Corrosion inspection methods

A particularly insidious failure mechanism that is commonly found in carbon-steel tubing is under-deposit corrosion. In many cases, corrosion products fomi a scab that can mask the presence of the pitting, making it difficult to quantitatively assess using conventional NDT methods. However, by combining proper cleaning procedures with laser-based inspection methods, the internal surface of the tubing can be accurately characterized and the presence of under-deposit corrosion can be confirmed and quantified. 

No one method for corrosion inspection is sufficient in itself and it is extremely dangerous to rely on data provided by one method only. A study is required of all methods available and the most suitable then chosen —... 

In reality, the identification of one or more forms of corrosion requires visual observation, nondestructive inspection methods, optical microscopic examination, and sometimes electron scanning microscopy, etc. The first study of the corrosion appearance of a case should divide corrosion into uniform and localized corrosion. Localized corrosion can be further identified as macroscopic or microscopic local corrosion. Microscopic attack refers to a minute amount of dissolved metal, accompanied by considerable damage, before the phenomenon becomes visible to the naked eye. 

General corrosion control, prevention, and monitoring should be planned for from the onset. To detect and monitor corrosion, various methods must be used, while corrosion protection would be done by the addition of inhibition and use of protection coatings [5]. Visual examination, laser methods, the replication microscopy liquid penetration testing method, magnetic particles testing, the eddy current inspection method, acoustic emission technique, thermal methods of inspection, and nondestructive methods are the various methods to be adopted when monitoring pipeline corrosion [5]. 

Inspection requirements and methods must be defined for the TBS. Inspection is assisted by the transparency of the molten salts, but still must be performed at elevated temperature. Visual inspection is assumed to be possible using actively cooled, submersible camera systems with sapphire viewing windows. Other inspection methods may also be adaptable to the TBS but have not as yet been evaluated. Here it is assumed that corrosion of carbon-based materials (particularly C/C 0-rings) will be sufficiently slow that inspection of flexible C/C seals between blocks will not be required over the life of the TBS. 

For those individuals who may desire to learn more about HTHA, please review items 4, 5, 6, and 7 within the technical findings section of the CSB report. It is paramount to understand the difficulty of inspection and identification of HTHA. The corrosive condition can be microscopic and may be present only in small, localized areas of equipment. Inspection methods are umeliable. There are few inspectors who have the techniques and expertise to identify HTHA. 

Traditional inspection methods have known limitations (such as lack of accessibility to difficult- to-reach areas), can involve personnel safety concerns, and are not always reliable (such as operational inaccuracies due to scale formation, noise, or vibration) all of these add a measure of uncertainty to the effectiveness of the inspection and raise concern about the possibility of undetected corrosion problems in areas that cannot be inspected. For a comparison of monitoring techniques, see Table 3.1. 

ASTM G 49, Recommended Practice for Preparation and Use of Direct Tension Stress-Corrosion Test Specimens— This presents the design, preparation, and use of ASTM standard tension test specimens for investigating susceptibiUty to SCC. Specimens are generally tested in a selected environment imder axial load conditions until fracture occurs. This standard describes the apparatus for providing the load to the stressed specimens along with exposure considerations and inspection methods. 

In general, the most widely used field technologies for inspection are ultrasonic thickness measurement, while for online methods they are corrosion test specimens, electrical resistance, and linear polarization probes. Both of the inspection methods and the first two online methods measure metal loss. The last method measures corrosion rate, but only in a sufficiendy conductive process environment, normally water. 

Measurement sensitivity divides the metal loss methods. Ultrasonics and radiography are usually considered as inspection methods. Typically, ultrasonics has a measurement resolution of around 50 pm (0.002 in.), and radiography 250 p,m (0.010 in.). Consequently, these types of measurements are typically made annually. Corrosion test specimens (coupons) assess metal loss typically over a one to three month interval. Electrical resistance probes, as an automatic coupon, assess metal loss typically over a few hours to a few weeks. 

The helicopter borne infrared sensors were employed [i4] to inspect power lines for aluminum corrosion. This method is suitable for detection of severe corrosion with many strands distorted and bulging. Eaurly corrosion damage cannot be effectively detected. 

Curing time and temperature requirements Inspection methods for coated surfaces Thickness control method Weather resistance Corrosivity Ease of repair... 

Acoustic leak detection methods such as illustrated in Fig. 8.4 may find already broken or damaged pipes [7]. However, corrosion monitoring possibly may identify areas where corrosion activity on pipes is likely while the remote field effect can inspect pipes to find damage before they fail. A complete diagnostic program is likely to use all these inspection methods. 

Inspection techniques for the detection and measurement of corrosion range from simple visual examination to nondestructive evaluation. Significant technological advances have been made in the last decade. For example, the combined use of acoustic emission (AE) and ultrasonics (UT) can, in principle, allow an entire structure to be inspected and growing defects to be quantified in terms of length and depth. Advanced corrosion monitoring methods have been developed that have both online capabihty and the abihty to detect problems at an early stage. The oil and gas production and petrochemical industries have assumed a... 

TABLE 6.5 Summary of Corrosion Mechanisms Detected by in-Service inspection Methods in LWR, BWR, and PWR systems... 

Projection radiography is widely used for pipe inspection and corrosion monitoring. Film digitisation allows a direct access to the local density variations by computer software. Following to a calibration step an interactive estimation of local wall thickness change based on the obtained density variation is possible. The theoretical model is discussed, the limitations of the application range are shown and examples of the practical use are given. The accuracy of this method is compared to results from wall thickness measurements with ultrasonic devices. 

Projection radiography has long been used for pipe inspection and corrosion monitoring. In this traditional tangential wall thickness estimation the distance of border lines of the projected wall shadows of a pipe onto the film is a direct measure for the wall thickness. This method is not considered here, newer developments can be found in / /. 

A method that would enable detection and classification of the bone content in 100% of the incoming frozen fish blocks in real time without delaying the production line was desired. An obvious solution to this was the use of X-ray inspection. X-ray inspection however requires the integration of a highly sensitive inspection equipment with the corrosive atmosphere associated with food processing as well as the tough restrictions imposed by the health authorities regarding irradiation of food. 

---