Cabinet testing

The CASS Test. In the copper-accelerated acetic acid salt spray (CASS) test (42), the positioning of the test surface is restricted to 15 2°, and the salt fog corrosivity is increased by increasing temperature and acidity, pH about 3.2, along with the addition of cupric chloride dihydrate. The CASS test is used extensively by the U.S. automobile industry for decorative nickel—chromium deposits, but is not common for other deposits or industries. Exposure cycle requirements are usually 22 hours, rarely more than 44 hours. Another corrosion test, now decreasing in use, for decorative nickel—chromium finishes is the Corrodkote test (43). This test utilizes a specific corrosive paste combined with a warm humidity cabinet test. Test cycles are usually 20 hours. 

Closed cabinet tests and their operation may be likened to a small lire burning in a closed room. The specimen is decomposed by a heat source and the resulting effluent accumulates within the cabinet. The decomposition system is either mounted within the cabinet as in the aircraft50 and maritime tests51 or may be outside, connected to the cabinet by a short duct, as in ASTM E 1678.52... 

FIGURE 12.48 Maximum smoke density developed during controlled combustion of acrylic, mod-acrylic, and other fiber types. Results are from NBS smoke cabinet tests. (From Fiber Producer, p. 44, April, 1984.)... 

Cabinet tests consist in tests in which specimens are placed in a testing apparatus where the exposure conditions can be controlled. By means these tests it is possible to examine the performance of materials exposed to corrosive atmospheres that may be encountered in various locations, including near sea or in industrial areas. These tests can include the effects of humidity (RH), salt, corrosive gases (e.g. SO, H S, CO ), temperature and UV radiation. 

The main purpose of cabinet tests is to accelerate material degradation simulating a long service and allowing to preview the corrosion occurring during in-service exposure. Cabinet tests are a useful indication of possible corrosion performance in natural environments and, in many cases, can be used to provide a relative comparison of material performance. 

As for immersion tests, also in cabinet tests the corrosion effects are evaluated by weight loss, appearance, metallographic examination, changes in electrical and mechanical properties, with evaluation criteria prescribed by standards. It is worthwhile to note that the evaluation of corrosion effects based on visual appearance often is subjective. Some objective method should be used, such as image analysis, with the aim of minimising the influence of human factor in the post-test evaluation. 

Corrosion test cabinets have evolved from homemade water-and-spray tanks that operated at fixed or ambient temperatures and humidity to computer-controlled, multi-environmental laboratory apparatus that are not only functional, but are also an attractive addition to the laboratory. Sizes range from small benchtop units to large walk-in and drive-in chambers capable of testing hill vehicles or other large specimens. ISO requirements have led to the use of NIST traceable devices for measurement of temperature, humidity, and/or other critical conditions to enhance the accuracy and repeatability of the test conditions. A Ust of commonly used cabinet tests is shown in Table 1. 

However, the results of cabinet tests should be considered in conjunction with field performance data. 

When it is impractical to test in service, laboratory simulations of the corrosive exposures may be necessary. Immersion tests, cabinet tests, and electrochemical measurements are in widespread use, but do not necessarily provide results that correlate with service exposures. However, relative rankings of selected materials under controlled testing conditions can be useful in materials development. 

Many so-called cabinet tests are used to detect lot-to-lot variation of materials. Cabinet tests involve exposure of a material to an aqueous fog that may contain a variety of corrosive species (e.g., chlorides, sulfates), either singly or in combination. The exposure conditions (e.g., temperature, solution pH, solution specific gravity, fog delivery rate, position of specimens) are carefully specified and controlled. However, extensive efforts to correlate cabinet test results with those of service performance tests for many materials, including steel, have failed. The inability of the cabinet to reproduce the exposure conditions experienced in service is the primary contributor to that limitation. The unique interplay of the environment with the relatively fragile passive film on steel makes proper selection of exposure conditions mandatory. Standardized cabinet tests that could be adapted for quality control of uncoated steel are described in the following standards ... 

Cabinet tests are the most commonly used laboratory tests in the automobile industry. These have been developed to simulate the effects of atmospheric corrosion 44. Normally, a test chamber is used and the desired environment is introduced under controlled conditions. A list of these tests is given in Table 6. The simplest of these is the humidity test whereby the temperature and relative humidity within a chamber are controlled. A condensing humidity chamber, which operates at 100 % relative humidity and 38°C, provides... 

The most widely used cabinet test is the neutral salt spray (Fog) test (ASTM B 117), which consists of a fog of 5 % sodium chloride within the chamber at 35 C [46. Controversy exists over the validity of B 117 as a performance test because corrosion mechanisms are not always the same as those observed in automobile service. Also, not all materials can be successfully evaluated in the test. However, the value of the salt spray test as a quality assurance test is well documented [46]. Several modifications to the salt spray test have been developed including acetic acid salt spray (ASTM G 85, Annex 1), copper accelerated acetic acid salt spray (ASTM B 368), acidified synthetic seawater fog (ASTM G 43, Method of Acidified Synthetic Seawater (Fog) Testing), and modified salt spray (ASTM G 85). ASTM G 85 also includes cyclic tests. 

Accelerated cabinet testing of plastic materials can be performed with essentially the same validity as when testing metallic materials. 

The oldest and most widely used cabinet test is ASTM B117 [Test Method of Salt Spray (Fog) Testing], a test that introduces a spray in a closed chamber where the test specimens are exposed at specific locations and angles [24]. The concentration of the sodium chloride (NaCl) solution has ranged from 3.5 to 20 percent. There is a wide... 

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