Corrosion
and protection of steel piles and sheet piles in soil and water
Excerpt
and translation of Report 93, Swedish
Commission on Pile Research, by Göran Camitz
Summary
This
investigation assumes that steel piles and sheet piles in
soil and water are expected to remain in place for a long
period of time without corrosion affecting their bearing capacity.
In many cases, the life span requirement in soil is in the
range of 100 years. The general aim of the investigation is
to provide information for taking corrosion into account when
designing steel piles and sheet piles.
The
basis for the investigation is an extensive literature survey
and acquisition of experiences, which have been processed
in considerable detail. The report thus summarises present
knowledge in the area of corrosion and corrosion protection
of steel piles and sheet piles in different types of soil
and water.
The
report begins with a theoretical presentation of the way in
which corrosion attacks steel piles and sheet piles and the
factors influencing this process. A comparison is made with
the corrosion resistance of unprotected piles of carbon steel
and piles of ductile iron.
This
is followed by a summary of the results of Swedish and foreign
field tests of corrosion of steel piles and sheet piles in
soil and water. The emphasis is on reporting the corrosion
rates measured in the tests.
Detailed
accounts are given of the various measures that can be used
to inhibit corrosion on piles and sheet piles. These are the
introduction of a corrosion allowance (i.e. oversized cross-sections
of piles), anti-corrosion painting, application of a polyethylene
coating (on steel tube piles), zinc coating, electro-chemical
(cathodic) protection and casting in cement mortar or concrete.
Detailed comments are given on the protection efficiency of
the anti-corrosion systems in various conditions. Information
is also provided on calculation of the corrosion allowance.
Finally, the approximate costs of the various protective measures
are stated.
The
corrosivity of different types of soils and waters in Sweden
towards unprotected steel piles and sheet piles has been classified
in appendix 1
of the report. The degree of corrosivity is expressed as the
average penetration in (mm/year). The purpose of this classification
is to serve as a guide in calculating corrosion allowances
or choosing other protection measures for such structures.
The
investigation has identified inadequate knowledge in certain
areas, which might be clarified through further research and/or
practical investigations.
The
report includes a corrosion glossary, together with a list
of standards and handbooks applicable to the area of corrosion
protection of steel piles and sheet piles.
1.
Factors influencing the corrosion of steel piles and steel
sheet piles
Corrosion
in soil, water or moist out-door environment is caused by
electro-chemical processes. The process takes place in corrosion
cells on the steel surface, which consists of an anodic surface
(where the corrosion takes place), a cathodic surface (where
oxygen is reduced) and the electrolyte, which reacts with
these surfaces. In the case of general corrosion, the surface
erosion is relatively even across the entire surface. Local
corrosion however is concentrated to a limited surface area.
Pronounced cavity erosion is rather unusual on unprotected
carbon steel in soil or water.
Electro-chemical reaction in one corrosion cell
The
rate of corrosion can be measured by micrometer or ultrasound
measuring devices and is usually expressed by the thickness
reduction (penetration) as a function of time (mm/year). Corrosion
rate can also be measured by weighing of samples before and
after exposure. Penetration is then expressed as the loss
of mass over an area as a function of time (g/m2 .
year), which then can be translated into an average erosion
value (mm/year).
Path of electric current between corrosion
cells
Different
types of corrosion can occur in soils, such as soil corrosion,
galvanic corrosion and current leakage corrosion. In the case
of steel piles and steel sheet piles, soil corrosion is most
important. Soil corrosion is caused primarily by diffusion
of oxygen from the soil to the steel surface. Since the groundwater
surface acts as a diffusion barrier, corrosion is most significant
in the unsaturated zone between the ground surface and a level
just below the groundwater level. Several factors have significance
in this zone, such as soil type and pH-value, but these are
of less importance at greater depth.
Corrosion
decreases with depth on piles or sheet piles in stationary,
fresh water or salt water, due to the reduced oxygen content.
