see my papers 

The Stern-Geary relationship is strictly only applicable to systems with both anodic and cathodic reactions obeying Tafel kinetics and both reactions distributed uniformly over the surface. Neither of these requirements apply to steel corroding in concrete, so that the best you can really do is to get an indication of relative rates of corrosion. Consequently it doesn't really matter too much what S-G coefficient you use (I don't like to call it a 'constant', because it isn't), and 52 mV is as good as any, on the basis that the cathodic reaction will be mass-transport limited oxygen reduction.

I didn't completely understand your question, but if you need a formula to calculate the Stern-Geary constant (B) from the Tafel constants, it is: B=ba.bc/(ln10 x (ba+bc)) where ba and bc are the Tafel constants (both positive) and B is the Stern-Geary constant. You may find more information in F. Mansfeld, “The Polarization Resistance Technique for Measuring Corrosion Currents”, in Advances in Corrosion Science and Technology, Fontana and Staehle Eds., Plenum Press, New York (1976).

I agree with Professor Robert Alan Cottis & Professor J.C.S. Fernandes .Also,I would recommend reading the following attached item;

Princeton Applied Research Application_Note_CORR-1.pdf

Greetings

in my experience we determine the rate of corrosion in the steel RP technique specifically applying those criteria B = 26 mV for active steels and B = 52 mV for steels liabilities obteniendos excellent results in more than 15 years of research. both using potentiostats or galvonostatos laboratory and digital corrosimetros (GECOR) in fieldwork.

anybody have a proper step by step solution to find out rate of corrosion by Linear polarization test and calculate corrosion after few year.

Is it uniform Corrosion in case of concrete carbonation and a decreasing pH-value it could be possible to use LPR or is it chloride induced pitting corrosion? If you will investigate chloride induced pitting corrosion by any polarization method you will get a problem by calculating the current density. You don't know the active corroding surface of your embedded reinforcement! You can use micro CT but without it is nearly imposible to calculate a corrrosion rate or mass los by using a Rp. In fact, your result out of LPR is just ohm not ohm cm² . And please do not forget, a real polarizartion resistance at a corrosion system with a passive layer does not exsist ;-)