the upper phase of the hot-dip galvanizing coating
is usually from 1/3 to ½ the coating thickness. It
depends on coating application conditions (zinc melt
composition, temperature, etc.), (Fig.1).
Diffusion coatings are obtained by chemical-thermal
treatment of products at a temperature of 380-
480 °C in zinc-based powder mixtures or optionally
electroplated or hot-dip galvanized coatings are
converted into diffusion coatings by appropriate heat
treatment. It should be noted that electroplated and
metallized zinc coatings do not contain intermetallic
compounds (phases) and consist of zinc of a certain
chemical composition.
Layers
(phases)
of zinc
coatings Hot-dip
zinced water
and gas
pipes Hot-dip
zinced +
thermally
treated
water and
gas pipes
η -phase 36-38
(353-372) 80-84
(784-824)
ζ - phase 260-265
(2550-2600) 285-292
(2795-2863)
δ - phase 440-460
(4315-4510) 450-460
(4413-4510)
Γ - phase 500-540
(4903-5296) 510-535
(5000-5246)
Table 1. Microhardness of layers (phases) of zinc
coatings on pipes and tubes of various purposes
Fig 2a
Two phases are clearly observed in the classical
diffusion zinc coating obtained by chemical-thermal
treatment in powder mixtures based on zinc at 430-
480 0C during 3-5 hrs.: the Г-phase which is a thin
(2-4 μm) dark layer located directly on the surface of
the coated article and a δ1-phase located above this
layer (Fig. 2). The structure of the δ1-phase on the
etched microsection looks like elongated (columnar)
crystals (Fig. 2, a). The Г-phase is an intermetallic
compound containing up to 28% iron. It borders with
the solid solution of zinc in iron (α-phase, Fig. 2, b)
at one side and the δ1-phase at the other side. The
latter phase is also an intermetallic compound with
iron content from 7 to 11.5%.
Fig 2b
In the case of diffusion galvanizing using zinc-
containing powder mixtures, the coating formed
on the product surface consists of not pure zinc
but of an iron-zinc alloy representing a series of
intermetallic iron-zinc compounds.
Diffusion zinc coatings obtained in powder
mixtures have an improved resistance to corrosion-
erosive action of a rapidly moving aggressive
aqueous medium due to the iron-zinc alloys of a
corresponding structure formed in the coating
having increased microhardness (Table 1).
Hot-dip zinced and diffusion coatings applied
from zinc-based powder mixtures have a similar
mechanism of formation, namely diffusion.
Therefore, in terms of classification, these zinc
coatings can be attributed to one type (class) of
coatings which is confirmed by their structure. In
accordance with the diagram of the state of the
iron-zinc system, a number of similar phases
(intermetallic compounds) are present in the
structure of these coatings. However, the general
structure of these coatings is still different and their
properties largely differ as well.
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TUBE NEWS June 2018
21