Atmospheric corrosion accelerated tests

The relative susceptibHity of several commercial aHoys is presented in Table 8. The index used is a relative rating based on integrating performance in various environments. These environments include the harsh condition of exposure to moist ammonia, Hght-to-moderate industrial atmospheres, marine atmosphere, and an accelerated test in Mattsson s solution. The latter testing is described in ASTM G30 and G37 (35,36) and is intended to simulate industrial atmospheres. The index is linear. A rating of 1000 relates to the most susceptible and zero designates immunity to stress corrosion. 

Industrial atmospheres usually accelerate the corrosion of zinc. When heavy mists and dews occur in these areas, they are contaminated with considerable amounts of acid substances such as sulphur dioxide, and the film of moisture covering the metal can be quite acid and can have a pH as low as 3. Under these conditions the zinc is dissolved but, as the corrosion proceeds, the pH rises, and when it has reached a sufficiently high level basic salts are once more formed and provide further protection for the metal. These are usually the basic carbonate but may sometimes be a basic sulphate. As soon as the pH of the moisture film falls again, owing to the solution of acid gases, the protective film dissolves and renewed attack on the metal occurs. Hudson and Stanners conducted tests at various locations in order to determine the effect of atmospheric pollution on the rate of corrosion of steel and zinc. Their figures for zinc are given in Table 4.34 and clearly show the effect which industrial contamination has on the corrosion rate. 

Almost all tests carried out to study the starting process of atmospheric corrosion have been performed in a surface without corrosion products however, in real conditions, the metal is covered with corrosion products after a given time and these products begin to play its role as retarders of the corrosion process in almost all cases. Corrosion products acts as a barrier for oxygen and contaminants diffusion, the free area for the occurrence of the corrosion is lower however, the formation of the surface electrolyte is enhanced. Only in very polluted areas the corrosion products accelerate the corrosion process. Water adsorption isoterms were determined to corrosion products formed in Cuban natural atmospheres[21]. Sorption properties of corrosion products (taking into account their salt content-usually hygroscopics) determine the possibilities of surface adsorption and the possibility of development of corrosion process... 

The most desirable data are those obtained for the material of interest in the intended conditions of exposure. Such data are not readily available in the literature. Published data on atmospheric corrosion should be used with caution since atmospheric conditions are changing with time, as for example acid rain as a variable factor. Accelerated testing, including electrochemical tests, should have a good link with the natural and practical conditions. Published data should be consulted because they are generally useful. Some published data are mentioned here as examples since they are useful in selecting materials or discussion of case histories ... 

Yu.N. Mikhailovsky, V.M. Popova and A.I. Marshakov. Field and accelerated tests of contact and volatile inhibitors of atmospheric corrosion on different metals. Protection of Metals, 2000, Vol. 36, No. 5, pp. 546-551. 

A laboratory test must be designed and performed so that the most important parameters from an atmospheric corrosion perspective are controlled. Reproducibility and the ability to mimic the atmospheric corrosion in different ambient environments are other important criteria of a laboratory test. Parameters to consider in accelerated tests are sample preparations prior to exposure, relative humidity, temperature, exposure time, corrodents, and corrodent delivery rate [1]. 

B.S. Skerry, C.H. Simpson, Accelerated test method for assessing corrosion and weathering of paints for atmospheric corrosion control. Corrosion 49 (1993) 663—674. 

Horton et al. [55] observed that when steels containing Cu and Ni are exposed in industrial and marine atmospheres, the Cu and Ni appear in the mst layers both in the loose outer and adherent inner mst on skyward and ground ward surfaces. Also it was shown by chemical analysis that Ni, Cu, Cr and Mn from weathering steel appear in the mst layer and provides protection. Presence of chlorides in the atmosphere accelerates corrosion of steels leading to the formation of basic Fe ", Fe chlorides and jS FeOOH. Townsend et al. [56] conducted 8-year atmospheric corrosion tests on weathering steel in mral, industrial and marine environments with different heated conditions and indicated that heat treatments have no effect on the corrosion resistance/performance of weathering steels. 

