Hexagonal martensite α′ gets special metallographical corrosion behavior: the inner microstructure of some grains shows unobvious contrast after corrosion, however, some other grains have obvious corrosion character, namely the inner vimineous α′ array orthogonally. In present work, the mechanism of this corrosion behavior was studied based on OM, AFM and EBSD analyses. It’s found that: there are three groups of orthogonal precipitation directions of α′ when transformed from original β phase, if the metallographical observation surface is parallel or near-parallel to any one of the three groups of orthogonal precipitation directions, then this group of α′ plates must array orthogonally or parallelly. Under this circumstance, α′ plates are perpendicular to the observation surface, thus the α′/α′ interfaces are deep, which leads to deep corrosion depth, and finally deep corrosion characters are shown. For a given grain, the two arrangement modes of α′ plates can exist simultaneously, which appears orthogonal array mode as a whole.