Where Machined Stainless Parts Actually Fail in Service

September 27, 2026

None of the failure modes below are machining defects. A part can hold every tolerance on the drawing and still fail in the field because the grade, the geometry or the residual stress was wrong for what it sits in. This is what to check before that happens.

Pitting, the one most buyers already know

Pitting starts on an open surface when chloride ions locally break down the thin chromium oxide film that makes stainless resist corrosion in the first place. Once a pit initiates, the chemistry inside it becomes self-sustaining and it grows faster than the surrounding metal corrodes.

Pitting resistance is what PREN estimates: chromium plus 3.3 times molybdenum plus 16 times nitrogen, from nominal composition. 303 sits near 18, 304 near 19, 316L near 24, 2205 duplex near 35. The comparison across grades is in 303, 304 and 316L compared and the full table is in the grade reference.

Crevice corrosion, the one that catches assemblies

A crevice is any narrow gap where a stainless surface meets something else: a gasket face, a washer under a bolt head, a lap joint, a threaded connection, a deposit sitting on the part. Inside that gap, oxygen depletes and chloride concentrates, and the same passive film breakdown that causes pitting starts at a lower chloride level and progresses faster once it begins.

This is why a part with a perfect open-surface finish can still fail at a joint. The geometry that creates the crevice is usually a design decision, not a machining one:

• A flat gasket face against a flange is close to unavoidable, so the grade has to carry the risk instead.
• A bolted joint with a washer creates a crevice at every washer face. Where the assembly sits wet, this is a real site to expect corrosion, not just theoretical.
• A weld with incomplete penetration leaves a crevice inside the joint. See machining stainless weldments.
• Threads, especially blind ones, trap moisture and concentrate chloride at the root. Threading and tapping stainless covers the geometry side.

What moves the risk. Grade first: 316L over 304 wherever a crevice cannot be designed out. Finish second: electropolishing removes some of the surface irregularity that nucleates crevice attack, covered in passivation against electropolishing. Geometry last, where the design allows it: open drainage paths, avoid horizontal ledges that hold liquid, and specify through-holes over blind ones on wetted parts.

Stress corrosion cracking, the one that needs three things at once

Stress corrosion cracking needs a susceptible material, a tensile stress and a specific corrosive environment, all three present together. Remove any one and it does not happen. This is why it is rarer than pitting but far more dangerous when it does occur, because a part can crack through with almost no visible warning.

The material. Austenitic grades, 304 and 316L included, are susceptible to chloride stress corrosion cracking, particularly above about 60 degrees Celsius. Duplex grades such as 2205 resist it substantially better because of their mixed austenitic-ferritic structure, which is one of the reasons duplex costs more than the corrosion resistance number alone would suggest. See 2205 duplex machining.

The stress. Applied stress from service load matters, but so does residual stress left in the part from cold work. Austenitic stainless work hardens under a machining tool, and aggressive cuts, particularly light finishing passes on an already-hardened surface, leave tensile residual stress right where a crack would start. Work hardening in stainless parts covers how that residual stress builds up during machining.

The environment. Chloride is the common trigger for austenitic grades, at concentrations well below what causes visible pitting. Hot chloride environments, including some clean-in-place chemistries used in food processing, are a known site for this.

What to do about it. Specify the more resistant grade when the environment is genuinely severe rather than trusting a passivation step to compensate. A stress relief anneal after heavy machining is sometimes specified on parts with a known stress corrosion history, though it is uncommon on standard commercial work. Passivation itself does not relieve residual stress, it only restores the chromium oxide film at the surface. See passivation against electropolishing for what each finishing route actually changes.

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What to write on the drawing or in the RFQ

Naming the failure mode you are worried about is more useful to a shop than naming a grade and hoping. A sentence such as bolted joint, wet service, want crevice resistance tells a shop to think about the washer face and the thread class, not just the alloy.

Environment: submerged in seawater intermittently, bolted flange joint with gasket Concern: crevice corrosion at the flange face and washer contact Grade: 316L minimum, 2205 duplex acceptable if cost allows

What to put in a stainless RFQ has the full list of what a shop needs to quote a part with an environment like this correctly the first time. The environment questions on the drawing are listed in our guide to stainless steel grade selection.

Frequently asked questions

Figures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.

Keep reading

• Stainless steel parts for cryogenic service: 304L against 316L, what impact tests do and do not prove and what to put on the order
• 316L for hydrogen service: why nickel content, cold work and heat history matter more than the grade name