Corrosion NDT technologies and Capacitive imaging

                                                                          State-of-the-art systems currently in commercial deployment  for corrosion detection.

                                                                          Current state-of-the-art (SoA) NDT techniques for detecting the presence of corrosion on  metallic surfaces, often suffer losses in effectiveness, or are even rendered completely  ineffective, by the presence of protective coatings such as paint. Even though the NDT  techniques employed in detecting CUP are mature, they still tend to be highly labour intensive and require extensive preparation beforehand. There is therefore a continuing  drive to increase the speed of inspection, to reduce the preparation required and, if  possible, to inspect without the need in some cases, to shut down operation. With some  notable exceptions, it is possible to find most types of defect. However, it is frequently  not cost effective to do the inspection, so driving down overall inspection costs is a major  research and development aim in the NDT industry. In the SoA overview below, the  suitability for detection of corrosion under paint is evaluated for each of the current  inspection techniques.

                                                                              1. Visual NDT. Visual and optical testing is by far the most common NDT method of  corrosion detection. An experienced inspector equipped with basic tools as a  magnifying glass and a hand-held torch can conduct a effective inspection. For  difficult to access areas, a boroscope (miniature camera attached to a fibre-optic  cable) can also be used. In maintenance operations, visual testing and optical  inspection is usually the first line of defence. After initial detection, secondary follow up techniques include Electronic Speckle Pattern Interference (ESPI) for early  detection of de-cohesion of paint coatings and optical surface topography systems such  as laser interferometry can be used. Such enhanced visual/optical methods mainly detect and measure deformations on surfaces which are caused by sub-surface  corrosion damage such as blistering and pitting.  

                                                                          Suitability for CUP Detection: Visual NDT inspection does not use complicated  equipment and is the usual first step in the detection of CUP by the location of  porosity, bead contouring and blistering. The limiting factor is the resolution ability of  the human eye which has difficulty determining porosity diameters less than 0.25 mm  or cracks less than 0.025 mm wide. Also, objects nearer than 150 mm - 250 mm are  hard to focus on and lighting conditions must also be adequate for good visibility. Only  surface discontinuities can be detected with this method and another major  disadvantage is that the surface must be cleaned of contaminants to prevent masking  of those surface discontinuities.

                                                                              2. Ultrasonic Inspection. Ultrasonic NDT is a very broad area but essentially most  ultrasonic inspection techniques use frequencies in the range (1-10) MHz, and as the  propagation speeds of various wave modes are a function of the material’s various  elastic moduli and density, this make ultrasound a very powerful technique for  materials characterisation. Strong reflections occur at boundaries where material  properties change make the technique highly appropriate for thickness measurements  and crack detection, both of which could be manifestations of corrosion.  

                                                                          Suitability for CUP Detection: Near surface ultrasonic inspection methods (such as  Lamb or Rayleigh) could work in most instances but usually require full intimate  contact and impedance-matching coupling with the inspected surface which is a very  time-consuming process . Moreover, reasonable specification ultrasonic flaw detectors  can cost anything upto €20K and require many days of expensive operator and/or  contractors time to cover even small areas. 

                                                                              3. Magnetic Particle Inspection (MPI). A magnetic field is applied to the painted metal  surface using a permanent magnet, electromagnet, flexible cables or hand-held  probes. If the material is homogenous then, most of the magnetic flux is concentrated  below the material's surface. However, if any material discontinuity is present, the  magnetic flux is distorted and locally 'leaks' from the surface in the immediate region  of the defect. Fine magnetic particles (usually powder) which are applied to the  surface of the specimen are attracted to the area of flux leakage producing a visible  indication of the flaw. The powders are usually black iron particles and red /yellow  iron oxides. In some instances, the iron particles are coated with a fluorescent  material to allow them to be viewed under a UV lamp for even easer detection.  

