Dr. D. B. Blücher, Prof. Dr. O. Ø. Knudsen (SINTEF) and L. Eliassen, V. Ellefskas (Tyco FPP)
Corrosion in water filled galvanized pipes
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Background
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The reason for this investigation was a pressure increase / blue flame when welding in
a galvanized steel piping filled with municipal water after about 3 years of service
White zinc corrosion products were seen after dissassembly
Galvanized steel has been used in a variety of industries for many years to prevent
corrosion of steel in atmospheric conditions. The galvanizing process involves the
application of a thin layer (about 20-100μm) of metallic zinc to the substrate base
metal which is typically mild steel.
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Experimental 1
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By the formation of these corrosion
products, hydrogen gas evolved causing a
pressure increase in the piping system
(from about 7 bars of municipal water to
about 18 bars)
A simple test proved the presence off
H2(g)
H2(g) from pipe
2H2(g)+O2(g)→2H2O(l)
Collecting H2(g) from pipe
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Experimental 2
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The surface topography of
received samples was measured
in an Alicona 3D infinite focus
microscope
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The pH of the system water was
analyzed by a pH Meter
Radiometer phm 210
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Results 1
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In the piping system, the water-fill-level is obvious.
Below this level, zinc corrosion products (white) can be seen.
Above this level is the gas phase contains air and hydrogen gas
Corrosion on mild steel
Water fill level
Water fill level
Corrosion on mild steel
Zinc corrosion products
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Results 2
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To further investigate the corrosion attack causing the hydrogen evolution and the
pressure increase, a section of the galvanized pipe wall facing downwards was
removed
In this section, the partially removed zinc layer on the inside is evidence of corrosion
attack
Hole for nozzle facing downwards
Pipe outer
Zn-layer
about 95µm
Zn-layer
corroded away
Pipe inside
(water-filled)
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Results 3
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Optical images of the area around the water-fill-level.
To the right, a topographical image of the same general area obtained by the confocal Alincona microscope is seen.
Water fill level
water
air
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Corrosion mechanism 1
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Zinc is a very active material. This is
illustrated in the stability diagram of Zn in
aqueous solution. The red arrow is the
hydrogen overpotential at with Zn corrodes
causing the evolution of hydrogen gas.
The corrosion of the zinc layer inside the
pipes is governed by the selective dissolution
of zinc to protect the steel.
The chemical reaction for this is:
(1) Zn → Zn2+ + 2eThe counter reaction being:
(2) O2 + 2H2O + 4e- → 4OHDissolved oxygen in the water is
consumed, or:
(3) 2H2O + 2e- → H2(g) + 2OHPourbaix diagram for Zn in aqueous solution [Pourbaix 1954]
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The hydrogen evolution will cause the
pressure to increase, as observed.
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Corrosion mechanism 2
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Although black steel pipes are used occasionally, new dry and water-filled installations
are almost exclusively constructed using hot dip galvanized pipes [Van Der Schijff
2013].
In general, the corrosion rate of zinc is lower in hard water than in soft water or
distilled water. This is attributed to the formation of a protective surface layer of zinc
carbonates in hard waters. As seen in the table below, the corrosion rate of zinc can
vary significantly, from as low as 15μm/yr to 142 μm/yr in different waters [Zhang, X.G
1996]:
Roughly corresponds to
Norwegian municipal water
Harder water in continetal
Europe
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The relative differences in the
corrosion rate of zinc between
using 98% nitrogen and
compressed air in a variety of
different applications was
compared to the right.
In every environmental condition,
98% nitrogen resulted in a lower
metal loss when compared directly
to compressed air
mil
Corrosion mechanism 3
Average metal loss (1 mil=25,4µm) of corrosion coupons for
98%N2(g) vs. compressed air over a 12 month study [Tihen,
J., 1974]
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Conclusions
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When the pipes are filled with water, oxygen in the water and the air inside the pipes
will cause corrosion of the zinc. The corrosion process will be slowed down when all
the oxygen in the water (originally about 8ppm) has been consumed by reaction (2). If
all air pockets are removed by ventilation, this will also decrease corrosion somewhat,
but not significantly.
The corrosion process can further be slowed down by removing all galvanic coupling
to bare steel, i.e. at cut edges and auxiliary piping/connections. However, the
corrosion will continue by hydrogen evolution from water. As long as there is Zn
remaining on the mild steel piping and the pipe is filled with water, the corrosion
process will proceed.
The corrosion may have two possibly detrimental effects to the system:
1. The hydrogen formed in the corrosion reaction may cause fractures or leaks due
to the high pressure. Hydrogen is also a highly flammable gas that is unwanted in
the system.
2. The zinc corrosion products may block the sprinklers.
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Solution
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A possible way to solve the problem with the hydrogen pressure increase is to install
evacuation valves at the highest points for the hydrogen gas to escape.
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Vent the system so that no air pockets can form
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Use of stainless steel or glass fibre re-inforced piping
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Dry piping over nitrogen gas
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The introduction of corrosion inhibitors is not suggested because of their possible
negative side effects, e.g. environmental aspects and health issues.
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Backup
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Conclusion
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Zinc corrodes quite fast in tap water, the result being Zn corrosion products (white)
and hydrogen evolution.
These corrosion products are gathered in the piping and the hydrogen gas is collected
in "pockets" above the water phase in the system. This causes a pressure increase in
the whole sprinkler system.
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Results
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The topography of the corroded pipe inner wall around the water-fill-level.
1mm
Water fill level
water
air
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