Fig.4. Ashop section with packages of diffusion zinced lifting pipes
The interest in electrodeposition of alloys
characterized by completely new physicochemical
properties absent in their components has
significantly increased. This makes it possible to
significantly reduce coating thickness and, in some
cases, reduce severity of the problem of non-ferrous
metal deficiency. Recommendations are known on
the use of zinc coatings doped with various metals
(Cd, Ni, Fe, Cr, Co, Mn) which can prolong service
life of the protected product without increasing
the coating thickness. The most promising types of
zinc-based coatings are electroplated coatings with
Zn-Ni, Zn-Fe, Zn-Mn alloys that, at certain ratios
of components, have high corrosion resistance,
solderability, weldability, phosphatizing ability and
polymer coating. To improve tightness, corrosion
resistance and durability of threaded joints of oil-
country tubular goods (drilling and casing tubes,
lifting pipes), various sealing and scuff-resistant
coatings are applied to the couplings threads. Mostly,
electroplated zinc coatings are used for these
purposes. Their thickness is 25-150 μm depending
on the product range and purpose of pipes and tubes.
Fig. 5a. Appearance of diffusion zinced rectangular
and round tubes for building structures
Smooth and uniform coatings having thickness
of 40±10 μm are obtained on couplings with a
triangular thread profile and 25±5 μm on couplings
with a trapezoidal thread profile. They provide high
tightness, corrosion resistance and mechanical
strength to threaded joints. Zinc-nickel coatings
ensure a 10-fold increase in the number of screwing-
unscrewing cycles without worsening thread
parameters and tightness of threaded joints.
Metallized coatings are applied by spraying
molten zinc with a hot stream of air or gas.
There are two types of metallizing: gas-flame
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TUBE NEWS June 2018
19