ASTM D7671 Standard Test Method for Corrosiveness to Silver by Automotive Spark-Ignition Engine Fuel - Silver Strip Method
10. Procedure
10.1 Procedure A (Involving the Use of a Pressure Vessel):
10.1.1 Place 30 mL of sample, completely clear and free of any suspended or entrained water (see 8.3) into a chemically clean and dry 25 mm by 150 mm test tube.
10.1.2 Within 1 min after completing the final preparation (polishing, 9.2), place the prepared silver strip inside a suitable suspension assembly, such as the cradle assembly shown in Fig. 2, and insert this assembly into the sample tube.
10.1.3 Place the sample tube and suspension assembly into the pressure vessel (Fig. 1) and screw the lid on tightly.
10.1.3.1 If more than one sample is to be analyzed at essentially the same time, it is permissible to prepare each pressure vessel in the batch before completely immersing each pressure vessel in the liquid bath at 50 °C +/- 1 °C (122 °F +/- 2 °F), provided the elapsed time between the first and last samples is kept to a minimum.
10.1.4 After 3 h +/- 5 min in the bath, withdraw the pressure vessel and immerse for a few minutes in cool water (tap water).
10.1.5 Open the pressure vessel carefully (Warning - Contents may be under pressure) in a well ventilated area, such as an exhaust hood, to safely vent any residual pressure that may have built up during the test.
10.1.5.1 Safe venting may be done by cutting a single notch through the threads on the container body only, not on the knurled cap. The notch need be only 1 mm wide, cut no deeper than the valley between threads, and cut carefully so as not to compromise the "O"-ring seal.
10.1.6 Immediately withdraw the silver strip from the suspension assembly with forceps and immerse in wash solvent. Withdraw the strip at once, dry it with ashless filter paper or cotton ball (by blotting not wiping) and inspect it for evidence of tarnishing or corrosion.
10.1.6.1 In handling the test strip during the inspection and comparison, the danger of marking or staining can be avoided if it is inserted in a flat glass tube (see Appendix X1), which can be stoppered with absorbent cotton.
10.2 Procedure B (Involving the use of a Vented Test Tube):
10.2.1 Place 30 mL of sample, completely clear and free of any suspended or entrained water (see 8.3) into a chemically clean and dry 25 mm by 150 mm test tube.
10.2.2 Within 1 min after completing the final preparation (polishing), secure the silver strip in a suitable suspension assembly, such as with a cable tie lanyard (see Fig. 3), as described in 10.2.2.1 - 10.2.2.5, and then insert into the test tube and stopper with a vented cork
10.2.2.1 Rest the strip on a clean sheet of filter paper.
10.2.2.2 Insert the tip of the cable tie into the grommet. Tighten slightly, forming a large loop.
10.2.2.3 Position the loop on the paper such that the loop surrounds the outer perimeter of the silver strip. Without touching the strip, carefully tighten the cable tie until it holds the strip securely at the four corners but does not contact any of the other surfaces. The grommet should be located on the short side of the silver strip.
10.2.2.4 Insert the strip into the test tube and pull the end of the cable tie up through the vent tube in the cork stopper.
10.2.2.5 Use the cable tie to raise or lower the suspended silver strip. Position the strip so that a minimum of 5 mm of sample is present above the top of the strip. Once the proper placement is achieved, fold the top of the cable tie over the edge of the vent tube and secure between the exterior of the glass insert and the inner wall of the stopper.
10.3 Place the test tube upright in a test tube rack inside the water bath. (Warning - Flammable vapor can be vented from the heated test tube.)
10.3.1 If more than one sample is to be analyzed at essentially the same time, it is permissible to prepare each sample in the batch before immersing in the liquid bath at 50 °C +/- 1 °C (122 °F +/- 2 °F), provided the elapsed time between the first and last samples is kept to a minimum.
10.4 After 3 h +/- 5 min in the bath, withdraw the test tube. Remove the stopper.
10.5 Immediately withdraw the strip with forceps and immerse in wash solvent. Withdraw the strip at once, dry it with ashless filter paper or cotton balls (by blotting not wiping) and inspect it for evidence of tarnishing or corrosion.
10.5.1 In handling the test strip during the inspection and comparison, the danger of marking or staining can be avoided if it is inserted in a flat glass tube (see Appendix X1), which can be stoppered with absorbent cotton.
11. Interpretation of Results
11.1 Interpret the corrosiveness of the sample by comparing the appearance of the test strip with a freshly polished one to give a classification based on that given in Table 1. All surfaces, including the edges, shall be taken into account.
11.1.1 The Color Standard for Tube Deposit Rating (referenced in Test Method D3241) shall be used to differentiate between the brown colorations mentioned in Classifications 1 and 2. Any brown coloration less than No. 4 on the Color Standard shall be rated Classification 1. Any coloration equal to or darker than No. 4 on the Color Standard shall be rated as Classification 2 or higher.
