M_(23)C_6 and the o phase decreased corrosion resistance in sample S1. So, these structures are not wanted. In order to remove these structures, heat treatment must be performed. The satisfactory result was obtained for a homogenized sample because of removing a great amount of structure that causes corrosion. The best result was obtained for a sample containing 1 wt. percent Nb. So, adding more Nb is unnecessary. Because the Nb-containing samples demonstrate similar behavior, addition of 0.5 wt. percent Nb will be sufficient. However, the addition of less amounts than 0.5 wt. percent. Nb must be investigated. The corrosion test results obtained from hydrochloric acid solution are as follows: adding not more than 1 wt. percent niobium may be useful in operating conditions. However there is no difference in adding niobium in the amount from 1 and 3 wt. percent. Therefore, the less amounts than 1 wt. percent Nb must be investigated. It was observed that heat treatment with insufficient period did not affect corrosion resistance for samples Nb-free but this period positively developed the corrosion resistance in Nb-containing samples. homogenization for 180 min improved corrosion resistance of the material. So, heat treatment for 180 min was needed for these types of steels.
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