JNTU B.Tech III INSTRUMENTATION PRACTICES IN INDUSTRIES Previous Question Paper (September - 2021 - R18)

Mercury Intrusion Porosimetry (MIP) is the primary technique for characterizing the porous structure of solids. It relies on the principle that mercury, a non-wetting liquid, will only enter pores under applied external pressure.

The relationship between the applied pressure and the pore diameter is governed by the Washburn equation:

Where D is the pore diameter, γ is the surface tension of mercury, θ is the contact angle, and P is the applied pressure.

• The sample is placed in a penetrometer and evacuated to remove air and moisture.
• Mercury is introduced into the chamber, and pressure is gradually increased.
• As pressure increases, mercury is forced into smaller pores. The volume of mercury intruded at each pressure increment is measured.
• The resulting data provides the pore size distribution, total pore volume, and bulk density of the material.

The Polarization Resistance (Rp) method is a non-destructive electrochemical technique used to measure the instantaneous corrosion rate of steel reinforcement in concrete structures within the cement industry.

The method involves applying a small potential perturbation (typically ±10 to 20 mV) to the steel reinforcement relative to a reference electrode (e.g., Ag/AgCl or saturated calomel electrode) and measuring the resulting current response.

The polarization resistance is defined as the slope of the potential-current density curve at the corrosion potential:

The corrosion current density (Icorr) is inversely proportional to the polarization resistance, calculated using the Stern-Geary equation:

Where B is the Stern-Geary constant, which depends on the anodic and cathodic Tafel slopes. By measuring Rp on-site, engineers can quantify the rate of metal loss in real-time, allowing for timely maintenance of concrete structures exposed to aggressive industrial environments.