BS ISO 18070:2015 pdf download – Corrosion of metals and alloys — Crevice corrosion formers with disc springs for flat specimens or tubes made from stainless steel.
6.3 Crevice former The crevice assembly is of the spring loaded type. Nut, bolt, and washers (rings) are made from titanium. The thickness of the washers is between 2 mm and 4 mm. Outer diameter of the washers shall be at least 20 mm and the inner diameter at least 5,2 mm. Bolt diameter shall be 5 mm (M5 threads) and the length is about 75 mm. A PVC (polyvinyl chloride) or PE (polyethylene) hose is mounted around the bolt, from the head of the bolt to the crevice former at the nut side, to prevent electrical contact between specimen and the assembly and to centre the rings, disc springs, and specimen. A schematic drawing of the assembly for flat specimens is shown in Figure 1 and for tubes is shown in Figure 2. A ring of inert polymeric material, like PTFE (polytetrafluoroethylene) is mounted to centre the disc springs, as shown in Figure 1 and Figure 2. The disc springs are made of titanium or nylon coated steel. The disc spring shall be according to DIN 2093 or with a calibrated deflection-force relationship. A set of four springs is used to obtain a load of ~0,9 kN (this corresponds approximately to a torque of 3 Nm, with the crevice assembly applied using DIN 2093 disc springs). The crevice formers are made of PVDF (polyvinylidene fluoride) and can be used up to 95 °C. The outside diameter is 20 mm and the inside diameter is 7 mm and height is 15 mm.
6.4 Crevice formers for tubes The specifications for crevice formers for tubes are given in 6.3. In addition, on one side, a curvature shall be machined with same radius as the outer diameter of the tube. The crevice former shall not be thinner than 5 mm. Examples of the calibration curves for the force and torque that shall be used for tubular specimens are shown in Figure 3 and Figure 4, respectively. The force from Figure 3 shall be used for calculating the corresponding deflection using the linear relationship between the force and deflection for the spring assembly. The tube shall be thick enough to ensure no deflection of the tube during the test.