Abstract: A sucker rod coupling pin comprises a hole into which the sucker rod is to be inserted, and has a plurality of annular depressions which surround the inner wall of the hole. As viewed crosswise, the annular depressions respectively have tapered surface portions arranged in series from the open end to the closed end of the coupling pin hole. The tapered surface portions whose cross sections progressively increase toward the closed end of the coupling pin hole have their respective lengths progressively shortened in relation to the preceding tapered portion, toward the closed end of the coupling pin hole, and the angle of inclination of the respective tapered surface portions progressively increases toward the closed end of the coupling pin hole.

Abstract: A composite coating steel sheet is disclosed which has good corrosion resistance, paintability and corrosion resistance after paint coating. The steel sheet comprises a plated base steel sheet, such as a galvanized, zinc alloy plated or aluminum plated steel sheet, and a chromate film formed on the plated base steel sheet. A composite silicate resin film is formed on the chromate film. The composite silicate film is composed of a reaction product of a colloidal silica, an organic resin and a silane compound.