GB2161721A

GB2161721A - Precoated corrosion-resistant steel pipe piles for marine use, and structure thereof

Info

Publication number: GB2161721A
Application number: GB08418586A
Authority: GB
Inventors: Motoo Shiraishi; Mansei Tanaka; Minoru Nakanura; Koichi Sato
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 1984-07-20
Filing date: 1984-07-20
Publication date: 1986-01-22
Also published as: MY100110A; AU554013B2; GB2161721B; GB8418586D0; AU3120684A

Abstract

A steel pipe pile for mains use is provided with outstanding resistance to corrosion and scratching by precoating with a polyethylene resin material, preferably including carbon black and an anti-oxidant, at least over the splash zone and tidal zone. The coating may be extended to the riprap layer or further. Otherwise electrical corrosion protection or a corrosion allowance may be provided beneath the coating.

Description

SPECIFICATION Precoated corrosion resistant steel pipe Background of the invention The present invention relates to precoated corrosion-resistant steel pipe piles for marine use which support structures in ports and harbors, sea and rivers.

Description of the prior art Steel pipe piles have been heretofore employed as foundation piles of a building on land and, harbor and river embankments and piers, and further, with regard to marine structures, have widely been used as steel building materials able to cope with deep water and poor ground.

In general, the steel pipe piles for use in harbor, sea, and rivers have been heretofore used without any covering. In recent years, however, harbor, sea and river structures are required to possess a durability for extended periods of 40 to 50 years.

It has thus become necessary to take anticorrosion measures enabling steel pipe piles used in such harshly corrosive environments to maintain their corrosion- resistance for extended periods of time.

Various methods for preventing the corrosion steel pipe piles have been known, such as use of a coating of tar-epoxy, electrical protection, and a mortar coating method employing fiber reinforced plastic (hereinafter referred to as FRP cover method).

However, the tar-epoxy coating method is troublesome because it has to be reapplied within a few years. Furthermore, extended corrosion resistance cannot be expected, as even if the steel pipe pile is coated with the tar-epoxy before it is driven into place, the coating is soft and tends to be scratched when being handled or driven. In addition, after having been driven into place, it may be struck by driftwood or the like, causing damage to the coating, making the pile more susceptible to corrosion at that point. Moreover, if the steel pipe pile is coated with the tar-epoxy after it is driven into place, it follows that only the part above the water will be protected from corrosion. If the underwater portion of steel pipe pipe is to be coated, the cost therefor would become very high because of the necessity of having to drain the water from around the pile.

On the other hand, however, the electrical corrosion protection measure is disadvantageous in that the electrochemical function is such that corrosion protection is difficult in the splash zone and the tidal zone, where steel corrosion develops most rapidly.

Corrosion of steel materials in harbors, seawater and rivers proceeds most rapidly in the splash zone and the tidal zone, and is slower underwater, and slower still on sea mud.

"Splash zone" in this specification refers to the portion above the mean high water springs obtained from the high point of the highest tide; "tidal zone" refers to the zone between the mean high water springs and the mean low water springs; and "seawater zone" refers to the portion below the mean low water springs.

According to a recent study on the corrosion rate of steel structures in harbors conducted by an official organisation, the average corrosion rate of steel pipe pile is 0.37 mm/year to 0.6 mm/year in the splash zone, and 0.35 mm/year to 0.5 mm/year in the tidal zone and thereabout. It was found that the mean corrosion rate in the seawater zone tends to gradually decrease as the depth of the seawater increases, and it is less than 0.05 mm/year.

It was also reported that the corrosion rate was 0.1 mm/year to 0.05 mm/year in the riprap layer, 0.05 mm/year in sea sludge, and 0.01 mm/year to 0 mm/year in the sea mud.

Assuming a mean corrosion ratio of 1.0 in the splash zone and the tidal zone, the corrosion ratio in the seawater zone amounts only to about 1/10, and to only about 1/50 in the sea mud.

It follows from the above that electrical protection of corrosion is hardly effective for the steel pipe pile in the splash zone and the tidal zone where protection against corrosion is most desired. Consequently, as a most effective means for preventing corrosion in the splash and tidal zones, where corrosion is most marked, the following method has recently been proposed.

Figure 1 of the accompanying drawings shows an embodiment of a conventional method of preventing corrosion.

In Figure 1, in a steel pipe pile 1 driven into the sea bed 3, a FRP tubular cover 9 encloses the splash zone 4, the tidal zone 5, and part of the outer surface of the pile just below the tidal zone 5, with the space between the tubular cover 9 and the steel pipe pile 1 being filled with mortar 10. The lower end of the mortar 10 is covered by an anticorrosion seal means 11. In Figure 1, 7 is a concrete structure and 8 is a riprap layer.

