The stoic Statue of Liberty was vulnerable to corrosion as a result of its placement in the humid, saline environment of the New York Harbor. Regular maintenance checks in the 1980s revealed that galvanic corrosion had taken place between the outer copper skin and inner wrought iron, resulting in serious rust and weakening the statue’s armature bars. Corrosion testing needed to take place to save the landmark from destruction. A type of corrosion test is a salt spray test and its test method is specified in ISO 9227:2022—Corrosion Tests In Artificial Atmospheres – Salt Spray Tests.
What Are Salt Spray Tests?
Salt spray testing is a laboratory simulation of a corrosive saline environment where a salt solution is sprayed throughout the test cabinet. It is used as an accelerated means of testing the ability of surface coatings to withstand atmospheric corrosion, examining the suitability of the coating for use a protective finish. The appearance of corrosion products (rust or other oxides) is assessed after a pre-determined time period. Test duration is dependent on the coating’s corrosion resistance, as the more corrosion resistant the coating is, the longer the period of testing before the appearance of corrosion or rust. Typically, the materials tested are metallic—although stone ceramics and polymers may also be tested—and finished with a surface coating that provides corrosion protection to the underlying metal.
What is ISO 9227:2022?
ISO 9227:2022 specifies the apparatus, reagents, and procedure to conduct neutral salt spray (NSS), acetic acid salt spray (AASS), and copper-accelerated acetic acid salt spray (CASS) tests. These tests are used to evaluate the corrosion resistance of metallic materials, with or without permanent or temporary corrosion protection. The standard also describes the method for assessing the corrosivity of the test cabinet environment.
It is important to note that the salt spray tests in ISO 9227:2022 are not intended to be used for comparative testing to rank different materials with respect to corrosion resistance or as means of predicting long-term corrosion resistance of the tested material.
What Is the Purpose of Salt Spray Tests?
ISO 9227:2022 states that salt spray tests are particularly useful for detecting discontinuities, such as pores and other defects, in certain metallic, organic, anodic oxide, and conversion coatings.
Additionally, the salt spray methods are all suitable for checking that the quality of a metallic material, with or without corrosion protection, is maintained. Here are the three salt spray test methods ISO 9227:2022 details:
1. The Neutral Salt Spray (NSS) Test
The NSS test is the test method in which a neutral approximate 5% sodium chloride solution is atomized under a controlled environment. The NSS test is applicable to the following:
- Metals and their alloys
- Metallic coatings (anodic and cathodic)
- Conversion coatings
- Anodic oxide coatings
- Organic coatings on metallic materials
2. The Acetic Acid Salt Spray (AASS) Test
The AASS test is the test method in which an approximate 5% sodium chloride solution acidified by the addition of acetic acid is atomized under a controlled environment. It is especially useful for testing decorative coatings of copper + nickel + chromium, or nickel + chromium. It also suitable for testing anodic and organic coatings on aluminum.
3. The Copper-Accelerated Acetic Acid Salt Spray (CASS) Test
The CASS test is the test method in which an approximate 5% sodium chloride solution acidified by the addition of acetic acid and with the addition of copper(II) chloride is atomized under a controlled environment. It is useful for testing decorative coatings of copper + nickel + chromium, or nickel + chromium. It is also suitable for testing anodic and organic coatings on aluminum.
ISO 9227:2022—Corrosion Tests In Artificial Atmospheres – Salt Spray Tests is available on the ANSI Webstore.