Corrosivity Test Methods for Polymeric Materials. Part 4- Cone Corrosimeter Test Method
Authors: Bennett James G. , Kessel Stephen L. , Rogers Charles E. 
Published: March 2007

This is the fourth in a series of papers to investigate corrosivity test methods published by the Polyolefins Fire Performance Council, an operating unit of The Society of the Plastics Industry, Inc.

In the first paper, 24 polymeric materials were evaluated for smoke corro sivity following the test method proposed by ASTM E05.21.70 which uses a ra diant combustion/exposure apparatus. The second paper discussed the evalua tion of the same materials using the CNET corrosion test method under 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 poly meric materials were evaluated using a modified DIN acid gas test method and the results were compared to both the previous ASTM E05.21.70 and CNET results. These commercially available polymeric materials cover a broad range of compositions used for wire and cable insulation and jacketing.

In this paper, the same polymeric materials were evaluated following the "Fire Response Standard for Determining the Corrosive Effect of Combustion Products Using a Cone Corrosimeter" proposed 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 ignite combustible vapors. A portion of the pro ducts of decomposition or combustion are channeled in a dynamic mode through an exposure chamber in which corrosion targets are placed until the specimen has lost 70% of its total available mass loss. The mass loss is deter mined from previous experiments at the recommended heat flux. When the specimen has lost 70% of its mass loss, the exposure chamber is sealed and iso lated. The corrosion of the target is determined by exposing the target to the now static combustion products for one hour measured from the start of the test. The target is then placed in an environmental chamber at 75% relative humidity at 23°C for 24 hours. The test method measures the increase in elec trical resistance of a metallic circuit. This increase is related to the decrease in conductive cross-sectional area resulting from metal loss due to corrosion. The increase in electrical resistance of each target is determined throughout the test and correlated to its metal loss. The 24 hour corrosion value is reported as metal loss in angstroms.

In this study, heat fluxes of 25 and 50 kW/m2 were used to simulate two dif ferent fire scenarios. All of the materials were run at 50 kW/m2 and 12 materials were run at 25 kW/m 2. Two targets, one with a span of 2,500 Å and the second with a span of 45,000 A were used during each test at each heat flux.

The results of this study indicate that the measured corrosivity of materials: (1) does not correlate consistent with the expectations based upon the known chemis try of their compositions (2) varies numerically with the heat flux under which the tests are run and on the target used to obtain the corrosion data and (3) although numerically different, loosely ranks the corrosive potentials of the materials in a consistent 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 recommending both in the appendix. As corrosion values are numerically de pendent on the conditions and target used to obtain the data, it is questionable how this test method can be used as a standard for determining and comparing the corrosion potentials of materials without requiring 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 corro sive potentials of the 24 materials using the four test methods will be made and one test method recommended for use as a global standard.