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The salt spray test developed by the American Society of Testing and Materials (ASTM) test B117 is one of the most widely adopted standards. Its use is internation and its provisions have been re-written into the national standards of other countries, ISO 9227 is the European derivative of this standard.
ISO9227 is an excellent reference document for the salt spray practitioner, with many helpful hints and tips contained in its useful appendixes. It is also regularly updated, so it is a standard that is continuously evolving. Practitioners applying the ISO 9227 test standard are responsible for checking they are using the most appropriate version for their application.
Test specimens are placed in an enclosed chamber and exposed to a continuous indirect spray of salt water solution (also referred to as fog or mist) which falls-out on to the specimens at a rate of 1.0 to 2.0 ml/80cm²/hour, in a chamber temperature of +35C. The chamber climate is maintained under constant steady state conditions. The test duration is variable.
NSS & AASS
Salt spray testing is performed by placing samples in a test chamber designed in accordance with Paragraph 4 (Apparatus) of ISO 9227 and operated in accordance with Paragraph 8 (Operating conditions) of ISO 9227.
The test solution to be sprayed is prepared by dissolving pure sodium chloride into pure water, in a ratio of 5% sodium chloride to 95% water, by weight. The purity for each should be as specified in ASTM B117.
The salt solution should be prepared according to the nature of the test in accordance with Paragraph 3 of ISO 9227 and the 3 options are summarised below;
NSS (Neutral Salt Spray) - The pH of the salt solution fall-out should be maintained at a neutral pH (pH 6.5 to 7.2)
AASS - A sufficient amount of glacial acetic acid should be added to the salt solution (should modify the pH to 3.1 to 3.3)
CASS - A sufficient amount fo copper chloride dihydrate should be added to the salt solution to produce a concentration of 0.26g/l +/- 0.02g/l
At the time the samples are placed into the test chamber, it should be pre-conditioned to the operating temperature of 35˚C and producing fall-out at the specified rate. The salt fog fall-out rate is verified by using a minimum of two 80cm2 funnels per salt spray atomiser, each inserted into a measuring cylinder graduated in ml. These collectors are located inside the chamber, nominally at the same height as the test samples, one located close to the atomiser and one located away from the atomiser, in accordance with the positions specified in ASTM B117, but also taking into account the chamber manufacturers defined work-space.
This test can be used to test the relative resistance to corrosion of coated and uncoated metallic specimens, when exposed to a salt spray climate at an elevated temperature.
The appearance of corrosion products (oxides – commonly referred to as white rust & red rust) is evaluated after a period of time. Test duration depends on the corrosion resistance of the coating; generally, the more corrosion resistant the coating is, the longer the period of testing before the appearance of oxides.