⚡ Key takeaways
- "Salt spray 500 hours" is not an acceptance requirement. It is missing two of the three elements: method, duration and pass criteria.
- NSS is a comparative test, not a life prediction. Continuous spray at constant temperature and pH cannot represent wet–dry cycling, pollutant deposition or UV.
- Three methods, three severities: NSS at 35 °C and pH 6.5–7.2, AASS at pH 3.1–3.3, CASS adding copper chloride at 50 °C.
- Hours are not comparable across methods. CASS 100 h and NSS 100 h are entirely different tests.
- White rust means the coating is being consumed; red rust means the substrate has been reached. Only one of them is a failure signal.
- Appearance is the pass mark, function is the entry requirement. A fastener that passes the rating but whose torque coefficient has drifted is still a reject.
- ISO 4042 gives minimum salt spray hours by chromate type and coating thickness — 8 μm yellow chromate, for example, is 72 h to white rust and 120 h to red rust.
- Cyclic salt spray correlates better with outdoor exposure, and is the right tool when NSS results do not match field failures.
1. The most common misreading: hours are not life
2. The three salt spray tests
3. Which standard system to use
4. A complete judgment chain: method, duration, criterion
5. What to judge: white rust, red rust and function
6. Zinc plating with chromate conversion: reference hours
7. Cyclic salt spray
8. Different materials need different methods
9. Six test traps
10. How to write an executable requirement
11. Sources
1. The most common misreading: hours are not life
"This bolt is 500 hours salt spray." The phrase appears in engineering correspondence every day, and it is not an acceptance requirement.
Two reasons. First, it lacks information. The same "500 hours" could mean neutral salt spray or copper-accelerated, continuous or cyclic, and judging appearance or function afterwards. Change any one of those and the conclusion can invert.
Second, and more important: neutral salt spray is a comparative and evaluative test, not a life prediction test. It holds the sample at constant temperature, constant pH and continuous spray, while the real atmosphere cycles wet and dry and adds differential aeration cells, pollutant deposition and UV ageing. The corrosion mechanisms differ fundamentally, so converting 96 hours into "X years outdoors" has no reliable basis.
It does have two real uses:
- Lateral comparison: under identical conditions and identical criteria, compare processes, suppliers and batches.
- Batch consistency control: monitoring production or incoming goods for stability, provided conditions and criteria do not change.
Keep within those two and salt spray data is genuinely useful. Push past them into life prediction and the conclusions will be wrong.
2. The three salt spray tests
| Parameter | NSS — neutral | AASS — acetic acid | CASS — copper accelerated |
|---|---|---|---|
| Sodium chloride concentration | 50 ± 5 g/L | 50 ± 5 g/L | 50 ± 5 g/L plus CuCl₂·2H₂O 0.26 ± 0.02 g/L |
| Collected solution pH at 25 °C | 6.5–7.2 | 3.1–3.3, adjusted with glacial acetic acid | 3.1–3.3 |
| Chamber temperature | 35 ± 2 °C | 35 ± 2 °C | 50 ± 2 °C |
| Fallout rate | 1.0–2.0 mL/(80 cm²·h) | As NSS | As NSS |
| Main application | Metals and alloys, metallic coatings, organic coatings, anodic films — the baseline, most widely used | Acidic service; decorative chromium, Ni-Cr and Cu-Ni-Cr | Rapid evaluation of highly corrosion-resistant coatings; anodic films on aluminium |
| Severity | Reference | Above NSS — the acid accelerates hydrogen depolarisation | Highest of the three |
CASS acceleration is not one adjustment but three at once: higher temperature, acidic pH, and divalent copper. Copper ions are strongly depolarising and markedly accelerate the cathodic reaction, which in turn accelerates anodic metal dissolution.
Three rules:
- Hours are not comparable across methods. CASS 100 h and NSS 100 h are not the same test.
- Always state the method with the number. Hours alone say nothing.
- Common durations are 24, 48, 96, 168, 336 and 672 h, with long runs at 720 h and beyond. The longer the run, the more demanding the chamber uniformity and sample placement.