                                                                          Suitability for CUP Detection: magnetic field-gradients associated with corrosion pitting on the surface of steel are usually far smaller than the magnetic field gradients associated with cracks and it is a well-observed empirical fact that even  with cracks, magnetic particle methods are of little use when said surface cracks are  covered in paint thicker than about 75 microns and most methods need a supply of  electricity and requires a skilled operator for interpretation of most measurements.

                                                                              4. Traditional Eddy Current Techniques. Eddy current testing (ET), uses alternating  currents applied to a conducting coil held in close proximity to the object under  inspection. In accordance with Lenz’s Law, the inspected object generates eddy  currents to oppose the alternating current in the coil. The eddy currents are then  detected by the same coil, separate coils, or magnetic field-sensors. Changes in the  induced eddy currents may be caused by changes in a material’s electromagnetic  properties, variations in material thickness or the sharp discontinuities caused by the  presence of cracks. Again, these are all phenomena which may also be associated with  the presence of CUP. Current state-of-the-art ET apparatus is now extremely portable and ET It is the second most common method specified for the routine NDT of aircraft.  Recent technological advances in this field which have rapidly increased both  efficiency and speed of detection, are multichannel sensors and the replacement of  search coils with Giant Magneto-resistive (GMR) magnetic field sensors (originally  developed for computer hard drives).  

                                                                          Suitability for CUP Detection: On aluminium surfaces for instance, using conventional  scanned eddy-current transducers is time consuming and ineffective as variations in  paint thickness overlaid on a corrosion-free section often produce “lift-off” signals  which are essentially indistinguishable from signals produced from paint layers of  uniform thickness which do conceal areas of corrosion on the underlying surface. Large  surface areas are also a problem and need some kind of area scanning device which  can be expensive and also very limiting in terms of speed of scan. Also the more  complicated the geometry of the surface becomes, the more difficult it is to  distinguish between real defects and corrosion.

                                                                              5. Other Eddy Current Techniques. Conventional eddy current instruments measure the  a.c. impedance of the detection coil, whilst pulsed eddy current instruments measure  the transient voltage signal with a frequency spectral content which ranges from  practically 0 to 100 KHz or higher. This broadband response enables various digital  signal analyses to be performed and to differentiate between subtle variations in  waveforms associated with the small fluctuations in geometry that are typical of CUP.  Remote field eddy current scanning is another variation of conventional eddy current  inspection that has found widespread use in attempting to detect hidden corrosion.
                                                                          Magneto-optical imaging of eddy currents is another relatively recent advance in  eddy currents applied to the detection of hidden corrosion. By using the Faraday  (which rotates the plane of polarisation of light) it is possible to image eddy currents and produce real-time images of to quickly detect subsurface corrosion  and cracking.  

                                                                          Suitability for CUP Detection: In general, eddy current techniques do not have as  much sensitivity and spatial resolution as say ultrasonic NDE techniques and can  only inspect relatively small scan areas. Although remote-field eddy current  scanning is not sensitive to liftoff - the downside is that an ultra-sensitive eddy  current system is also required to handle the low amplitude signals obtained from  remote field eddy current probes. Magneto-optical imaging of eddy currents shows  great promise but the current state-of-the-art with this approach is that it is far  more sensitive to cracks below paint than the actual surface corrosion itself  Another disadvantage is that these sensors are based on a rigid substrate and  impossible to bend and so cannot be used very easily to detect corrosion and  cracks under paint on structures having curved surfaces which makes it unsuitable  for inspection of structures such as bridges and piping.

