Galvanic corrosion atmospheric tests

The test assembly used originally by Subcommittee Vlll of ASTM Committee B-3 in its comprehensive studies of atmospheric galvanic corrosion had the disadvantage that it depended on paint coatings to confine corrosion to the surfaces in actual contact with each other. In interpreting the results, it was frequently difficult to decide how much corrosion was due to galvanic action and how much to a variable amount of normal corrosion through failure of the paint system. 

Test method for assessing galvanic corrosion caused by the atmosphere... 

Galvanic corrosion testing under atmospheric exposures has been standardized and bears the titles ISO 7441-84, ASTMG 104-89 for plate tests and ASTM G116-93 for wire-on-bolt tests.21,34,35 A form of test assembly for galvanic corrosion testing is shown in Figure 2.5. 

ASTM G116, Standard Practice for Conducting Wire-on-Bolt Test for Atmospheric Galvanic Corrosion, West Conshohoken, PA, 1993. 

ASTM G104-89, Standard Test Method for Assessing Galvanic Corrosion caused by the Atmosphere, Philadelphia, PA, 1989. 

Nevertheless, there were reliable reports of corrosion occurring with molybdenum disulphide in films and in greases. Several such reports arose from the US Army , originating with a salt fog test of a missile launcher in which all parts coated with solid film lubricant rusted badly. Subsequent reports described galvanic corrosion of various metals with molybdenum disulphide in moist atmospheres. 

Hot dip galvanizing has also been used on calorized steel. Mokhamed and Kukharev (1987) said that the best corrosion performance in a simulated atmospheric test was on samples calorized at 850°C for 4 hours in the mixture of ferroaluminum containing 40% Al followed by galvanizing at 450 C for 4 hours( ). 

It is occasionally desirable to evaluate galvanic corrosion effects from atmospheric exposures (see ASTM G 116, Conducting Wire-on-Bolt Test for Atmospheric Galvanic Corrosion and ASTM G 149, Conducting the Washer Test for Atmospheric Galvanic Corrosion). Several different types of sj)ecimens have been used for this purpose. One approach is... 

Unlike galvanic corrosion testing in electrolytes, galvanic corrosion testing in the atmosphere can be standardized because special circumstances exist that limit the number of test variables. 

Three factors that are different in atmospheric testing from most other galvanic corrosion testing are the presence of a limited amount of electrolyte, the need for a very long duration of exposure, and the importaince of specimen... 

Finally, the time that a specimen is wet depends on sunlight heating, prevailing winds, and specimen geometry. Specimen orientation is, therefore, much more critical in atmospheric galvanic corrosion testing. Standard orientations are specified in ASTM G 50 as 30° to the horizontal facing south or the nearest source of corrodent (the sea). 

Three standardized atmospheric galvanic corrosion tests are the plate test, the wire-on-bolt test, and the disk test. The plate test, described in ISO Standard IS07441, Determination... 

The wire-on-bolt test, described in ASTM G 116 (Practice for Conducting the Wire-on-Bolt Test for Atmospheric Galvanic Corrosion), has been used with standard materials as an atmospheric corrosivity test under the names of the CLIMAT and ATCORR tests. This test consists of wrapping a 1.0-m length of wire of the anodic material around a threaded bolt or rod of the cathodic material. Post-test evaluation is typically done by mass loss only. Since the wire diameter is much smaller than the 0.5-cm galvanic interaction distance in the atmosphere, the effective tmode-to-cathode area ratio is well below 1 1, making this a fast test. A typical exposure duration is 90 days. The short duration is... 

FIG. 16—Cross section of disk test assembly for atmospheric galvanic corrosion. 

Only mass loss information is obtained from the standard atmospheric galvanic corrosion tests, with the exception of the plate test. The plate test specimens can also be evaluated by metallography, visual examination, probe-type attack depth gages, or mechanical tests such as tensile strength. 

Standard Practice for Conducting the Washer Test for Atmospheric Galvanic Corrosion Standard Practice for Selecting and Characterizing Weathering Reference Materials Used to Monitor Consistency of Conditions in an Exposure Test... 

The test specimens must be physically separated from each other and supported on inert racks. When supported on metal racks the test specimens must be insulated from the metal rack and from each other to prevent galvanic action. Atmospheric corrosion coupons are usually held with ceramic electrical insulators as shown in Figure 15.2. The coupons are usually mounted either at an angle of 20-30 from the horizontal or vertically (see Figure 15.3), Atmospheric corrosion tests usually last at least 1 year and very often extend to 3-5 years. 

Atmospheric Corrosion Resistance Based on 13-year atmospheric test results (see Figure 1), it is estimated that GALVALUME sheet will outlast galvanized by two to four times in marine, industrial and rural atmospheres. When compared to aluminum coated sheet steel, GALVALUME sheet has superior corrosion resistance at sheared edges. 

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