Corrosion evaluation and monitoring practices

Abstract

Corrosion tests are aimed at to evaluate and select suitable materials for process, evaluation of corrosivity of environment, and to check the effectiveness of the applied corrosion control measures. These tests may be performed in laboratory, in actual process plant, or in the field such as atmosphere tests. The laboratory tests are generally carried out using small specimens, small volume of solution (simulated synthetic or from actual process). These are useful and economic to screen out the most useful or useless materials for the process orto test further in plant environment. The actual in-plant test needs plant availability for the desired test and may take certain time and cost. There are several methods being used for evaluating corrosion. Laboratory test methods may be categorized as an electrochemical, which are based on the measurements of current evolves from electrochemical reactions, while non-electrochemical techniques are based on the overall weight lost by the specimen or surface features due to corrosion such as pit, crevice, crack or microstructural changes appeared after the test. However, for choosing appropriate method one needs to know about the type of corrosion against which resistance of materials is being sought. For instances, to determine the thinning rate of a steel pipe, methodologies for general/uniform corrosion such as weight loss, polarization resistance etc. will be more useful than the others. The knowledge of a parameter of concern that defines the resistance of materials against specific form of corrosion is also important. For example, pitting and protection potentials (obtained in electrochemical experiments) are characteristic features (parameters of concern) to illustrate the pitting susceptibility of any passivating alloy in a corrosive environment. Several methods being practiced to evaluate different types of corrosion are discussed in forthcoming sections.