volunteer as (something)
1. To choose or offer to assume or fulfill some role or position without being asked, directed, or required to do so. I don't know why everyone thinks I volunteered as class secretary, because I definitely didn't! She volunteered as the spokesperson of the group.
2. To perform some role or duty without pay, as for charity. I volunteer as a firefighter on the weekends. Why don't you volunteer as a reader down at the retirement home? They would really appreciate your help.
volunteer for (something)
To choose or offer to assume or undertake some position, role, or duty without being asked, directed, or required to do so. I know you didn't volunteer for this responsibility, but you've done an admirable job filling in after Jack left the company. She volunteered for the role, not realizing how much additional work it entailed.
volunteer as something
to submit oneself as a person ready or willing to do something. Would you be willing to volunteer as a marcher? I will volunteer as a helper in the hospital.