Salt spray testing is conducted in a chamber where specimens are continuously exposed to a fine mist of saltwater solution. This accelerated test is commonly used to evaluate corrosion protection, especially for coated metals.

Purpose and Methodology of Salt Spray Testing

Salt spray testing (typically per ASTM B117 or ISO 9227) is an accelerated corrosion test that exposes materials to a constant fog of salt solution in a heated chamber. The standard neutral salt fog uses about 5% sodium chloride at ~35 °C, with specimens placed at an angle so that a salt mist continuously settles on them. The test runs for a set duration (often in multiples of 24 hours) before examining the samples for corrosion products (like rust or pitting). This method is quick, repeatable, and well-standardized, making it popular for quality control and comparative evaluation of protective coatings. For example, manufacturers might specify that a zinc-plated fastener must survive 96 hours with no red rust as a criterion. In essence, the salt spray test’s primary purpose is to rank relative corrosion resistance under severe chloride exposure – originally to check coatings for defects or porosity – rather than to predict exact service life.

Test methodology: In a typical test, specimens (such as fasteners) are cleaned, placed in the chamber, and continuously misted with saline fog. There are neutral (plain NaCl) tests and modified versions (such as acetic acid or copper-accelerated tests for coatings). The results are usually reported as hours to first sign of corrosion (e.g. rust or staining). More corrosion-resistant materials or coatings will endure longer before showing damage. Standards do not fix a maximum duration, but common test periods range from 24 up to 1000+ hours depending on the expected resistance. Importantly, this test creates a constantly wet, high-chloride atmosphere that is much harsher than many real-world environments (even more aggressive than natural seawater in some respects). It’s a useful comparative tool under controlled conditions, but it is not a direct simulation of actual outdoor exposure, which usually involves dry periods, UV radiation, and other factors absent in a salt fog chamber.

Meaningful or Misleading Results for Stainless Steels?

When it comes to stainless steel fasteners, salt spray test results must be interpreted with caution. Stainless steels resist corrosion by forming a thin passive oxide layer. A continuous salt mist can attack this passive film and induce localized corrosion (like pitting or crevice attack), but the test conditions do not always reflect real service conditions. Key considerations include:

• Overly Severe Conditions: The salt fog environment is extremely aggressive for many stainless grades. In fact, the ISO 9227 salt spray standard itself warns that results often cannot be correlated to long-term behavior in real service. Stainless parts that might endure decades in a natural environment could show failure (pitting or rust stains) after days or weeks in ASTM B117 testing. For example, a 304 stainless component may start to pit or discolor in a salt chamber even though, in intermittent wetting outdoors, it might remain largely rust-free with occasional maintenance. The test essentially exaggerates chloride exposure, sometimes producing corrosion that would never occur under normal use.
• Lack of Real-World Factors: Salt spray cabinets maintain 100% relative humidity and constant salt deposition. Actual service environments (even coastal sites) typically involve wet-dry cycles, rinsing by rain, formation of protective scale, temperature fluctuations, and UV exposure – all of which affect corrosion processes. None of these factors are present in a steady-state salt fog. As a result, salt spray can be a poor predictor of behaviors like patina formation or time to first maintenance. It tends to promote worst-case pitting but not the full range of atmospheric effects. Critical forms of attack for stainless – such as stress-corrosion cracking or intergranular corrosion – will not be triggered by a salt fog test alone, because those require specific conditions (tensile stress, elevated temperature, or acidic chemicals) that the B117 test does not impose. Thus, a stainless fastener susceptible to stress cracking or sensitization might still sail through a salt spray test with no issues, giving a false sense of security.
• Surface Condition and Design Effects: The outcome for stainless steels in salt spray can vary widely with surface finish, fabrication, and part geometry. Rough surfaces or scale can harbor chloride droplets longer, accelerating attack, whereas a smooth passivated surface sheds water faster. Crevices (like under washers or threads) will trap salt solution, often leading to crevice corrosion in the test. A polished, well-passivated 316 bolt might survive far longer than a rough-finished 316 casting due to such differences. Similarly, contaminants (e.g. iron particles from tooling) on stainless steel will cause rapid rust spotting in salt spray. In practice, salt fog testing is sometimes used to verify proper passivation of stainless parts – if brown rust appears quickly on a supposed stainless fastener, it often indicates surface iron contamination or a lesser grade alloy. Clean, high-grade stainless should only exhibit minor localized pitting or tea-staining at most, even after many hours.
• Pitting and Crevice Corrosion in Salt Fog: Austenitic stainless grades (304, 316) are prone to chloride pitting, and the salt spray test will reveal differences in pitting resistance. For instance, Type 316 (with molybdenum) generally lasts significantly longer before pitting than Type 304 in the same test. A 316 stainless fastener may withstand on the order of 100+ hours with no visible rust, whereas 304 might show rust spots much sooner in full-strength salt fog. Lower-chromium martensitic grades like 410 or 420 stainless fare even worse – these can start corroding within a day or two. One industry test report noted that 410/420 stainless pins showed staining after ~48 hours and severe pitting by 300 hours of salt spray exposure. By contrast, a high-alloy 300-series stainless (if properly passivated) can endure hundreds or even thousands of hours before significant corrosion. Duplex stainless steels (e.g. 2205), with their higher chromium and nitrogen content, perform better still – often outlasting 316 in salt fog tests due to their superior pitting resistance (PREN). However, while these rankings (410 < 304 < 316 < duplex) match expected corrosion resistance trends, the absolute test hours can be misleading. Continuous salt spray tends to exacerbate pitting beyond what would occur in a natural intermittent exposure. In service, stainless fasteners might only see wet salt conditions occasionally, giving them time to rebuild their passive film; the test offers no such reprieve.

