Corrosion can have significant long-term effects on the structural resistance of steel piles in ground. There is a possibility that this effect can be intense if ground contains chlorides, sulphites or other aggressive chemicals. Also de-icing salt (NaCl) used on roads during winter maintenance may have effect on the corrosion of steel piles. In this research project a series of site investigations and laboratory tests have been conducted to investigate the effect of de-icing salt for corrosion on steel piles. Totally 12 sites were investigated, partly from locations where de-icing salt has been used and partly where de-icing salt has not been used. Most of the piles were part of bridge structures, but the study includes also data from a few special test piles. Investigated steel piles have been in ground for five to 24 years. The results show that the corrosion and its rate are low. The detrimental effect of de-icing salt on corrosion is negligible.