Corrosion performance of Post Zinc and Perfect Post Zinc and their comparison
with other zinc plating methods.
Zinc Plating and the Daiwa Z Process
In general, the zinc plating is made up of the following two layers; the
base layer formed by combining steel and zinc (metal alloy layer) and the
layer formed by pure zinc. Since iron is contained in the base metal alloy
layer, this layer becomes corroded when the pure zinc layer becomes eroded.
Therefore, it is said that the corrosive resistance of the coating depends
on the thickness of the pure zinc layer.
Our Post Zinc (PZ) and Perfect Post Zinc (PPZ) steel tubes are manufactured
by our "Daiwa Z Process" which Galvanize the tubes at the same time
as they are made.
The feature of this manufacturing process is that it reduces the thickness
of the alloyed layer by shortening the submersion time in galvanizing bath, thus keeping the thickness of the alloyed layer
to a minimum.
*The electrolytic zinc grade specified by JIS H 2107 (Zinc purity: 99.995% or
above) is used for the coating on the PZ and PPZ.
Generally, the ground zinc used in Hot dip Galvanized steel tube is Level
1 prime western grade zinc (Zinc purity: 98.5% or above).
Difference between the coating on the Post Zinc/Perfect Post Zinc and dip
coating.
The difference between our coating process and the dip coating process
is that dip coating process has a thinner pure zinc layer and a thicker metal alloy
layer than ours. Conversely, the coating using our process has a thicker
pure zinc layer and a thinner metal alloy layer.
Consequently, the post zinc and the perfect post zinc tubes can retain
their anti-corrosion qualities by using only a small amount of zinc.
Moreover, the post zinc and the perfect post zinc tubes hardly contain anti-environmental
elements such as lead and cadmium, and therefore are kind to the environment.
The difference between post zinc, perfect post zinc, and pre-coating.
The coating on the welded seams of pre-coated tube are burnt during the
welding process at tube making.
To repair the burnt area, coating is redone on this area by metal thermal spraying. However,
the spraying over the burnt area is not as uniform as when coating is done
directly on the base metal. As a result, the corrosion on the sprayed areas
proceeds faster than the other areas. Corrosion also results in the blistering
of color painted coatings over these areas.
No such problems exist on the Post Zinc and Perfect Post Zinc tubes where
the coating is all uniform.
Anti-corrosion properties of zinc plating
Zinc plating protects the steel base from corroding with its "protective
filming" and "self-corroding" qualities.
Protective filming quality
The thin oxide film created on the surface of the zinc plating does not
let air nor water pass to the layer below, and therefore protects the pure
zinc and alloyed layers, as well as the steel base.
Self-corroding quality
If the pure zinc layer happens to be scratched for some reason and the
steel base becomes exposed to the open air, the pure zinc starts to erode
faster than the steel, thus electro-chemically protecting the steel base.
This is not the case for other steel tubes that contain less quantities
of zinc and more quantities of aluminium. Their self-corroding qualities
are less and therefore the corrosion of the exposed steel base occurs faster.
