A detailed tabulation of the chemical resistance of materials is given in Appendix 3 of this volume. A more condensed summary of the corrosion resistance of some widely used materials is also given in Table 8.19a. In selecting the materials of construction for instruments, one should also keep in mind that most process fluids are not pure and that the rate of corrosion is also a function of flow velocity and of dissolved oxygen content. Nondestructive testing, such as ultrasonic scanning, is also used in inspecting storage tanks, pressure vessels, and piping. If the plant’s atmosphere contains corrosive gases, it is also desirable to enclose, purge, or otherwise protect the more sensitive instruments or their components. In such installations it is advisable to protect the stems of control valves by boots. Corrosion monitoring can be based on 30- or 60-day coupons, which provide data only on the average rate ofcorrosion. Mobile monitoring laboratories can provide spot checks, or in the more critical cases, permanent monitoring equipment can be installed to provide continuous corrosion rate and pitting tendency readings. In cooling systems, where chlorine is used to control biological deposition, corrosion monitoring is needed because if the chlorine residual is too high, corrosion will occur.Coupons are the simplest and most frequently used devices in the monitoring of corrosion. Coupons are small pieces of metals, usually of a rectangular shape, which are inserted in the process stream and are removed after a period for study.