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Corrosion and protection of steel piles and sheet piles in soil and water
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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



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