Failure analysis of 316L stainless steel corrugated pipe

316L stainless steel is a kind of austenitic stainless steel with good corrosion resistance, widely used in power plants, refineries, chemical plants, etc. However, the metal bellows manufactured by 316L stainless steel have caused a corrosion perforation phenomenon in the Cl−environment, which will affect the safety of equipment operation. In this paper, by observing the corrosion morphology, examining and analyzing the material composition, corrosion products, working conditions, and production process of the metal bellows of a power generation company. The results show that the main cause of failure of stainless steel corrugated pipe is pitting corrosion perforation caused by high concentration chloride aqueous solution in the medium.


Introduction
During electricity production, chloride ion is widely considered to be the most harmful reactive anion in the electric generator.A small amount of chloride ions is enough to destroy the passive film on the metal surface [1] .It even causes serious safety accidents such as corrosion perforation of water-cooled wall tubes [2][3] , corrosion cracking of heat transfer tubes of nuclear power steam generators [4] , and corrosion fracture of steam turbine blades [5] .
316L stainless steel corrugated pipe is a flexible pressure fitting used to compensate for the mutual displacement of connecting ends such as pipelines.316L stainless steel is an ultra-low carbon steel [6] , which has good corrosion resistance and is widely used in power plants, oil refineries, chemical plants, etc.However, it is prone to pitting corrosion, crevice corrosion, and stress corrosion in relatively harsh environments such as Cl corrosion, with pitting corrosion being the most severe [7] .Zhang Sheng et al. found in the seawater immersion experiment of stainless steel that the residual chloride ions in seawater were the main reason for the formation of pitting corrosion perforation in 316L stainless steel [8]   .Compared with 317, 254, and 3127 stainless steel in the Cl -environment, 316L stainless steel has the lowest pitting corrosion potential, and the highest possibility of pitting corrosion [9,10] .Zhao et al. found that the main phase composition of 316L stainless steel is austenite, but there is a small amount of ferrite.The corrosion of the ferrite phase is the main reason for the decrease in corrosion resistance of 316L stainless steel [11] .Du et al. found that as the residual stress in the corrugated pipe increases, the martensite content in its matrix increases, leading to an increase in the stress corrosion sensitivity of stainless steel [12] .Statistics show that about 97.4% of corrugated pipes in the Beijing and Tianjin regions suffer from corrosion damage, with stress corrosion cracking being the main factor causing corrosion damage to corrugated pipes [13] .From this, it can be seen that the production of martensite induced by plastic deformation and the action of corrosive chloride ions from the outside are some of the reasons for the pitting corrosion failure of stainless-steel corrugated pipe materials.Through the inspection and analysis of the material composition, corrosion products, and working conditions of metal bellows, this paper determines the failure reason of stainless-steel corrugated pipe and puts forward improvement measures.

Test analysis
The stainless-steel corrugated pipe (specification 1'1/4-inch, design material 316L) of a gas fuel pipeline in a gas turbine power plant leaked.Scanning electron microscopy (SEM) was used to observe the morphology of inner and outer walls and corrosion pits, and the composition of surface corrosion products was determined by energy dispersive spectroscopy (EDS) combined with back-scattered electron imaging.

Macro analysis
As shown in Figure 1, after the longitudinal section of the bellows, it is observed that obvious corrosion occurs on one side, the corrosion products adhere to the inner wall surface, and the other side is smooth and free of any corrosion products or dirt.
As shown in Figure 2, a large area of yellow-brown corrosion products is attached to the inner wall of the corrugated pipe on the corrosion side, the corrosion in the groove is the most serious, and many corrosion pits and perforations are found on the inner wall.After the inner wall is enlarged, many corrosion pits (not penetrating the pipe wall) are found.It can be determined that corrosion first occurs from the inner wall.The corrosion morphology has typical pitting characteristics.The perforations are all located on the side where corrosion occurs and are distributed linearly along the axis.As shown in Figure 3, there are perforations on the outer wall of the corrugated pipe on the corrosion side, and there are yellow and white crystals near the perforation position.

Elemental analysis
The results of the elemental composition analysis of the bellows are shown in Table 1.According to DL/T 715-2015 "Selection guide for the metallic material of fossil power plants", the similar grade of 316L is 022Cr17Ni12Mo2.According to GB/T 20878-2007 "Stainless steel and heat-resisting steels-Designation and chemical composition", the element composition of the pipe meets the requirements of 316L material.

SEM analysis
As shown in Figure 4 and Figure 5, a scanning electron microscope was used to analyze the corrosion perforated section and corrosion pit (without penetrating the pipe wall) after cleaning.The microscopic morphology was observed, and no microcrack was found.

Energy spectrum analysis
Figure 6 shows the inner wall corrosion product (powder), mainly containing Fe, O, Na, Cl, Ca, Cr, S, Ni, and other elements.Figure 7 is a yellow-white crystal at the outer wall, mainly containing elements such as Na, Cl, O, Ca, etc. From this, it can be inferred that there is liquid and corrosive medium chlorine in the corrugated pipe.Due to the inclined arrangement of the pipe, after the corrosion perforation, the liquid containing corrosion material drips from the perforation position to the outer wall of the pipe under the action of gravity, and then crystallizes and remains on the outer wall.The energy spectrum analysis results of the corrosion products at the inner wall corrosion pit (without penetrating the pipe wall) are shown in Figure 8, mainly containing Fe, O, Cr, Ni, P, Ca, Cl, and other elements.

