© 2006 by Taylor & Francis Group, LLC

118 Corrosion Control Through Organic Coatings
thicknesses were tested at R dry = 0, 50, and 93.8%. For all four substrates, the highest
amount of steel weight loss was seen at R dry = 50%.
In summary, corrosion on both steel and zinc-coated steel substrates is slower
if no drying occurs. This finding seems reasonable because, as the electrolyte layer
becomes thinner while drying, the amount of oxygen transported to the metal surface
increases, enabling more active corrosion [11, 12]. A similar highly active phase
can be expected to occur during rewetting under cyclic conditions.
Readers interested in a deeper understanding of this process may find the works
of Suga [13] and Boocock [14] particularly helpful.
7.2.3.2 Zinc Corrosion — Atmospheric Exposure vs. Wet
Conditions
A drying cycle is an absolute must if zinc is involved either as pigment or as a
coating on the substrate. The corrosion mechanism that zinc undergoes in constant
humidity is quite different from that observed when there is a drying period. In field
service, alternating wet and dry periods is the rule. Under these conditions, zinc can
offer extremely good real-life corrosion protection — but this would never be seen
in the laboratory if only constant wetness is used in the accelerated testing. This
apparent contradiction is worth exploring in some depth.
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 accelerated
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 cause the growth of red rust and
undercutting from the scribe [1, 6, 18-21]:
1. The first reaction is a galvanic cell located at the scribe. The anode is the
metal exposed in the scribe, and the cathode is the adjacent zinc layer
under the paint.
2. The second reaction is located not at the scribe but rather at the leading
edge of the zinc corrosion front. Anodic dissolution of zinc occurs from
the top of the zinc layer and works downward to the steel.
Ito and colleagues have postulated that the magnitudes and the comparative ratio
of these two mechanisms changes with the amount of water available. When they
repeated their experiments with R dry on painted, cold-rolled and galvanized steels,
© 2006 by Taylor & Francis Group, LLC