In summary, salt spray results on stainless steels are at best a rough comparative indicator of pitting corrosion resistance, not a definitive predictor of real-world durability. Passing a high-hour salt spray test does not guarantee a stainless steel fastener will never corrode in the field, and failing (rusting) in a salt spray test doesn’t always mean the material is unsuitable for service. Thus, while one can perform ASTM B117 on stainless fasteners, the data must be used cautiously. Industry experts often note that this test “overstresses” stainless steels, potentially giving misleading results if taken as a direct measure of service performance. It is most appropriate when used for quality control or comparing alloy grades, rather than as an absolute qualification for use in chloride environments.

Industry Guidance and Standards Perspective

Several industry standards and publications emphasize that salt spray testing has limited applicability for stainless steels beyond comparative assessment. The ASTM B117 standard itself is a neutral practice (it doesn’t dictate failure criteria), but other guidelines caution against over-reliance on this test:

• ISO 9227 / ASTM B117 Warnings: The international salt spray standard (ISO 9227) explicitly states that results from salt spray testing often cannot be used to predict long-term corrosion behavior because the test conditions differ significantly from natural exposure. This aligns with ASTM’s long-held position that salt fog results are primarily for relative comparisons. In practice, a requirement like “316 stainless must pass 1000 hours in salt spray” should be understood only as a quality benchmark, not as “will survive X years in service.” Real-world corrosion rates can diverge greatly.
• Stainless Steel Industry Advice: Organizations like the British Stainless Steel Association (BSSA) note that salt spray can be used to compare stainless grades in a chloride-rich setting, but caution that using such lab results to guarantee field performance is inappropriate. They point out that test outcomes are sensitive to specimen geometry, surface condition, and test specifics, making it unreliable to generalize results to all situations. The International Stainless Steel Forum (ISSF) has even published guidance titled “The Salt Spray Test and its Use in Ranking Stainless Steels – The Test and its Limits,” underscoring that there are clear limits to what B117 tests can tell us about stainless durability.
• Common Practice for Fasteners: In the fastener and coatings industry, salt spray (ASTM B117) is widely recognized as a convenient yardstick for comparing finishes or materials under severe salt exposure. It’s part of many specifications for coated fasteners (e.g. galvanized or plated bolts must hit a certain hour threshold). However, for bare stainless steel fasteners, there is no universal salt spray requirement because high-performance stainless (especially 304/316 and above) is generally expected to resist corrosion in ambient conditions by nature, and lower-grade stainless (like 410) is known to rust if exposed to salt. Some manufacturers will still perform salt spray tests on stainless parts to verify lot quality or surface passivation. If a stainless fastener shows unusual early corrosion in a salt fog, it may indicate a problem (wrong grade, improper heat treatment, surface iron contamination, etc.). Thus, the test can serve as a screening tool. But industry experts often advise that more service-realistic tests be used for design or material selection purposes, rather than relying on B117 alone.
• Not Suited for Certain Corrosion Modes: For stainless steels, specific corrosion modes like intergranular corrosion (due to sensitization) are addressed by standards such as ASTM A262 practices, which involve boiling acid tests – salt spray would not reveal those susceptibilities. Likewise, chloride stress corrosion cracking is typically evaluated by tests like U-bend exposures in hot chloride solutions (e.g. ASTM G36 or NACE TM0177) rather than by a mild 35 °C salt fog. These industry practices reflect the understanding that each form of corrosion demands a tailored test. Salt spray primarily targets surface pitting/general corrosion in a marine atmosphere scenario and is not a catch-all for every corrosion mechanism.

In essence, standards bodies and stainless experts support using salt spray tests with great care for stainless steel. They serve as a convenient comparative test, but many publications caution that one should not reject or accept a stainless steel for a given application based solely on salt spray hours. More indicative assessments are recommended for a true measure of performance in intended environments.

