# Application of Metal Magnetic Memory Testing Technology to the Detection of Stress Corrosion Defect
## 2 Experimental Section
### Description of Corrosion and Detection System  
The schematic of the set-up for the corrosion system is shown in Figure 1(a). The electrochemical method was used to accelerate the corrosion of the specimen. The working electrode is the specimen connected to the positive electrode of the direct current (DC) power source (UTP3303, produced by UNI-TREND Technology (China) CO. LTD). The copper sheet was served as a counter electrode connected to the negative electrode of the DC power source. There is no direct contact between the electrolyte and the specimen. The electrolyte contacts the surface of the specimen placed on a plastic block above the electrolyte level through a specially shaped sponge (Corrosion area = 175 mm\({}^{2}\)). The electrolyte for the corrosion process was 3.5% (wt.%) NaCl solution. In corrosion experiments, the specimen is corroded by galvanostatic corrosion of 0.4 A. Two [PERSON]'s laws of electrolysis govern the mass of anodic material corroded or consumed when the current passes through the circuit. \[\vartriangle m=KQ=Klt \tag{1}\]  
Figure 1: Geometry and dimensions of the specimen used in the experiments and scanning lines (in mm). Figure 2: Schematic of the electrochemical corrosion system and the 3D Magnetic Flux Leakage (MFL) detection system. \[K=\frac{M}{nF} \tag{2}\]  
Hence, the mass of substance (\(\Delta m\), theoretical):  
\[\triangle\ m,\ \text{theoretical}=\frac{3600 Mlt}{nF} \tag{3}\]  
where \(K\) is electrochemical equivalent; \(n\) is the valence of the substance; \(M\) is the atomic mass or the molecular weight of the substance; \(F\) is [PERSON]'s constant, 96,494 C/mole; \(I\) is the current in the electrochemical reaction; \(t\) is time in hours. The corrosion experiments were divided into four stages: Accelerated corrosion was performed by the galvanostatic corrosion method using 0.4 A current for 2 h in the first corrosion stage, and the corrosion products of specimens were rinsed off carefully. Then the specimens were dried in warm air and weighed with an electronic balance to calculate the mass loss \(\Delta m\). The geometric dimensions (width and depth) of corrosion defects at every scanning line of specimens were measured to calculate the average value of them. After that, the specimens were corroded again for 2 h and repeated the above procedure until the specimens were corroded for 8 h.  
Corrosion at different stages are given in Table 2. Photographs of the corroded specimen, at various corrosion stages during the experiment, and the schematic of corrosion defect are shown in Figure 3. Variations of mass loss of the specimen (\(\Delta m\), measured and \(\triangle m\), theoretical) and defect feature (depth \(h\)) with corrosion time (\(t\)) are shown in Figure 4. It is found that the defect depth is associated with mass loss (\(\triangle m\), measured). With the increase of corrosion time, the defect depth and mass loss (measured and theoretical) have the same variation tendency. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multicolumn{4}{c}{**Corrosion Stage**} \\ \cline{2-5}  & **I** & **II** & **III** & **IV** \\ \hline Current \(I\) (A) & 0.4 & 0.4 & 0.4 & 0.4 \\ Corrosion time \(t\) (h) & 2 & 4 & 6 & 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Corrosion at different stages. Figure 3: (**a**) Photographs at various corrosion stages of the corroded specimen: (I) 2 h after an experiment is started; (II) 4 h after an experiment is started; (III) 6 h after the start of the experiment; (IV) 8 h after the start of experiment; (**b**) schematic of corrosion defect. The three-dimensional (3D) MFL detection system involves three main parts: a 3D scanning console system, a commercial EMS2003 metal magnetic memory testing apparatus, and a non-magnetic scanning platform. A Hall probe with a resolution of 1 A/m is a part of the testing apparatus, which was fixed on a non-magnetic 3D scanning console system controlled by a computer moving automatically at a uniform detection speed (90 mm/min).