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THIRTEENTH MEETING PANEL ON FIRE RESEARCH AND SAFETY, MARCH 13-20, 1996 VOLUME 1
Kellie Ann Beall, Editor June 1997 Building and Fire Research Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899
U.S. Department of Commerce William M. Daley, Secretary Technology Administration Gary R. Bachula, Acting Under Secretary for Technology National Institute of Standards and Technology Robert E. Hebner, Acting Director Corrosion from Combustion Products - An Overview
Pravinray D. Gandhi Senior Staff Engineer
Underwriter Laboratories Inc. 333 Pfingsten Road, Northbrook, IL 66062 (847-272-8800 Ext. 43354) Corrosion from Combustion Products - An Overview
Abstract
There has been a keen focus to understand corrosion from combustion products as it is related to potential damage from their deposition on equipment. Several test methods are available to assess the potential for corrosion damage from combustion products. These include indirect methods using the pH and conductivity measurements, and also direct methods that measure loss of metal on a target. These are discussed in this presentation. A recent development of determining the reliability of electronic equipment when exposed to combustion products uses an interdigitated target and measures leakage current after the exposure. Data developed using this technique is presented and discussed.
Introduction
There has been a keen focus to understand corrosion from combustion products as it is related to potential damage from their deposition on equipment. Of specific interest is the impact on the reliability of electrical and electronic equipment. Since the effluents can be carried away by the buoyancy of the gases, the potential for damage exists remotely from the fire. For this reason, this phenomenon is also termed "non-thermal fire damage." Another feature of this phenomenon is that in many instances, the damage may become evidence after significant passage of time. The time delay for the impact of corrosive deposits to become evident on equipment performance depend upon the rate of chemical reactions taking place, and the availability of the appropriate conditions of temperature and relative humidity.
Three forms of corrosion have been identified. These may be defined as follows:
1. Metal loss due to electrolytic and chemical attack on metal; 2. Leakage current due to increased surface conductance; and 3. Increased contact resistance due to deposition of combustion product and subsequent chemical reactions.
The loss of metal results in reduction in the strength of structural members, and increase in electrical resistance of exposed metal parts. However, the exposed metal components on electronic circuit boards is limited to contacts. The second form of corrosion from the deposition of combustion products on electronic circuit boards, may lead to increased leakage currents to cause malfunction of circuitry. The third form of corrosion influences the electrical resistance between contacts for equipment such as relays and switches. Corrosion between contacts may lead to increased electrical resistance and thus cause a malfunction.
In this paper, the methods currently used to assess corrosion of combustion products, and a new approach to determine the impact of combustion products to the reliability of electronics are presented.
Methods
Several techniques have been devised to determine the corrosiveness of combustion products. These techniques may be divided into indirect measurements, and direct measurement methods.
Indirect Methods of Measurement
The indirect methods of measurement consist of measuring the either halogen acid gases, or the change in pH or electrical conductivity of a solution through which combustion products have been bubbled. Some of the methods have been standardized as shown in Table 1.
Table 1 - Indirect Methods of Measuring Corrosion
From Table 1 it may be observed that these methods are material tests. Thus, it does not have the ability to determine the influence of finished product construction. IEC 754-2, however, does test the individual materials separately and then provide a weighted average of pH and conductivity of the constituent material of the finished cable. Further, the corrosion potential is inferred from the amount of acid gases (halogen gases), change in pH or electrical conductivity and do not provide a direct measurement the three modes of corrosion.
Direct Measurement Methods
In the US and internationally, some effort has been focused on developing a method that provides a direct measurement of corrosion. In the US, ASTM E05, and ASTM D-9 considered corrosion methods using metal loss targets. In consideration of the method, several objectives for the test were developed as follows:
The last objective may have been identified on the prolonged post-exposure time to measure loss of metal from coupons exposed to combustion products from shipboard fires. Thus, the need for fast experiments was related to the sensitivity of the measurement technique used.
National Telecommunications laboratory of France (CNET) developed a corrosion test that measured the change in electrical resistance of a printed wiring board. Since the corrosion would reduce the metal loss thickness, the resistance of the target is expected to increase. The test method consists of burning a mixture of 600 mg of test sample, and 1000 mg of polyethylene in a 20 liter closed chamber, which is initially kept at 50ºC. The ignition is accomplished by using an electrically heated coil at a temperature of 800ºC. The combustion products deposit on a water-cooled printed wiring board. The temperature of the target is maintained at 40ºC during the test. The change in resistance is measured at the end of 1 hour. The test, with some modifications, have now been standardized as ISO 11907-2.
The test is conducted in two steps. In the first step, the weight loss characteristics of the sample is determined by conducting two tests at the desired radiant flux level. The corrosion tests are then conducted with the gas sampling until 70% of sample weight loss determined in the first step is attained. The exposure chamber is then sealed, and the target is exposed to the combustion products for a total of 60 minutes. There is an additional 24 hr. post-test exposure at 75% RH, and 75ºF.
