Salt Spray Testing

What is salt spray testing (ASS and NSS)?

Salt spray testing is a form of corrosion testing used to evaluate the durability and corrosion resistance of protective coatings, such as powder coatings, on metal substrates. The test simulates corrosive conditions to which the coated mental may be exposed, such marine, industrial, or urban atmospheres.

Aluminium is tested with acetic acid salt spray (AASS)  – typically at a pH of ~3.1–3.3. Mild steel and galvanized steel are tested with neutral salt spray (NSS) at a pH of XXXXX

Tests are conducted in accordance with BS EN ISO 9227, which is the UK’s recognised standard for neutral, acetic acid, and copper-accelerated salt spray testing.

Why are aluminium and steel treated differently?

Coated aluminium and coated steel are treated differently because the two metals corrode in a different way.

Aluminium: On exposure to air and moisture aluminium naturally forms a thin oxide layer. This offers some protection to corrosion – but not for ever. In order to put this layer to the test over an accelerated period of time,  acetic acid salt spray is used as it is more aggressive and infiltrates this protective oxide layer. It simulates the effect that polluted environments or acid rain would have over a few years of exposure.

Steel: This metal corrodes far more quickly. The main corrosion path of steel is to form iron oxide (red rust) when it is exposed to chlorides – i.e. salt. This is not a protective layer and the integrity of the metal immediately begins to suffer. Salt is present in marine and coastal conditions, also along roadways and will begin to cause rust formation. The salt used in the tests does not need any acid to accelerate the process – the neutral salt spray is harsh enough.

How is the test carried out?

Coated metal sections are placed at a 15–30° angle in a test chamber which sprays a fine mist of salt solution at 35°C (±1°C). 

Inspection of the metal is made at regular intervals to check for:

• Blistering
• Undercutting at scribe lines
• Rust or corrosion
• Adhesion loss

A standard test is 1000 hours, but tests can be carried out for longer, depending on the specification or other requirements.

What is being tested?

The property being tested is the barrier protection afforded by the powder coating. 

And also:-

• The adhesion of the powder coating to the substrate.
• The corrosion creep at edges or damaged areas.
• The suitability of pre-treatment and primers e.g. TiZr, zinc-rich primers

How are the tests interpreted?

For aluminium (AASS) we are looking for:-

• Pitting, and under-coating corrosion
• Lifting of the coating
• Visual grading of the coating

A coated aluminium panel might be expected to survive 1000 hours without any of these occuring

For powder coated steel (NSS) the measurement is the time to:-

• First appearance of red rust (substrate corrosion)
• Blistering, flaking, delamination

A powder-coated steel panel might be expected to survive 1,000 hours with no blisters or corrosion at scribe marks.

Find out more about Powdertech's AASS and NSS Testing Services