This paper discusses the corrosion behavior of two coagulants used in water treatment, ferric chloride (FC) and polymeric ferric sulfate (PFS). Corrosion tests were performed to compare the corrosivity of these two coagulants on aluminum 6061 and steel 4140 specimens. Results showed that both temperature and concentration of the coagulants substantially impact corrosion rates. The corrosion rates increased with the increase of temperature and concentration. The results from a scanning electron microscope (SEM) showed that chloride caused more serious pitting than sulfate anion on both aluminum and steel specimens. Although SEM confirmed the existence of pitting corrosion, the results of weight loss indicated that the uniform corrosion predominate the corrosion mechanism, and pitting corrosion played a less important role. The test proved that PFS was less corrosive than FC, which may lead to the large-scale application of PFS in waste treatment in the near future. (C) 2003 Elsevier B.V. All rights reserved.