Phosphoric Acid will do the job just as well. Easy to neutralise with a mild alkali like bicarbonate of soda or just proper water flushing.

 

I first clean the steel by whatever mechanical means is possible and then paint it with phosphoric acid. What follows the acid depends on the area and its ultimate treatment. If it's not superficial bodywork, I'll not generally worry about neutralising the acid.

 

What is left behind is pretty stable/converted.