Guide
Does a Salt Spray Test Prove a 316 Part Will Survive at Sea?
A salt spray test is a laboratory corrosion test in which parts sit in a heated chamber filled with a fog of salt solution, as described in ASTM B117 and ISO 9227. Short answer: no, it does not prove a 316 part will survive at sea, because the standard itself says results have seldom been correlated with natural-environment performance; on 316 hardware it is useful as a screen for contamination and poor passivation, and the pitting and crevice questions need other tests.
By Saola Manufacturing engineering team · Published · Updated
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What does a salt spray test measure, and what does it leave out?
It produces relative corrosion information for metals and coated metals under one fixed fog, and the standard itself warns that predicting performance in natural environments has seldom been correlated with its results.
The scope of ASTM B117-26a says the practice covers the apparatus, procedure and conditions needed to create and maintain the test environment, that its purpose is relative corrosion resistance information for metals and coated metals, and that it does not prescribe the type of specimen or the exposure periods for a specific product (ASTM B117-26a). It adds that correlation and extrapolation should be considered only where appropriate long-term atmospheric exposures have been run alongside. So the hours figure on a purchase order is not coming from the test standard; it comes from whoever wrote the part specification.
ISO 9227 is described by one test laboratory as the European derivative of ASTM B117. Its neutral salt spray option uses 5 percent sodium chloride solution at a chamber temperature of +35 °C with pH held between 6.5 and 7.2, and the test duration is variable (Ascott Analytical, ISO 9227 page). Both tests are the same idea: a standard fog, a variable duration, a comparison between samples.
Why does a long hours number say so little about 316 at sea?
Because 316 fails at sea by localized attack, pitting and crevice corrosion in warm chloride water, and a fog chamber is not designed to rank or reproduce that.
Atlas Steels grade data describe 316 as subject to pitting and crevice corrosion in warm chloride environments and as not resistant to warm seawater, with brown surface staining common in marine use (AZoM, 2001). Those are the failures that matter on a cleat base, a through-bolt washer or a hinge pin sitting in a wet crevice. Test methods built for them exist and are separate from salt fog: ASTM G48 ranks stainless steels and related alloys for pitting and crevice resistance in ferric chloride solution (ASTM G48-11(2020)e1), ASTM G150 finds an electrochemical critical pitting temperature for alloys from Type 316 up to 6 percent molybdenum grades (ASTM G150-18), and ASTM G78 is a guide to crevice-corrosion testing of stainless alloys in seawater and other chloride-containing waters (ASTM G78-20 (2025)).
The practical reading is simple. A part can pass a very long salt fog and still crevice-corrode in service under a washer, and a part can show rust stain in a short fog because of surface contamination while the alloy underneath is sound. The hours number alone cannot separate those two cases.
What is salt spray good for on 316 hardware?
It is a reasonable screen for surface problems: embedded iron, contamination from tooling, and poor passivation, because those show up as rust staining in a wet chloride fog.
An H&G Houston technical FAQ on stainless castings in salt spray links rust on stainless to pitting from salt water exposure, and says that where the cause is embedded iron contamination, passivation may eliminate the unsightly rust (H&G Houston FAQ, accessed 2026). The same answer says that resistance to chloride pitting rises with alloying content, particularly chromium and molybdenum, so a fog test can also reveal material that is under-alloyed for the chlorides involved. ASTM A967 goes further and names a salt spray test as one of seven practices for confirming that passivation was effective, alongside water immersion, high humidity, copper sulfate, potassium ferricyanide, damp cloth and boiling water (ASTM A967/A967M-17).
Use it, then, as a lot-release or process check on the surface condition, not as proof of service life. The table matches each question to the test that is built for it.
| Question to answer | Test that fits | What it tells you | What it does not tell you |
|---|---|---|---|
| Is there free iron or contamination on the surface after machining? | Salt spray (ASTM B117 or ISO 9227) or another A967 practice | Rust staining shows surface contamination | Service life of the alloy at sea |
| Was passivation effective? | Practices in ASTM A967, including its salt spray practice | Pass or fail on the surface condition | Crevice or pitting resistance of the alloy |
| Is the alloy resistant enough to pitting? | ASTM G48 or ASTM G150 | Ranks alloys by pitting initiation or critical pitting temperature | Performance of a specific machined joint |
| Will a joint crevice-corrode in seawater? | ASTM G78 crevice guide, or a design review | Compares crevice resistance under defined conditions | Cosmetic staining from surface iron |
| Does a plated or coated part meet its coating specification? | Salt spray per the coating standard, with its own hours and criteria | Lot-to-lot comparison of the coating | Behavior of the 316 substrate in a crevice |
| Does the part survive a harsher, wet-dry environment? | Modified or cyclic salt fog such as ASTM G85 annexes | A more corrosive comparison than the neutral fog | A guaranteed life; G85 does not set exposure periods |
What should you ask for instead of a long hours number?