                                                                              6. Thermography. Thermal imaging cameras are the most common sensing devices used  for this technique. However, passive imaging will not normally reveal areas of CUP if  there are no leakages or other localised sources of heat and the area under inspection  is in a state of uniform thermal equilibrium with its surroundings, meaning that no hot  or cold spots are visible. In order to be able to detect CUP, external heating or cooling  is applied in the form of the short energy pulses. Flash thermography techniques  usually use quartz lamps and thermal perturbation is then followed by a differential  time-resolved infrared image analysis. Coating defects such as blistering and sub 

                                                                          surface corrosion spots are detected in infrared images due to the differences in the  thermal diffusivity between the defective and non-defective areas. A temperature  difference of a fraction of a degree is sufficient for reliable detection of CUP. Another  significant recent advancement in thermal imaging to detect CUP is Thermal Stress Analysis (TSA) which uses mechanical energy (in the form of vibration) to stimulate  very localised heating at sub-surface discontinuities, such as corrosion-induced cracks  in metals. This has opened up a new field of application for the IR method.  

                                                                          Suitability for CUP Detection: Thermographic inspection has the advantage that it has  the potential for large areas to be inspected in real-time and thermal wave imaging  techniques are a very promising prospect for the detection of CUP. However, if water is  present under a layer of paint which is not actually covering corrosion, false  indications occur. Major limitations are that if no “hot spots” or leakages are present  then the temperature differences that produce thermal image contrast are too small  and any CUP features can become indistinguishable from good regions. To overcome  this, it is necessary to induce localised temperature differentials and the accessories  required for flash thermography (halogen lamps and hot air guns) are relatively large  and cumbersome, restrict inspection to relatively small areas and can also restrict  access in difficult to reach places. The technique is quite costly and the purchase price  of thermal imaging equipment also tends to be relatively high and often outside the  reach of all but large organisations and government agencies.

                                                                              7. Radiography. Current state-of-the-art radiography used in NDT falls into two broad  categories: x-ray radiography and isotope radiography. Both types of inspection have  the advantage that they can detect both surface and hidden flaws and that results can  be obtained with the minimum amount of sample preparation. The range of detectable  flaws extends to some of those surface features associated with corrosion and which  may also be covered by layers of paint, such as pores, inclusions and cracks.  

                                                                          Suitability for CUP Detection: X-ray radiography requires bulky and expensive  equipment, presents safety hazards to the users and other personnel and is highly  directional and sensitive to flaw orientation. Moreover, it takes a high degree of skill  and training on the part of the operator. Isotope radiography is less expensive than x rays and inherently more portable but its use is heavily legislated and requires a high  degree of safety and legal compliance and also requires highly trained and experienced  operators. Another major disadvantage with regards to detection of CUP is that  neither of these two radiographic techniques can be readily used in the back-scattered  (reflectance) mode which is necessary for single sided inspection of painted surfaces.  Cost is another issue portable x-ray equipment can be costly and spare parts such as  x-ray tubes can cost several thousand euros at a time to replace.  

                                                                              8. Microwave NDT. State-of-the-art microwave NDT inspection is currently a niche  market and theoretically microwave methods should offer several advantages for  detection and evaluation of corrosion under paint: they can penetrate low-loss  dielectric materials and are sensitive to changes associated with variations in  dielectric properties and boundary interfaces – all of which would suggest that they  are suitable for detecting the presence of layers such as corrosion under paint.  Moreover, microwave techniques are non-contact, one-sided (i.e. reflection) and  relatively fast as compared with other NDT techniques.  

                                                                          Suitability for CUP Detection: Despite great early promise, it was found that  detection of CUP based upon comparison of the amplitude and/or phase of a reflected  microwave signal with an initial calibration value was highly susceptible to errors  caused by variations in paint thickness, and sensor drift. The sensitivity to these errors  is because the mode of operation is based on the pressing up of the horn antenna  against (and in direct physical contact with) the painted surface and the formation of  a standing wave between the microwave transceiver and metal surface being  inspected. The presence of dielectric materials such as corrosion or paint shifts the  standing wave, causing a change in the amplitude and phase of the response.  Unfortunately, shifts can also result from changes in the effective altitude above the  metal surface caused by variations in paint layer thickness and the effects of dielectric layers such as corrosion and paint, and altitude above the metal surface are difficult  to separate. Also, being a relatively low-volume product, units can be difficult to  source and tend to be costly as compared to the more common types of NDT  equipment.

