Coating type
Corrosion test duration, hr
2200 (3 months) 4400 (6 months) 8760 (1 year)
Average
corrosion
rate,
g/m2 hr Corrosion
depth
index,
μm/yr Average
corrosion
rate,
g/m2 hr Corrosion
depth
index,
μm/yr Average
corrosion
rate,
g/m2 hr Corrosion
depth
index,
μm/yr
33.4 0.0173 21.1 0.0060 7.3
4.8 0.0024 2.9 0.0022 2.6
193.4 0.1321 146.9 0.0923 102.6
Hot-dip zinc
0.0274
coating (in a zinc
melt)
Diffusion zinc
coating
(in powder
mixtures)
0.0039
Non-coated steel 0.1740
nipples
(for comparison)
Table 3. Results of corrosion tests of zinc coatings in a moving medium of hot water supply systems
Diffusion zinced lifting pipes were dipped for
industrial tests in deep gaslift oil wells and
monitored during the test period. The test program
included periodic lifting of the pipe columns for
inspection and taking samples of pipes and couplings
for examining state of the diffusion zinc coating, its
thickness change (Table 4) and physico-mechanical
characteristics of pipes and couplings.
Inspection of the pipes showed that the surface of
the diffusion-zinced lifting pipes did not have any
corrosion damage or any damage to the coating
layer. After taking samples for laboratory testing, the
column of diffusion zinced lifting pipes was again
dipped into the well to continue industrial tests.
Thus, even when the lifting pipes were operated for
65 months in complicated oil and gas production
conditions, reduction in the coating thickness was
15% less on the external surface and up to 50% less
on the inner surface subjected to crrosion-erosion
wear.
Time from
the test
start, mos.
0
Coating
39-56
thickness
on the
outside
surface, μm
Coating
35-65
thickness
on the
inside
surface, μm
21 mos. 40 mos. 65 mos.
34-48 34-47 35-40
25-50 15-35 15-30
Table 4. Data of metallographic studies of
thickness change of diffusion zinc coating on
lifting pipes during long-term industrial tests in
an oil well.
Further studies have established that during
operation of diffusion zinced products in corrosive
media, dense layers of corrosion products were
formed on their surface. They possessed protective
properties and protected the underlying coating
layers from destruction (self-protection effect).
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TUBE NEWS June 2018
23