C5212 is a suitable material for “spring materials”, but there is an explanation in the standard that “phosphor bronze for springs” should be used especially when high performance spring properties are required. C5191 has a lead content of 5.5-7.0%, and C5212 has a lead content of 7.0-9.0%. Increasing the lead increases the strength of the metal, but decreases its conductivity. Also, when ductility is required, the material itself is hard, so it becomes less and less suitable as the amount of Sn increases.