                                                                          Summary of Technical Limitations of Existing NDT Methods  and Techniques

                                                                          Many techniques are available for detection and monitoring defects as corrosion and  cracks under paint and coating. There is however limitations for the usability for the  purpose of cost-effective detection of corrosion under paint. From the summary of SoA  above there are limitations as described below.  
                                                                                    • Visual techniques is limited by the resolution ability of the human eye restricting the  detection of defects with diameter 0.25 mm or cracks less than 0.025 mm wide. Only surface  discontinuities can be detected and the surface must be cleaned of contaminants before  inspection.  
                                                                                  • Ultrasonic inspection require full intimate contact and impedance-matching coupling with  the inspected surface which is a very time-consuming process, and costly in terms of  equipment cost and in labour when contracting specialist inspection services which is the  usual business model. 
                                                                                  • Magnetic Particle Inspection are of little use when surface cracks are covered in paint  thicker than about 75 micron, difficult to interpret without expensive specialist consultancy.
                                                                                   • Eddy Current Techniques are time consuming and general do not have as much sensitivity  and spatial resolution as say ultrasonic NDE techniques in addition to that relatively small  areas cab be scanned. Even though magneto-optical imaging of eddy currents shows great  promise they are currently far more sensitive to cracks below paint than the actual surface  corrosion itself and do not work so well on large areas or curved surfaces – which tends to  limit its usefulness in inspecting larger structures
                                                                                  • Thermography has the major limitation that if no “hot spots” or leakages are present then the  temperature differences that produce thermal image contrast are too small and any CUP  features can become indistinguishable from good regions. The purchase price of thermal  imaging equipment is high and often outside the reach of most SMEs.  
                                                                                • Radiography. X-ray radiography requires bulky and expensive equipment, presents safety  hazards to the users and other personnel and is highly directional and sensitive to flaw  orientation. Another major disadvantage with regards to detection of CUP is that neither  radiographic techniques can be readily used in the back-scattered (reflectance) mode which is  necessary for single sided inspection of painted surfaces  
                                                                                • Microwave NDT suffers from that detection was highly susceptible to errors caused by  variations in paint thickness, and sensor drift and being a low-volume product is difficult to  source and hence quite costly. 

                                                                          Innovative Features of Capacitance Imaging Device

                                                                          The Capacitance Imager overcomes the shortcomings of existing SoA technologies and is  essentially unaffected by any of the factors which inhibit and limit the performance of all  the existing NDT technologies currently used in the field today.  

                                                                          This new NDT system represents a truly disruptive technology which goes well beyond  the state-of the-art in its ability to readily detect the presence of both deep sub-surface  hidden corrosion as well as even minor occurrences of surface scaling and corrosion under  paint. Moreover, the system operates at area scanning rates which are orders of magnitude  more rapid than existing state-of-the-art and at a fraction of the cost of existing systems. (It is even possible that one embodiment of this CI new system could also work in a stand off fashion underwater and would for instance, do away with the expensive and time consuming need for ocean-going vessels to go into dry docks to be inspected).  

                                                                          The overall innovation in this system is that it is unaffected by factors inhibiting and  limiting existing NDT technologies with scanning rates which are orders of magnitude  faster than scanning than current state-of-the-art and unlike ultrasound, eddy currents  and radiography etc, does not require intimate contact/ fixed distance with the surface  itself.                                                                          Example                                                                                                                                                         
                                                                          For example, one of the fastest non-contact scanning rates for the detection of corrosion  under paint is Eddy Current Array (ECA) scanning. One such device is an Olympus “ECA  SAA-112-005-032”. This is a 32 element array probe with one of the largest commercially  available scanning heads (112 mm wide). It scans at 5mm distant from the surface at an  average linear scanning rate of 30 mm per second. This equates to scanning a 1m2 area  every five minutes (306 seconds) .  

