Please tell about the available techniques in your lab then I suggest  the methods for measuring corrosion rate of a coated mild steel sample according to the limitation you specified in your question.

Dear Dawood Ansari Haji Sheik,

In your case volumetric methods for estimation of corrosion rate could be appropriate for corrosion of steels. As an example see the next link:

https://www.researchgate.net/publication/43463007_An_Hydrogen_Evolution_Method_for_the_Estimation_of_the_Corrosion_Rate_of_Magnesium_Alloys 

Article An Hydrogen Evolution Method for the Estimation of the Corro...

Mohammed Hussein j. H. Al'Atia

Dear sir, Thank you for the reply. I will get back to you with the necessary information.

Valerii Kotok

Dear sir, Thank you for your valuable suggestion. I will go through the text.

Yes dear I think it is so good to have potentiostat.  Now, the corrosion rate of a coated mild steel sample is strongly related to the integrity of the coating layer in terms of their defects, mainly connecting porosity which is responsible for the transport of the aggressive environment through these pores to reach the underlying steel surface where the corrosion process proceed. The potentiostat method will give you all electrochemical parameters values that facilitate the calculation of the coating protective property and then the corrosion rate of steel substrate vs. the integrity of the coating in terms of the coating porosity percent. This means that the verification of coating integrity is essential for the studying the underlying steel surface corrosion rate changes. This is a simple and affordable method by potentiostat for measuring corrosion rate of a coated mild steel sample after the gravimetric method.

Best regards