properties to the steel products operated in corrosive
media. In such coatings, electrochemical protective
effect of the zinc coating is combined with the
waterproofing protective effect of paints and
varnishes. Application of combined coatings can be
considered as a kind of electrochemical method of
corrosion protection when zinc coating performs
not only the role of an effective ground layer but also
provides electrochemical protection of the steel base
in the places of potential damage (scratches, cracks,
spalling) of the top coating layer. Combination of the
zinc coating that forms alloy with the substrate steel
and the organic coating has a synergistic effect. The
degree of protection provided by the duplex system
is higher than the sum of the protection degrees of
zinc and paint coatings taken separately.
Conclusion
As set forth above, zinc coatings applied by various
methods differ substantially in uniformity, structure
(microstructure), density, chemical composition
which ultimately determines their protective
properties, primarily corrosion resistance in various
media. Therefore, based on comparison of the
aggregate of properties and the results of corrosion
tests, rational fields of application of various zinc
coatings are resumed below:
1) Electroplated (electrolytic) coatings. These
coatings are widely used in machine building and
production of home appliances to protect the
products against corrosion and improve decorative
appearance of various parts and products in low-
aggressive environments. The electrolytic method
has found wide application in zincing mainly the
external surface of 6 to 15 mm diameter pipes
and tubes used in motor industry. Such coatings
are recommended for corrosion protection of steel
sheets and small diameter pipes. Thickness of the
electroplated alloy layer is 2-3 μm. For example,
pipes with a multilayer coating (a 1 μm thick layer
of copper, a 1-2 μm thick layer of Zn-Ni alloy (8-
14% Ni) and a 14-20 μm thick zinc layer) have high
corrosion resistance in brake and fuel systems of cars
and tractors providing a 3 to 4-fold longer service life
and reliability of these systems.
2) Metallized
coatings are used
in construction, in
repair of various
large-sized facilities
(containers, metal
structures, external
surfaces of pipelines,
welded seams of
pipelines, etc.).
3) Hot-dip zinc
coatings are most
widely used in
metallurgy (in production of galvanized strips and
wires), in pipe and tube industry, in construction of
industrial and civil facilities as well as in protection
of various metal products using continuous or batch
zinc coating process.
4) Diffusion coatings are used for corrosion
protection of various metal products operating in
severe corrosion-erosion conditions. Diffusion zinced
products are successfully used in civil and industrial
engineering, in coke and oil industry, shipbuilding,
primarily to protect various pipeline systems and
technological devices as well as fasteners, e.g. bolts,
nuts, etc., especially high-strength products. The
diffusion zincing method is becoming more and more
widespread in zincing long steel products (pipes,
components of road railings, power transmission
lines, valves, etc.) (Figs. 4, 5) which is due, first of all,
to high chemical and physicomechanical properties
of diffusion zinc coatings.
5) Zinc-filled coatings are applied on the steel
strip used at automated production lines of the
automotive industry (various car body parts), to
protect steel products and structures such as tanks,
pipelines, piers, ship hulls, bridge structures, in
repair works, etc.
6) Combined coatings are widely used in construction,
shipbuilding, mechanical engineering and other
industries where a combination of anti-corrosion
properties with a decorative appearance of the product
is required. In industrialized countries, up to 90% of
sheet material is produced double protected.
.
TUBE NEWS June 2018
25