ASTM B117 Salt Spray Testing for Industrial Covers: What the Numbers Actually Mean

A procurement manager from a Rotterdam port facility sent us a cover last March. It had been on a dockside conveyor motor housing for 11 months , 150 meters from the North Sea. The fabric was intact.

The 316 stainless buckles were clean. But the zipper had seized solid. Not from dirt. From chloride pitting on the zinc-alloy slider the factory substituted without telling anyone.

He had paid €47 per cover. The covers he had bought before from a different factory cost €31.50. The difference was €15.50 per unit. He had 240 units. The replacement cost , labor, shipping, downtime documentation , ran to roughly €11,000.

That slider was the cheapest part on the cover. Maybe 35 cents. It failed first because nobody specified a salt spray rating for the hardware. They specified it for the fabric. They forgot the fasteners.

This is where ASTM B117 salt spray testing enters the conversation — not as a lab curiosity, but as the single spec line that separates a cover that survives 11 months in a salt-laden environment from one that doesn't.

If you buy or specify industrial covers for coastal facilities, marine equipment, or any installation within 5 kilometers of salt water, the B117 rating on your purchase order is the difference between a maintenance budget you can predict and one you cannot.

What Is ASTM B117 and Why Should a Cover Buyer Care?

ASTM B117 is a standard practice for operating a salt spray (fog) apparatus. First published in 1939, it is the most widely referenced accelerated corrosion test standard in the world.

The current edition, B117-26, defines the chamber conditions, solution chemistry, and specimen placement that create a repeatable corrosive environment.

It does not define pass/fail criteria. It does not tell you how many hours to test. It does not predict how long your cover will survive on a dock in Rotterdam.

What it provides is a reproducible baseline , a way to say "this material corroded at 200 hours under these exact conditions, and that material did not corrode until 1,000 hours under the same conditions."

For a cover buyer, the practical question is simpler: if two factories quote you a cover at different prices, and one has tested its hardware to 1,000 hours B117 while the other tested to 200 hours, the price difference starts to make sense.

The €15.50 gap between a cover that lasts and one that does not is often hidden in a single line of the spec sheet labeled "salt spray rating."

Our facility in Ningbo runs a B117 chamber continuously. We test fabric swatches, coated samples, and assembled hardware , zippers, buckles, grommets, drawstring toggles , as complete sub-assemblies. Testing the fabric alone is insufficient. The failure usually starts at the smallest component.

For more on how we structure our overall QC process, see our quality control standards guide.

How the Salt Spray Test Actually Works

The test chamber holds specimens at a 15 to 30-degree angle from vertical. An atomizer sprays a continuous fog of 5% sodium chloride solution , roughly the salinity of the North Atlantic , at 35°C plus or minus 2°C.

The fog collection rate must be 1.0 to 2.0 milliliters per 80 square centimeters of horizontal collection area per hour.

The salt is not table salt. ASTM B117 requires sodium chloride with total impurities below 0.3% by mass, halides other than chloride below 0.1%, and copper below 0.3 parts per million.

Anti-caking agents, which are common in food-grade salt, act as corrosion inhibitors and are prohibited. The pH of the collected solution must land between 6.5 and 7.2 when measured at 23°C.

Three parameters matter for reproducibility: fog uniformity across the chamber, solution chemistry, and specimen placement. Fog must reach every part of every specimen without dripping from one part onto another. A single chamber can hold 20 to 40 specimens depending on size.

The standard requires at least two fog collection points to verify distribution.

The test runs continuously. No cycling. No drying periods. No UV exposure. This is the first thing to understand about B117: it is a single-stress test that never stops spraying salt fog.

Real environments cycle between wet and dry, hot and cold, salt and sun. We will come back to why this matters.

How Many Test Hours Equal How Many Years in the Field?

There is no linear conversion.

ASTM B117 itself states plainly that "prediction of performance in natural environments has seldom been correlated with salt spray results when used as standalone data."

The corrosion rate inside a chamber at constant 35°C with continuous salt fog has no fixed relationship to corrosion on a factory floor in Hamburg with seasonal temperature swings and intermittent exposure.

That said, decades of industry practice have produced rough reference points that procurement managers use for ballpark comparisons:

These are approximations, not guarantees. A cover that passes 500 hours B117 can fail in 18 months on a Florida dock if the UV simultaneously degrades the coating.

The B117 number tells you about chloride corrosion resistance specifically — not about UV stability, abrasion resistance, or cold-crack performance. For a broader look at how covers fail across multiple failure modes, see our analysis of common cover failure causes.

