Salt spray testing
Controlled corrosion exposure: NSS/AASS/CASS distinctions, specimen preparation, chamber verification and limits on interpreting test hours.
Tags: inspection, corrosion, coatings
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What the test establishes
Salt spray compares specimens under a specified chamber exposure. Q-Lab distinguishes screening and qualification from predicting service life, which requires evidence such as outdoor exposure data. Q-Lab, introduction to corrosion
Original acceptance example: suppose a fictional requirement calls for three scribed panels after 500 hours NSS, each with mean scribe creep no greater than 2.0 mm. Panel results of 1.2, 1.7 and 2.4 mm give a set average of 1.77 mm, but the set fails the stated individual-panel limit. These invented limits illustrate why averaging rules matter; they are not a coating qualification standard. No service-life estimate follows from either the exposure hours or this calculation.
A useful requirement identifies the method and edition, exposure duration, specimen configuration, evaluation method and acceptance criteria. “Passed 1,000 hours salt spray” omits much of that information. A test on an intact flat panel, a scribed panel and an assembled fastener can probe different failure paths even when chamber exposure is identical.
Test completion and acceptance are separate. The exposure method establishes the laboratory environment; the product or customer specification must define the required duration and acceptable corrosion, blistering, adhesion loss or other endpoint. Agree these before exposure so that a visually striking result is not assessed against an improvised rule.
Identify the exposure precisely
Q-Lab summarizes ASTM B117 as continuous 5% sodium chloride fog at 35 °C with neutral pH. Those headline conditions are not a complete operating instruction: salt and water quality, collected solution, deposition, specimen placement and other provisions still need the specified standard. Use the applicable revision rather than a supplier webpage as the contractual procedure. Q-Lab, continuous salt spray overview
The ISO 9227 family includes distinct environments:
| Designation | Environment distinction |
|---|---|
| NSS | Neutral salt spray |
| AASS | Acetic-acid salt spray |
| CASS | Copper-accelerated acetic-acid salt spray |
Ascott's Issue A: 2019 method statement explicitly addresses ISO 9227:2017, including a higher chamber temperature for CASS than NSS/AASS. These are different procedures, not interchangeable labels for severity. Its historical edition reference must not be treated as confirmation of the current contractual edition. Ascott, ISO 9227 method statement, pp. 1–3
Continuous salt fog also differs from cyclic corrosion tests that introduce wetting, drying, controlled humidity or other exposures. A cyclic method may better represent a particular mechanism, but selecting a harsher or more elaborate cycle does not automatically validate a service-life prediction. Q-Lab explains that correlation and prediction require relevant outdoor evidence. Q-Lab, test selection and interpretation
Specify the coating system and specimen
Record substrate alloy and surface condition, pretreatment, coating layers, thickness, cure/conditioning and specimen age. Decide whether the test represents routine production, a laboratory formulation or a deliberately damaged system. Preserve specimen identity through preparation and evaluation so the result remains linked to the actual coating process.
Substrate choice matters. Q-Lab's Q-PANEL application guide, updated 2023, recommends steel for coatings used on steel and aluminium for coatings used on aluminium. For steel panels it also recommends protecting the back to prevent extraneous corrosion from contaminating the test. Define edge/back protection and any excluded evaluation zones in the agreed specimen preparation. A coating name alone does not make different substrates equivalent. Q-Lab, LP-0867 applications guide
When required, a scribe deliberately breaches the coating to assess corrosion or loss of adhesion spreading from damage. Q-Lab notes that scribe depth and geometry affect results, particularly with galvanized substrates, and recommends identifying the tool used. Retain the initial scribe geometry and the governing evaluation method. A casual scratch is not a reproducible substitute for the specified preparation. Q-Lab technical bulletin LF-8144, specimen preparation
Use handling and cleaning methods that preserve the intended surface. Photograph initial defects, exposed edges and masked areas before testing. An unexplained pre-existing holiday or handling mark can otherwise be attributed to the coating's corrosion response.
Demonstrate that the chamber delivered the exposure
A running timer and a temperature setpoint do not prove the required corrosive environment. Ascott's method statement describes monitoring temperature, solution concentration, pH and collected fog. It also calls for loading that avoids specimen shadowing or droplets falling from one specimen onto another. Collectors need suitable locations without contamination from specimen runoff. Ascott, chamber monitoring and loading
Record actual conditions and deviations, including interruptions and corrective actions. Use the selected standard's requirements for apparatus verification, chamber corrosivity checks and permitted interruptions. Q-Lab describes performance verification tools including mass-loss coupons, collection measurements and independent environmental monitoring. These checks address different aspects of exposure and should not be conflated with the product's pass/fail result. Q-Lab, verifying corrosion-test performance
As a bounded equipment example, Q-Lab identifies Q-FOG SSP for continuous salt spray and its CCT/CRH models for additional cyclic capabilities. Confirm the installed chamber's actual capability for the required cycle and transitions; a salt-fog cabinet is not necessarily equipped for a controlled-humidity automotive test. Q-Lab, chamber capabilities
Evaluate the specified failure modes
Decide in advance whether evaluation concerns substrate red rust, coating corrosion products, blistering, scribe creep, delamination or adhesion after exposure. Document the prescribed rinse, drying/recovery and any removal of loose coating or corrosion products. Measurements before and after such treatment are not automatically comparable.
Q-Lab's cyclic-corrosion bulletin discusses evaluation of coated specimens using methods such as ASTM D1654 and stresses reporting test interruptions and specimen handling. It supports treating exposure and evaluation as separately controlled operations. Apply the method actually specified for the product, including edge exclusions, measurement locations and individual-specimen rules. Q-Lab, LF-8144 evaluation guidance
Why test hours are not service years
The American Galvanizers Association explains a specific limitation for zinc: normal atmospheric exposure permits protective zinc corrosion products to develop, whereas continuous salt spray can accelerate a different mechanism. Rankings between dissimilar protective systems can consequently mislead. This is a mechanism-specific warning against using salt-spray results as a universal comparison of outdoor durability. AGA, salt spray testing
Use field evidence or a validated correlation appropriate to the material, failure mode and service environment when predicting life. Do not convert hours to years with a generic multiplier. Likewise, salt-spray resistance alone does not establish UV colour retention, mechanical durability or compliance with an entire coating approval scheme.
Report a reproducible result
Retain specimen and coating-system details, preparation/scribe records, method revision, chamber identification and verification, actual exposure conditions, interruptions, evaluation timing and method, individual results and photographs with scale. State the acceptance requirement and disposition separately. A useful conclusion identifies exactly which specimens met which criteria under which exposure, rather than describing the coating as universally corrosion-proof.