ASTM D7671 Standard Test Method for Corrosiveness to Silver by Automotive Spark-Ignition Engine Fuel - Silver Strip Method
5.1 Silver Strip Corrosion Pressure Vessel (Procedure A), constructed from stainless steel or similar strong and non-corroding metallurgy according to the dimensions as given in Fig. 1, that is the apparatus described in Test Method D130. The vessel shall be capable of withstanding a test pressure of 700 kPa gauge (100 psi).
5.1.1 Alternative designs for the vessel's cap and synthetic rubber gasket may be used provided that the internal dimensions of the vessel are the same as those shown in Fig. 1. The internal dimensions of the pressure vessel are such that a nominal 25 mm by 150 mm test tube can be placed inside the pressure vessel.
5.2 Test Tubes, of borosilicate glass of nominal 25 mm by 150 mm dimensions. The internal dimensions shall be checked as acceptable by use of a silver strip (see 6.3). When 30 mL of liquid is added to the test tube with the silver strip in it, a minimum of 5 mm of liquid shall be above the top surface of the strip.
5.3 Stoppers (Procedure B), such as cork, to accompany test tubes in 5.2. Each stopper shall have a vent hole to equilibrate pressure that may build up in the test. The stoppers should be drilled through the center and fitted with a length of 3.2 mm (1/8 in.) OD glass or Nalgene vent tube.
5.4 Silver Strip Suspension Assembly, such as examples shown in Fig. 2 (Procedure A) or Fig. 3 (Procedure B), capable of suspending the silver strip in approximately the center of the sample-filled test tube during the test, such that the strip is kept in an upright and vertical position.
5.4.1 For Procedure A, a cradle of glass, polytetrafluoroethylene or other inert material connected to a holder of sufficient length and width as shown in Fig. 2 has been found suitable to use.
5.4.2 For Procedure B, a cable tie (see 6.2) wrapped around the edges of the strip and inserted through the stopper has been found suitable to use for this purpose as shown in Fig. 3.
5.4.3 Other assemblies or designs capable of meeting the functional requirement may also be used in Procedures A or B.
5.5 Test Bath:
5.5.1 General - The test baths shall be able to maintain the test temperature to within +/- 1 °C (2 °F) of the required test temperature (normally 50 ºC).
5.5.2 Liquid Bath Used for Submerging Pressure Vessel(s) (Procedure A) - The bath shall be deep enough to submerge one or more pressure vessels (see 5.1) completely during the test. As the bath medium, use water or any liquid that can be satisfactorily controlled to the sample test temperature. The bath shall be fitted with suitable supports to hold each pressure vessel in a vertical position when submerged. The use of a solid block bath has been found as a suitable alternative to the liquid bath.
5.5.3 Liquid Bath Used for Vented Test Tube Method (Procedure B) - The bath liquid level shall be at a level that is higher than the liquid level in the test tube at the test temperature. It is recommended that the bath be placed inside an exhaust hood.
5.5.4 The use of a solid block bath has been found to be a suitable alternative to the liquid bath.
5.6 Temperature Sensing Device (TSD), capable of monitoring the desired test temperature in the bath to within accuracy of +/- 1 °C (2 °F), measured in the middle of the liquid bath. The ASTM 12C (12F) (see Specification E1) or ASTM E2251 (see Specification E1) or IP 64C (64F) total immersion thermometers have been found suitable for use in the test. If used, no more than 10 mm (0.4 in.) of the mercury should extend above the surface of the bath at the test temperature.
5.7 Polishing Vise, for holding the silver strip firmly without marring the edges while polishing. Any convenient type of holder (see Appendix X1) may be used provided that the strip is held tightly and that the surface of the strip being polished is supported above the surface of the holder.
5.8 Viewing Test Tubes, (optional) flat glass test tubes, are convenient for protecting corroded silver strips for close inspection or storage (see Appendix X1 for the description of a flat-glass viewing tube). The viewing test tube shall be of such dimensions as to allow the introduction of a silver strip (see 6.3) and made of glass free of striae or similar defects.
5.9 Forceps, with either stainless steel or polytetrafluoro-ethylene tips, for use in handling the silver strips, have been found suitable to use.
5.10 Timing Device, electronic or manual, capable of accurately measuring the test duration within the allowable tolerance.
5.11 ASTM Silver Strip Corrosion Standards, consist of reproductions in color of typical test strips representing increasing degrees of tarnish and corrosion, the reproductions being encased for protection in plastic and made up in the form of a plaque. See Table 1.
5.11.1 Keep the plastic-encased ASTM Silver Strip Corrosion Standards protected from light to avoid the possibility of fading. Inspect for fading by comparing two different plaques, one of which has been carefully protected from light (for example, new plaque). Observe both sets in diffused daylight (or equivalent) first from a point directly above and then from an angle of 45°. If any evidence of fading is observed, particularly at the left-hand end of the plaque, it is suggested that the one that is the more faded with respect to the other be discarded.
188.8.131.52 Alternatively, place a suitably sized opaque strip (for example, 20 mm (3/4 in.) black electrical tape) across the top of the colored portion of the plaque when initially purchased. At intervals remove the opaque strip and observe. When there is any evidence of fading of the exposed portion, the standards shall be replaced.
5.11.2 The plaques are full-color reproductions of typical strips. They have been printed on aluminum sheets by a 4-color process and are encased in plastic for protection. Directions for their use are given on the reverse side of each plaque.
5.11.3 If the surface of the plastic cover shows excessive scratching, it is suggested that the plaque be replaced.
6. Reagents and Materials
6.1 Ashless Filter Paper or Disposable Gloves, for use in protecting the silver strip from coming in contact with the individual during final polishing.
6.2 Cable Tie - Inert ties such as nylon ties of approximate dimensions 2 mm x 208 mm.
6.3 Silver Strips - Use strips 12.5 mm to 12.7 mm wide, 2.5 mm to 3.0 mm thick, and 17.0 mm to 19.0 mm long assaying at 99.9 % (m ⁄m) Ag minimum, which are the same specification requirements described in IP 227. The strips may be used repeatedly but should be discarded when the strip's surface shows pitting or deep scratches that cannot be removed by the specified polishing procedure, or when the surface becomes deformed.
6.4 Surface Preparation/Polishing Materials, silicon carbide grit paper or cloth of varying degrees of fineness including 53 µm to 65 µm (240 grit) grade; also a supply of 105 µm (150 mesh) size silicon carbide grain or powder and absorbent cotton (cotton wool). A commercial grade of absorbent cotton is suitable, but pharmaceutical grade is most commonly available and is acceptable.
6.5 Wash Solvent - 2,2,4-trimethylpentane (isooctane) of minimum 99.75 % purity. (Warning - Extremely flammable. See 7.1.)
7.1 Wash Solvent - 2,2,4-trimethylpentane (isooctane): Harmful if inhaled. Use with adequate ventilation. Keep away from heat, sparks, and open flame. Avoid prolonged or repeated skin contact. Consult MSDS for additional information.
7.2 Automotive Spark-Ignition Fuel (typically gasoline) - Harmful if inhaled. Use with adequate ventilation. Keep away from heat, sparks, and open flame. Avoid prolonged or repeated skin contact. Consult MSDS for additional information.