ASTM B799-95 Standard Test Method for Porosity of Gold and Palladium Coatings by Sulfurous Acid/Sulfur Dioxide Vapor

The standard "ASTM B799-95 Standard test method for porosity of gold and palladium coatings by sulfurous acid/sulfur dioxide vapor" published by the American Society for Testing and Materials (ASTM) describes equipment and a test method for determining the porosity of gold and palladium coatings, particularly electroplatings and plated metals used in electrical contacts.

This test method is designed to indicate whether the level of porosity is less or greater than a value that the user, based on experience, considers acceptable for the intended application.

There are other porosity test methods in practice, such as “ASTM B735-16 Standard Test Method for Porosity of Gold Coatings on Metal Substrates by Nitric Acid Vapor”, “ASTM B741-95 Standard Test Method for Porosity of Gold Coatings on Metal Substrates by Paper Electrography”, “ASTM B798-95 Standard Test Method for Porosity of Gold or Palladium Coatings on Metal Substrates by Gel-ball Electrography”, and “ASTM B809-95 Standard Test Method for Porosity of Metallic Coatings by Moist Sulfur Vapor (Flowers of Sulfur)”. For the selection of porosity tests for electrocoatings and related metallic coatings, “B765-03 Standard Guide for Selection of Porosity and Gross Defects Tests for Electrocoatings and Related Metallic Coatings” has also been published.

Many terms used in this test method are defined in ASTM B542-13 Standard Terminology for Electrical Contacts and Their Uses, and terms related to metallic coatings are defined in ASTM B374-21 Standard Terminology for Electroplating. The following definitions specific to this standard apply:

• Corrosion products are reaction products resulting from pores that protrude from or are otherwise attached to the coating surface after steam test exposure.
• The measurement area (or critical surface) is the surface being examined for the presence of porosity. Critical surfaces or measurement areas of the part to be tested shall be indicated on the drawing of the part or by the provision of suitably marked specimens. For specification purposes, critical surfaces or measurement areas are generally defined as those parts of the surface that are important to the serviceability or function of the part, for example, contact properties or may be a source of corrosion products or stain films that interfere with the function of the part.
• Metallic coatings include coatings or other metallic layers applied to the substrate. Coatings may consist of a single metallic layer or a combination of metallic layers.
• Porosity is the presence of any discontinuity, crack or hole in the coating that exposes a different underlying metal.
• The subplate is a metallic coating layer between the substrate and the topmost layer or layers. The thickness of a subplate is typically greater than 0,8 µm (30 µin).

The test method uses concentrated sulfurous acid which emits sulfur dioxide gas according to the equilibrium reaction. Exposure times may vary depending on the degree of porosity to be revealed. The reaction of the gas with a corrodible base metal in the pore spaces produces reaction products that appear as discrete dots on the gold or palladium surface. The discrete dots are counted with the aid of a magnifying glass or low-power stereo microscope.

This test method is suitable for coatings containing 95 percent or more gold or palladium on substrates composed of copper, nickel and their alloys commonly used in electrical contacts.

This porosity test involves corrosion reactions in which products identify defective areas in coatings. Since the chemistry and properties of these products may not be similar to those found in natural or service environments, this test method is not recommended for predicting the electrical performance of contacts unless correlation with service experience is first established.

Gold coatings are often specified for contacts of detachable electrical connectors and other devices. Electrodeposit is the most commonly used form of gold in contacts, but it is also used as an intrinsic or plating metal and as a weld on the contact surface. The intrinsic nobility of gold enables it to resist the formation of insulating oxide films that would inhibit reliable contact operation.

Palladium coatings are sometimes specified as gold alternatives, both as electrodeposited and as inlay or plated metal on electrical contacts and similar electrical component surfaces. This test method is particularly suitable for determining porosity in palladium coatings since the reactive atmosphere used will not attack palladium if the specified test conditions are followed. In contrast, palladium coatings are attacked by nitric acid and other strong oxidizing agents, so ASTM B735-16 cannot be used to determine porosity in such coatings.

Among the numerous testing, measurement, analysis and evaluation studies that our organization has provided for businesses in various sectors, it also provides testing services defined in the standard “ASTM B799-95 Standard test method for porosity in gold and palladium coatings with sulfurous acid/sulfur dioxide vapor” with an educated and expert staff and advanced technological equipment.