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Test Method Comparison

NSS, CASS, and AASS

Choosing the Right Salt Spray Test Method

The test method is defined by your substrate and your product standard — not your preference. Using the wrong method can invalidate supplier approval test results and miss real-world failure modes. This guide explains when each method applies and what distinguishes them technically.

Side-by-Side Comparison

The Three Standard Salt Spray Methods

FeatureNSSCASSAASS
Solution5% NaCl5% NaCl + CuCl₂ + acetic acid5% NaCl + acetic acid
pH6.5 – 7.23.1 – 3.33.1 – 3.3
Temperature35°C50°C35 – 50°C (spec-dependent)
AggressivenessModerateHighestHigh
Primary substratesAll metals, galvanised, zinc/nickel platingAluminium alloys, decorative chromeOrganic coatings on metals
StandardASTM B117 · ISO 9227 NSSISO 9227 CASS · ASTM B368ISO 9227 AASS
Typical duration96 – 500 h16 – 96 h96 – 240 h

Method Selection

When the Standard Specifies Each Method

NSS

When to Use Neutral Salt Spray

  • Zinc, nickel, and cadmium electroplatings (ASTM B633, B841)
  • Hot-dip galvanised steel (ASTM A123, A153)
  • Powder coat and paint qualification (OEM specs, ASTM D1654)
  • Default method when the product spec does not specify NSS, CASS, or AASS explicitly
  • Baseline corrosion resistance comparison across coating systems
CASS

When to Use CASS

  • Decorative chrome on plastic trim (automotive exterior, appliances)
  • Anodised aluminium — CASS is more discriminating than NSS for this substrate
  • Aluminium alloy castings for automotive and aerospace
  • When faster test results are needed for aluminium (CASS is much more aggressive than NSS on aluminium)
  • Where ISO 9227 CASS or ASTM B368 is explicitly called out in the spec
AASS

When to Use AASS

  • Organic coatings (paints, lacquers, primers) on metal substrates
  • Military and aerospace organic finish qualification
  • Specifications referencing DIN 53167 or ISO 7253
  • Where AASS is required instead of NSS for painted steel
  • Situations where acetic acid acceleration is needed but copper contamination (CASS) is undesirable

Equipment

Can One Chamber Run All Three Methods?

Yes — with the right chamber specification and thorough cleaning between method changes. The chamber must meet these requirements for multi-method operation:

  • Maximum temperature ≥50°C (required for CASS — many NSS-only chambers are limited to 35°C)
  • Acid-resistant interior lining — required for AASS and recommended for CASS
  • Accessible drain and cleaning ports to flush the system between acid and neutral test runs
  • Separate solution reservoirs or a thorough flush protocol when switching between NSS and acid methods
Indeecon's chamber supports NSS, CASS, and AASS

Common Mistakes

Mistakes When Selecting a Test Method

01Defaulting to NSS when the product standard explicitly requires CASS — common with decorative chrome and anodised aluminium parts.
02Running CASS at 35°C instead of 50°C. CASS requires 50°C — a chamber that only reaches 35°C cannot run a valid CASS test.
03Not adjusting pH between methods. CASS and AASS require pH 3.1–3.3; NSS requires 6.5–7.2. Using the wrong solution will invalidate the test.
04Comparing NSS results directly with CASS results. The methods are not equivalent — shorter CASS hours do not linearly convert to longer NSS hours.
05Not cleaning the chamber thoroughly between method changes. Copper residue from CASS testing can contaminate an NSS test run.

FAQ

Technical Questions