ASTM G96-90(2013)

5.1 General corrosion is characterized by areas of greater or lesser attack, throughout the plant, at a particular location, or even on a particular probe. Therefore, the estimation of corrosion rate as with mass loss coupons involves an averaging across the surface of the probe. Allowance must be made for the fact that areas of greater or lesser penetration usually exist on the surface. Visual inspection of the probe element, coupon, or electrode is required to determine the degree of interference in the measurement caused by such variability. This variability is less critical where relative changes in corrosion rate are to be detected.

5.2 Both electrical test methods described in this guide provide a technique for determining corrosion rates without the need to physically enter the system to withdraw coupons as required by the methods described in Guide G4.

5.3 Test Method B has the additional advantage of providing corrosion rate measurement within minutes.

5.4 These techniques are useful in systems where process upsets or other problems can create corrosive conditions. An early warning of corrosive attack can permit remedial action before significant damage occurs to process equipment.

5.5 These techniques are also useful where inhibitor additions are used to control the corrosion of equipment. The indication of an increasing corrosion rate can be used to signal the need for additional inhibitor.

5.6 Control of corrosion in process equipment requires a knowledge of the rate of attack on an ongoing basis. These test methods can be used to provide such information in digital format easily transferred to computers for analysis.