Question 4 of 8: Two galvanic corrosion problems

Question 4: Two galvanic corrosion problems (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Given. Two dissimilar-metal assemblies in service: (a) a brass (Cu–Zn) faucet threaded onto an iron (carbon-steel) water pipe, wetted internally by tap water; (b) plain steel screws fastening aluminium siding, exposed to the weather, in which the screws corroded severely.

Find. For each couple, which member is the anode and why; and for (b), whether the observed outcome is what the galvanic series would predict, with an explanation of the mechanism that produced it.

Both parts are galvanic-corrosion problems, so both are governed by the same three questions: which metal is more active in the relevant environment, is there a continuous electrolyte path and a metallic path between them, and what is the ratio of cathode area to anode area? The first question sets the direction of attack, the third sets its intensity, and part (b) turns on both.

(a) Brass faucet on an iron pipe. In the galvanic series for natural waters, copper and its alloys sit well toward the noble (cathodic) end, while iron and steel sit toward the active (anodic) end; the open-circuit potential difference between them in aerated tap water is of the order of 0.3–0.5 V. Connect them metallically at a threaded joint and immerse the joint in water, and a short-circuited galvanic cell is complete. The iron pipe is the anode and corrodes; the brass faucet is the cathode and is protected.

The half-reactions are the familiar pair: at the anode, iron dissolves as Fe → Fe²⁺ + 2e⁻; at the brass cathode, the electrons are consumed by oxygen reduction, O₂ + 2H₂O + 4e⁻ → 4OH⁻, since ordinary tap water is aerated and near neutral. Attack is not spread evenly along the pipe: it concentrates within roughly the first pipe diameter or two of the joint, because that is where the ionic-path resistance through the water is lowest. In practice the result is a wall that thins locally and eventually weeps at the thread root. Two aggravating details are worth noting. First, dezincification of the brass itself is a separate risk in soft or chloride-bearing waters and is the reason plumbing codes call for dezincification-resistant grades. Second, copper ions released from upstream copper tube can plate out on the steel and set up innumerable tiny local cells, an effect quite independent of the joint. The standard remedy is to break the metallic path with a dielectric union or a plastic-lined nipple, so that even though both metals still contact the water there is no electron path between them.

(b) Steel screws in aluminium siding. No, this is not what the galvanic series alone would lead you to expect. Aluminium is thermodynamically far more active than iron — its standard potential is about −1.66 V against −0.44 V for iron — so on first principles the siding should have been the anode and the screws should have been cathodically protected. Yet the screws are what corroded. Three effects, acting together, explain the reversal.

The first and most important is passivity. Aluminium exposed to air instantly grows a tenacious, adherent Al₂O₃ film a few nanometres thick. What the galvanic series ranks is not the thermodynamic potential of bare metal but the measured potential of the surface actually present, and a passivated aluminium surface sits far closer to the noble end than bare aluminium does — in near-neutral aerated water it can sit noble to steel. The couple therefore reverses polarity: the passive aluminium becomes the cathode and the steel screw becomes the anode. The second effect is the area ratio, and it is what turns a mild problem into a severe one. The corrosion rate at the anode depends on current density, not total current. Here a very large cathode — a whole siding panel — is coupled to a very small anode, the exposed head and shank of one screw. All the cathodic current generated over square metres of aluminium must be balanced by anodic dissolution over a few square millimetres of steel, so the local current density, and with it the rate of metal loss, is magnified by that same area ratio. This is the classic “small anode, large cathode” configuration and the single worst arrangement in galvanic design.

The third effect is the local geometry. The screw hole is a crevice: water is retained there by capillarity long after the exposed surfaces have dried, and oxygen inside the crevice is consumed and not replenished. That sets up a differential-aeration cell which drives the oxygen-starved crevice interior anodic, and the hydrolysis of the dissolved metal ions acidifies the trapped water and accelerates the attack further. Driving the screw also scrapes off whatever zinc or organic coating it carried, exposing bare steel exactly where the crevice is. The correct design response is straightforward: use fasteners at least as noble as the sheet — stainless steel or, better still, aluminium or hot-dip galvanised screws with a sealing washer — use a non-conductive or sealant-bedded washer to break the electrical path and exclude water from the hole, and never accept a small-anode/large-cathode geometry when the reverse is available.