In flowing ground water, corrosion can be significant also
at greater depth. Piles in fresh water, rich in carbonate
deposits experience less corrosion than in flowing water with
a low pH-value. Sea-water causes more corrosion due to the
high salt content than fresh water. Salt water can be mixed
with fresh water, especially in harbors and river deltas.
Vegetation on steel can influence the rate of corrosion.
The
most important factor corrosion is the duration of exposure
of the steel surface to water (wetting phase) and the presence
of corrosion-stimulating agents in the moist zone. If the
duration of exposure and the environment are known, it is
possible to estimate the corrosion rate.
2.
Corrosion investigations on steel piles and steel sheet piles
Corrosion
of steel piles and steel sheet piles in soil and water has
been investigated relatively detailed by several systematic
long-term tests, with measurements on piles and sheet piles
which have been exposed to fresh water and salt water over
long periods.
Six
major investigations using piles or steel rods, which have
been installed and observed a number of years in undisturbed
soil with well-defined testing conditions. In additions, reference
can be made to two additional investigations, where steel
samples had been placed in excavated and refilled soil. One
test included carbon steel plates, and the other carbon steel
rods. These tests are also of relevance for the corrosion
effect on steel piles and sheet piles. The test data are comprehensive
and include a large number of observations.
Corrosion situation of unprotected piles in marine environment
The
results from the investigations in soil show consistently
that the location of the ground water level, and thus the
access to air has a dominating influence on the corrosion
of steel piles. Below the ground water level, the average
corrosion is normally small, rarely in excess of 20 mm/year and often significantly lower. Further, local corrosion is usually
low some distance below the ground water level.
In
and above the ground water zone the average corrosion is somewhat
higher, but rarely more than 40 mm/year. However,
the corrosion rate in and above the ground water level can
locally be significantly higher than the average value. In
two Swedish investigations on driven steel piles and embedded
steel rods, respectively, the corrosion rate was in the deepest
zone about 200 mm/year in one investigation and 80 mm/year in the other
one. The number of observations is, however, too small to
determine typical values with sufficient reliability. This
aspect deserves further investigation.
From
the investigations of piles in water, it can be concluded
that in all cases the most severe corrosion effect could be
found at or in the vicinity of the water level. Below the
water level, corrosion is usually less deep. At the bottom
sediment level, corrosion is small and of the same order of
magnitude as in soil on land. In the airated zone, corrosion
is typically lower, but not without significance.
|
Conditions
|
Salt
water
|
Fresh
water
|
|
Water
at surface:
|
100
mm/year
|
50
mm/year
|
|
Salt
water in splash zone:
|
300
mm/year
|
200
mm/year
|
|
Below
the water level:
|
100
mm/year
|
100
mm/year
|
|
Bottom
sediment:
|
50
mm/year
|
20
mm/year
|
Average
values of average corrosion penetration on steel piles and steel
sheet piles
3.
Measures against corrosion
Due
to the requirement of long life time of most structures, it
is important to include corrosion aspects in the case of steel
pile and steel sheet pile foundations in soil and water. This
can be achieved either by accounting for the corrosion effect
when choosing the wall thickness, or by protecting the piles
or sheet piles against corrosion. The two methods can also
be combined. One, or a combination of the following measures
can achieve corrosion protection:
-
corrosion protection paint,
- PE-cover
(in case of steel tube piles),
- zinc
coating,
- electro-chemical
(cathodic) protection
- cement
or concrete cover.
In
the case of steel piles in soil, the most common method is
to account for the expected corrosion penetration. Corrosion
protection measures are used rarely on piles in soil.
Steel
tube piles can be protected effectively by application of
a PE-cover of a few millimeter thickness. This cover can be
applied in the factory and is usually placed on a coating
of epoxy. Steel tube piles in water, where the mechanical
wear is low, can in this way be protected for long time periods.
When the steel tube piles with the PE-cover are driven into
coarse-grained soil, the effect of damaging the protection
layer must be taken into consideration.
