These are very basic questions about aqueous corrosion, and they show a lack of understanding of the mechanisms of oxidation.  I STRONGLY suggest you get a good textbook and try to learn about the thermodynamics and kinetics of aqueous corrosion, the Pourbaix diagram and the Tafel model.  To get you started though:

1. In acidic conditions (below about pH 5, depending on the conditions), both iron and chromium are in active corrosion regions.  Assuming that the rate of corrosion is not sufficient to significantly alter the ionic concentrations in the electrolyte, there is nothing to make corrosion more 'difficult' as time progresses.  The corrosion rate remains constant with time and the weight loss with time is linear.

2. The open circuit voltage (potential difference) is the difference in the two standard electrode potentials.  Once the current drawn from the cell is greater than the exchange current density of one of the electrodes, the cell voltage (potential difference) will be reduced - this is evident from the Tafel diagram.  The corrosion potential is a potential, not a potential difference, so it must be measured relative to an external standard - usually the standard hydrogen electrode (SHE).  It is the potential at the point where the two Tafel slopes cross - that is, where the two cell reactions are operating at the same rate.  The two are not the same, and there is no reason they should be equal.

3. If a metal is truly passive, it makes no sense to measure its anodic Tafel branch.  The passive current density will be independent of the corrosion potential, providing passivity is maintained.  If you want to measure the Tafel slope, you must do it with the metal in an active corrosion region.

there is one method known as small amplitude voltammetry to measure nano corrosion currents

Give a small current and follow over potential vs time  curve - go on substituting R values  -

see a good book on Corrosion - passive metal corrosion - Try EIS

OCP is the corrosion potential

 what yoget from extrapolating  of Tafel lines is the psuedo E corr ( as the surface is changed during polarization)

good only try cathodic branch for analysis

These are very basic questions about aqueous corrosion, and they show a lack of understanding of the mechanisms of oxidation.  I STRONGLY suggest you get a good textbook and try to learn about the thermodynamics and kinetics of aqueous corrosion, the Pourbaix diagram and the Tafel model.  To get you started though:

1. In acidic conditions (below about pH 5, depending on the conditions), both iron and chromium are in active corrosion regions.  Assuming that the rate of corrosion is not sufficient to significantly alter the ionic concentrations in the electrolyte, there is nothing to make corrosion more 'difficult' as time progresses.  The corrosion rate remains constant with time and the weight loss with time is linear.

2. The open circuit voltage (potential difference) is the difference in the two standard electrode potentials.  Once the current drawn from the cell is greater than the exchange current density of one of the electrodes, the cell voltage (potential difference) will be reduced - this is evident from the Tafel diagram.  The corrosion potential is a potential, not a potential difference, so it must be measured relative to an external standard - usually the standard hydrogen electrode (SHE).  It is the potential at the point where the two Tafel slopes cross - that is, where the two cell reactions are operating at the same rate.  The two are not the same, and there is no reason they should be equal.

3. If a metal is truly passive, it makes no sense to measure its anodic Tafel branch.  The passive current density will be independent of the corrosion potential, providing passivity is maintained.  If you want to measure the Tafel slope, you must do it with the metal in an active corrosion region.

Yes Now i remember -- You can use  "CHARGING curve method " follow overpotential - vs Time -

the sizes and the quantity of the pitting corrosion points are growing in time, so one should expect raising weight loss/ time unit . Linearity  of the pocess is to be proven. I rather expect an exponential process.

To Michel Moor

It empirique result so it is proved