Effect of Phosphorus on Phosphatability and Perforation Corrosion Resistance of Ultra-low Carbon Steel Sheet

Abstract

The influence of phosphorus on both the phosphatability and perforation corrosion resistance of ultra low-carbon Al-killed steel sheets were investigated. Results are summarized as follows;
(1) Phosphorus in ultra low-Carbon steel sheets was beneficial in refining the grain and increasing the coverage area of a phosphate layer on the steel surface. The steel sheet containing 0.07 wt% phosphorus has excellent phosphatability.
(2) Phosphorus in steel accelerated the pickling rate of the steel in acid solutions. The rest potential of P-containing steels in a zinc phosphating bath moved to noble direction more rapidly than the steels without phosphorus. This fact means that the former shows the higher formation rate of the phosphate layer than that of the latter. The effect is possibly caused by the depolarization due to phosphorus in steel.
(3) The increase in the phosphorus content of steel reduced the weight loss of both bare cold-rolled steel sheets and phosphated ones in cyclic corrosion tests. The addition of phosphorus to steel improved perforation corrosion resistance. This effect was more pronounced in phosphated steel sheets.
(4) Phosphorus in steel also improved perforation corrosion resistance after painting.