When considering a monitoring technique to track corrosion, the most significant parameters related to corrosion in a given application must be selected. In some cases, tracking of changes to the environ- ment may be sufficient, while in others direct knowledge of structural corrosion may be preferable. Corrosion sensing methods may be divided into two main categories; those that react to environmental corrosivity, and those that respond to actual degradation of a structure of interest. This paper reviews work presented over the last decade in the research and development of sensors for monitoring and measuring both environmental corrosivity and structural corrosion, along with some examples of field applications.
Efforts have been ongoing for decades to monitor structural corrosion and environmental corrosivity. Many types of sensors have been used in a variety of applications in order to better understand and track parameters related to corrosion events. Such parameters include pH, temperature, humidity, oxygen levels, ionic concentrations (atmospheric or fluid chemistry), corrosion potential, corrosion current, electrochemical impedance, electrochemical noise, and others. The type of sensor measure- ment(s) selected depends on the structure of interest, the environment in which it exists, and the ex- pected forms of corrosion to which it might succumb.
Sensors and multi-sensor units have found mili- tary and commercial monitoring applications in a wide variety of settings including, but not limited to, vehicles (e.g. ships, aircraft), infrastructure (e.g. bridges, tunnels), utilities (e.g. water, electricity), and industry (e.g. chemical processing, oil and gas production). Sensors can be divided into two main categories: those that determine corrosivity of the environment in contact with the structure of interest, and those that directly measure corrosion of the structure itself. In the first case, sensors return valuable information about the corrosivity of the local environment, either by measuring parameters that indirectly relate to corrosion phenomena, or by measuring self- corrosion of an element within the sensor unit itself. Sensors of these types may be used to monitor the environment of structures in immersion in natural or industrial waters, buried underground, embedded in concrete, exposed to atmosphere, or exposed to specialized industrial environments. In the second case, information about the actual condition and corrosion behavior of a structure, such as corrosion potential, coating condition, or loss of wall thickness, can be obtained. Certain corrosion sensor tech- nologies and combined sensor units have the capability of monitoring both the structure and its envi- ronment. A series of examples of corrosivity and corrosion sensor developments of the last decade is discussed here. The list is not intended to be exhaustive but rather to provide a brief overview of de- velopments in sensor and monitoring technology.
CORROSIVITY MONITORING
Indirect Corrosivity Measurements
A variety of sensors have been developed to monitor environmental characteristics known to affect corrosion. Such characteristics can include temperature, relative humidity, oxygen levels, dissolved or atmospheric ionic contamination, and solution pH. Although there have been many methods over the years to monitor these types of system characteristics, improvements are still being made. The use of new "smart" materials such as advanced ceramics, composites, and polymers, for example, have led to successful improvements in monitoring of a number of corrosivity parameters in the oil and gas indus- try. 1 Research has shown that the direct monitoring of moisture i