Figure 2 is an enlarged view of the principal portion of Figure 1. To carry out the work of Figure 1, a specialist, such as a diver, is required, and since the work depends considerably on waves, tides, and other such marine conditions, the method is disadvantageous in that the resultant working efficiency is so poor that there is insufficient waterproofness along the boundary between the concrete structure built onto the top of the steel pipe pile and the mortar filling. Moreover, the cost is high.

Summary of the invention It is an object of the present invention to provide a precoated corrosion-resistant steel pipe pile for marine use which has outstanding resistance to corrosion and scratching.

It is another object of the invention to provide a marine structure in which the precoated corrosionresistant steel pipe pile is employed.

Other and further objects of the-invention will become apparent from the following description made with reference to the accompanying drawings.

Brief description of the drawings Figure 1 is a side view of a conventional method for the prevention of corrosion of steel pipe piles; Figure 2 is an enlarged view of principal parts of Figure 1; Figure 3 is a side view of a steel pipe pile according to this invention; Figure 4 is an enlarged cross-sectional view through the line A-A of Figure 3; Figure 5 is a longitudinal sectional view of part of an embodiment of the present invention wherein the steel pipe pile of the invention is used to support a concrete structure; and Figure 6 is a longitudinal sectional view of part of another embodiment using the steel pipe pile of the invention.

Detailed description of the invention The present invention relates to a precoated corrosion-resistant steel pipe pile, and a structure employing the steel pipe piles, wherein the steel pipe pile to be driven into the sea bed is previously coated with a polyethylene resin material hereinafter referred to as "coating material") to provide corrosion-resistance at least in the tidal zone and splash zone.

The present invention will be described in detail with reference to the embodiment shown in the drawings.

Figures 3-5 shows a first embodiment of the invention.

The splash zone 4, tidal zone 5, and a portion of the surface extending slightly these below of a steel pipe pile to be driven into the sea bed 3 are all covered with the coating material 2. The lower end of the precoated corrosion-resistant steel pipe pile 6 thus constructed is driven into the sea bed 3. A concrete structure 7 consisting of concrete slabs, such as a pier or jetty, is then constructed on the top of the precoated corrosion-resistant steel pipe piles 6. That is, after the concrete structure has been built up, the upper part of the coating material 2 of the precoated corrosion-resistant steel pipe piles is located inside the concrete structure 7.Thus unlike the conventional methods of preventing corrosion of steel pipe piles, the precoated corrosion-resistant steel pipe pile of the present invention is coated in the factory with the coating material 2 to the extent required by the design. Thus, the steel pipe pile can be proofed against corrosion surely and easily. In particular, the portion around the steel pipe pile and the concrete structure can be prevented from any corrosion resulting from the splashing of seawater by coating the head of the steel pipe pile which is inserted into the concrete structure.

Figure 6 shows a second embodiment of the invention. The coating material 2 is previously applied over the steel pipe pile 1 from where it locates into the concrete structure 7, down through the splash zone 4 and the tidal zone 5, to the riprap layer 8 on the sea bed. The other parts of the construction are the same as those of the first embodiment.

In accordance with the present invention, the thickness of the coating material 2 is preferably from 1.5 to 4.5 mm, and more preferably, 2 to 3 mm, The first embodiment of the invention shows an example of use of electrical corrosion protection for the portion a of the steel pipe pile between the part coated with the coating material 2 and the sea bed 3.

Experience has shown that for the remaining part b below the coated part 2, electrical corrosion protection is sufficient. Also, the steel pipe pile may be coated over the whole length thereof with the coating material 2. Or the limit of the coating of the coating material 2 may be only somewhat below the sea bed 3. In this case, however, neither electrical protection treatment nor the corrosion allowance is required.

"Corrosion allowance" refers to a method of preventing corrosion in which an extra thickness is added to the wall thickness of the steel pile required on the basis of design, to allow for an assumed amount of corrosion.

In the second embodiment of the invention, coating material 2 is applied along the whole length of the steel pipe pile 1, where it locates in the concrete structure 7, through the the splash zone 4 and tidal zone 5 and down to just below the riprap layer 8 or the sea bed 3. In this case the pile can be permanently proofed against corrosion from the concrete structure 7 right down to just below the riprap layer 8 or sea bed 3. In addition, the coating material 2 has excellent resistance to acids and chemicals, hence the steel pipe pile is protected from corrosion even if industrial wastes containing acidic substances and chemicals are disposed of around it.

That is, the steel pipe pile according to this invention can withstand the corrosive action of acidic soils of pH 5 or iess, and also has high resistance to the soils wherein anaerobic bacteria, such as sulphate reducing bacilli are present.