Ask for the test that matches the failure you fear, written as a named method with a pass criterion, plus a design that avoids crevices in the first place.
For solid 316 hardware that means three layers. First, material evidence: the mill certificate and PMI to confirm the grade, since the chloride resistance comes from chromium and molybdenum. Second, surface evidence: a passivation requirement and, if you want proof, one of the A967 verification practices rather than an open-ended fog. Third, design evidence: open joints, drainage, and no stagnant crevices, because no hours number rescues a trapped washer. If the application is truly severe, ask for a pitting or crevice method from the table, run on material from the same heat.
For plated, PVD or powder-coated parts, the hours do matter, but they come from the coating specification and not from the stainless. The guides on zinc plating salt spray hours and salt spray for anodize and powder coat cover those numbers. This guide is about what the fog means on the 316 itself.
Is a salt spray requirement different for coated 316 hardware than for bare 316?
Yes: a coated part is tested against the coating's own specification, while bare 316 has no standard hours figure to meet, so the pass criterion must be written out.
A coating test compares lots of a finish against a number the finish standard defines, and a failure points to the coating, the pretreatment or the process. Bare or passivated 316 has no such number in B117, which does not prescribe exposure periods at all. When a customer asks for a fog test on bare stainless, the only defensible approach is to write the duration, the surface condition, the sample plan and the reject criterion yourself and to treat the result as a screen.
Where a finish is added to 316 hardware, the substrate and the coating both enter the picture. A failed fog on a coated part is a question for the finishing process before it is a question for the alloy. The surface finishing page lists the finishes Saola manages through partner plants.
How should the RFQ and acceptance clause read?
Name the standard and edition, the duration, the specimens and condition, the inspection method and the reject criterion, and say what the result will be used for.
A usable clause has six parts, each left blank for the owner of the specification to fill:
- Standard and edition, for example ASTM B117 or ISO 9227 neutral salt spray, so both sides run the same fog.
- Duration, taken from the coating or product specification, because the test standard does not prescribe one.
- Specimens: finished parts from each lot in the as-shipped condition, with the number of pieces stated, not coupons.
- Reject criterion: for example no rust staining or pitting visible without magnification, and how pass or fail is read.
- Purpose: lot release or process verification, so nobody reads a pass as a service-life guarantee.
- Report: who runs the test, the chamber conditions and the photographs, supplied with the shipment.
Stainless bar is not among the materials listed on the CNC turning and CNC milling pages, so confirm material and finishing scope at drawing review rather than assuming it. Test reports come from whoever ran the test; the guide on passivation of machined stainless parts covers the surface callout.
Frequently asked
Is a thousand hours of salt spray good for 316?
The number means nothing without the standard, the pass criterion and the part. ASTM B117 does not prescribe exposure periods for a product (ASTM B117-26a), and the standard warns about correlation to natural environments.
Is ISO 9227 the same as ASTM B117?
They are closely related. One laboratory describes ISO 9227 as the European derivative of B117, and its neutral salt spray option uses 5 percent sodium chloride at +35 °C (Ascott Analytical, ISO 9227 page). Name the standard and edition on the drawing so both sides run the same test.
Why does 316 show rust spots in a salt fog?
Often embedded iron from tooling or handling rather than the 316 itself. The H&G Houston FAQ says passivation may eliminate rust that comes from embedded iron contamination (H&G Houston FAQ, accessed 2026). See the guide on rust spots on stainless steel.
What is cyclic salt fog testing?
ASTM G85 sets conditions for five modified salt spray (fog) tests, including a cyclic acidified salt spray and a seawater acidified test, for when a harsher comparison than neutral fog is wanted; it does not specify the specimen, exposure periods or how to interpret results (ASTM G85-19).
Can a passing salt spray report be used as a marine service guarantee?
No. It shows a surface or process condition under one fog. Service at sea depends on grade, design details, crevices, temperature and maintenance, none of which the fog reproduces.
Sources
- ASTM B117-26a, Standard Practice for Operating Salt Spray (Fog) Apparatus (store page, scope and significance), ASTM International, 2026
- ISO 9227 salt spray test standard, Ascott Analytical Equipment, accessed 2026
- ASTM A967/A967M-17, Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts (store page), ASTM International, 2017
- ASTM G48-11(2020)e1, Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution (store page), ASTM International, 2020
- ASTM G78-20 (2025), Standard Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing Aqueous Environments (store page), ASTM International, 2025
- ASTM G85-19, Standard Practice for Modified Salt Spray (Fog) Testing (store page), ASTM International, 2019
- ASTM G150-18, Electrochemical Critical Pitting Temperature Testing of Stainless Steels and Related Alloys (store page), ASTM International, 2018
- Stainless castings in salt spray environment, technical FAQ, H&G Houston, accessed 2026
- Stainless Steel: Grade 316, supplied by Atlas Steels, AZoM, 2001