Monitoring weight loss coupons

The techniques used for monitoring corrosion in a process plant are (i) visual inspection (ii) weight loss coupons (iii) electrical resistance probes (iv) measurement of corrosion potential (v) linear polarization measurement (vi) hydrogen probes (vii) thickness measurement and crack detection (viii) visual inspection (ix) sentinel holes. 

Long-term corrosion impact is generally indicated by system integrity studies in which maintenance and inspection teams are tasked to find leaks and failures. Some large utilities have a specialized corrosion team to monitor the water quality, using corrosion loops in which treated water circulates over weight-loss coupons. These coupons are often made from different materials and exposed to various water flow rates. The coupons would be periodically measured to determine average corrosion rates. 

In addition to classical methods such as weight loss (via coupons), solution analysis for dissolved metal, monitoring of gas evolution (in the case of H2) and change in metal resistance, a select number of electrochemical techniques are widely accepted. This section briefly describes these methods. Weight-loss coupons are still the traditional, accepted baseline for comparisons electrical resistance (ER) and linear-polarization resistance (LPR) are the most widely used electrical and electrochemical techniques. Electrode potential monitoring is sometimes a valuable way of following active/passive transitions. There continues to be an emphasis on the use of non-destructive testing (NDT) techniques (particularly ultrasonic types). 

Proper field monitoring and inspection techniques can be used to measure the real corrosion rate, to find safety risks on time and to avoid corrosion accidents. The field monitoring and inspection techniques normally include weight-loss coupons, electrical resistance probes, ultrasonic wall thickness measurements, infrared thermal imaging systems, etc. Ultrasonic wall thickness measurement is widely used to monitor the general and erosion corrosion rates of the equipment and pipelines in crude oil processing plants. 

If changes have been made to the process (e.g. if incoming water quality cannot be maintained or other uncertainties arise concerning the corrosion behavior of the construction materials) it is possible to incorporate coupons or probes of the material into the plant and monitor their corrosion behavior. This approach may be used to assist in the materials selection process for a replacement plant. Small coupons (typically, 25 x 50 mm) of any material may be suspended in the process stream and removed at intervals for weight loss determination and visual inspection for localized corrosion. Electrical resistance probes comprise short strands for the appropriate material electrically isolated from the item of plant. An electrical connection from each end of the probe is fed out of the plant to a control box. The box senses the electrical resistance of the probe. The probe s resistance rises as its cross-sectional area is lost through corrosion. 

The most common method of corrosion monitoring is the direct weight loss method, via small metal coupons. In addition, there are a number of electrical methods, including developed potential measurement,... 

Corrosion coupons (intrusive). In what is perhaps the simplest form of corrosion monitoring, small specimens are exposed to an environment for a specific period of time and subsequently removed for weight loss measurement and more detailed examination. Even though the principle is very simple, there are numerous potential pitfalls, which can be avoided by following the recommendations of a comprehensive ASTM guide (ASTM G4 standard). 

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