Resource-constraint nature of Wireless Sensor Network (WSN) turns the security issue certainly into a big challenge. An efficient key management scheme is the pre-requisite to ensure security in WSN. In this paper we present two versions of a secured key management protocol (SecCOSEN) adopted on COSEN, a chain oriented sensor network, which is used for periodic data collection. SecCOSEN uses partial key pre-distribution and symmetric crypto-graphy techniques. Where as one version of SecCOSEN protocol uses shared partial keys in a sensor chain the other version uses private partial keys. Both versions of SecCOSEN show high resilience to different security attacks. The protocol outperforms other random key pre-distribution protocols in the sense that it requires lower space, lower communication overhead and offers very high session key candidates. A comparative discussion between the two versions is also provided.