Galvanic corrosion — the same two metals, in different proportions

Drag upward to give the stainless more of the wetted area and the aluminium less; drag down for the reverse. The arrow keys do the same, a step at a time. End goes straight to the worst case; Home to the mildest. Nothing else changes — not the metals, not the water. Time is compressed. Sound is off until you turn it on.

What you are looking at

Two metals in contact in seawater make a battery. The less noble one — here the aluminium — gives up electrons and dissolves; the more noble one, the stainless, does not corrode but provides somewhere for those electrons to go, by reducing dissolved oxygen at its surface.

That reaction is limited by how fast oxygen can reach the metal, so each square centimetre of stainless can only pass so much current. The total current the couple runs is therefore set by how much stainless is wetted — and nothing else.

All of that current has to come out of the aluminium. If the aluminium is large, the current is spread thinly over it and each part loses very little. If the aluminium is small, exactly the same current is concentrated into that small area, and it is eaten through quickly. Halve the anode while the same current is still arriving and you double the rate; the damage follows the ratio of the two areas, not the identity of the metals. Note that the control here does something stronger than that: it splits a fixed total area, so shrinking the anode also enlarges the cathode driving it, and the rate climbs faster than doubling.

The two things worth taking away

A small anode is the dangerous arrangement. An aluminium rivet in a stainless plate is a different object from a stainless rivet in an aluminium plate, even though a materials list would describe them identically. The first concentrates the whole couple current into the rivet.

This is why a coating on the anode can be worse than no coating at all. Paint the aluminium and leave a scratch, and you have not reduced the driving force — you have reduced the anode to the size of the scratch. The same current now leaves through a pinhole. Coat the cathode instead and the couple current falls with it.

What is not verified

Stated plainly, because a piece whose value is trustworthiness should not overclaim. The first list is the part that would change how you read the result. The second is scope — true, worth saying, and not load-bearing.

• No engineer has reviewed this. If you are one and would check it, REVIEW.md says what would actually help and is meant to cost about fifteen minutes. The model is the standard oxygen-limited galvanic couple and the built-in checks confirm it reproduces the closed-form area-ratio law exactly — but that check is the author's own, not a corrosion engineer's.
• The cathode is assumed fully oxygen-limited, at one current density for still, aerated seawater. Real limiting currents vary several-fold with temperature, flow and fouling, and a couple can be under mixed control instead, in which case the ratio law is weaker than the strict proportionality drawn here.
• Resistance of the electrolyte path is ignored. In seawater that is a fair simplification; in fresh water or damp concrete it is not, and distance from the couple starts to matter a great deal.

Scope, for completeness. True of the model, and none of it changes the answer above.

• The attack is drawn as uniform thinning. Real galvanic attack on aluminium is usually pitting, which perforates sooner than an averaged depth suggests — so the real case is worse than the one drawn, not better.
• No passivation, no protective film, no polarity reversal. Aluminium in particular depends on a passive film that this model does not represent, and some couples reverse polarity with temperature.
• Time is compressed heavily, and it has to be: the mild settings take decades. What you watch in a second is about a week.

Commissions

The subject changes; the method does not. If you have a failure mode, a mechanism, or a process that people keep getting wrong because the diagram cannot move, that is the shape of problem this suits. You get one file, the checks that hold it honest, and the notes explaining every decision including the ones that were reversed. Commissions start at £4,000 and take three to four weeks from an agreed brief. yinxueshi1017@gmail.com

Not commissioning anything? Still worth an email. If your version of this failed differently — other geometry, other numbers, a mechanism this piece leaves out — tell me in one line. Corrections are the main thing that decides what gets built next.