How Different Cover Materials Perform Under ASTM B117

The material hierarchy under salt spray breaks into two categories: the fabric body and the hardware. Buyers who specify only the fabric rating walk into the Rotterdam zipper problem every time.

The hardware column is the one most buyers overlook. Zinc-plated steel buckles start showing white rust at 72 hours and red rust by 200 hours.

In a coastal installation, that translates to a seized buckle within six months.

304 stainless extends the window to 200 to 600 hours , workable for inland outdoor use but insufficient within 5 kilometers of salt water. 316 stainless, with its 2 to 3% molybdenum content, passes 1,000 hours without red rust and often reaches 2,000-plus hours.

The molybdenum is what blocks chloride pitting.

For the fabric body, the coating matters more than the base weave. A 600D Oxford with quality PU coating at 5 to 6 grams per square meter will outlast a heavier fabric with a thinner coating.

We have seen 210D Oxford with PVC backing delaminate at the edges within 120 hours , the salt fog attacks the interface between coating and fabric where the cut edge is exposed.

For covers used in marine environments specifically, see our saltwater environment covers guide.

What ASTM B117 Does Not Tell You

The standard's own text contains the warning.

Section 3.2 states: "Prediction of performance in natural environments has seldom been correlated with salt spray results when used as standalone data."

The correlation coefficient between B117 results and real-world atmospheric corrosion is approximately 0.11 according to a NACE corrosion study , meaning the test explains about 1% of the variance in actual field performance.

Why the low correlation? Because real environments cycle. A cover on a generator in a Scottish harbor experiences salt spray during storms, UV exposure on clear days, freeze-thaw cycles in winter, and mechanical abrasion from wind flapping.

The B117 chamber provides exactly one of those stresses , continuous salt fog at constant temperature , for the entire test duration.

Cyclic corrosion tests address this gap. ISO 12944-9, originally developed as ISO 20340 for offshore coating performance, combines 72 hours of UV exposure, 72 hours of salt spray, and 24 hours of freezing at -20°C in a repeating weekly cycle.

Its correlation coefficient with real-world marine corrosion reaches 0.71 to 0.79 , roughly seven times better than B117. For covers that will experience combined salt, sun, and freezing conditions, specifying ISO 12944-9 in addition to B117 provides a more complete picture.

The practical takeaway: B117 is useful for comparing materials within a single test run in the same chamber. It tells you that 316 stainless outperforms 304 stainless, which outperforms zinc-plated steel.

It does not tell you that a cover that passes 500 hours B117 will survive five years on your site. For that prediction, you need cyclic testing, field data, and an understanding of your specific environment.

How to Write a Salt Spray Spec Into Your OEM Cover Order

Six lines to add to your next RFQ or purchase order. Each one addresses a specific failure mode we have seen in the field:

1. Fabric body: ASTM B117, minimum 240 hours without coating delamination or visible substrate corrosion. This is the baseline for any outdoor industrial cover. For coastal installations within 5 kilometers of salt water, raise it to 500 hours.

2. All hardware: 316 stainless steel, ASTM B117 tested to 1,000 hours minimum without red rust. This covers buckles, D-rings, zipper bodies, grommets, snaps, and drawstring toggles. Do not accept "stainless steel" without the grade number. 304 is not 316.

3. Zipper tape and slider: specify the slider material separately from the fabric body. A common factory substitution is a zinc-alloy slider on a polyester zipper tape. The tape survives B117. The slider seizes at 200 hours.

Require that the assembled zipper , not the components , pass the specified B117 duration.

4. Thread: bonded polyester with UV stabilizer, tested to match the fabric B117 rating. Nylon thread absorbs 4 to 6% moisture by weight in humid conditions, swells, and loses tensile strength under UV. Bonded polyester does not.

The cost difference is about 12 to 18 cents per cover.

5. Stitch holes and cut edges: require hem sealing method. Salt fog enters the fabric through stitch holes and cut edges via capillary action. A folded hem with a single stitch line wicks moisture into the fabric core.

A welded hem or double-fold with thermal sealing blocks this path. This adds 20 to 30 cents per cover on standard sizes.

6. Test certificate: require a dated lab report from an ISO 17025 accredited laboratory. A factory claiming "tested to ASTM B117" without a certificate has not tested anything.

The certificate should show the specimen description, test duration, chamber conditions, and photographic evidence at minimum 168-hour intervals.

For a complete walkthrough of the OEM specification process from RFQ to container, read our custom cover OEM spec guide. And for covers deployed in high-temperature environments where silicone-coated fiberglass becomes the default material, see our high-temperature insulation covers guide.

Frequently Asked Questions