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SAA AS 2331.3.5

2001 Edition, November 23, 2001

Complete Document

METHODS OF TEST FOR METALLIC AND RELATED COATINGS METHOD 3.5: CORROSION AND RELATED PROPERTY TESTS - SULFUR DIOXIDE/HYDROGEN SULFIDE POROSITY TESTS



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Description / Abstract:

This Standard sets out methods for assessing the degree of porosity in electroplated metallic coatings, such as gold, palladium and rhodium, that cover undercoats and basis metals that form visible compounds due to coating porosity, when exposed to sulfur dioxide-hydrogen sulfide atmospheres.

Electroplated coatings of non-tarnishing precious metals, such as gold and rhodium for engineering purposes, are required to retain their design characteristics over long periods of storage and use. These characteristics include the following:

(a) Low contact resistance.

(b) Solderability.

(c) A stable surface suitable for carrying high frequency currents.

Impairment of performance may result from—

(i) the formation of corrosion products at discontinuities in the coating; or

(ii) corrosion of the coating itself due to its composition not being adequate to withstand the environmental conditions.

Environments containing sulfur compounds can cause these types of corrosion on precious metal coatings.

The test methods described in this Standard employ the use of hydrogen sulfide (H2S) which will affect silver and copper but not nickel or tin, and, for thicker coatings, the additional use of sulfur dioxide (SO2) which will produce visible effects on any basis metal or undercoat.

The test chosen should be capable of revealing pores that expose the basis metal or an undercoat. For normal service conditions, the thickness of any undercoat specified should safeguard against the penetration of corrosion into the basis metal. However, when the coating is subjected to exceptionally severe environmental conditions, the development of special tests may be required.

NOTE: Special tests are a matter for agreement between the purchaser and the supplier.

Another type of porosity test for precious metal coatings is given in AS 2331.3.6.

NOTE: The test procedures described in this Standard do not necessarily include all of the precautions required to satisfy health and safety aspects. Care should be taken to ensure that the procedures are carried out only by people who have received suitable training. Guidance in the handling and use of hazardous chemicals is given in AS/NZS 2243.1 and AS/NZS 2243.2.