Schematic drawings showing the nondestructive QC method for measuring cylinder coating quality. The coating thickness is measured by inserting a probe into the small opening of a gas cylinder using a specially designed tool so that a hard contact can be made with the coated cylinder surface, as shown in Figure 7. The measurement method is based on magnetic principles and uses the substrate's magnetic properties to determine the coating thickness. Figure 8 provides typical results for the coating-thickness component of the test. In that instance, visual observation had indicated that all of the cylinders in the lot being tested were smooth and had no visible defects.
Results shown represent the average thickness over the length of each cylinder.
The coating-porosity component of the QC test is based on electrochemical measurements and is illustrated in Figure 7. The test yields a quantitative number for porosity that is normalized to an arbitrary scale from 1 to 100. A cylinder with a coating porosity value of less than 3 is considered to have negligible porosity and would be accepted, but any cylinder with a porosity value greater than 3 would be rejected and returned for reprocessing. As was the case for the coating-thickness test results shown in Figure 8, several of the cylinders had to be rejected. The consistently low impurity levels that have been achieved provide further evidence of the corrosion resistance of the treated and qualified cylinders.