Several studies have been conducted to compare the corrosion results obtained from the ISO 11907-2, and the ASTM D5485 test protocols. The data from one of the studies are presented in Figure 5, and Figure 6 respectively.
It may be observed that a trend is apparent between the data obtained in the CNET and the ASTM D5485 test apparatuses. The CNET test apparatus is a static chamber test. However, the mass of the test sample is small enough that here is more than sufficient oxygen for combustion of the sample. On the other hand, the ASTM D5485 test, the sample burns under well-ventilated conditions. Figure 7 shows a comparison of the data presented in Figure 5 and Figure 6.
One of the advantages of the ASTM D5485 method is that it permits testing portions and products such as cable samples. This is important since test items may be manufactured with more than one material and co-combustion of materials constituents can influence the corrosion results. Further, product testing also allows determining the influence of construction. Figure 8 depicts some corrosion data obtained from cable samples using ASTM D5485.
The direct measurement tests that have been developed use metal loss target. This measurement however does not provide information on the impact of deposited combustion products on electronics. In Central Equipment Offices, it has been observed that deposition of dust over a period of time can lead to malfunction of circuit boards under appropriate conditions of humidity and temperature. The dust has many ionic substances that cause an increased electrical surface conductance. Further, investigation of this phenomenon has shown that the surface conductance, or leakage current increases with % Relative Humidity. In fact, it has been observed that below a critical humidity level, the leakage current is relatively insignificant. Increase in humidity beyond the critical level results in marked increase in leakage current. Figure 9, adapted from Comizzoli et. al, shows the influence of relative humidity on the leakage current from a number of dust samples. It may be noted that below relative humidity of less than 40% the leakage current is negligible, except for dust from the Kuwaiti oil well fires (labeled K10). The high leakage current at low humidity levels of the Kuwaiti samples is suspected to be due to the presence of graphitic carbon resulting from the hydrocarbon fires at the end of the Gulf War.
Plastics contain fire retardants, fillers, and other additives that may release ionic particles when they are involved in a fire. The thermal plume of a fire then can carry these particles to remote areas where they can deposit on electronic circuitry in an analogous manner to dust particles.
In order to study the influence of combustion products on the leakage current on electronic circuitry, a series of tests were conducted using a tube furnace apparatus similar to that described in IEC 754-2².
Leakage Current Experiments
The combustion tube furnace consisted of furnace, silica tube, combustion boat, air supply system, and a mixing chamber for the combustion products. The tube furnace had an inside diameter of 60.3 mm and a heating zone of 300 mm. The test temperature was controlled by an electronic temperature controller. The silica tube was 1600 mm long, 47.5 mm inside the diameter and had a wall thickness of 2.75 mm. The silica tube was placed in the tube furnace such that it extended 400 mm from the rear end of the furnace. The rear end of the tube was ground and was fitted with a glass adapter connected to an air supply from a dry compressed air cylinder. A porcelain combustion boat, 97 mm in length, was used to hold the test sample during the test.
The mixing chamber was made from polymethyl methacrylate (PMMA), with dimensions of 310 x 310 x 340 mm. A stainless steel plate was attached to the other side of part of the chamber connected to the silica tube. The purpose of the plate was to protect the PMMA surface from flames emanating from the silica tube. The top of the mixing chamber was a blowout panel to release excessive pressure. The chamber had a 6.3 mm opening at the bottom of one of the sides to permit exhaust of combustion products to a smoke abatement system. The mixing chamber was placed 385 mm away from the end of the tube furnace, such that 55 mm of the silica tube protruded inside the chamber.
Test Samples
Test samples were obtained commercially. Six materials were used in this investigation. In the combustion tube furnace experiments the IEC 754-2 test specifies a sample weight of 1,000 ± 5 mg. The samples are described in Table 2.
Table 2 - Test Samples
The samples were conditioned for at least 16 h at a temperature of 23 ± 2°C, and relative humidity of 50 ± 5%.
Future Work
This study provides information for understanding the smoke Corrosivity behavior of cable jacket and insulation materials. We are anticipating continuing this investigation to refine the test protocol and correlate to large scale testing. Some areas of future work include: 1) comparison of data to metal loss behavior, 2) comparison of data to pH and conductivity data, 3) expanded materials list for testing, 4) conduct SEM/EDAX analysis of test patterns, 5) analysis of smoke particulate, 6) small and large scale product testing.
Discussion
Patrick Pagni: Which of the two do you thin is the more serious problem: condensate on particles or gas phase material as a corrosive agent? Pravinray Gandhi: In my opinion, the particle condensation is the most problem. This is not based upon any measurements I have made, but is made based upon all the observations I've had in studying the targets after exposition to corrosion particles.
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