Alternative Corrosion Tests for Stainless Steel Fasteners

Given the limitations of salt spray for stainless steels, various alternative test methods are preferred when evaluating corrosion performance of stainless fasteners:

• Cyclic Corrosion Tests: Unlike continuous salt spray, cyclic tests simulate wet/dry and temperature cycles to better reproduce natural conditions. Tests such as ASTM G85 Annex 5 (Prohesion) or automotive cyclic tests (e.g. SAE J2334, ISO 16701) expose samples to alternating salt mist, drying periods, humidity, and sometimes UV. These cycles allow protective oxide films to reform and corrosive salts to concentrate, which often correlates more realistically with outdoor exposure. Stainless steels often perform relatively better in cyclic tests than in constant fog, and such tests can reveal patterned behaviors (e.g. the tendency for crevice corrosion during the damp phase, etc.). For fasteners intended for outdoor or marine use, a cyclic corrosion test can give a more accurate comparative ranking of materials and coatings in terms of expected field performance.
• Immersion and Soak Tests: For applications involving immersion or splashing (e.g. fasteners in submerged structures or tanks), simple immersion tests or alternate immersion (like ASTM G44, cycling between salt water immersion and air) can be used. These tests focus on uniform corrosion rates and pitting under controlled chemistry. For example, a set of stainless samples might be immersed in seawater or chloride solutions at various temperatures to measure weight loss or pitting depth over time. This can identify if a material is prone to crevice attack under deposits or if elevated temperature exacerbates corrosion. Immersion tests are more directly related to environments like submerged coastal fasteners, though they still accelerate time compared to field exposure.
• Electrochemical Tests (Laboratory Accelerated): Modern electrochemical methods allow rapid evaluation of a stainless alloy’s pitting and crevice resistance. Tests like the Critical Pitting Temperature (CPT) per ASTM G48 (a ferric chloride exposure) or ASTM G150 (electrochemical determination of CPT) directly measure the temperature or potential at which pitting initiates. Similarly, cyclic potentiodynamic polarization tests can determine pitting potentials and re-passivation ability in a given environment. These quantitative metrics (CPT, critical crevice temperature, pitting index, etc.) are very useful for comparing stainless grades (e.g. confirming that duplex stainless has a higher CPT than 304/316, meaning it resists chloride pitting to higher temperatures). They essentially probe the same vulnerability that salt spray exploits, but in a more controlled and material-specific manner. For fastener materials selection, such tests provide scientific insight into pitting resistance that a simple salt fog hour count cannot.
• Stress and Sensitization Tests: If the concern is stress-corrosion cracking (as can occur in some stainless under tensile stress in hot chloride environments), standards like NACE TM0177 or ASTM G36 (boiling MgCl₂ test) are more appropriate. These subject the material to simultaneous mechanical load and corrosive medium to see if cracking occurs. Likewise, intergranular corrosion susceptibility is evaluated by tests such as ASTM A262 Practices A-E (using boiling acids to attack sensitized grain boundaries). Fasteners (especially welded or heat-treated ones) might need these tests to ensure long-term integrity. Salt spray would not trigger these failure modes, so relying on it could overlook such critical issues.
• Field Exposure Testing: Ultimately, real-world exposure trials provide the most confidence. Placing stainless fasteners in the intended service environment (or a similar aggressive location, such as an outdoor marine test site) for an extended period can reveal actual performance. For instance, fastening assemblies can be mounted at a seashore test station to see if crevice corrosion occurs under bolt heads over a year. While time-consuming, field tests capture all variables (sunlight, pollutants, biological factors) and often validate the acceleration tests. They are sometimes used to qualify materials for the most demanding applications (e.g. fasteners for coastal bridges or offshore platforms), in conjunction with accelerated lab tests.

In choosing an alternative method, it’s important to match the test type to the expected corrosion challenges for the fastener. A combination of tests may be used: for example, do a quick salt spray or immersion test to check for any obvious issues or compare candidate alloys, use electrochemical tests to quantify pitting resistance, and perform a longer-term cyclic or field test for final validation. This multifaceted approach gives a much clearer picture of how stainless steel fasteners will perform in practice, covering not just simple chloride attack but also any other relevant corrosion mechanisms.

Conclusion

Salt spray (ASTM B117) testing can be applied to stainless steel fasteners and will differentiate their relative corrosion resistance (often dramatically so between low and high alloy grades). However, it should not be the sole basis for material selection or performance guarantees for stainless steels. The test’s aggressive, unrelenting nature tends to produce results that, while useful for rough rankings and quality control, may be misleading in terms of real-world corrosion behavior. Stainless steels have complex corrosion modes – pitting, crevice, stress cracking, etc. – that are better evaluated by targeted methods and more realistic simulations. Industry standards and experts recommend using salt spray primarily as a supplementary, comparative test, and turning to cyclic tests, specialized corrosion tests, and field exposure to truly assess a stainless fastener’s suitability for its intended environment. In summary, when dealing with critical applications, one should view salt spray test results on stainless steels as one data point among many – helpful for detecting weaknesses or verifying batch quality, but to be weighed alongside more representative corrosion testing to ensure a reliable, durable fastener performance.