Salt spray test

Salt spray testing is popular because it is relatively inexpensive, quick, well standardized, and reasonably repeatable.

Although there may be a weak correlation between the duration in salt spray test and the expected life of a coating in certain coatings such as hot-dip galvanized steel, this test has gained worldwide popularity due to low cost and quick results.

For example, pre-treated + painted components must pass 96 hours Neutral Salt Spray, to be accepted for production.

The principal application of the salt spray test is, therefore, enabling quick comparisons to be made between actual and expected corrosion resistance.

For this reason, the salt spray test is most often deployed in a quality audit role, where, for example, it can be used to check the effectiveness of a production process, such as the surface coating of a metallic part.

Other test solutions have other chemicals added including acetic acid (often abbreviated to ASS) and acetic acid with copper chloride (often abbreviated to CASS) each one chosen for the evaluation of decorative coatings, such as electroplated copper-nickel-chromium, electroplated copper-nickel or anodized aluminum.

ASTM G85 Annex A1 – Acetic Acid Salt Spray Test (non-cyclic) This test can be used to determine the relative resistance to corrosion of decorative chromium plating on steel and zinc based die casting when exposed to an acetic acid salt spray climate at an elevated temperature.

Test specimens are placed in an enclosed chamber, and exposed to a changing climate that comprises the following 3 part repeating cycle.

0.75 hours exposure to a continuous indirect spray of salt water solution, prepared in accordance with the requirements of the test standard and acidified (pH 2.8–3.0) by the addition of acetic acid.

Test specimens are placed in an enclosed chamber, and exposed to a changing climate that comprises the following 2 part repeating cycle.

First, a 30 minute exposure to a continuous indirect spray of synthetic seawater solution, prepared in accordance with the requirements of the test standard and acidified (pH 2.8–3.0) by the addition of acetic acid.

The entire test cycle is at a constant chamber temperature of 49 °C (may be reduced to 24–35 °C for organically coated specimens).

The first climate cycle comprises a continuous indirect spray of neutral (pH 6.5–7.2) salt water/synthetic seawater solution, which falls-out on to the specimens at a rate of 1–2 ml/80 cm2/hour.

The second climate cycle comprises 0.5 hours of continuous indirect spray of neutral (pH 6.5–7.2) salt water/synthetic seawater solution, which falls-out on to the specimens at a rate of 1–2 ml/80 cm2/hour.

First, a 1-hour exposure to a continuous indirect spray of salt water solution, prepared in accordance with the requirements of the test standard and acidified (pH 3.1–3.3) by the addition of acetic acid.

Daily checking of testing parameters is necessary to show compliance with the standards, so records shall be maintained accordingly.

For example, a typical electroplated zinc and yellow passivated steel part lasts 96 hours in salt spray test without white rust.

Hot-dip galvanizing produces zinc carbonates when exposed to a natural environment, thus protecting the coating metal and reducing the corrosion rate.

The zinc carbonates are not produced when a hot-dip galvanized specimen is exposed to a salt spray fog, therefore this testing method does not give an accurate measurement of corrosion protection.

ISO 9223 gives the guidelines for proper measurement of corrosion resistance for hot-dip galvanized specimens.

Painted surfaces with an underlying hot-dip galvanized coating can be tested according to this method.