Faster Corrosion during the Wet-Dry Transition

The drying condition was defined as 35°C and 60% RH the wet condition was defined as 35°C and constant 5% NaCl spray (i.e., salt spray conditions). is the total time, wet plus dry, of one cycle, and is the amount of time at 60% RH, 35°C during one cycle. Cold-roUed steel and galvanized steels with three zinc-coating 

Readers interested in a deeper understanding of this process may find the works of Suga [13] and Boocock [14] particularly helpful. 

Although this is a book about paints, not metallic corrosion, it becomes necessary at this point to devote some attention to the corrosion mechanisms of zinc in dry versus wet conditions. The reason for this is simple zinc-coated steel is an important material for corrosion prevention, and it is frequently painted. Accelerated tests are therefore used on painted, zinc-coated steel. In order to obtain any useful information from acceloated testing, it is necessary to understand the chemistry of zinc in dry and wet conditions. 

In normal atmospheric conditions, zinc reacts with oxygen to form a thin oxide layer. This oxide layer in turn reacts with water in the air to form zinc hydroxide (Zn[OH]2), which in turn reacts with carbon dioxide in air to form a layer of basic zinc carbonate [15-17]. Zinc carbonate serves as a passive layer, effectively protecting the zinc underneath from further reaction with water and reducing the amount of corrosion. 

When zinc-coated steel is painted and then scribed to the steel, the galvanic properties of the zinc-steel system determine whether, and how much, corrosion will take place under the coating. Two mechanisms canse the growth of red rust and undercutting from the scribe [1, 6, 18-21]  

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