Corrosion Monitoring

Abstract

Now we know that it is impossible to prevent corrosion of many metals used by us because they are thermodynamically unstable in the environment. If corrosion of metals is inevitable, we want, of course, to know how long equipment or structure will serve safely and efficiently under particular conditions. In other words, if we know the corrosion rate and corrosion type, how may we predict the life longevity of the equipment?When should we carry out shutdown, repair, or replace it? To answer these questions, corrosion monitoring is carried out. Most corrosion damage occurs during relatively short periods of time, usually during startup, changes, upsets, or shutdowns. Therefore, corrosion monitoring methods must supply the realtime (immediate) information. To use the metallic constructions without corrosion monitoring is similar to living and working with closed eyes and ears. Why else do we have to monitor corrosion of metal equipment and structures in industry? Various electronic devices are used. Their reliable functioning depends on the quality of electric contacts made of different metals and alloys. If corrosive species are present in the environment, electric contacts may corrode, reliability of the data received by these devices diminishes, and corrosion may result in malfunctioning and unpredicted disasters. This is especially important in aircraft, space vehicles, military technique, etc. Therefore, we should monitor corrosive aggressiveness of the environment where electronic or other devices are in contact with it. The third reason for corrosion monitoring relates to preventive anti-corrosion measures. Various methods are used for corrosion control. Corrosion inhibitors, neutralizers, anti-scaling inhibitors, biocides, demulsifiers, and oxygen scavengers are used for prevention of or decreasing corrosion.We must know the efficiency of these anti-corrosive chemicals. When cathodic protection is used for corrosion control of underground and underwater equipment and structures, we should know the efficiency of cathodic protection. If coatings are used for corrosion control, we have to know their efficiency and protective properties. In other words, corrosion monitoring helps us to define the efficiency of the corrosion control methods. To sum up, there are three important questions in industry connected with corrosion monitoring. The first question is a “qualitative” one. Does corrosion occur? Is there any equipment or structure suffering from corrosion? Many side issues exist.What is the form of the corrosion? Is the corrosion uniform or localized? Here is a wide spectrum of corrosion phenomena: pitting, crevice, SCC, galvanic corrosion, dealloying, intergranular corrosion, erosion, cavitation, MIC, hydrogen damages, and fretting corrosion. We have to define the form of the corrosion in order to take correct preventive measures. The second question relates to the previous one.What are the reasons for and factors influencing corrosion? Certainly this question is connected with the selection of anticorrosionmeasures. The third question is a “quantitative” one.What is the corrosion level? What is the corrosion rate? The latter is the corrosion of material occurring in the unit of time. This is not always the corrosion product quantity. Corrosion may change with time and with distance on a metal surface. Even if we know the corrosion level, that is if we received a corrosion rate of 0.01 mm/year, or 0.1 mm/year, or 1 mm/year, the following questions remain. Is this value low or high? Is the situation dangerous or may we exist with such a corrosion rate? May we continue to use the equipment? Or it will be better to stop using it, to repair, or to take measures for corrosion control? Certainly, the attitude to all these things is always relative. If one hair remains on our head, this is little. If one hair is floating in a plate with soup, this is many! A similar situation exists with the evaluation of corrosion rate. Sometimes a corrosion rate of 0.1 mm/year is a high value. Sometimes a corrosion rate of 0.5 mm/year is allowable. For example, for heat exchanger tubes made of carbon steel with a wall thickness of 2.3 mm and a life design period of 15 years, a corrosion rate of 0.1 mm/year is a critical or allowable value. For carbon steel pipes with a wall thickness of 9 mm and a life design period of 15 years for water supply, a corrosion rate of 0.5 mm/year is an allowable value. This means that the third (“quantitative”) question concerns the classification of corrosion rates for a particular industry, for specific particular equipment and structures. We may find different classifications for maximum corrosion rates in the chemical and oil industries. For example, the allowable (acceptable, maximum, critical) corrosion rates shown in Table 5.1 are recommended for particular equipment in the chemical industry.