1. 研究目的与意义（文献综述包含参考文献）

Our industry today is highly concern with corrosion as a major problem, since it involves a high amount of mechanical failure and huge sums of money loss are encountered. In a laboratory way, corrosion has been simulated and modelled as pure metal loss with defined shapes or regions. However, real corrosion has chemical and physical changes, along with electrical and thermal property changes in the coating layer that should be taken into account in corrosion characterization [1]. A review of current literature shows that thermography is applicable to a wide range of materials [2]. Thermography in non-destructive testing and evaluation (NDTE) technique can be divided into two categories, which are; the passive and active approaches. The passive approach tests materials and structures which are naturally occurring at different (often higher) temperatures than the ambient background, while in the case of active thermography, an external heating stimulus is used to induce relevant thermal contrast. The active approach to thermography has numerous applications in NDTE. Moreover, since the characteristics of the required external stimulus are known, i.e. heating time applied to the sample, quantitative characterization becomes possible. The two categories of heating techniques applicable to NDTE defect detection are; principal component analysis (PCA) and independent component analysis (ICA) [2, 4, 6] . Pulsed Eddy current thermography involves the application of a high frequency (typically 50#8211;500 kHz) electromagnetic wave to the material under inspection, and the ability to acquire quantitative information about defect geometry, i.e. angle, depth and length, are important for the accurate description of a defect [3]. A good number of techniques can be used in metal surface corrosion characterization. Electrochemical impedance spectroscopy (EIS) is a technique, that can be used to test corrosion rate and to investigate the protective properties of a coating [5] and can mostly be used on metals with a very high melting point such as steel. However, this pulsed excitation simultaneously generates numerous frequencies on the sample during the testing. According to the inspection frequency and skin-depth relationship, PEC can penetrate different depths versus different frequencies comparing to EC with single frequency sinusoidal and providing several useful parameters such as defect size and location [6]. Eddy current methods are sensitive to surface and sub-surface defects, whereas the detection range is restricted by penetration depth [7]. These are reasons why thermography is one emerging method, with advantages over the previous methods of being a fast, non-contact, real-time measurement method over large areas. However, it is currently applied on a larger base to samples in the lab instead of it being used on in situ structures, and the high heat absorption rate of the surface and specific excitation techniques are required for different applications. From these points it can be seen that generally, different NDTE methods have different advantages and disadvantages. As such the combination of complementary non-destructive methods can achieve a better performance. In this paper, the underlying phenomena of eddy current pulsed thermography technique is prevailed due to the signature temperature characteristics. Moreover, the system associated with the technique and related heating parameters is discussed, provided with the illustration of system setup. Finally, a brief overview about the approaches of the technique as a viable NDTE Technique for industrial inspection has been discussed following the conclusion of the review towards the characterization based on eddy current pulsed thermography for metal surface corrosion.

2. 研究的基本内容、问题解决措施及方案

Under the condition of no prior knowledge, the Skewness is quantified to define the extent to which a distribution differs from a normal distribution of the data obtained from a sample and hence will be used to classify the specimens with different corrosion stages, and the skewness results will be theoretically analyzed. The rationality of the skewness algorithm is explained by combining the physical principles. Corrosion is a big problem in many industries, and ECPT technology has broad application prospects in the field of material surface/subsurface defect detection. It has great potential to apply it to the research on corrosion state.