friendly kind: because it is even and steady, you can measure its rate (millimetres or mils lost per year) and simply design in a corrosion allowance, extra wall thickness meant to be sacrificed over the part's life. A tank corroding at 0.1 mm per year needs about 3 mm of extra steel to last 30 years, and you can inspect it and predict its end. The dangerous forms are the ones that hide — that bore a needle-hole while the surface stays bright — and the rest of this guide is a tour of them.

Pitting is localized corrosion at its most deceptive. On a metal that leans on a thin protective film, a pit usually starts where a chloride ion punches a microscopic hole through that film: a single tiny spot turns anodic while the whole shielded surface around it stays cathodic. Attack then drives downward into that spot, drilling a narrow pit that can perforate a tank wall while less than a gram of metal is gone and the surface still looks clean. It is the cavity in an otherwise healthy-looking tooth — a small opening over a growing hollow. Judging such a part by its total weight loss is dangerously misleading, because all the damage is concentrated in the depth of one hole.

1. A small spot loses its defence — a chloride ion cracks the protective film, or a sheltered pocket runs out of oxygen — so that spot becomes the anode, dissolving metal: M goes to M(n+) plus n electrons.
2. The large, oxygen-rich surface all around it becomes the cathode (oxygen plus water plus electrons goes to hydroxide), so it stays protected and funnels every bit of attack into the one small spot.
3. Positive metal ions pile up inside the pit, so negative chloride ions migrate in to balance the charge — the pit fills with concentrated metal chloride.
4. That chloride solution reacts with water (hydrolysis) to make acid, so the inside of the pit turns acidic and even more aggressive, dissolving metal faster still.
5. Faster dissolution makes more ions, which pull in more chloride, which make more acid — the loop feeds itself (it is autocatalytic), so the pit bores deep while the outside looks untouched.

Crevice corrosion runs on the very same self-feeding chemistry, but its trigger is geometry rather than a broken film. Anywhere the electrolyte goes stagnant — under a gasket, a washer, a bolt head, a barnacle, a pile of dirt — the trapped liquid soon uses up its dissolved oxygen and cannot get more, while the open surface just outside stays freshly oxygenated. That difference alone builds a cell: recall the oxygen concentration cell from guide 1, in which the oxygen-starved region is forced to be the anode. The sheltered crevice corrodes, the exposed face is protected, and the same acidifying loop drills it deeper. The defence is to design crevices out: prefer welds to lapped or bolted joints, seal or avoid stagnant pockets, and let assemblies drain.

Attack that follows the microstructure

Some corrosion ignores the surface and follows the material's own internal seams. Intergranular corrosion attacks preferentially along grain boundaries — the mismatched seams where two crystal grains meet, like floor tiles laid at slightly different angles — while the grain interiors are barely touched, so grains can literally drop out and a part crumbles with little overall metal loss. The textbook case is sensitized stainless steel. Stainless resists because dissolved chromium (above about 12 percent) builds a protective film; but hold an austenitic stainless at 500 to 800 degrees C — exactly what the heat-affected zone beside a weld experiences — and chromium carbides precipitate right along the grain boundaries, robbing the thin strips next to them of the chromium they need. Those chromium-depleted strips lose their protection and corrode as narrow anodes. It is called weld decay, and it is why welded stainless can rot in a line a few millimetres from a perfectly sound weld.

The cures follow straight from the cause: use a low-carbon grade like 304L so there is barely any carbon to form carbides, use a grade stabilized with titanium or niobium that seize the carbon before chromium can, or solution-anneal and quench to redissolve the carbides. A quieter cousin of this structural attack is selective leaching, or dealloying, where the environment dissolves one element out of an alloy and leaves the rest as a weak, porous skeleton. In the dezincification of brass, zinc is leached out and the copper is left as a spongy, low-strength mass that keeps the original shape but has lost its strength; in the graphitic corrosion of gray cast iron, the iron dissolves and leaves a fragile graphite network. The trap is that the part looks whole — same size, same outline — right up until it fails, because the damage is in what is missing, not in what you can see.

Finally, erosion-corrosion is the alliance of flow and chemistry. A fast or turbulent liquid mechanically scours away the protective film — or the corrosion products that would otherwise slow attack — and lays bare fresh metal for the electrolyte to eat, so the two act far faster together than either could alone. It carves horseshoe grooves in pump impellers, thins the outer wall of pipe elbows where the stream slams into the bend, and pits condenser tubes; a violent version, cavitation, does the same when collapsing vapour bubbles hammer the film off. Slower flow, smooth bends, a more film-tenacious alloy, and generous entry radii all help. The thread running through every one of these forms is the same: corrosion concentrates wherever the geometry, the flow, or the microstructure lets one small region become and stay the anode.

Stress plus environment: cracking and hydrogen

The most insidious forms need two things at once. Stress-corrosion cracking appears when a sustained tensile stress and a specific corrosive environment act together on a susceptible alloy — and neither the stress alone nor the environment alone would do any harm. Fine, branching cracks grow slowly through the metal, and here is the unsettling part: they look brittle even in an otherwise tough, ductile alloy, so a part can snap with no warning and no visible corrosion. The pairings are maddeningly specific: chlorides crack austenitic stainless steel, ammonia cracks brass (the old 'season cracking' of cartridge cases stored in stables), caustic soda cracks carbon steel. Think of it through the fracture mechanics of the failure rung — corrosion opens and sharpens a crack, the tensile stress concentrates at its tip, the tip corrodes forward, and the crack ratchets across the section until what is left can no longer carry the load.

Hydrogen embrittlement is the other two-part attack, and its agent is the smallest atom of all. Atomic hydrogen — released by a corrosion reaction, by acid pickling, by electroplating, or by over-vigorous cathodic protection — is tiny enough to diffuse straight into the steel lattice, where it collects at flaws and slashes the metal's toughness, so a component can crack under a stress well below its yield strength. The cruel twist echoes a lesson from the failure rung: the strongest steels are the most vulnerable, because the very high-strength microstructures that resist yielding are exactly the ones hydrogen embrittles most. It differs from stress-corrosion cracking in mechanism — hydrogen entering and weakening the metal, rather than a surface dissolving forward — though in practice the two can be hard to tell apart, and both are reminders that strength bought at the cost of toughness is a bargain the environment can call in.