Corrosive gas tests

Corrosive gas tests are often a combination of a gas introduced into a high humidity atmosphere and typically used as a porosity test. A few examples of these tests are ... 

The Mixed Flowing Gas Test, ASTM B 827, is used extensively in the electronics industry. As the name implies, the test method uses a combination of various gases at controlled temperatures and humidities to produce the desired conditions. The MFG test requires equipment of a more complex nature than other corrosive gas tests. A schematic of a typical chamber is shown in Fig. 3b, as depicted in ASTM B 827. 

Experience has shown that our best efforts to predict potential problem areas frequently miss the fatal flaws. For this reason it is advisable to perform at least one broad, severe corrosive gas test and one similarly harsh particle test, even if no obvious failure scenario can be envisioned that would be expected to be manifested through these tests. It is also helpful to stress the material or product sufficiently to produce significant corrosion or malfunction, even if this requires unrealistically harsh conditions. Such a test helps ensure that the designer understands the materials limitations and may indicate that the product is over-designed. Less resistant, less expensive materials may be sufficient for the intended use. 

Corrosive gas testing became commonplace in the mid-1970s as a result of the need of the electronics industry to reduce the cost of materials used as electrical contacts on connectors and as conducting pathways on various kinds of insulating substrates. Studies at IBM [75-78] and Battelle [79] played a key role in the development of this technology. Studies at both institutions compared laboratory data to field data and achieved results in the laboratory that accurately reproduced field results. Field exposures of 20 years can be simulated in 5-30 days in the laboratory. Based on metal coupon exposures and simultaneous air px>llutant measurements from over a hundred locations around the world, a four-level environmental classiflcation scheme for environmental severity, which is now in common use around the world, was developed by the industry-sponsored Battelle group. 

The greatest difficulty in using corrosive gas tests comes in deciding how to test entire assemblies of equipment, rather than just the metallization schemes used for connectors, and in evaluating equipment for those indoor environments where the RH may reach close to 100 %. The Battelle, Bellcore, and Siemens test conditions are intended for simulations of metals that will be used in office buildings, homes, and other air-conditioned environments. Enclosed environments such as sheds, utility huts, and non-air-conditioned manufacturing environments are more like outdoor than indoor environments with respect to RH. Standardized test conditions are not available. At this time, the test engineer has little alternative but to use the conditions that work for metals in indoor environments to cover all "enclosed" environments. 

Whether these corrosive gas tests are reaUstic for materials other than those used for connectors or for operating electronic equipment is not clear. The test should be carried out, but the observation of no failures should not be taken to mean there will be no field failures in typical urban environments. Similarly, any failures that are observed should be carefully evaluated to ensure that the same mechanism would he operative in field situations. Connectors tire a somewhat unique part of an electronic assembly in that the active part is frequently a noble mettil and the sensitivity of the mated surfaces to failure may be lower thtin many other parts of electronic assemblies. Most failures in electronic assemblies attributable to the environment are due to ionic particle contamination in conjunction with atmospheric moisture. In 20 years of evaluating field failures in the United States, the author has never seen a failure that could be attributed to the effects of SOj, has seen a few caused by H2S or HCl, has heard of a few caused by NOx, and has seen several hundred that were caused by ionic contamination. Clearly, valid accelerated testing of electronic components, circuit boards, and assemblies must include ionic contamination. Emerging methods are discussed in the Fine Particle Testing section in this chapter. 

The determination of the first bending critical speed is well established however, there is also concern with regard to the rotor support system s sensitivity to exciting forces. These come from unbalance and/or gas dynamic forces arising during operation in service. Operation with dirty corrosive gas will soon cause rotor unbalance. The rotor dynamics verification test is concerned with synchronous excitaticm, namely unbalance. The test must also verify that the separation margins are to specification. 

Hydraulic (Liquid Seal) Flame Arresters Hydraulic (liquid seal) flame arresters are most commonly used in large-pipe-diameter systems where fixed-element flame arresters are either cost-prohibitive or otherwise impractical (e.g., very corrosive gas or where the gas contains solid particles that would quickly plug a conventional arrester element). These arresters contain a liquid, usually water-based, to provide a flame barrier. Figure 23-62 shows one design. Realistic tests are needed to ensure performance, as described in EN 12874 [15]. Note that hydraulic flame arresters may fail at high flow rates, producing a sufficiently high concentration of gas bubbles to allow transmission of flame. This is distinct from the more obvious failure mode caused by failure to maintain adequate liquid level. 

Fluoropolymers also low flammability. The fully fluorinated polymer polytetrafluoroethylene, for example, burns only in 95% oxygen under the test conditions of LOI. The burning, however, produces a highly toxic and corrosive gas hydrogen fluoride. 

Place 0.2 g of solid sodium bromide, NaBr, weighed on a beam balance, in a 16 X 150-mm test tube. Have at hand the materials and reagents to carry out all of the gas tests in the next paragraph, and then move to the hood. Working at the hood, add 1 mL of concentrated (18 M) sulfuric acid, H2SO4, solution. CAUTION Concentrated (18 M) H2SO4 is very corrosive and... 

If 1,1,1-trichloroethane is not properly stabilized it forms hydrochloric acid in the presence of aluminum. HCl corrodes aluminum. The presence of free water invalidates the result of this test. An aluminum coupon is scratched beneafli flie surface of a solvent The coupon is observed for 10 min and 1 h and flie degree of corrosion is recorded in form of pass (no reaction) or fail (gas bubbles, color formation, or metal corrosion). The test is important to cleaning operations because aluminum should not be used for parts of machines (pumps, tanks, valves, spray equipment) in contact with corrosive solvent. 

The equipment needed to conduct any of the above tests is typically a traditional salt spray chamber that meets the requirements of B 117, with additional electronic control packages for cycling air, solution, and/or corrosive gas flow, while monitoring time and temperature. 

Suggested test conditions that cover both "indoor and "outdoor environments are given in Table 7. It can reasonably be argued both for and against using the same humidity for indoor and outdoor testing. Since most chambers have been set up to operate stably at 70 or 75 % RH, one possible approach is to do all corrosive gas exposure testing at this RH but, for outdoor simulations, to follow the... 

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