Making Salt Spray Tests Comparable: Six Things That Change the Result

Two suppliers test the same part, one reports 240 h and the other 96 h. Both used "neutral salt spray". The difference is almost never the coating — it is the test.

1. Standard and chamber parameters

Neutral salt spray (NSS) is nominally 5 % NaCl at 35 °C, pH 6.5–7.2. In practice, chambers differ in:

• Salt concentration drift as the reservoir empties and is refilled.
• pH drift from dissolved CO₂ and from contamination.
• Collection rate (should be 1–2 mL/h per 80 cm²) and distribution across the chamber.
• Recovery time after the lid is opened; frequent opening disturbs the fog.

Ask for the chamber's calibration record and the collection-rate data, not just the report.

2. Specimen preparation

A part cleaned with a solvent behaves differently from one handled with bare hands. Fingerprints are chloride and oil. Degreasing method, drying method and handling between sealing and testing all matter, and none of them appear in a typical test report.

3. Scribing

Scribed panels test the coating's ability to resist undercutting from a deliberate defect — a very different question from an intact coating. Whether the scribe is applied, how deep, and with what tool changes the reported hours dramatically. If the customer's specification mentions scribing, follow the exact method.

4. Specimen angle and position

Standard practice is 15–30° from vertical for panels, with parts positioned so that salt runs off rather than pooling. A part laid flat collects puddles and fails early; a part hung vertically with a recess at the bottom collects condensate at the recess.

Position within the chamber matters too. Corners and the area near the nozzle see different fog density. Rotating positions between runs is the simplest control.

5. Rinsing and evaluation

Corrosion product has to be removed before judging, and the rinsing method can remove more than intended. Agree in advance:

• Rinse temperature and duration, and whether mechanical action is allowed.
• Whether the assessment is done wet or after drying.
• The area-percentage method and the reference photographs.
• Who judges — one person, or two independently.

6. Time zero

Does the clock start when the part leaves the sealing bath, after a specified ageing period, or after 24 h of conditioning? Passivation films continue to develop for hours to days. Testing immediately after drying can under-report performance; testing two weeks later can over-report it if the part was stored badly.

A one-page comparison protocol