Due to its excellent corrosion resistance, 304 stainless steel is widely used in equipment and parts requiring good comprehensive properties (corrosion resistance and formability), and is widely used in chemical equipment, pressure vessels and other industries.
Related reading: Stainless Steel Grades
The sulfuric acid pipe of a fertilizer plant is used to connect the outlet (0.82MPa) of the sulfuric acid pump with the reactor.
The pump has a flow of 14m3/h, a head of 63m, a sulfuric acid concentration of 93.5%, and a service temperature of normal temperature.
The pipeline was replaced in 2016.
After two years of use, there was liquid leakage at the weld of the discharge pipe at the inlet and outlet of the pump and the high neck flange at the pressure gauge interface.
The pipe wall was cleaned and penetrant tested, and cracks were found (see Fig. 1).
According to the original data, the steel pipe is made of 304 stainless steel, the pipe diameter is DN50, and the wall thickness is 3.5mm.
After welding, the penetration test was carried out, and the result was acceptable.
After cutting and sampling the steel pipe, it is found that the seepage is located in the weld area and cracks are found.
Fig. 1 Cracking Position and Morphology of Sulfuric Acid Pipe
In order to find out the cause of corrosion cracking failure of stainless steel pipe and avoid the risk of reoccurrence, this article plans to analyze the chemical composition, metallographic microscope and scanning electron microscope of the failed steel pipe, so as to find out the cause of failure and propose preventive measures.
1. Test method
(1) Chemical composition analysis
ARL-4460 direct reading spectrometer is used to detect the chemical composition of the base metal and weld of stainless steel pipe to determine whether the chemical composition meets the standard requirements.
(2) Metallographic microscopic analysis
Cut the sample from the liquid penetration point (see Fig. 1c).
The sample includes base metal, weld and heat-affected zone.
Carry out pre-grinding, rough grinding, fine grinding and polishing on the sample.
Use OLYMPUS-GX51 metallographic microscope to observe the non-metallic inclusions in the sample.
Then, use ferric chloride hydrochloric acid aqueous solution to etch it.
Observe the structure of the sample under the metallographic microscope (see Fig. 2).
Fig. 2 metallographic sample
(3) SEM analysis
Tear the sample along the crack with hydraulic pliers, scan and observe the crack surface of the sample with Hitachi S-3400 thermal field emission scanning electron microscope, and conduct energy spectrum analysis with EDAX energy spectrometer.
2. Results and discussion
(1) Chemical composition analysis
Table 1 shows the chemical composition of base metal and weld of stainless steel pipe.
It can be seen from Table 1 that the chemical composition of the stainless steel pipe purchased by the company is lower than that of the standard in the base metal and weld metal, and the content of other elements meets the standard requirements.
Cr is the main corrosion-resistant element in stainless steel.
If the content of Cr is low, the corrosion resistance of stainless steel will decrease.
Table 1 Chemical Composition of Stainless Steel Pipe Materials (Mass Fraction (%)
(2) Metallographic microscopic analysis
Firstly, the specimen was mechanically polished, and the distribution of non-metallic inclusions in the specimen was observed under a microscope without etching.
Through observation, it is found that there are few non-metallic inclusions, but there are single large size inclusions, rated as Ds2 (see Fig. 3a).
The existence of non-metallic inclusions will destroy the continuity of the matrix, reduce its mechanical properties, and make the matrix more prone to fracture.
The non-metallic inclusions will make the passive film (oxide film) formed on the surface of stainless steel substrate thinner, causing the junction between non-metallic inclusions and the substrate to be corroded first, and then the local corrosion at the interface will extend to the substrate, resulting in pitting corrosion.
The existence of non-metallic inclusions also easily leads to grain boundary embrittlement and intergranular corrosion, which reduces the corrosion resistance.
Fig. 3 Microstructure of Specimen Fracture after Corrosion
The polished sample was chemically etched and its structure was observed under a metallographic microscope.
Fig. 3b shows the micrograph of the base metal of the sample.
The structure is single-phase austenite (with twins), no abnormality is found at the grain boundary, and the average grain size of the metal is grade 7.
Fig. 3c shows the microstructure of the fusion zone (left weld, right heat affected zone).
This zone has normal structure, good fusion, no cracks, pores, and no other welding defects.
The metallographic microstructure is observed near the crack (welding heat affected zone), as shown in Fig. 3d.
