Steel is the most common material that used to building and construction fields. For example, AISI 1045 of medium carbon steel which has a low price with good quality. However, the metal will decrease quality. This is caused by the effect of corrosion. To slow down the corrosion phenomenon and give effect to the desired material properties, it can be treated by using heat treatment. The purpose of this researching was to determine the corrosion rate of the material without heat treatment and the material given heat treatment with variations of annealing, normalizing, quenching and tempering using AISI 1045 material with seawater electrolyte. The method used in testing the corrosion rate is electrochemical, according to the ASTM G59-97 (2009) standard. From the results of the study, the highest corrosion rate values in the test specimens without heat treatment were 0.068461, 0.069042 and 0.069597 (mmpy). While the lowest corrosion rate values in the annealing heat treatment specimen were 0.012147, 0.014623 and 0.017637 (mmpy). There are two things that affect the corrosion rate, the first is internal stress. If this stress is not removed, it causes stress corrosion cracking. The second, the pearlite phase. Pearlite has an arrangement of cementite and ferrite. When connected to an electrolyte, the two phases will experience microgalvanic corrosion. Because the microgalvanic effect causes the test specimen with more perlite phase to corrode faster.