Steel
piles and steel sheet piles can be covered by corrosion protection
paint according to standard procedures. Modern paints consist
normally of pure epoxy or epoxy modified by resin. Also in
the case of paint-protected piles it is necessary to take
care when piles are driven into granular soils.
Cathodic
protection with electric current applied to steel sheet pile
wall. Rod-type anodes are connected directly with steel sheet
pile
Properly
executed anti-corrosion measures, using high-quality methods
can protect steel piles in soil or water over periods of 15
to 20 years. PE-cover in combination with epoxy coating can
achieve even longer protection times.
Using
cathodic protection, corrosion of steel piles can be monitored
with high reliability. Protection can either be achieved by
using a “sacrifice” anode, or with applied electric current
from an active anode. Optimal shielding effect is obtained
when the cathodic protection is combined with corrosion protection
paint or, in the case of steel tube piles, of PE-cover. Steel
piles in soil should be provided with a suitable cover if
cathodic protection is used. In this case, the protection
current will flow to the damaged area and shield the exposed
steel surface, thereby reducing the required supply of electricity.
Steel sheet piles can be equipped by cathodic protection without
cover and the protection anodes can be mounted directly on
the sheet piles.
Hot
zinc-coating of steel piles in soil can achieve normally long-lasting
protection, provided that the zinc layer has sufficient thickness.
In some soils, especially those with low pH-values, the corrosion
of zinc can be high, thereby shortening the protection duration.
Low pH-values occur normally in the airated zone above the
lowest ground water level. In such a case, it is recommended
to apply protection paint on top of the zinc layer.
|
Soil
type
|
Corrosion
penetration in zinc
(mm/year)
|
|
Clay
and gyttja (organic clay)
|
10
– 30
|
|
Peat
|
10
– 30
|
|
Sand
|
2
– 4
|
|
Sandy
till, pH-neutral
|
2
– 3
|
|
Sandy
till, low pH
|
8
|
Corrosion
steel plates with hot-coated zinc cover in different soils (placed
soil)
Piles
in water will experience higher corrosion, when the flow velocity
is in excess of 0,5 m/s. The durability of o zinc in water
depends to a high degree on the pH-value, the chemical composition
and the flow velocity of the water. Outside the pH-interval
7-12, the resistance of zinc is low. “Soft” water can attack
zinc while hard water is less aggressive.
Casting
of steel piles in concrete achieves high and long-lasting
corrosion protection, especially due to the high pH-value.
However, concrete or mortar should not contain any chlorides.
Steel tube piles, which are filled with concrete, do not show
any corrosion.
Quay
walls constructed of steel sheet piles or steel piles can
be protected effectively in the zone around the medium water
level by casting the concrete head beam to some depth below
the water level. In the case of a new quay wall it is possible
to account for the corrosion penetration effect below the
water level, or by using cathodic protection. Older, existing
quays can also be provided with cathodic protection.
The
optimal solution for steel piles in soil is to account for
the loss of wall thickness due to corrosion penetration. The
optimal solution for steel tube piles in corrosive water,
where accounting for corrosion penetration may not be sufficient,
is probably the use of PE-cover. If the PE-cover is combined
with cathodic protection and sacrifice anode, the additional
cost is often marginal.
Cathodic
protection with electric current applied to sheet pile wall.
Rod-type anodes are placed in the bottom sediment some distance
from the steel sheet pile wall.
Cathodic
protection of steel tube piles in water, using “sacrifice
anodes”. The anodes are suspended in-between the piles
Cathodic
protection with “sacrifice anode” (zinc anode) for steel piles
in a pier. Two sacrifice anodes are mounted on each steel
pile
Cathodic
protection with electric current applied to steel piles in
water. Anodes rigidly mounted in protection tubes between
piles
Arrangement
of anodes for cathodic protection with electric current applied
to steel piles in water. Anode mounted in a support on the
sea bottom
|
