# Application of Metal Magnetic Memory Testing Technology to the Detection of Stress Corrosion Defect
## 4 Quantitative Evaluation Parameters  
To further analyze the relationship between the MFL signals and corrosion degree, two characteristic magnetic parameters \(S_{\rm n}\) and \(S_{\rm t}\) were defined. The parameters \(S_{\rm n}\) and \(S_{\rm t}\) were extracted from the MFL signals of representative scanning line (Line 3) in experiment 3 (corrosion after loading), and the calculation methods of magnetic characteristic parameters are shown in Figure 13. For the  
Figure 12: Schematic diagram of MFL signals in three experiments: (**a**) experiment 1 (static tensile); (**b**) experiment 2 (corrosion without loading); (**c**) experiment 3 (corrosion after loading). normal component presented in Figure 13a, the parameter \(S_{\rm n}\) can be obtained directly. The parameter \(S_{\rm n}\) is calculated by taking the signal difference between the peak and trough of the \(H_{\rm n}\). However, the tangential component \(H_{\rm t}\) presented in Figure 13b and the parameter \(S_{\rm t}\) cannot be obtained straightforwardly. Figure 13c shows the gradient of \(H_{\rm t}\), the position of the peak and trough of the gradient, A and B, can be obtained directly. Therefore, the parameter \(S_{\rm t}\) can be calculated by taking the distance from the peak position of \(H_{\rm t}\) to line AB. Variations of characteristic magnetic parameters (\(S_{\rm n}\) and \(S_{\rm t}\)), mass loss of the specimen (\(\triangle m\), measured and \(\triangle m\), theoretical), and defect feature (corrosion depth \(h\)) with corrosion time (\(t\)) are shown in Figure 14. It is found that magnetic characteristic parameters, mass loss, and corrosion depth have the same variation tendency. \(S_{\rm t}\) is considered to be the best parameter for quantitatively evaluating the degree of the corrosion among the four characterization parameters. Figure 13: The calculation methods of magnetic characteristic parameters: (**a**) calculation method of \(S_{\rm n}\); (**b**) calculation method of \(S_{\rm t}\); (**c**) gradient of \(H_{\rm t}\). The relationship between the parameter \(S_{\rm t}\) and the corrosion time \(t\) is shown in Figure 15. It is found that the parameter \(S_{\rm t}\) and the corrosion time \(t\) have established a good linear relationship. After linear fitting, the equation defining the dependence of \(S_{\rm t}\) on \(t\) was found, and the correlation coefficient R\({}^{2}\) = 0.999:  
\[S_{\rm t}=-0.0476+1.3047t \tag{5}\]  
The direct dependence of mass loss (\(\Delta m\), theoretical) on the parameter \(S_{\rm t}\) was obtained by Equations (3) and (5) as follows:  
\[\triangle\,m,\ \mbox{theoretical}=\frac{3600(S_{\rm t}+0.0476)}{1.3047 nF}MI \tag{6}\]  
At the same time, the width change of the corrosion defect is small in the corrosion test, so the area of the corrosion area \(S\) is considered unchanged. The corrosion depth \(h\), theoretical can be calculated from the mass loss \(\triangle m\), theoretical:  
\[h,\ \mbox{theoretical}=\frac{\Delta m,\ \mbox{theoretical}}{\rho S} \tag{7}\]  
where \(\rho\) is the density of the specimen. The direct dependence of corrosion depth (\(h\), theoretical) on the parameter \(S_{\rm t}\), \(h\), theoretical \(=f(S_{\rm t})\), was obtained by Equations (6) and (7) as follows:  
\[h,\ \mbox{theoretical}=\frac{3600(S_{\rm t}+0.0476)}{1.3047 nF\rho S}MI \tag{8}\]  
Figure 14: Variations of characteristic magnetic parameters (\(S_{\rm n}\) and \(S_{\rm t}\)), mass loss of the specimen (\(\triangle m\), measured and \(\triangle m\), theoretical), and defect feature (corrosion depth \(h\)) with corrosion time (\(t\)). Figure 12:0 The relationship between the parameter \(S_{\rm t}\) and the corrosion time \(t\).