Ad hoc networks are important pillars for IoT (Internet of Things) development. In order to support IoT traffic transmission in ad hoc networks, it is of great importance to design effective routing protocols to relieve traffic congestion while ensuring data secrecy. To this end, in this paper we first propose a physical layer security-aware congestion relief routing protocol (PCRR). Through sensing the eavesdroppers' location, hop count and congestion information of available paths, PCRR can select an appropriate secure route for data transmission. Based on PCRR and combining the advantages of single-path and multi-path routing strategies, we further develop a mixed-path PCRR protocol (MPCRR), which is more capable of counteracting eavesdroppers by utilizing the simultaneous transmissions from multiple paths. According to the current network state information, M-PCRR can flexibly decide whether or not to execute the multi-path scheme. Finally, we conduct extensive OPNET simulations to validate the performance of proposed routing protocols and show how their route selection is conducted. The results indicate that compared with AODV, PCRR and M-PCRR can improve the network throughput, reduce the hop count and energy consumption, while ensuring the delay performance and control overhead.