The microcracks distributed along the grain boundary can be clearly seen, and there are carbide of network grain Cr, thus forming a chromium poor zone, as shown in Fig. 4.
When the chromium content (mass fraction) is more than 12%, the passivation effect is obvious, which can significantly improve the corrosion resistance of stainless steel.
When the chromium content is less than 12%, the passivation state is destroyed, the potential drops, and the passivation state remains in the crystal, thus forming a micro galvanic cell with small anode (chromium poor area in the grain boundary area) and large cathode (matrix), which accelerates the corrosion of the grain boundary.
The precipitation temperature of Cr23C6 carbide is 450-850 ℃, which is the sensitization temperature range of stainless steel intergranular corrosion, also known as the dangerous temperature range.
The above morphological features show that there is sensitization in this area after welding, which leads to intergranular corrosion in the heat affected zone of the weld and reduces the intergranular corrosion resistance of the stainless steel heat affected zone, which is one of the reasons for the cracking of the stainless steel pipe.
(3) Fracture scanning observation
Put the processed fracture sample into the scanning electron microscope, and conduct microscopic observation and analysis by using secondary electron imaging.
As shown in Fig. 4, it can be found that the fracture is uneven, there are many corrosion products and cracks, and the cracks are distributed in a dendritic manner.
These cracks are characterized by secondary cracks, and the cracks have penetrated into the material matrix.
These characteristics indicate that the failure of 304 stainless steel pipe is stress corrosion cracking.
The thermal conductivity of stainless steel is poor.
After welding, the steel pipe will generate residual stress due to high temperature.
The corrosion microcracks of stainless steel pipe will accelerate under the effect of residual stress and form stress corrosion cracking.
Fig. 4 SEM observation of fracture morphology
(4) Energy spectrum analysis
The corrosion products on the fracture surface of stainless steel pipe are analyzed by energy spectrometer. Fig. 5 shows the results of energy spectrum analysis.
From the diffraction peak spectrum, it can be seen intuitively that the chlorine content is very high, indicating that the stainless steel pipe is in a chlorine containing corrosion environment.
The cracked steel pipes of the fertilizer plant are placed in the open air.
The plant site is located in the coastal zone, only 1.1km away from the coast, which belongs to a typical marine atmospheric environment.
When the temperature and humidity are high, a large amount of seawater will evaporate to produce salt fog, which will cause high chloride ions in the air.
The water containing chloride ions will be adsorbed on the outer wall of the stainless steel pipe, forming a corrosive medium, which will continuously corrode the stainless steel pipe.
Austenitic stainless steel will form a dense passivation film (oxide film) on its surface in ordinary atmospheric environment.
This passivation film insulates the atmosphere from direct contact with the stainless steel surface, protects the stainless steel and makes it have excellent corrosion resistance.
Even if the passive film is damaged, it will be regenerated and repaired in time.
However, chloride ion is very easy to destroy the passivation film of austenitic stainless steel, forming pitting or pits on the surface, and accelerating the corrosion of stainless steel.
The corrosion cracking of stainless steel in this case cannot be simply attributed to one aspect, but the cracking of stainless steel pipe is caused by the joint action of multiple factors.
(1) The non-metallic inclusions will damage the integrity of the passive film on the metal surface and reduce the corrosion resistance of stainless steel.
Therefore, the non-metallic inclusions should be strictly controlled below Level 1.5.
(2) The Cr content in the base metal and weld metal is low, and Cr deficiency reduces the compactness of the chromium passive film on the stainless steel surface.
Therefore, the quality requirements of steel pipes and welding materials should be improved, and the incoming components should be strictly tested to ensure that the weld metal composition is not weaker than the base metal.
During the welding process, the welding parameters shall be strictly controlled, and the welding heat input shall be as small as possible to prevent sensitization from causing Cr to precipitate along the grain boundary and generate Cr23C6, so as to avoid intergranular corrosion of stainless steel.
(3) The chemical fertilizer plant is located in the marine atmospheric environment, where the chloride ion content in the air is high, and there is an appropriate temperature and humidity to accelerate corrosion, which causes the oxide film on the stainless steel surface to be easily damaged, thus causing electrochemical corrosion on the stainless steel surface.
The corrosion microcracks accelerate to expand under the effect of residual stress, causing stress corrosion cracking.
Therefore, the site air environment shall be strictly controlled, and the salt fog environment shall be isolated (such as painting or adding protective layer) to prevent the damage of chloride ions.