The durability of the coating material 2 to weather, corrosion or scratching can be further improved by the addition of carbon black, which intercepts ultraviolet rays and an anti-oxidant consisting of phenolic or sulphur compound.

Examples of the composition of the coating material 2 are shown as follows: Example 1 Polyethylene resin 97.2 wt% Carbon black 2.6 wt% Phenolic anti-oxidant 0.2 wt% Example 2 Polyethylene resin 96.9 wt% Carbon black 2.8 wt% Phenolic anti-oxidant 0.3 wt% The properties of the polyethylene resin are: density 0.915 -0.970 g/cm3; melt index 0.05 -0.5 g/10 min. Use of a copolymer comprising vinyl acetate, butene, hexene, and polyethylene is preferable.

The precoated corrosion-resistant steel pipe pile of this invention is produced as follows. The coating material of this invention is continuously extruded in a semi-molten strip from the T-die of an extruding machine. The material is spirally twined around the steel pipe pile which has been preheated and coated with an adhesive material, the winding of the covering material being such as to overlap across the width thereof. Next, the pile is subjected to a finish using pressure forming rolls in order to attain a uniform thickness over every part of the pile.

In accordance with the present invention, the steel pipe pile 1 can be effectively protected from corrosion at least in the tidal zone and the splash zone for an extended period of time by the use of the coating material 2 with its waterproofness, resistance to corrosion and scratching. Moreover, since the steel pipe pile 1 has been already coated on its outer surface with the coating material 2 before it is driven into place at job site, quality maintenance can be fully satisfied. Further, as the coating material 2 is strong, it is not easily damaged during handling or by the impact of floating driftwood.

The steel pipe pile of this invention can be applied in conventional pile-driving as such, hence no underwater work for anti-corrosion is required after the pile has been driven. In addition, there are economical advantages such as that the coating material 2 is highly durable and thus requires no maintenance.

Claims

1. A precoated corrosion-resistant steel pipe pile for marine uses in which the exposed surface of said steel pipe pile is coated partly or entirely with a polyethylene resin material.

2. A pile according to Claim 1, in which the polyethylene resin coating material is applied so that, when the pile is in use, the coating extends downwards from substantially the top thereof far enough to give protection only for the tidal zone and the splash zone of the pile.

3. A pile according to Claim 1, in which the polyethylene resin coating material is applied so that, when the pile is in use, the coating extends downwards through the splash zone and the tidal zone of the pile for a distance such that it will extend to a riprap layer on the sea bed.

4. A pile according to Claim 2 or Claim 3, in which the polyethylene resin coating material is extended to the top of the pile for location in a concrete structure to be supported by the pile when in use.

5. A pile according to any one of the preceding claims, in which electrical corrosion protection is applied to at least a portion of the pile that extends beneath the polyethylene resin coating when the pile is in use.

6. A pile according to any one of Claims 1 to 4, in which a corrosion allowance is provided in the thickness of the steel wall along at least a part thereof that extends beneath the polyethylene resin coating material when the pile is in use.

7. A pile according to any one of the preceding claims, in which the thickness of the coating material is between 1.5 and 4.5 mm inclusive.

8. A pile according to Claim 7, in which the thickness of the coating material is between 2 and 3 mm inclusive.

9. A pile according to any one of the preceding claims, in which the steel pipe is such that it can withstand the corrosive action of acidic soils of pH5 or less.

10. A pile according to any one of the preceding claims, in which the steel pipe is such that it has a high resistance to soils in which anaerobic bacteria are present.

11. A pile according to any one of the preceding claims, in which an agent which is an anti-oxidant is incorporated in the coating material.

12. A pile according to any one of the preceding claims, in wh1ch an agent that intercepts ultra-violet rays is incorporated in the coating material.

13. A pile according to any one of the preceding claims, in which the coating material is as herein described with reference to the accompanying Example 1.

14. A pile according to any one of Claims 1 to 12, in which the coating material is substantially as herein described with reference to the accompanying Example 2.

15. A marine structure for ocean and harbour use comprising concrete slabs supported above a sea bed by corrosion-resistant steel pipe piles, each constructed according to any one of the preceding claims.

16. A method of forming a corrosion-resistant steel pipe pile according to any one of Claims 1 to 11, the method comprising extruding a semi-molten strip of polyethylene resin material from an extruding machine, spirally winding the strip round the steel pile, when this has been preheated and coated with an adhesive material, the edges of the spiral winding being disposed so that they overlap and subjecting the wound steel pile to forming rolls in order to reduce the wound coating to uniform thickness.