CORROSIVITY TEST METHODS FOR POLYMERIC MATERIALS .4. CONE CORROSIMETER TEST METHOD

This is the fourth in a series of papers to investigate corrosivity te st methods published by the Polyolefins Fire Performance Council, an o perating unit of The Society of the Plastics Industry, Inc. In the fir st paper, 24 polymeric materials were evaluated for smoke corrosivity following the test method proposed by ASTM E05.21.70 which uses a radi ant combustion/exposure apparatus. The second paper discussed the eval uation of the same materials using the CNET corrosion test method unde r consideration by ISO TC61/SC4/WG2 and IEC TC89/WG3 and compared the CNET results with the ASTM E05.21.70 results. In the third paper, the 24 polymeric materials were evaluated using a modified DIN acid gas te st method and the results were compared to both the previous ASTM E05. 21.70 and CNET results. These commercially available polymeric materia ls cover a broad range of compositions used for wire and cable insulat ion and jacketing. In this paper, the same polymeric materials were ev aluated following the ''Fire Response Standard for Determining the Cor rosive Effect of Combustion Products Using a Cone Corrosimeter'' propo sed by ASTM D09.21.04. In this test method, a specimen is subjected to radiant heat at the recommended heat flux using a spark igniter to ig nite combustible vapors. A portion of the products of decomposition or combustion are channeled in a dynamic mode through an exposure chambe r in which corrosion targets are placed until the specimen has lost 70 % of its total available mass loss. The mass loss is determined from p revious experiments at the recommended heat flux. When the specimen ha s lost 70% of its mass loss, the exposure chamber is sealed and isolat ed. The corrosion of the target is determined by exposing the target t o the now static combustion products for one hour measured from the st art of the test. The target is then placed in an environmental chamber at 75% relative humidity at 23-degrees-C for 24 hours. The test metho d measures the increase in electrical resistance of a metallic circuit . This increase is related to the decrease in conductive cross-section al area resulting from metal loss due to corrosion. The increase in el ectrical resistance of each target is determined throughout the test a nd correlated to its metal loss. The 24 hour corrosion value is report ed as metal loss in angstroms. In this study, heat fluxes of 25 and 50 kW/m2 were used to simulate two different fire scenarios. All of the materials were run at 50 kW/m2 and 12 materials were run at 25 kW/m2. Two targets, one with a span of 2,500 angstrom and the second with a s pan of 45,000 angstrom were used during each test at each heat flux. T he results of this study indicate that the measured corrosivity of mat erials: (1) does not correlate consistent with the expectations based upon the known chemistry of their compositions (2) varies numerically with the heat flux under which the tests are run and on the target use d to obtain the corrosion data and (3) although numerically different, loosely ranks the corrosive potentials of the materials in a consiste nt manner at both heat fluxes and with both targets. The test protocol does not specify either the heat flux or the targets to be used recom mending both in the appendix. As corrosion values are numerically depe ndent on the conditions and target used to obtain the data, it is ques tionable how this test method can be used as a standard for determinin g and comparing the corrosion potentials of materials without requirin g that both the specific heat flux and the target be specified in the test protocol as well as be reported with the results. To complete the review of corrosion test methods, a comparison of the corrosive poten tials of the 24 materials using the four test methods will be made and one test method recommended for use as a global standard.