Analyzing Stainless Steel Flanges in LNG Gasification Stations (Part two)

1.3 Analyzing the impact on marine and atmospheric environment
The LNG gasification station is located near the coast, which is 1 km away from the coast and belongs to the marine atmospheric environment. The station is located in a low-lying area surrounded by mountains, and the air humidity is relatively high. When the temperature of the ocean environment is high, a large amount of seawater produces salt fog, and the content of chloride ions in the air is relatively high.
 
The surface of the stainless steel is in contact with air, and a dense oxide film is mainly oxide formed by Fe and Cr on the surface. This oxide film isolates the direct contact between the external environment and the stainless steel, so that even if the oxide film on the surface is damaged, it can form again. However, a solution or vapor in a special environment containing Cl- will cause damage to the oxide film. Microelectrodes are easily formed, which form a galvanic cell when there is a potential difference and an electrolyte. A reduction reaction occurs at the positive electrode of the primary battery, and oxygen is reduced as shown in formula (1).
 
Because the oxide film has a certain resistance, the current is weak, and the action of the micro-battery is slow. The metal or metal oxide is oxidized and loses electrons to form metal ions, which enter the electrolyte solution. With the dissolution of the oxide film, its resistance gradually decreases, and the current increases, which accelerates corrosion. When the concentration of O2 and Cl- in the salt spray enters the bare metal surface, the corrosion is accelerated. The greater the concentration of Cl- in the salt spray is, the stronger the conductivity of the electrolyte solution and the greater the corrosion rate become. Of course, this is also closely related to the good permeability of acid. When the metal dissolves, the electrons cannot be reduced, which inhibits the further dissolution of the metal, and the metal surface is repaired to form an oxide film, showing a certain anti-rust ability. When the dissolution rate of metal is higher than the reduction rate of O2, O2 has no time to form the Fe3O4 oxide layer. The electrolyte solution directly contacts the bare metal surface, and accelerates the corrosion of stainless steel.
 
1.4 The analysis of the operating environment
The LNG gasification station needs to supply gas during the peak period of gas consumption every day, and is in a state of shutting down during the low peak period of gas consumption. When the gas is supplied, the gasifier starts to work. At this time, the gasifier is in a low-temperature state, especially the temperature of the liquid phase inlet, equipment and pipelines is particularly low. When the gasifier is not working, the equipment is at normal temperature. Because the gasifier is repeatedly in the switching state of working and not working, the stainless steel lap joint flange connected to the gasifier is always in a state of alternating cold and hot temperature loads. The temperature alternating stress generated by the periodic change of temperature will cause fatigue damage to equipment and pipelines. When the fatigue damage reaches a certain level, stress corrosion cracking will occur in the stress concentration area, leading to accidents.
 
1.5 The influence of welding on weldment
Welding thermal stress refers to the stress caused by temperature differences inside the weldment in the welding process. The internal stress generated in the weldment in the welding process will cause the shape and size of the weldment to change. The thermal conductivity of austenitic stainless steel 304 is small (about 33.3% of low carbon steel). The linear expansion coefficient is great (50% greater than that of low carbon steel). The shrinkage stress is great during cooling, so the welding stress is also great. In addition, if the welding current is too great or the welding speed is slow in the welding process, the welding seam will be overheated, resulting in greater welding stress in the heat-affected zone. Tensile stress is easily generated in the welding process, which aggravates the generation of thermal cracks.
 
2. Conclusion
The reasons for the failure of corrosion cracking in the heat-affected zone of the stainless steel lap joint flange of the LNG gasification station are as follows based on the above analysis:
① The material of the stainless steel flange does not meet the requirements. The C content exceeds the standard, and the Cr content is lower than the standard value. As a result, in the welding process, the diffusion of carbon is great, and the consumption of chromium is great, which intensifies the lack of chromium in the heat-affected zone in the welding process, and accelerates the tendency of intergranular corrosion in the heat-affected zone of stainless steel. In addition, the excessive Mn content leads to a sharp drop in the thermal conductivity of the stainless steel lap joint flange, and an increase in the linear expansion coefficient, which causes great internal stress to be formed during rapid heating or cooling. The cracking tendency of the workpiece increases. The failed stainless steel lap joint flange has been in an environment with alternating cold and hot temperature loads for a long time, and the temperature alternating stress generated by the periodic change in temperature has caused corrosion and cracking in the heat-affected zone of the stainless steel lap joint flange.
 
② The LNG gasification station is in a marine atmospheric environment, and the Cl- content in the nearby air is high, which makes the oxide film on the surface of the stainless steel easily destroyed, thus causing electrochemical corrosion on the surface of the stainless steel. In addition, the content of Ni element in stainless steel flange is lower than the standard value, and the corrosion resistance of steel parts is reduced. The increase in the Cl- content in the air and the decrease in the Ni content in the flange material double accelerate the formation of corrosion cracking in the heat-affected zone of the stainless steel lap joint flange.
 
In summary, the root cause of corrosion and cracking in the heat-affected zone of the flange of the LNG gasification station is that the material of the flange does not meet the requirements, resulting in a large amount of carbide precipitation (Gr23 C6) between the grains in the welding process, resulting in the occurrence of intergranular chromium deficiency. It has been in the state of alternating cold and hot loads and marine atmospheric environments for a long time, resulting in corrosion and cracking in the heat-affected zone.
 
6. Suggestions
To ensure the safe and stable operation of the LNC gasification station and avoid corrosion and cracking of austenitic stainless steel equipment, the following points should be paid attention to during the construction and operation of the LNG gasification station in the future:
When steel products arrive for inspection and acceptance, there must be a quality certificate for the product provided by the manufacturer. Before using the steel, it is necessary to entrust a third-party inspection unit to conduct a sampling inspection of the steel to test the physical and chemical properties of the steel to ensure that the various indicators of the steel meet the requirements.

Before the material is welded, the welding process must be evaluated, and the appropriate welding parameters and welding speed must be selected to avoid overheating of the welding seam and minimize the thermal stress and residual stress in the welding heat-affected zone.
 
When installing flanges or welded pipes on site, they need to be assembled before welding, and the last welding joint must be centered. Forced alignment is not allowed to avoid installation stress during installation.

During the operation of the gasifier, the frequency of switching the gasifier should be reduced as much as possible to prevent the equipment from being repeatedly subjected to alternating loads of cold and heat.

Before supplying gas, the pipeline and gasifier must be fully pre-cooled. Avoid opening the liquid outlet valve of the storage tank when it is not fully pre-cooled, resulting in sudden cooling of the pipeline and equipment.