They key to tanking properly with zarya is in 3 parts,

1) use your shield to intercept projectile based attacks that are incoming to ally (yes that means sometimes walking into the path of fire because your ally cant/won't move out of the way, examples of heroes with such projectiles to intercept are ming, art, chromie)

2) Try to cycle your shields so that as your shield is coming off of a "high risk target" (low hp about to get hit by a major attack, heroic or stun) as you are shielding yourself and then placing Zarya as much between incoming dps and allies as u can for the length of her shield, before retreating to mid range and unloading grenades, by the time your shield has run out shield ally should be ready to use again or just about ready.

3) never stop firing the grenades if u can hit a target DO IT! with near instant release and a quick recharge time regardless if zarya has her bonus dps up yet hitting the low hp back-line consistently with your grenades will pressure the enemy team back, as you preform your shield cycle with allys, your grenades will only scale in power.