coating that is sprayed on, the hot-dip galvanizing process dips fabricated steel components into a bath of molten zinc at 850 °F (454 °C), where the surface of the steel reacts with the liquid zinc to form a metallurgically bonded alloyed coating. During the galvanizing process, a series of zinc/iron alloy layers are formed on the base steel. The initial layer, which is 75% zinc and 25% iron, bonds with the surface of the base steel and is referred to as the Gamma layer; the second layer, the Delta layer, is 90% zinc and 10% iron; the third layer, the Zeta layer, is 94% zinc and 6% iron; and the top layer, the Eta layer, is 100% zinc. Because the steel is totally immersed in the zinc, all surfaces, including edges, are uniformly coated. As soon as the hot-dip galvanized component is exposed to the atmo- sphere, the pure zinc top layer starts to NWZU I XI[[Q^M []ZNIKM ÅTU SVW_V I[ I zinc patina, which is a protective barrier that prevents moisture and chlorides from corroding the underlying steel. When the patina starts to develop, an QVQ\QIT TIaMZ WN bQVK W`QLM BV7 Y]QKSTa forms as the zinc reacts with oxygen in the air. When this ZnO layer is exposed to moisture, it converts into a thin layer of zinc hydroxide [Zn(OH)2 the Zn(OH)2 dioxide (CO2
]. Over time,
layer reacts with carbon ) in the air and converts
into a dense, insoluble layer of zinc car- bonate (ZnCO3
) that slows corrosion of
the underlying zinc. Since zinc is anodic to steel, hot-dip
OIT^IVQbQVO IT[W IK\[ I[ I [IKZQÅKQIT IVWLM if the galvanized coating is damaged physically. If individual areas of underly- ing steel become exposed, the surround- ing zinc will provide cathodic protection to unprotected sites as large as 0.25 in (6 mm) by corroding preferentially and [IKZQÅKQVO Q\[MTN \W XZW\MK\ \PM JIZM [\MMT This protection continues as long as zinc remains in the immediate vicinity of the exposed steel. Contact Kevin Irving, AZZ Galvanizing— e-mail: KevinIrving@AZZGALV.com
NACE International, Vol. 51, No. 11 November 2012 MATERIALS PERFORMANCE 21
References
1 ASTM A896/A896M-09, "Standard Practice for Conducting Case Studies on Galvanized Structures" (West Con- [PW PWKSMV 8)" );<5 1V\MZVI\QWVIT
2 ASTM A123/A123M-12, "Standard ;XMKQÅKI\QWV NWZ BQVK 0W\ ,QX /IT^I- nized) Coatings on Iron and Steel 8ZWL]K\[º ?M[\ +WV[PWPWKSMV 8)" ASTM International).
—K.R. Larsen