3. Which standard system to use
| Standard | Purpose | Note |
|---|---|---|
| GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests | The Chinese main standard, defining NSS, AASS and CASS conditions | Corresponds to ISO 9227; first choice for domestic projects |
| ISO 9227:2017 | The international main standard | Use where export or a foreign drawing cites it |
| ASTM B117 | US neutral salt spray | Conditions match NSS at 35 ± 2 °C and 1.0–2.0 mL/(80 cm²·h); if the customer cites it, meet its document requirements too |
| ASTM B368 | Copper-accelerated acetic acid salt spray | CASS within the US system |
| GB/T 2423.17-2024 | Environmental testing of electrical and electronic products — salt mist | In force from 2025-03-01, replacing the 2008 edition; applies to electronic products and their parts |
| GB/T 10587 | Specification for salt spray test chambers | This is an equipment standard for chamber acceptance and calibration and cannot replace a test method standard |
| GB/T 6461 / ISO 10289 | Rating of specimens after corrosion testing | The basis for protective rating Rp and appearance rating RA |
| GB/T 20854 / ISO 16701 | Cyclic salt spray testing | See section 7 |
Solution preparation has hard requirements that are easily overlooked: use distilled or deionised water at conductivity ≤ 20 μS/cm; use analytical grade sodium chloride with heavy metals ≤ 0.005 %; and never reuse the solution. These three decide whether the result is reproducible at all.
4. A complete judgment chain: method, duration, criterion
An acceptable salt spray requirement has to state three things, all of them:
| Element | What to write |
|---|---|
| Test method | NSS, AASS, CASS or cyclic? Include the standard number and year |
| Duration | How many hours, and whether that is continuous spray or cyclic; if cyclic, give the cycle programme |
| Pass criterion | Which phenomenon, rated to which standard, at what level does it pass? Are functional properties assessed? |
Compare two real formulations and the problem is immediate:
| Formulation | What is wrong |
|---|---|
| "Salt spray 500 h" | All three elements missing. On the same shipment, choosing NSS or CASS and judging appearance or function can produce opposite conclusions — and neither party has breached anything |
| "Neutral salt spray (NSS) continuous, 500 h per GB/T 10125-2021; after test, rated per GB/T 6461, protective rating Rp ≥ 9 and no substrate corrosion (red rust) permitted" | All three elements present: method with a standard number, an explicit duration and an executable criterion. This one can be accepted against |
5. What to judge: white rust, red rust and function
The criterion must separate two phenomena whose severity differs enormously:
| Phenomenon | Meaning | Severity |
|---|---|---|
| White rust | Corrosion product of the zinc coating — the coating itself being consumed | Relatively minor. It shows sacrificial protection is working; as long as the coating is not penetrated, the substrate is safe |
| Red rust | The steel substrate has begun to corrode | A failure signal. The coating has been penetrated locally and the barrier has gone |
Beyond white and red rust, judge pitting — count, depth and distribution — blistering, cracking, flaking, discolouration, adhesion and coating discontinuity. Some of these are quantifiable, pitting depth by metallography or profilometry; others can only be graded.
Easily omitted but critical: function. For a fastener, at least three things have to be re-checked after salt spray: whether tightening torque and torque coefficient are still in specification, whether tightening behaviour is normal, and whether locking torque on locking types still meets its value. Corrosion resistance can pass while the part will not turn, or while the torque coefficient has drifted, and on an assembly line that is a reject.
Test practice points:
- Samples are normally mounted at 15–30° from vertical.
- Samples must not shadow each other, and in particular one sample must never drip onto another. This is the most common source of non-reproducible results.
- Edges and cut faces interfere with the result; mask them where necessary.
- After the test, rinse gently in running water, dry and assess immediately. Do not wipe — wiping removes the corrosion products and with them the evidence.
6. Zinc plating with chromate conversion: reference hours
ISO 4042 gives minimum salt spray times for zinc electroplating with different chromate conversion treatments. It is one of the most frequently cited tables in practice:
| Chromate treatment | Code | Appearance | Coating thickness (μm) | White rust, minimum (h) | Red rust, minimum (h) |
|---|---|---|---|---|---|
| Clear, bluish (standard) | B | Clear with a blue tinge | 8 | 24 | 72 |
| Yellow chromate | C | Pale yellow iridescence to brown-yellow | 3 | 24 | 24 |
| C | 5 | 48 | 72 | ||
| C | 8 | 72 | 120 | ||
| Olive chromate | D | Olive green to olive brown | 3 | 24 | 24 |
| D | 5 | 72 | 96 | ||
| D | 8 | 96 | 144 | ||
| Black chromate | BK | Dark brown to black, decorative | 3 | — | — |
| BK | 5 | 12 | — | ||
| BK | 8 | 24 | 72 |
Three things to read carefully:
- These are minimum times, not guarantees. What is actually achieved depends on plating quality, passivation process and whether a sealing or post-treatment is applied — sealing in a wax-containing drying solution, for example, improves corrosion resistance markedly.
- Black chromate is decorative and the weakest, which is exactly what the dashes mean. Choose it and accept the trade-off; you cannot have both a black appearance and a long salt spray result. Note also that black chromate applied by barrel plating tends to wear off at the edges of cross recesses, exposing the lighter zinc beneath — a process characteristic, not a defect.
- Thicker coating means longer times, but thickness is limited by thread tolerance. Corrosion resistance cannot be solved by plating ever thicker, because the ceiling is set by the thread tolerance band — see the coating thickness section of the fastener parameter quick-reference.
