Flower of Sulfur (FoS) tests are used to examine the susceptibility of test articles to sulfur-based corrosion. Defined by ASTM-B809-95 (2018), initially approved by ASTM in 1990, the Standard Test Method for Porosity in Metallic Coatings by Humid Sulfur Vapor ("Flower-of-Sulfur") was developed to test the porosity of metallic coatings on silver, copper, and copper-alloy substrates. With the advent of lead-free electronics, variations of the original FoS test have been developed and reported in the literature. In particular, the test has been used to successfully demonstrate silver corrosion weaknesses that have escaped mixed-flowing-gas (MFG) tests.
The traditional FoS test uses a glass or acrylic desiccator to define the control volume. A solution of KNO3 and deionized water to create the humidity source, and powdered (granular) sulfur as the sulfur source. A dish of granular sulfur is placed in the bottom of the desiccator along with a dish of KNO3 solution. Test specimens are suspended over the dishes within the desiccator, and the desiccator is placed in the oven to control the ambient temperature. The desiccator lid must be sufficient to create a moisture seal and maintain a controlled humidity level within the desiccator. A schematic of the test is shown below.
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