The performance of protective coatings can be determined through accelerated laboratory tests but these are not the true representation of actual service conditions. Hence field tests are carried out to get reliable performance data on uncoated and coated steels. The protective systems and their corresponding corrosion data are available in the relevant overseas codes for reference. These data are not applicable in Indian specific context as atmospheric corrosion is location specific. Atmospheric corrosion is the frontier research area where limited work has been carried out as it comprises three phases (sohd/atmospheiic/liquid environment). Therefore it is significant to study the rust on MS and WS as well as coated steels in a given atmospheric environments. This is important for the... 

For all these reasons, no accelerated corrosion test can imitate atmospheric corrosion, which in itself is a complicated, unstandardized process. 

One-year test results on zinc and cadmium are in Table 2.22. The results of long-term atmospheric corrosion tests on zinc and cadmium coatings (Fig. 2.35) are compared with those of accelerated corrosion tests (Fig. 2.36) on the same coatings in three cabinets warm and humid, SO2 gas, and sea mist (Strekalov and Berukshtis, 1965). 

Strekalov and Berukshtis concluded that sea fog and warmth and humidity cabinets satisfactorily reproduce the nature of corrosive destruction of anodic coatings on steel, as observed under natural conditions in nonindustrial zones. Accelerated tests in SO2 gas cabinets were too harsh, however, and did not reliably reflect the behavior of zinc and cadmium coatings in a natural industrial atmosphere. 

Strekalov, P., and Benikshtis, G. (I%5). Atmospheric corrosion of zinc and cadmium coatings on steel and the changeover from accelerated to actual testing conditions. Korroz. Met. Splavov. Sb. (2), 264-278 (in Russian). 

The less dense exterior part of the rust layer contains mostly the phases y-FeOOH and a-FeOOH. The latter has the higher thermodynamic stability of the two but forms more slowly. For this reason, accelerated atmospheric corrosion tests yield lepidocrocite (y-FeOOH) primarily. 

The described mechanisms of atmospheric corrosion demonstrate the importance of pollutants and of humidity cycles. Unfortunately, the complexity of the underlying phenomena, makes it difficult to predict atmospheric corrosion behavior Irom accelerated testing. Outdoor exposure tests, which generally require several years, therefore remain popular. Different norms and recommended practices fix in detail the conditions to be applied in atmospheric corrosion tests. The following accelerated test methods are most commonly used in the laboratory ... 

In a salt spray test the sample is brought in contact with a saline fog in a closed chamber for a certain number of hours. The fog is formed by atomizing a NaCl solution by means of a nozzle. The high corrosivity of the artificial fog simulates that of a marine atmosphere, but the method does not offer a realistic model of more general atmospheric corrosion conditions, because it involves neither the presence of SO2 nor humidity cycling. In order to accelerate the corrosion, acetic acid or copper chloride is sometimes added to the saline solution. 

Laboratory corrosion tests for exfoliation corrosion susceptibility are a necessary tool for research and quality control engineers however, the validity of such accelerated tests depends on their relationship to realistic service conditions and their sensitivity to various degrees of susceptibility. The tests must be discriminating and yet not so severe as to be unrealistic. For the majority of engineered structures, exposure to outdoor atmospheres provides a baseline that is representative of many service conditions, except for structures that are subjected to unusual chemical environments. Experience has shown that seacoast conditions are more corrosive to aluminum alloys than inland urban and industrial conditions (see Fig. 2), and seacoast atmospheric exposure tests have been particularly useful for the validation of accelerated exfoliation tests [9]. 

Accelerated tests do not precisely predict long-term corrosion behavior however, answers are needed quickly in the development of new materials. For this resison, accelerated tests are used to screen candidate alloys before conducting atmospheric exposures or other field tests. They also are used for production control of exfoliation-resistant heat treatments for the AA2XXX, AA5XXX, and AA7XXX aluminum alloys. ASTM has standardized several laboratory tests for susceptibility to exfoliation corrosion in recent years. 

Lifka, B. W. and Sprowls, D. O., Relationship of Accelerated Test Methods for Exfoliation Resistance in 7XXX Series Aluminum Alloys with Exposure to a Seacoast Atmosphere, Corrosion in Natural Environments, ASTM STP 558, ASTM International, West Conshohocken, PA, 1974, pp. 306-333. 

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. 

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