ASTM D7671 Standard Test Method for Corrosiveness to Silver by Automotive Spark-Ignition Engine Fuel - Silver Strip Method
3. Summary of Test Method
3.1 This test method covers two procedures. Procedure A involves the use of a pressure vessel (to prevent the loss of volatile components in the sample), whereas Procedure B involves the use of a vented test tube. In both procedures, a freshly polished silver strip is suspended in 30 mL of sample which is heated to 50 °C +/- 1 °C for a duration of 3 h +/- 5 min. At the end of the heating period, the silver strip is removed, washed and the color and tarnish level assessed against the requirements in Table 1.
4. Significance and Use
4.1 Crude petroleum contains sulfur compounds, most of which are removed during refining. However, of the sulfur compounds remaining in the petroleum product, some can have a corroding action on various metals and this corrosivity is not related to the total sulfur content. In addition, fuels can become contaminated by corrosive sulfur compounds during storage and distribution. The corrosive effect can vary according to the chemical types of sulfur compounds present.
4.2 The silver strip corrosion test is designed to assess the relative degree of corrosivity of a petroleum product towards silver and silver alloys.
4.3 Reactive sulfur compounds present in automotive spark-ignition engine fuels under some circumstances can corrode or tarnish silver alloy fuel gauge in-tank sender units (and silver-plated bearings in some 2-stroke cycle engines). To minimize or prevent the failure of silver alloy in-tank sender units by corrosion or tarnish, Specification D4814 requires that fuels shall pass the silver strip corrosion test.