                                                                          In comparison, a CI unit is immune to small variations in stand-off distance and irregular  surfaces and scans at a rate at least 10 times faster.                                                                                                  frame every two seconds.  

                                                                          Overview of Operation Front-sided Capacitance Imaging  

                                                                          The basic configuration is to “open up” a standard parallel-plate capacitor so that both  electrodes are now in the same plane i.e. co-planar capacitor. An AC voltage is then  applied and an electric field distribution is established.                                                                                                                                      An object between the electrodes will affect this electric field pattern and any localised  change in sample properties will change the field distribution pattern and modulate the output signal. Correlating these changes in signal with localised surface information will therefore produce an image as the probe is moved over the surface of the object. The major practical advantage of this approach is that it is non-invasive, non-contact and only requires single sided access to the object being examined.  


                                                                          Co-planer copper electrodes are simple to fabricate via standard printed circuit board  (PCB) techniques. Aside from geometry considerations, probe performance is also  determined by other factors such as the gap between the electrodes and stand-off  distance between the probe and the surface of the inspected object. Optimum probe design for any particular application is derived from both analytical models and the results of experimental trials. Overall, and  as a general rule, it is found that field penetration depth is determined mainly by the  geometry of the electrodes and that there is a trade-off between probe size and image  resolution. And the major practical advantage for NDT applications is that it is completely  non-contact and requires access to only one side of the inspected object. 

                                                                          Corrosion and Other Defects Under Paint.  
                                                                          It is possible to image through an insulating barrier to detect things hidden on a  conducting surface underneath – as is the situation with corrosion under paint (CUP) and  the schematic of the configuration of the probe and its interaction with the specimen in  different circumstances is illustrated below.  

                                                                          Electric filed distribution through painted surface with no corrosion or cracks

                                                                          Electric field distribution for a metal sample covered with an insulating layer of paint. (b)  a metal surface containing a notch or deep crack, and (c) a simulated delamination of the insulator from the metal of a type typically associated with corrosion. 

                                                                          Baseline measurements on defect-bearing metal surfaces covered by insulating layers 
                                                                          For baseline measurement purposes and to test the validity of the models, the first test  sample was an aluminium plate, containing four separate machined 20 mm square flat bottomed holes of different depths (2 mm, 4 mm, 6 mm and 8 mm). Capacitive images of  aluminium plate, taken at stand-of distances of (a) minimal air gap – practically intimate contact , (b) 2 mm, (c) 4 mm and (d) 6 mm and (e) 8 mm. The scan was performed at the surface containing the flat-bottomed holes. 
                                                                          Scans of surface corrosion under paint. The next figure  shows a steel plate which was immersed  into brine (seawater) to half its depth and held at a high temperature for 10 days to accelerate aging. A capacitive imaging scan of this surface readily detected the main areas of where rust was present. This is almost certainly due to the change in electrical impedance properties of the oxide layers and a small contribution from the fact that the topographic profile and surface height  changes slightly too. To the left, a photograph of artificially aged corroded plate and to  the right is the capacitive image scan of same corroded steel plate.
                                                                          A second sample, which differed slightly from the first in as much as the unexposed surface was galvanized, was also treated to the same  accelerated aging process over half its surfaceand the results are presented in Figure ??. The  results are similar to the previous example the  utmost right image shows the result of a scan taken of the corroded surface through a 5mm layer of insulation and the hidden corrosion is still readily detectible and identifiable. This is a  photograph of a galvanized steel plate, immersed in brine for 10 days to cause  accelerated rusting in the lower section shown  to the left (a). In the centre (b), capacitive image taken in air, highlighting the main areas of rusting, and on the right (c) is a similar image taken through insulating coating (i.e. paint).  

                                                                          Laser Process Parameters And Surface Roughness
                                                                          Laser Process Parameters