Corrosion perforation mechanism analysis
The local corrosion that often occurs in stainless steel mainly includes stress corrosion, galvanic corrosion, intergranular corrosion, pitting corrosion, and crevice corrosion.Pitting corrosion is referred to as pitting, also known as hole corrosion or pit corrosion.Pitting corrosion is usually a form of localized corrosion that is highly concentrated in the anode region caused by small anode and large cathode corrosion cells that occur in easily passivated metals or alloys in the presence of aggressive anions and oxidants [14] .Many corrosion pits (not penetrating the pipe wall) are found on the inner wall, so it can be determined that the corrosion first occurs from the inner wall.Its morphology is consistent with the characteristics of pitting corrosion.
Natural gas contains a small amount of H 2 S and CO 2 .H 2 S has a high solubility in liquid water [15] .When the water inside the pipeline only exists in the gaseous state, it will not cause serious internal corrosion.Liquid water is a necessary condition for severe internal corrosion.Under certain conditions, the water vapor in the pipeline will liquefy to form a water film adsorbed on the surface of the pipeline.The inner wall of the pipe is cut open to check that there is corrosion on one side and no corrosion on the other side.According to the macroscopic corrosion morphology, it is speculated that liquid water will accumulate in the corrugated pipe.There is a large amount of Cl element in the corrosion product, and it can be speculated that the main corrosion element is Cl.The activation of chloride ions has a great influence on the establishment and destruction of stainless-steel oxide film.Niu Lin [16] studied the effect of halogen ions on iron passive film in alkaline solution, and considered that the order of pitting corrosion caused by halogen ions is Cl ->Br ->F -.In oxidizing medium, a dense oxide film is formed on the surface of stainless steel, which makes stainless steel have better corrosion resistance.However, in the medium, if chloride ion exists, the passive film of stainless steel will be destroyed.According to the adsorption film theory, in the medium containing chlorine, the ability of chloride ions to be adsorbed by metals is much stronger than that of oxygen molecules and metals to form a passive film.Chloride ions can squeeze out the oxygen atoms that have formed the passive film and form soluble metal chlorides with metal cations.Then it dissolves and peels off, forming small pitting pits of tens of microns, exposing the metal to become an anode.However, the surface electrode potential of the remaining passive films that have not been damaged is much lower than that of the bare metal.Therefore, the undamaged passive film constitutes a large cathode surface.This forms the structure of a large cathode and a small anode.Once the primary cell is formed, the dissolved chloride forms in the hole, and the passive film formed on the metal surface cannot be exchanged.As a result, the concentration of chloride in the hole is increasing, the acidity is gradually enhanced, and the pitting holes are deepening, eventually forming multiple small and deep pitting holes [17] .H 2 S can accelerate the anodic dissolution of 316L stainless steel, destroy the passive film, and cause pitting corrosion.The reaction product is SO 4 2-.When the concentration of SO 4 2-is low, Cl -on the stainless-steel surface is the main adsorbed ion.Due to the competitive adsorption of Cl -and SO 4 2-, Cl -is locally concentrated on the surface of stainless steel, resulting in the increase of Cl -concentration here and accelerating the occurrence of pitting corrosion of stainless steel [18] .

Conclusion
(1) The corrugated pipe material shall meet the material requirements of 316L.
(2) Corrosion occurs from the inner wall and expands to the outer wall until the perforation leaks.The corrosion morphology has typical pitting characteristics.There are no micro-cracks at the corrosion perforation part and the corrosion pit (without penetrating the pipe wall).The corrosion products contain Cl, P, S, Na, Ca, and other elements.
(3) Austenitic stainless steel has strong corrosion resistance due to a dense chromium oxide (Cr 2 O 3 ) passive film on the surface.It has good corrosion resistance in clean and dry conditions.However, when the electrolyte liquid and chloride ions exist at the same time, due to the adsorption film theory, pitting holes are formed, resulting in corrosion perforation.
(4) There is corrosion on one side of the inner wall of the pipe and no corrosion on the other side.This means that the liquid exists on one side of the tube and does not fill the entire tube cavity.In addition, after the pipe is cut, the half piece is erected vertically, and the water slowly flows into the end.It is found that the water will remain in the grooves at all levels.This also explains the phenomenon that the corrosion perforations in the pipe are linearly distributed along the axial direction on the side of the pipe where the corrosion occurs.
In summary, the main cause of failure of a stainless-steel corrugated pipe is pitting corrosion perforation caused by a high concentration of chloride aqueous solution in the medium.

Figure 2 .
Figure 2. Macro-morphology of the inner wall of corrugated pipe.Figure 3. Macro-morphology of the outer wall of corrugated pipe.

Figure 3 .
Figure 2. Macro-morphology of the inner wall of corrugated pipe.Figure 3. Macro-morphology of the outer wall of corrugated pipe.

Figure 5 .
Figure 5. SEM morphology of corrosion pit (without penetrating the pipe wall).

Figure 6 .
Figure 6.Energy spectrum analysis of corrosion product (powder) of the inner wall.

Figure 7 .
Figure 7. Energy spectrum analysis of yellow-white granules with the outer wall.

Figure 8 .
Figure 8. Energy spectrum analysis of corrosive substances attached to the corrosion pit (without penetrating the pipe wall).

Table 2
Energy spectrum analysis results of corrosion products on inner and outer walls.

Table 3
Energy spectrum analysis results of corrosive substances attached to the corrosion pit (without penetrating the pipe wall).