Stainless steel owes its name to a passive film a few nanometres thick. Pitting and crevice corrosion are what happens when that film fails in one small place while surviving everywhere else — which is why a tank that is 99.9% pristine can still leak, and why the failure is always in the spot you could not inspect.
Pitting: the self-drilling hole
Pitting starts where the passive film breaks locally — typically at a chloride-adsorbing site, an inclusion or a surface defect. Once a pit initiates, it becomes self-sustaining: the confined solution inside acidifies and concentrates chloride, dissolving metal faster, which deepens the confinement. The result is attack that drills rather than spreads — pits with small mouths and disproportionate depth, easily missed by visual inspection and devastating to pressure boundaries.
Crevice corrosion: pitting with a head start
A crevice — under a gasket, a deposit, a clamp, a washer, a lap joint — provides ready-made confinement, so the same acidified chemistry develops without needing to initiate a pit. That is why crevice attack begins at lower temperatures and milder chemistry than open-surface pitting, and why the damage hides exactly where the geometry prevents looking: under the very feature that caused it.
What decides susceptibility
- The alloy. Resistance to localised attack scales with composition — chromium, molybdenum and nitrogen. A grade that performs in fresh water can pit rapidly in warm brackish service.
- Temperature. Localised corrosion has threshold behaviour: small temperature increases can move an alloy from immune to attacked.
- Chloride and oxidants. Concentration by evaporation matters more than bulk values — the film of seawater that dries on a hot surface is far more aggressive than the sea.
- Surface condition. Embedded iron, weld heat tint and rough finishes all lower the initiation barrier.
- Stagnation. Flow refreshes the surface chemistry; dead legs and shutdowns let local cells establish.
What the investigation establishes
Sectioning pits to read their morphology; analysing pit contents and deposits for the driving chemistry; checking the alloy against its specification (under-specified or substituted material is a recurring root cause); and reconstructing the local conditions — temperature, wetting, stagnation — that crossed the threshold. The output is a fix matched to the true driver: a more resistant grade, a design change that removes the crevice, surface treatment, or operating and inspection changes. The corrosion-investigation method in full →
When to contact MTIS
When a "corrosion-resistant" component leaks or pits; when localised attack keeps returning after repairs; or when you need to know whether the rest of the system shares the conditions that caused the first hole. Keep pitted samples wet-or-dry as found and uncleaned, and request an investigation. Related service: Corrosion & Integrity →