7. Cyclic salt spray
Continuous spray is stable and reproducible, but it departs from natural corrosion mechanisms. Outdoors, wet–dry cycling creates differential aeration cells and concentrates corrosion products during the dry phase, and neither effect appears in continuous spray.
Cyclic salt spray compensates through a programmed cycle, typically salt mist → dry → humid hold. It sits closer to real marine or road environments and usually correlates better with outdoor exposure.
| Item | Comment |
|---|---|
| Common standards | GB/T 20854, ISO 16701; the automotive industry has its own cyclic corrosion specifications |
| Application | Durability assessment for automotive components, building structures and other long-term outdoor products |
| Another use | When NSS results do not match field failures, cyclic testing often reproduces the real failure mode |
| Cost | Long cycle, complex parameters, poorer inter-laboratory comparability than NSS — the cycle programme must be written into the contract in full |
8. Different materials need different methods
| Material or coating system | Preferred method | Reason |
|---|---|---|
| Zinc plating with chromate | NSS | Conventional assessment with mature criteria, as in section 6 |
| Decorative chromium, Ni-Cr and Cu-Ni-Cr | CASS | Strong acceleration; the industry convention for this system |
| Anodic films on aluminium | CASS or AASS | NSS discriminates poorly between film qualities within a practical duration |
| Stainless steel | Generally unsuitable to judge by NSS alone | Stainless often shows "no obvious reaction" in neutral salt spray while real chloride pitting and crevice risk remain; use dedicated methods such as ferric chloride pitting tests |
| Long-term outdoor parts | Cyclic salt spray | Better correlation with natural exposure |
| High-strength steel fasteners, class 10.9 and above | Complete the hydrogen embrittlement assessment first | If electroplating is used, hydrogen embrittlement is the primary risk, not corrosion. Passing salt spray says nothing about it — see why high-strength bolts should not be plated casually |
9. Six test traps
| Trap | Consequence |
|---|---|
| Comparing hours across different methods | CASS 100 h is not NSS 100 h; the comparison is void |
| Stating hours without a criterion | The result cannot be accepted, both sides argue, and responsibility cannot be assigned |
| Using the equipment standard GB/T 10587 in place of a method standard | Chamber acceptance is not method correctness — a clear mismatch of purpose |
| Samples shadowing each other or dripping into one another | Results are not reproducible and inter-laboratory data cannot be compared |
| Judging appearance only, never function | The corrosion rating passes while torque coefficient or locking torque is already out of specification, and the line still fails |
| Converting salt spray hours into outdoor years | Extrapolating continuous-spray acceleration to a wet–dry natural environment has no reliable basis |
10. How to write an executable requirement
Everything above condensed into a template that can be copied directly:
| Item | Example wording | Which link of the chain it fills |
|---|---|---|
| Surface treatment | Zinc electroplating with yellow chromate conversion (code C), coating thickness 8 μm | Fixes the precondition; without it the method choice has no basis |
| Test method | Neutral salt spray (NSS), continuous, per GB/T 10125-2021 | Element one: method |
| Duration | 120 h | Element two: duration |
| Appearance criterion | Rated per GB/T 6461; no substrate corrosion (red rust); white rust area not exceeding X % | Element three: criterion, appearance part |
| Function criterion | After test, torque coefficient still within 0.12–0.18 and tightening behaviour normal | Element three: criterion, function part |
Note that the 120 h in that example is not invented: per the ISO 4042 table in section 6, yellow chromate (code C) at 8 μm corresponds to a minimum of 120 h to red rust, which makes "no red rust at 120 h" an executable criterion with a standard behind it.
In three closing sentences:
- Salt spray hours are a complete requirement only when method, duration and criterion are all present.
- It is for comparison and batch control, not for predicting life.
- For fasteners, appearance is the pass mark; function is the entry requirement.
11. Sources
- GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests; ISO 9227:2017: solution concentration, pH, chamber temperature, fallout rate and applicability for NSS, AASS and CASS.
- ASTM B117 for neutral salt spray and ASTM B368 for copper-accelerated acetic acid salt spray.
- GB/T 6461 and ISO 10289: rating of specimens after corrosion testing, protective rating Rp and appearance rating RA.
- GB/T 20854 and ISO 16701: cyclic salt spray corrosion testing.
- GB/T 2423.17-2024, in force 2025-03-01, replacing the 2008 edition; GB/T 10587, specification for salt spray test chambers, an equipment standard.
- ISO 4042, Fasteners — Electroplated coating systems: minimum salt spray times for zinc coatings with each chromate conversion type, and coating thickness limits.
- Company technical handbook: surface treatment processes, chromate codes, appearance and corrosion characteristics.
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