maybe some useful reduction potentials:
Compare the oxidative power of chloride and oxygen:
Cl2 (g) + 2 e− <--> 2 Cl− E0 = +1.36 V
O2(g) + 2 H2O + 4 e− <--> 4 OH−(aq) E0 = +0.40 V
see what happens to carbon (non-diamond) in presence of oxygen and acid (in alkaline carbon is even easier oxidized):
C (s) + H2O <--> CO (g) + 2 H+ + 2e- E0 = +0.517 V
CO (g) + H2O <--> CO2 + 2 H+ + 2e- E0 = -0.106 V
Now reason what would happen to carbon in the presence of chlorine