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The intergranular corrosion susceptibility of heat affeceed zone(HAZ) for HR3C steel after aging at 600degC was investigated using electrochemical double-loop potentiokinetic reactivation. Experimental results show that HAZ has intergranular corrosion susceptibility after aging at 600degC. At the initial stage precipition of M23C6 along grain boundaries occurs rapidly, which leads to formation of Cr-depleted in the grain boundary and the increase of intergranular corrosion susceptibility. With the increase of aging time, the Ra value of HAZ rapidly increase. Intergranular corrosion of HAZ after aging occur due to the precipitation of M23C6 along grain boundaries, not only the precipitation of M23C6 along grain boundaries caused poor Cr, but the precipitation of M23C6 inside the grains also results in the formation of Cr-depleted zone and subsequent corrosion.