Galvanic corrosion atmospheric

Bimetallic corrosion in atmospheres is confined to the area of the less noble metal in the vicinity of the bimetallic joint, owing to the high electrolytic resistance of the condensed electrolyte film. Electrolytic resistance considerations limit the effective anodic and cathodic areas to approximately equal size and therefore prevent alleviation of atmospheric galvanic corrosion through strict application of the catchment area principle. 

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. 

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

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... 

Baboian, R., Final Report on the ASTM Study Atmospheric Galvanic Corrosion of Magnesium Coupled to Other Metals, ... 

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. 

Baboian, R., Final Report on the ASTM Study Atmospheric Galvanic Corrosion of Magnesium Coupled to Other Metals, Atmospheric Factors Affecting the Corrosion of Engineering Metals, ASTM STP 646, S. Cobum, Ed., 1978, ASTM International, West Conshohocken, PA, pp. 17-29. 

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... 

Teeple H.O. (1956), Atmospheric Galvanic Corrosion of Mg Coupled to Other Metals, ASTM, STP 175, pp 89-115. 

Atmospheric galvanic corrosion will always be limited to the contact area. Under appropriate conditions, it may lead to severe damage roofing that is perforated around bolts or screws, electrical components that are corroded at contacts with components made... 

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