Industrial salt spray testing

The reproducibility of test results between labs using the neutral salt spray tests has not been consistent, but the repeatability, within one lab, is better, and the test has value in comparing variations in coating systems. Correlation of hours of exposure in the salt spray test to actual performance of the plated part in service, even in marine atmospheres, is not consistent and usually avoided. A classic example is that cadmium deposits outlast zinc deposits on steel in salt spray tests and clean marine atmospheres, yet zinc outlasts cadmium when exposed to real, industrial atmospheres, because of the presence of sulfur-bearing corrodents in industrial environments. An important variable in salt spray testing is the position of the surface to be tested. Whereas the surface of test panels is specified to be 15—30° from the vertical (40), when salt spray testing chromated zinc-plated specimens, this range has appeared excessive (41). 

The main value of salt-spray tests is in the evaluation of the effectiveness of phosphate coatings in restricting the spread of rust from scratches or other points of damage in a paint film. This feature is of particular interest to the motorcar industry, as vehicles are often exposed to marine atmospheres and to moisture and salt when the latter is used to disperse ice and frost from road surfaces. Great care is needed in the interpretation of a salt-spray test, as it has been found to favour thin iron phosphate coatings more than is justified by experience with natural weathering. In the motorcar industry the present custom is to use zinc phosphate coatings on the car bodies and all other parts exposed to the outside atmosphere. 

Table 15.13. This specification follows good industrial practice, with additional safeguards in rinsing to remove residues to treatment solutions. Nonaccelerated treatments must be followed by a single rinse which may contain chromate accelerated treatments must be followed by three rinses—cold water, hot water and a final chromate rinse. Table 15.14 shows the salt-spray test requirements for phosphate coatings with various finishes without formation of rust the paints and lacquer have the additional requirement that no rust shall be visible beyond 0-2 in (5 mm) from the deliberate scratches and no blistering, lifting or flaking beyond 0-05 in (1-27 mm) from the original boundaries of the scratches.

The solution was applied to the surface of cold-rolled steel, dip-galvanized steel sheet, and aluminum strip for degreasing and passivating in a single operation at 40°C for 90 s. The surfaces were then lacquered (85-100 pm thickness) and tested in a salt-spray test for 480 h without showing subsurface migration and blistering. The method is especially useful in the automobile industry for coated sheet [191]. 

The salt spray testing has been defined by some industrial standards such as JIS, ISO, MIL, etc. The specimen is set in a chamber and sprayed intermittently for a certain period of time. Eor example, neutral 5 % sodium chloride at 50 °C, is sprayed onto the specimen. The size of the specimen is fixed and the spraying amount is usuaUy fixed. 

Japanese Industrial Standard (JIS) 2371 (2000) Method of salt spray testing... 

The automobile industry showed at Galvatech 92 that it is now aware of the limitations of salt spray testing, and evaluation of alternative tests formed the substance of several papers noted below. 

The salt spray test was adopted by ASTM in 1939 after considerable developmental work by the National Bureau of Standards [2]. It has widespread use in many industries, including automotive, aerospace, paints and coatings, etc. ASTM B 117 can be found as a reference in many material and performance standards as a stand-alone test or as part of other environmental conditions to which a specimen is subjected prior to acceptance. 

Although reams of salt spray data exist, their use must be approached with care. As with ferric chloride, few industries actually operate in continuous salt spray (coastal regions and ships being notable exceptions). A number of industries have found that the standard salt spray testing not only does not reproduce the damage observed in service, but also is not particularly accelerating. Thus, the auto industry has developed a more comphcated cycHc test known as GM 9540 [70]. The reasons for the discrepancies between service experience and these laboratory tests are not clear, and users of the test should beeir them in mind. 

Salt spray testing is popular because it is cheap, quick, well standardized and reasonably repeatable. There is, however, only a weak correlation between the duration in salt spray test and the expected life in service, since the corrosive processes involved are very complicated and can be influenced by many external factors. Nevertheless, salt spray test is widely used in the automotive, construction and aerospace industries. 

Three cyclic acidified salt spray tests have been widely used in the aluminum and aircraft industries. These are covered by the procedures described in Annexes A2, A3, and A4 of ASTM G 85, Practice for Modified Salt Spray (Fog) Testing. This standard does not prescribe the particular practice, test specimen, or exposure period to be used for a specific product, nor does it define the interpretation to be given to the test results. These considerations are prescribed by specifications covering the material or product being tested or by agreement between the purchaser and the seller. 

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. 

If suitable field sites are not available or lack controlled conditions, then corrosion tests must be conducted in the laboratory. Cabinets are constructed in which the atmosphere is controlled and high humidity and temperature can be used to help accelerate the tests. Marine environments are simulated by salt spray and industrial environments by sulphur dioxide or nitrogen dioxide. Figure 18 shows a salt-spray cabinet and the arrangement of test panels. Periodic changes of temperature within the cabinet can be used to simulate night and day. Addition of other aggressive salts or acid into the sprayed solution is further used to accelerate the test. 

Specific tests frequently used are (a) neutral 5 % Sodium Chloride salt spray (ASTM B 117, Test Method of Salt Spray (Fog) Testing), (b) 3.5 % Sodium Chloride by alternate immersion (ASTM G 44, Practice for Evaluating Stress Corrosion Cracking Resistance of Metals in 3.5 % Sodium Chloride Solution), and (c) exposure to various outdoor atmospheres. Guidelines for outdoor exposure are contained in ASTM G 50, Practice for Conducting Atmospheric Corrosion Tests on Metals. Generic types of atmospheres used are seacoast, industrial, urban, and rural. Sometimes specific geographical locations or local chemical conditions are important because they can produce unique results [2i],... 

This approach is useful if the environment is well characterized and a few weeks or months have been allocated for the test examples include exposure to salt spray to simulate sea coastal areas, exposure to 3.5 wt% NaCl solution for submarine environment, Kesternich chamber test (humid SO2 and CO2 mixture) for industrial environment [6], and mixed flowing gas tests for a range of environments. 

In the absence of a better testing method, industrial and military specifications often require the use of a 300-l--h salt spray exposure method to test anodized aluminum coupons and evaluate the quality of the anodizing processes. Not only is the salt spray (fog) exposure test time-consuming, but its results are highly interpretive and therefore relatively imprecise. The long duration of the test itself makes it... 

Finally, it is important to understand the limitations of each testing method [90], and that no one method will provide all of the necessary information. Thus, it is important to confirm rapid screening techniques with more traditional and industry-accepted methodologies such as weight loss tests or salt fog spray tests. For example, zinc phosphate has been shown to perform poorly in accelerated tests but generally well in exposure testing [47]. 

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