What is the passivation threshold of steel?

I'd think it's the limit at which there is no longer any iron that can be drawn by the passivation solution. Certain alloys have higher iron concentrations...those with high iron levels would require longer passivation times than alloys with low iron levels. Either way, there is a point reached at which no more iron can be removed. If I remember correctly, pipelines would be passivated with a hot acetic acid solution. A detector was used to measure the iron that was freed up. Once the iron levels decreased to a certain point, passivation was considered complete.