Corrosion cracking failure analysis of 304 stainless steel flange connection double-head studs in coastal atmospheric environment

Corrosion cracking failure analysis of 304 stainless steel flange connection double-head studs in coastal atmospheric environment

The 304 stainless steel flange fastening double-headed stud under the safety valve at the top of the gas separation tank in the process of service corrosion cracking, through macroscopic examination, chemical composition testing, metallographic analysis, scanning electron microscopy and energy spectrum analysis, etc., the 304 stainless steel double-headed stud cracking causes were analyzed. The results show that there are two main reasons for the cracking of the stud, one is that the material is not qualified, Cr content is lower than the standard value; the second is due to more S, Cl elements in the coastal industrial atmosphere, resulting in stress corrosion cracking of the double-headed studs at work. For the failure of the cause of the corresponding preventive measures and recommendations.

Austenitic stainless steel has become an important engineering material due to its outstanding corrosion resistance, mechanical properties and formability [1], and is widely used in chemical, pharmaceutical, transportation and nuclear energy fields [2,3,4]. Among them, 300 series austenitic stainless steels have more excellent corrosion resistance due to their higher chromium-nickel content [4] and are used in a large number of applications in petrochemical equipment. Nevertheless, austenitic stainless steels are highly susceptible to corrosion in chlorine-containing environments. Stress corrosion cracking is a typical form of cracking of austenitic stainless steel in hazardous environments and is one of the most important causes of performance failure of austenitic stainless steel. Stress corrosion cracking is a brittle cracking phenomenon of sensitive materials under the synergistic action of tensile stress and corrosive media below the strength limit of the material. In the petrochemical industry, the sensitive media causing corrosion of stainless steel are mainly chloride ions and hydrogen sulfide [5,6,7].

The petrochemical industry as a high-risk industry, its process equipment in the presence of flammable, explosive, toxic media. Once the leakage failure of containers, pipelines, short connections and other equipment occurs, it will have an important impact on personal safety, environmental safety and enterprise economic benefits. Therefore, to ensure the normal operation of equipment, economic development and environmental protection has great significance. The failure analysis of the various types of failures that have occurred, to find out the causes of failure, is the basis for good failure prevention measures. The use of a series of physical and chemical inspection methods, the analysis of equipment failure components, to determine the main causes of component failure, and put forward a reasonable protection program, the actual production has an important practical guidance significance.
In this paper, we analyze the failure of a cracked 304 stainless steel double-headed stud, identify the causes of failure and propose preventive measures. The cracked double-headed studs for a petrochemical company in the top of the gas separation tank safety valve flange fastening double-headed studs. The material is 304 austenitic stainless steel, the specification is M14×90mm, the performance level is A2-70. this double-headed stud is exposed to air for a long time, the working temperature is room temperature. The double-headed stud lost its tightening force causing the flange to seal poorly and the medium to leak. During the maintenance process of the plant, it was found that a large number of stainless steel double-headed studs used in the hydrogen refining unit were in a similar situation one after another, which was potentially more dangerous. Therefore, this paper analyzes and studies the cracking phenomenon of double-headed studs.

1. Physical and chemical examination

Combined with the actual service situation of the double-headed stud, focusing on the failure analysis of the fractured double-headed stud from the macroscopic morphology, chemical composition, microstructure, fracture characteristics, etc.

1.1 Failure macroscopic analysis

Cracking location for the double-headed stud of the light pole parts, the crack and the axis is a certain angle spiral expansion, there is a certain shear damage. The crack range is large, basically through the light rod parts. At the same time, multiple pockmarks can be observed in the middle of the crack, as shown in Figure 1(a). Multiple small cracks perpendicular to the axial direction can be seen near the crack, as shown in Figure 1(b). The double-headed stud section is separated and ultrasonically cleaned, and it can be found that the section is generally flat, and the surface is covered with yellow-brown and gray-black corrosion products.

