Method for surface treatment of metal pipes

1. A method for surface treatment of at least a portion of the outer peripheral surface of a metal pipe, comprising:

introducing a treatment liquid into a second distribution chamber having a flat bottom surface, and introducing the treatment liquid into a first distribution chamber through holes formed in the bottom surface of the second distribution chamber, the first distribution chamber having a flat bottom surface, and then dropping the treatment liquid onto at least a portion of the outer peripheral surface of a metal pipe through holes formed in the bottom surface of the first distribution chamber, the treatment liquid flowing in a downward direction when dropped from a respective hole in the bottom surface, the metal pipe further comprising a steel pipe having a threaded connecting portion formed on the exterior of at least one of its ends, and wherein the threaded connecting portion is the portion of the metal pipe which is subjected to surface treatment.

2. A method as claimed in claim 1 including maintaining a distance between the bottom surface of the first distribution chamber and the metal pipe constant as the outer diameter of the pipe varies.

3. A method as claimed in claim 2 including maintaining the distance constant in a range of 5 -100 mm.

4. A method as claimed in claim 2 including maintaining the distance constant by varying a height of the first distribution chamber in accordance with the outer diameter of the metal pipe.

5. A method as claimed in claim 1 including introducing the treatment liquid into the first distribution chamber from one or more nozzles mounted on a supply pipe.

6. A method as claimed in claim 5 including maintaining a distance between a bottom surface of the nozzles and a top surface of the treatment liquid in the first distribution chamber constant.

7. A method as claimed in claim 1 including introducing the treatment liquid into the second distribution chamber from one or more nozzles mounted on a supply pipe.

8. A method as claimed in claim 7 including maintaining a distance between a bottom surface of the nozzles and a top surface of the treatment liquid in the second distribution chamber constant.

9. A method as claimed in claim 6 including maintaining the distance between the bottom surface of the nozzles and the top surface of the treatment liquid in the distribution chamber constant by raising and lowering the nozzles.

10. A method as claimed in claim 1 including simultaneously dropping the treatment liquid from the first distribution chamber onto at least two metal pipes.

11. A method as claimed in claim 1 including introducing the treatment liquid into the first distribution chamber such that the treatment liquid accumulates in the first distribution chamber to a depth of 10 -20 mm.

12. A method as claimed in claim 1 wherein the surface treatment is phosphate chemical conversion treatment.

13. A method as claimed in claim 12 , wherein the diameter of the holes in the bottom surface of the first distribution chamber is 3 -5 mm.

14. A method as claimed in claim 13 wherein a plurality of pipes having different outer diameters are subjected to surface treatment, and the bottom surface of the first distribution chamber has at least 144 holes in an area measuring D X D mm 2 , wherein D (mm) is the outer diameter of the smallest metal pipe to be treated.

15. A method as claimed in claim 1 , wherein substantially all of the treatment liquid is supplied to the distribution chamber.

16. A method as claimed in claim 1 , wherein the distribution chamber extends along an axis of the pipe to be treated and the treatment liquid is dropped along said axis during said treatment.