Galvanizing, Sherardizing and
Other Protective Zinc Coatings:
Main Present-day Systems of Pipe
and Tube Corrosion Protection
Among the multitude of types of coatings used to protect steel and
cast iron pipes and tubes and other metal products from corrosion,
zinc coatings are the most effective and economically feasible. In
zinc coatings, barrier effect of corrosion protection due to shielding
of the product surface and electrochemical (protective) effect are
most effectively combined.
The variety of galvanizing processes, their
relative simplicity, the possibility of
mechanization and automation have led to the
fact that in terms of volume and nomenclature
of products protected from corrosion, zinc
coatings are peerless to other metal coatings.
In the present-day technical literature, various
galvanizing processes, properties of zinc
coatings, and the field of their application are
described in great detail but there is practically
no description of the mutual comparison
of various galvanizing methods and the
characteristics of zinc coatings obtained by
these methods.
It should be noted that different types of zinc
coatings are often perceived in practice as a single
coating with the basic properties characteristic of
pure zinc which does not contribute to the correct
choice of the zinc coating type for specific products
taking into account coating service conditions and
inhibits development of new galvanizing processes
aimed at obtaining zinc coatings with improved
properties.
When evaluating corrosion resistance of zinc
coatings, the type of coating determined by the
method of its formation (application) is important.
18 TUBE NEWS June 2018
The results of large-scale studies and industrial tests
of various types of zinc coatings conducted in recent
years have shown that it is the method of obtaining
zinc coatings that is the main factor determining
their structure and properties, first of all, corrosion
resistance.
Analysis of numerous published and practical data
has shown that six types of zinc coatings can be
distinguished. They differ in the mechanism of
formation and physico-chemical characteristics:
electroplated, metallized, hot-dipped, diffusion, zinc-
filled and combined.
Electroplated coatings are applied to the product
surface in electrolyte solutions under the influence
of electric current. Adhesion and structure of
the electroplated coating depend significantly on
the nature of its growth on the substrate surface,
especially in the initial period of electrodeposition.
Usually, conditions are created for obtaining denser
and less porous deposits. Such conditions are
achieved by heating, stirring the electrolyte, reducing
the current density, applying non-stationary
electrolysis modes. Evergrowing needs of the
industry in protected metal products put forward the
task of developing processes for applying new types
of coatings that are much more corrosion resistant
than pure zinc.