Figure.1 Cracked photo of the failed stud

1.2 Chemical composition detection

The specimens were taken away from the threaded section of the double-headed stud fracture area for spectral analysis, and the chemical composition was measured using a handheld direct-reading spectrometer as shown in Table 1. Comparison of the results with GB/T 1220-2007 “stainless steel bar” shows that the Cr content of the alloy is lower than the standard value, and the chemical composition of the failed double-headed stud is not qualified.
Table.1 Chemical composition of the failed stud (mass fraction, %)

Element	C	Si	Mn	P	S	Cr	Ni
Measured value	0.07	0.72	1.3	0.035	0.024	16.81	8.38
GB/T 1220-2007 standard value	≤0.08	≤1.00	≤2.00	≤0.045	≤0.030	18.00 – 20.00	8.00 – 11.00

1.3 Metallographic analysis

The Leica DMi8 metallographic microscope was used to conduct metallographic analysis of the failed double-headed stud. Figure 2 shows the microstructure of the axial and transverse sections of the cracked end of the double-headed stud. The microstructures are austenite and partial martensite, with intracrystalline twins and more granular carbides, some of which are distributed along the grain boundaries, resulting in the electrochemical inhomogeneity of the grain boundaries and grains. It can be found that the martensitic organization of the axial section of the double-headed stud is significantly more than that of the transverse section, which may be related to the double-headed stud forming process [8]. The grains are approximately equiaxially distributed with a relatively uniform size; the cracks are typical of cracking along the crystal; in addition to the main cracks, there are microcracks, which are also characteristic of cracking along the crystal.

Figure.2 Microstructure of the cracked end of the failed stud
(a) transverse section; (b) axial section

1.4 Fracture SEM and EDS analysis

Figure 3 shows the SEM morphology of the cleaned double-headed stud fracture. The fracture is typical of rock-like fracture along the crystal, and there are several secondary cracks. The EDS spectral analysis of the corrosion products of the original fracture of the double-headed stud, as shown in Figure 3(e,f), the specific results are shown in Table 2. compared with the chemical composition of the matrix material energy spectrum analysis (Table 1), a large number of sulfur, chlorine and other elements very harmful to austenitic stainless steel. Analysis of Table 2 shows that the highest point of chlorine content at the fracture is about 0.5%, and the highest point of sulfur content of 5%. And chlorine-containing substances are usually austenitic stainless steel stress corrosion cracking is extremely sensitive to the medium, which provides a specific corrosive environment for 304 stainless steel double-headed studs.

Figure.3 Failure stud fracture SEM morphology and corrosion EDS analysis
(a,b,c,d) fracture morphologies; (e) content peak of Cl; (f) content peak of S
Table.2 Results of corrosion product energy spectrum analysis of failed stud fracture

The highest point of Cl content			The highest point of S content
Element	Mass fraction/%	Atomic fraction/%	Element	Mass fraction/%	Atomic fraction/%
C	16.84	32.26	C	7.25	17.03
0	31.83	45.78	0	28.94	51.05
Si	1.03	0.84	Al	0.73	0.77
S	1.38	0.99	Si	1.22	1.23
Cl	0.49	0.32	S	5.11	4.5
Ca	0.17	0.1	Cl	0.34	0.27
Cr	8.53	3.78	Cr	1.97	1.07
Mn	0.47	0.2	Fe	7.46	3.77
Fe	29.97	12.35	Ni	0.67	0.32
Ni	3.14.	1.23	Zn	46.32	20
Zn	6.15	2.17
Total	100	100	Total	100	100

2. Analysis and discussion

Stress corrosion cracking of metal materials must simultaneously meet the specific conditions of material, environment and stress. From the inspection and test results:

