Salt spray accelerated exposure testing is a universally accepted technique to evaluate the level of corrosion resistance of a material, whether it is a coated metallic substrate, hybrid material, or a raw metallic substrate. Though not a true indication of real life corrosion exposure, salt spray has become the standardised method for accelerated corrosion testing. The test involves exposing the materials to a sodium chloride (NaCl) fog environment in an enclosed cabinet, with increased humidity and temperature. The salt solution is produced at specific concentrations, temperatures and pH, set out by international standard guidelines, such as ISO 9227 (2017), or ASTM B117 (2019). The test can be conducted for a set duration as required by industry. Testing can be performed on full parts, or on specific exposed faces or areas. In such cases, the sample may be prepared either with additional surface protection to prevent additional reactions, or with a scribe to accelerate the corrosion.

During exposure, the test pieces are placed at specific angles (determined by application, standard, or client) to allow the salt spray to settle/interact on the test face. Inspections can be carried out at regular intervals to detect the onset of corrosion and subsequent progression.

CREST has 3 Salt Spray Chambers in its laboratories, each with a capacity of 1000 L:

                                                               

Ascott S1000 iP                                     Liebisch S1000 M-TR                              Liebisch STR1000

There are four types of salt spray based testing available at CREST; neutral tests (NSS), cyclic (prohesion) acidic (AASS), and copper chloride (CASS) salt spray. NSS is performed continuously while prohesion, AASS and CASS are available upon request.

Neutral Salt Spray

Neutral Salt Spray testing is conducted according to ISO 9227:2017, ASTM B117:2019, or MIL-STD-810 Method 509.4. The test conditions consisted of a Neutral salt fog atmosphere generated from 5 wt. % aqueous NaCl solution at 35°C (±1).

The neutral salt spray (NSS) test particularly applies to

  • Metals and their alloy,
  • Metallic coatings (anodic and cathodic)
  • Conversion coatings
  • Anodic oxide coatings
  • Organic coatings on metallic materials

Prohesion

Prohesion, a method of cyclic accelerated corrosion testing, is also available according to ASTM G85 . This is a particularly useful for simulating exterior exposure, for paints on steel substrates.

Acidic Salt Spray

Corrosion testing is performed in an acidified salt spray environment, with a solution of sodium chloride acidified using acetic acid, as specified in ASTM G-85, and ISO 9227:2017. The acetic acid salt spray (AASS) test is especially useful for testing

  • Decorative coatings of copper/nickel/chromium,
  • Nickel/chromium.

Copper Accelerated Acetic Acid Salt Spray

The copper accelerated acetic acid salt spray is performed as specified in ASTM B368, ISO 9227:2017. The method is used to evaluate the corrosion performance of materials such as

  • Decorative copper/nickel/chromium on steel
  • Nickel/chromium coatings on steel
  • Zinc alloys
  • Aluminium alloys
  • Plastics/organic coatings (for severe service)
  • Anodized aluminium