# Application of Metal Magnetic Memory Testing Technology to the Detection of Stress Corrosion Defect
## 1 Introduction  
Many types of equipment manufactured with ferromagnetic materials in service are working under the condition of stress corrosion. Most of this equipment is susceptible to corrosion defects. The effect of pitting corrosion on pipeline steel has been studied by many scholars [1; 2; 3; 4]. [PERSON] and [PERSON] reported the failure of oil and gas pipelines due to stress corrosion cracking [5; 6]. It is necessary to conduct Non-Destructive Testing (NDT) for stress corrosion defects. The common NDT method for the quantitative evaluation of corroded defects, such as Electrochemical Noise Analysis (ENA), has been studied by many scholars [7; 8; 9]. It was used to investigate the corrosion behavior of mild steel (Q235) [10], and [PERSON] reported that pitting corrosion of high strength steel is quantitatively evaluated by combining 3-D measurement and image-recognition-based statistical analysis [11]. However, the reliability for assessing the sizes of corrosion defects needs to be further discussed. The traditional magnetic testing technologies [12; 13; 14], such as magnetic particle testing [15; 16], magnetic flux leakage testing [17; 18; 19], usually require a large external magnetic field [20; 21; 22], which is inconvenient to operateand even impractical under some conditions. Although the non-destructive testing of stress corrosion defects has been widely studied, the traditional NDT methods have some limitations. Therefore, it is necessary to develop a more simple and effective NDT technology.  
The Metal Magnetic Memory Testing (MMMT) technology, which was first presented at the 50 th International Welding Conference in 1997 by a Russian researcher, is an emerging testing method in the field of non-destructive testing technology [23; 24]. It is a weak-field detecting method, which uses the geomagnetic field instead of an artificial magnetic field as the stimulus source. MMMT technology is used to diagnose the early damage by measuring the self-magnetized leakage field on the surface of ferromagnetic materials [25; 26]. MMMT technology has the advantages of simple equipment, simple operation, and no treatment on the surface of the tested part. Nowadays, the MMMT technique has attracted the attention of scholars in many industry fields, such as machinery, pressure vessels, pipelines, etc. [27; 28]. It is widely studied by many scholars for its advantages over other magnetic testing methods [29; 30; 31], and the stress-magnetism effect has also been widely studied [32; 33; 34]. The impact of stress or strain on Magnetic Flux Leakage (MFL) signals of ferromagnetic materials has been researched for assessing stress concentration [35; 36] and fatigue damage [37; 38; 39]. The previous study [40] showed that the ambient stress conditions during corrosion defect formation would affect the MFL signals based on magnetic flux leakage testing. In recent years, MMMT was applied to the detection of rebar corrosion in concrete and the commonly used steel strand in structural engineering to locate the corrosion area and evaluated the corrosion degree [41; 42]. However, there is still a lack of research on stress corrosion. Therefore, it is very promising to detect stress corrosion defects by MMMT technology.  
In this paper, three experiments were performed to study the effect of corrosion on magnetic leakage signals. The electrochemical corrosion system was designed for corrosion experiments. The distribution regularity of MFL signals during the corrosion process was investigated. The feasibility of evaluating the degree of corrosion of ferromagnetic materials using the MFL signals was discussed.