Corrosion

Corrosion is not a manufacturing defect: it's a chemical reaction between metal and environment, and every alloy faces it differently. Understanding which mechanism is at play — surface oxidation, pitting corrosion, galvanic attack — changes how you choose a material and maintain it.

Among steels, the reference parameter is PREN (Pitting Resistance Equivalent Number), a numerical index that measures how well an alloy resists localized pitting corrosion. Steel 316L — the mid-to-high range standard — owes its resistance to the presence of molybdenum; the L indicates low carbon, which reduces the risk of carbide precipitation at grain boundaries. 904L adds nickel and molybdenum in greater quantities: Rolex has used it since the Submariner 168000, and it's more expensive to machine because it saturates tools rapidly. Grand Seiko developed Ever-Brilliant Steel with PREN 1.7 times higher than classical 316L, with the added advantage of a whiter surface that holds Zaratsu polishing better. Panerai's 316LVM — AISI 1.4441 — undergoes vacuum arc remelting (VAR) that further increases alloy purity and resistance to chemical attack.

Titanium starts from a different position: it spontaneously forms a titanium oxide layer on the surface that makes it naturally more resistant to salt water than any standard stainless steel. Grade 2 (99.8% purity) offers maximum resistance to marine corrosion, but cannot be polished and remains brushed by nature. Grade 5 (Ti-6Al-4V, also called ELI in the surgical version) is harder and more workable, so more common in sports cases. Citizen solves the poor surface hardness of raw titanium with the Duratect treatment; PVD adds a further protective layer without appreciable weight.

Naval bronze — CuSn8 or similar alloys — has resisted marine corrosion since antiquity: the Navy used it for bilge pumps before stainless steel existed. The paradox is that it oxidizes visibly, but that patina is a protective layer, not damage. Tin (97% alloy with copper and antimony) works the same way: it ages gracefully rather than degrading.

An interesting case from the field is the Easy Clean System by Grands Fonds: a slot on the bezel with a through channel that allows fresh water to circulate between bezel and case after use in the sea. It's a mechanical solution to the problem of salt deposits that, if neglected, cause crevice corrosion — one of the most insidious types because it advances under the surface before becoming visible. From the bench I often see dive watches with seized bezels for this reason: frequent rinsing with fresh water is not an option, it's routine maintenance.