Neutral salt spray

Sa.lt Spray Tests. One of the older accelerated corrosion tests is the salt spray test (40). Several modifications of this imperfect test have been proposed, some of which are even specified for particular appHcations. The neutral salt spray test persists, however, especially for coatings that are anodic to the substrate and for coatings that are dissolved or attacked by neutral salt fog. For cathodic coatings, such as nickel on steel, the test becomes a porosity test, because nickel is not attacked by neutral salt fog. Production specifications that call for 1000 hours salt spray resistance are not practical for quahty acceptance tests. In these cases, the neutral salt spray does not qualify as an accelerated test, and faster results from different test methods should be sought. 

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). 

Evaluation of cabinet corrosivity. In order to determine the corrosivity of the corrosion cabinet environment eight tests were performed [5] according to the standard method of the neutral salt spray test (Table 1). The results of corrosion rate of RS and the main statistical parameters such as the number of reference samples n, average RS mass m and RS mass loss Am of each RS, average RS surface area S and surface area of each RS Sn, mean averages of all eight experiments and their standard deviations are presented in Table 2a and 2b. 

Ordinary statistical analysis of the corrosivity tests showed that the mean of corrosion rate v=lll g/m2of eight experiments means and its standard deviation j-4.1 g/m2 estimate of neutral salt spray test cabinet corrosivity as 111 3 g/m2 (confidence 95%) shows that corrosivity of our corrosion cabinet meets the requirements of ISO 9227, which states that the average value and its data scattering should be 140 40 g/m2. 

Uncertainty of mass loss measurement. The standard method of the neutral salt spray test does not indicate the mass of RS. Mass loss was found as a difference between the RS prepared for the corrosion test and the RS after the corrosion test and corrosion product stripping as well as protective coating removal from the RS (Table 1). Such a mass loss determination is based on three components (1) mass loss determination by weighing (accuracy 0.5 mg and standard deviation 0.3 mg) before the neutral salt spray test and after it, (2) determination of difference and (3) cor-... 

Fig. 1 Main uncertainty sources of corrosivity measurement as rate of mass loss in a neutral salt spray cabinet expressed as v=Am/S...

It was assumed that the uncertainty of corrosion test duration comprised two components (1) accuracy which was indirectly indicated in standard method description and (2) our experimental possibility to keep the total duration of corrosion process in the neutral salt spray environment within limits of 0.2 h. 

Additional specifications from other corrosion test standards [2, 3] are required for the standard method of the accelerated corrosion test in the neutral salt spray test cabinet at 35 2 °C within 96 h. For the evaluation of corrosion data quality, the test should be performed according to the requirements of contemporary standards such as ISO/IEC 17025 [9] and, therefore, the corrosion test data uncertainty must be determined. 

ISO (1996) Paints and varnishes - determination of resistance to neutral salt spray (fog). ISO 7253, ISO, Geneva, Switzerland... 

International Standards Organization (1976). ISO 3768, Metallic coatings—Neutral salt-spray tests (NSS test), 3 pp. 

Paints Varnishes—Resistance to Neutral Salt Spray... 

Corrosion in Artificial Atmospheres Cosmetic Corrosion Test Metiiod for Automotive Parts Methods of Neutral Salt Spray Testing Salt Spray Test... 

Applicable test methods include [28] Continuous Salt Spray Tests (ASTM B 117), Neutral Salt Spray Test (ASTM B 117), Acetic Acid Salt Spray Test, Copper-Accelerated Salt Spray Test (CASS) (ASTM B 368), Cyclic Salt Spray Tests, the Copper Development Association (CDA) Test, the Hitachi Salt Spray Test, Climate Tests, The Humidity Test, The International Electrotechnical Commission/Intemational Organization for Standardization (lEC/ISO) Test and Mud Test. 

Table 4 summarizes electrochemical test data for 316L sintered in hydrogen and under vacuum [24]. In these examples, inferior corrosion resistance, documented also by neutral salt spray data, can be attributed to greater degrees of reoxidation of the surface of a specimen during cooling... 

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. 

This test can be as brief as 16 h, although it normally ranges from 144 to 240 h or more. As in the neutral salt spray test, a 5% NaCl solution is used, but the solution is adjusted to a pH range of 3.1 to 3.3 by the addition of acetic acid, and again, the temperature of the salt spray cabinet is controlled to maintain 35 -l- 1.1 or —1.7 °C (95 -H 2 or — 3 °F) within the exposure zone of the closed cabinet. 

Table 5.1 Corrosion resistance of aluminium panels anodized in inhibited sulphuric acid baths evaluated by neutral salt spray test. (Smith eta/., 1996)...

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