Special Report

203Chemical Weekly July 9, 2013

Ultrasonic testing for detecting corrosionTechnical advances in ultrasonic test methods open up newperspectives for damage analysis

T he increasing use of 3D and digi- tal non-destructive test methods supplies increasingly precise

information on flaws and defects. Us-ing a stirred tank reactor (STR) as a case study, TÜV SÜD Chemie Service presents aspects that must be taken into consideration. In the case at hand, leakage caused by corrosion occurred at a junction sheet welded onto the outer jacket.

Plant operation can give rise to conditions that were not considered in the design phase and in material selec-tion. Given this, in spite of a high level of thoroughness, corrosion is repea-tedly encountered in practice – even in stainless steel and enamel-coated components. The chemical industry is particularly vulnerable to corrosion, as aggressive operating fluids and at-

mospheres challenge the resistance to chemicals of piping, tanks and valves. Subsequent changes of operating conditions to improve production ef-ficiency are a further factor promot-ing corrosion. To this end, owners/operators often increase operating para- meters such as pressure and tempera-ture. However, a 10°C increase in tem-perature will double the rate of a chemi-cal reaction and thus also accelerate the corrosion processes. Unfortunately, stakeholders do not always give suffi-cient consideration to all these facts.

The phased-array ultrasonic test method offers one possibility of iden-tifying and analysing flaws such as corrosion in solid components. This method provides reproducible results for detailed analysis of the material structure or welding joints. The phased-

array technique is a state-of-the-art pulse-echo method, which uses ultra-sound probes made up of a number of individual elements that can function separately as transmitter or receiver. By introducing a time delay in the excita-tion of the elements, test experts can steer and focus the ultrasound beam.

The test data can be displayed as A-scans, B-scans and C-scans (see Figures 2 and 3), i.e. in horizontal, vertical and top view, enabling the results to be displayed in 2D and 3D. The phased-array technology offers better reproducibility of results than manual pulse-echo techniques. Com-pared to other non-destructive test methods such as digital radiography, it also does not involve any radia-tion protection measures, which often prove cost-intensive.

Case study: stirred tank reactor (STR)

The experts at TÜV SÜD Chemie Service have successfully applied the phased-array method in areas such as damage analysis at a manu-facturer of speciality chemicals. At this manufacturer, leaks had occurred in the outer jacket of a steam-heated enamel-coated STR. The tank, built in 1971, is equipped with an agitator and

Dipl.-ing. Jens KeilHead of Materials TechnologyEmail: jens.keil@tuev-sued.de

Dr.-ing. Manuel scholzTest Engineer Materials & Testing Email: manuel.scholz@tuev-sued.deTÜV SÜD Chemie Service GmbHwww.tuev-sued.de/chemieservice

Fig.1: The phased-array procedure can be used to detect and assess flaws such as corrosion in solid components

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Chemical Weekly July 9, 2013204

processes the raw materials used for plastics manufacturing. It is made of high-temperature pressure-vessel steel P265 GH. This material is suitable for the design load – in this case, tem-peratures of between 155 and 170°C at a steam pressure of 4.5 to 5 bar (450 to 500 kPa) and service water for cooling. This type of steel generally offers good cold and hot forming properties and suit-able minimum yield strength for STR used in this segment of industry.

The leaks in the tank reactor oc-curred in the upper part of a junction sheet welded to the tank’s outer jacket. The junction sheet forms part of the support ring. Reliable root-cause analy-sis was particularly important given that the manufacturer operated further STRs of the same type under similar conditions at various locations. TÜV SÜD’s experts first developed a stra-tegy that promised successful non-de-structive testing for investigating pos-sible corrosion damage with the help of the phased-array method. One option was the immersion phased-array tech-nique (based on HydroFORM). Water coupling offers optimised coupling conditions even on rough surfaces, and

Fig. 2: The test data displayed as C-scan

A comparison of NDT methods

TOFD technique (Time of Flight Diffraction)This ultrasonic test method is particularly suitable for recognising volumi-

nous defects. The technique analyses diffracted signals rather than reflected signals and is thus highly suitable for determining the size of inhomogeneities in the material.

Digital radiographyThis radiation technique permits testing of components with coatings,

enamelled parts and systems that cannot be accessed from the outside. Special evaluation systems provide immediate results, making time-intensive develop-ment of X-ray films a thing of the past.

Phased arrayUltrasonic testing of welds increasingly relies on the phased-array method.

The key advantage of the method is the 2D and 3D display of faults in real time – also in high-speed testing. Use of the test method requires expert and qualified assessment of a broad array of information.

thus has the advantage of not requiring removal of any coatings.

Technical advancement has opened up new perspectives for imaging, and thus also for finding tailored solutions and actions that offer safety and relia-bility while ensuring a high degree of availability. Today’s experts increas-

ingly use 3D and digital methods that deliver highly accurate results even at high test speeds. However, these state-of-the-art technologies also make high demands on the expertise of test ex-perts, who require extensive knowhow in handling test equipment on the one hand and in interpreting the displayed results on the other. After all, as pre-

Special Report

205Chemical Weekly July 9, 2013

cision and resolution improve, the number of alleged flaws displayed in-creases. Experts must therefore be able to differentiate between critical results and results that pose no risk for compo-nent integrity.

Interaction between people and technology

Incorrect interpretation of results may lead to defects being overlooked or excessive refurbishment being un-dertaken. This equally applies to com-ponents that are vulnerable to corro-sion, structural components and parts exposed to dynamic loads. Imperfect interaction between the new techno-logy and the qualification of the ex-perts may cause considerable follow-up costs. Matters are further complicated by loss of testing know-how and test-ing experience at the manufacturer’s company, e.g. based on spin-off of this field of activity or increasingly fast in-novation cycles in the research of NDT applications.

To return to our STR case study, phased-array examination provided a clear result. Further material testing

Fig. 3: The test data image enabling the results to be displayed in 2D and 3D

revealed that the defect was caused by strain-induced corrosion cracking. But what exactly does this mean? At higher temperatures, a magnetite layer may form when water – and water steam in particular – attacks the iron in the steel. The magnetite layer acts as a protective coating, shielding the underlying material against further corrosion. The detected flaws were lo-cated in the areas exposed to the high-est mechanical and thermal loads and the strain induced thereby. There, the magnetite layer had been damaged to some extent, leaving these points vul-nerable to corrosive attacks caused by the cooling or service water. In the case at hand, the damaged areas re-peatedly recovered under service con-ditions. The micrographs showed that the sides of the crack were covered by a layer of magnetite. However, in the case at hand the existing magnetite layer had been repeatedly damaged to some extent, so that corrosion pro-gressed until leakage occurred.

An appropriate solution must take a multi-faceted approach. TÜV SÜD’s experts firstly recommended

that improvements should be made to the design of the support device. A change in junction-sheet design helps to avoid load peaks, and thus strain. On the other hand, the mode of operation of the STR should be changed to ensure smaller tempera-ture gradients and thus reduce ther-mal loads (and consequently strain). Based on these findings, the manu-facturer took action, replaced the junction sheets on a further six STRs and changed the service parameters of the tanks. These measures have now significantly reduced the prob-ability of corrosion and have proved that early involvement of a third-par-ty service provider saves costs and improves plant availability. How-ever, to reap these benefits, experts with long-standing experience must use the right test methods and inter-pret the results correctly.

The testing laboratory of TÜV SÜD Chemie Service offers flexible ac-creditation, enabling the laboratory to validate innovative test methods itself and provide for their fast application in practice.