ASTM D7671 Standard Test Method for Corrosiveness to Silver by Automotive Spark-Ignition Engine Fuel - Silver Strip Method
8.1 In accordance with Practice D4057 or D4177, or both, it is particularly important that all types of fuel samples be collected in clean, amber glass bottles, plastic bottles, or other suitable containers that will not affect the corrosive properties of the fuel. Avoid the use of tin plate containers for collection of samples, since experience has shown that they may contribute to the corrosiveness of the sample.
8.2 Collect samples and close the container immediately after taking the sample. Adequate headspace in the container is necessary to provide room for possible thermal expansion during transport. It is recommended that volatile samples be filled between 70 % and 80 % of the container's capacity. Take care during sampling to protect the samples from exposure to direct sunlight or even diffused daylight.
8.3 Test the sample as soon as practical after sampling. If samples are not to tested immediately, store in a refrigerator at a temperature below 5 °C.
8.4 If suspended water (that is, haze) is observed in the sample, dry by filtering a sufficient volume of sample through medium rapid qualitative filter paper, into the prescribed clean, dry test tube. Carry out this operation in a darkened room or under a light-protected shield.
8.4.1 Contact of the silver strip with water before, during or after completion of the test run will cause staining, making it difficult to evaluate the strips.
9. Preparation of Test Strips
9.1 Surface Preparation - Remove all surface blemishes from all six sides of the strip obtained from a previous analysis (see Note 1). Use silicon carbide paper or cloth of such degrees of fineness as are needed to accomplish the desired results efficiently. Finish with 53 µm to 65 µm (240 grit) silicon carbide paper or cloth, removing all marks that may have been made by other grades of paper used previously. Immerse the strip in 2,2,4-trimethylpentane from which it can be withdrawn immediately for final preparation (polishing) or in which it can be stored for future use.
9.1.1 As a practical manual procedure for surface preparation, place a sheet of silicon carbide paper or cloth on a flat surface and moisten it with wash solvent. Rub the strip against the silicon carbide paper or cloth with a circular motion, protecting the strip from contact with the fingers by using ashless filter paper or wearing disposable gloves. Alternatively, the surface of the strip may be prepared by use of motor-driven machines using appropriate grades of dry paper or cloth.
9.1.2 Only final preparation (see 9.2) is necessary for commercially purchased pre-polished strips.
9.2 Final Preparation - For strips prepared in 9.1 or new strips being used for the first time, remove a strip from the wash solvent. To prevent possible surface contamination during final preparation, do not allow fingers to come in direct contact with the silver strips, by wearing disposable gloves or holding the strips in the fingers protected with ashless filter paper.
9.2.1 Polish first the edges and then the sides with the 105 µm (150 mesh) silicon carbide grains or powder picked up with a pad of cotton (cotton wool) moistened with wash solvent. Polish the edges with straight strokes, rotating the coupon clockwise on edge to polish all four edges. Then repeat the process counter-clockwise to make sure all the edge surfaces are polished. Wipe vigorously with fresh pads of cotton (cotton wool).
9.2.2 Handle the step without touching the surface with the fingers. Forceps have been found suitable to use. Clamp in a vise and polish the main surfaces with silicon-carbide grains on absorbent cotton. Do not polish in a circular motion. Rub in the direction of the long axis of the strip, carrying the stroke beyond the end of the strip before reversing the direction.
9.2.3 Clean all metal dust from the strip by rubbing vigorously with clean pads of absorbent cotton until a fresh pad remains unsoiled. When the strip is clean, immediately immerse it in the prepared sample.
NOTE 1 - It is important to polish the whole surface of the strip uniformly to obtain a uniformly stained strip. If the edges are worn (surface elliptical), they will likely show more corrosion than at the center. The use of a vise will facilitate uniform polishing.
NOTE 2 - It is important to follow the order of preparation with the correctly sized silicon carbide material as described in 9.1 and 9.2. The final preparation is with 105 µm silicon carbide grains or powder. This is a larger grain size than the 53 µm to 65 µm paper used in the surface preparation stage. The reason for this use of larger silicon carbide grains in the final preparation is to produce asperities (controlled roughness) on the surface of the silver, which act as sites for the initiation of corrosion reactions.