• 1) Fracture flange connection double-headed stud chemical composition failed, Cr content is lower than the standard value, and 18/8 stainless steel must contain more than 18% Cr and more than 8% Ni content to maintain its inherent corrosion resistance, so the double-headed stud material failed, resulting in its corrosion resistance reduced.
• 2) Despite the low chloride content detected by EDS, a large number of stainless steel chloride stress corrosion cracking cases show that stress corrosion cracking usually occurs in austenitic stainless steel in media with very low chloride ion content if other conditions are met [9], and the plant is adjacent to the coast and in the marine atmosphere, the alternating dry and wet atmospheric conditions as well as hot and cold changes can cause chloride ions in the craters on the surface of the double-ended stud concentration by deposition [10], thus satisfying the concentration required for cracking. In addition the industrial environment atmosphere contains more elemental sulfur. And the combined action of sulfur and chlorine is more likely to cause stress corrosion cracking of stainless steel than its individual action [11]. Sensitized 304 stainless steel and specific media constitute a stress corrosion sensitive system.
• 3) Combined with the actual service conditions, double-headed stud fasteners are subjected to preload and internal working pressure forces at work, double-headed studs are in a state of tensile stress, with the conditions of stress corrosion stress state occurs.

From the macroscopic characteristics of the crack in Figure 1, it can be seen that there are small dents on the surface near the middle section of the crack caused during transportation and the residual scratches after processing, and these dents and scratches form a dense group of pits. Electrochemical corrosion theory believes that the existence of surface pits and scratches will make the material surface properties discontinuous and uneven, reducing its electrode potential. Once the metal with lower potential encounters acidic medium, it will be dissolved as anode first corrosion. The carbide particles observed along the crystal precipitation in the metallographic of the fractured double-headed stud cause electrochemical variability at the grain boundaries, near the grain boundaries and between the grains, and this electrochemical inhomogeneity causes unequal dissolution of the grain boundaries and grains when encountering a sensitive medium containing sulfur and chlorine, thus causing intergranular corrosion [12], which leads to the formation of stress corrosion cracks. Through metallographic and fracture analysis, it can be seen that the crack is in the form of cracking along the grain, and the fracture is brittle fracture without obvious plastic deformation, which is a typical granular feature and consistent with stress corrosion characteristics [13].
According to the above analysis, the double-headed stud is considered to be stress corrosion cracking under the joint action of sulfur, chlorine elements and stress in coastal atmospheric environment.

3. Conclusions and recommendations

• 1) The failure mode of the fractured double-headed stud is chloride and sulfur ion-triggered stress corrosion cracking along the crystal.
• 2) The internal cause of the failure of this double-headed stud is the substandard chemical composition with Cr content lower than the standard value, resulting in the reduced corrosion resistance of the alloy.
• 3) The Cl element and S element etc. in the coastal atmosphere were absorbed by the double-headed stud, and the sensitive material cracked rapidly along the crystal under the combined effect of tensile stress and the coastal industrial atmosphere. And the double-headed stud in the transportation process has led to the formation of surface dents, which in turn formed a dense area of craters, accelerating the expansion of cracks, causing the double-headed stud fracture failure.

Corresponding preventive measures and recommendations are as follows:

• 1) To ensure that the chemical composition of stainless steel double-headed studs meet the requirements and adequate solution treatment is an important measure to resist stress corrosion cracking in the coastal atmosphere. Therefore, the corresponding double-headed studs should be strengthened incoming material supervision, appearance inspection, to avoid damage to the double-headed studs in the work process.
• 2) For the coastal atmospheric environment of the region, can consider the use of duplex stainless steel, ultra-pure ferritic stainless steel and other stress corrosion-resistant stainless steel materials. And the factory regularly carry out double-headed stud condition inspection to prevent the failure of double-headed studs or even fracture situation.

Authors: Wanli Chai, Qingqiang He, Fengxu Wang, Jiamun Xie

Source: Stainless Steel Studs Manufacturer – Yaang Pipe Industry Co., Limited (www.metallicsteel.com)

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