SHORT ANSWER
Crevice corrosion starts where seawater sits trapped and starved of oxygen: under washers, inside turnbuckle threads, beneath cleat bases and along the chainplate slot. The passive film that protects stainless cannot renew inside the gap, so the metal attacks itself locally while the outside stays bright. A better alloy raises the threshold; it does not remove the geometry.
When a fitting cannot simply be remade in a better alloy, a barrier coating is the other route: it changes the surface the crevice sees. The deposit grades, hardness figures and salt-spray acceptance tests used on plated marine hardware are documented in the corrosion and wear reference kept by Hardface Notes, which names the public standard behind each number.
Three checks before you act
- 01Inspect the gap, not only the bright face of the fitting.
- 02Separate the alloy question from the coating question before ordering parts.
- 03Record the test name and its conditions behind any corrosion claim.
What is crevice corrosion and where does it hide on rigging hardware?
Stainless steel resists rust because a thin passive film repairs itself in contact with oxygen. Inside a narrow gap that film starves: seawater enters, oxygen cannot renew, the trapped water turns acidic and chloride-rich, and the metal begins to dissolve at the tightest point. The attack is local and self-feeding, which is why a fitting can look serviceable on deck while it thins out of sight.
On a sailing boat the usual crevices are the turnbuckle body around its threads, the shackle pin inside the bow, the chainplate where it crosses the deck sealant, the underside of a cleat or stanchion base, and the winch drum against its shaft. Bedding compound that traps salt water counts as a crevice too. Note rust weeping from a joint, staining that returns after cleaning, or a fitting that feels rough at its contact faces. The attack hides at the interface, so the inspection has to open the interface.
When does a coating on the part beat upgrading the alloy?
Moving toward a more resistant alloy raises the threshold but keeps the same geometry: the crevice is still there. An upgrade pays when the whole part is being replaced anyway and the water stays moderate. It pays less when the part is large, threaded, oddly shaped or expected to sit in warm trapped water, because the gap defeats the alloy before the alloy defeats the gap.
A barrier layer answers a different question. It puts a controlled surface between the steel and the water, and a properly applied deposit can coat recesses and blind holes that a mechanic cannot polish or inspect. Coating earns its place when wear accompanies corrosion, when the part shape favors an even deposit, and when a verified layer costs less than remaking the fitting in a premium alloy. The trade is real: a coating adds a process step, a thickness to account for on threads, and a new failure mode if the layer is scratched through.
How do you read a corrosion test result before trusting it on deck?
A quoted salt-spray figure counts hours in a cabinet under one standard, usually a continuous neutral fog such as ASTM B117. The number ranks finishes against each other inside that cabinet; it does not translate into seasons at sea. Before using the figure, write down the test standard, the coating thickness, the substrate, the surface preparation and what counted as failure. Two results are only comparable when all five lines match.
For the alloy route, pitting and crevice tests report temperatures or indices rather than hours, and a higher number means the metal tolerated a harsher bath before attack began. Match the test geometry to the part: a pass on a flat coupon says little about the thread roots of a turnbuckle. Keep the test name, date and condition in the log beside the fitting, and let inspection intervals, not the brochure, decide when the part is next questioned.
BOUNDARY
A note cannot clear a loaded fitting
Crevice attack on a chainplate, a shackle pin or a turnbuckle is a structural question, not a cosmetic one. Deep pitting, a crack line, a seized thread or any doubt about section loss means the part leaves service or goes to a rigger before the next sail.
Extend the same record to the rig
Check the fittings against the mast tuning baseline, photograph interfaces the way the wear diagnostic records sail damage, and keep the part list beside the reefing hardware map so replacements match the boat.
SOURCES AND DATE
Checked 11 September 2026. Cabinet and laboratory results rank finishes under stated conditions; they do not predict the life of a specific fitting on a specific boat.