Research on corrosion behavior of safety valve spring in liquid chlorine tank

The corrosion behavior of the safety valve spring in the liquid chlorine tank is investigated in this study. The chemical composition, metallographic structure and hardness of the spring are also analyzed. The higher hardness of spring steel is caused by lower tempering temperatures. The corrosion product is grey with many cracks, spalling and pitting is also found. The phenomenon that where there is higher chloride, there is higher carbon is observed. Chloride ion is adsorbed by carbides and then pitting preferentially nucleate at carbides.


Introduction
The safety valve is one of the most widely used and important safety accessories for pressure vessels.When the pressure in the pressure vessel exceeds a pre-set value, the safety valve will open automatically and the pressure inside falls quickly to prevent explosion accidents caused by overpressure.Meanwhile, when the pressure in the pressure vessel is reduced to the allowable value, it automatically closes to avoid waste caused by continuous leakage of media.The spring of the safety valve is the key component under alternate loading over a long period.Moreover, the spring of the safety valve may suffer from chemical corrosion by media.The literature has several works about spring failures, and spring failures may be due to fatigue [1][2][3], corrosion [4], manufacturing flaws [5][6], or improper heat treatment [7].
During the offline verification process of the safety valve of the liquid chlorine tank, it was found that the safety valve was corroded seriously, the setting pressure was low, and the sealing pressure also dropped.After compression of the tightening spring within the pressure level, the setting pressure was still insufficient, so it was scrapped.The corrosion behavior of the safety valve spring is investigated with the scrapped safety valve.

Macroscopic examination
The safety valve especially the spring is corroded seriously.The spring is russet, which is present in Figure 1.Some of the corrosion layers have fallen off.As shown in Figure 1

Chemical composition
The composition of the spring steel is tested with Thermo Fisher-ARL4460 direct reading spectrometer, which is shown in Table 1.According to the inspection results and Chinese standard GB/T1222-2016, the spring is made of 80 steel, which is used in the manufacture of non-essential larger-size springs.

Metallographic structure and hardness
As shown in Figure 2, the metallographic structure of the spring is tempered troostite, which is a mixture of ferrite and a large number of dispersed cementite.The needle-like form has gradually disappeared, but it is still faintly visible.Generally, the heat treatment process of spring steel is quenching and medium-temperature tempering, the tempering temperature is 350℃ -500℃ with hardness of 35 -45 HRC.Nevertheless, the Vickers hardness of springs A and B is larger than 450 HV, which is displayed in Figure 3.That is, the hardness of spring steel is larger than 45 HRC.The tempering temperature has a significant influence

Cross-section morphologies and element distributions
The cross-section morphologies of the spring steel are shown in Figure 4, the corrosion product is grey with many cracks and spalling occurs.The thickness of the corrosion product is up to about 200 μm.The actual thickness of the corrosion product should be larger than 200 μm due to the corrosion products having fallen off.Liquid chlorine itself is not very corrosive, but it is highly corrosive when exposed to water.Liquid chlorine reacts with water to form hypochlorous acid and hydrochloric acid which react with metals to form ferrous chloride and hydrogen gas.Pitting is observed in Figure 4 and Figure 5, and the depth of pitting in Figure 5 is about 100 μm.As presented in Figure 5, element distributions of corrosion product was analyzed.The oxygen content and chloride content of the corrosion product are higher than those of the metal matrix.The chloride content is particularly high at the bottom of the pitting.Another interesting phenomenon is that where there is higher chloride, there is higher carbon.The research argued the characteristic of chloride ions is that there are obvious corrosion pits at the interface between the oxide layer and metallic matrix, and adsorption theory is usually used to explain the mechanism of chloride ion corrosion [8].Therefore, we can theorize that chloride ion is adsorbed by carbides and then pitting preferentially nucleate at carbides.

Conclusion
Based on the results and discussions above, the conclusion is as below: (1) The safety valve spring is made of common 80 steel.The higher hardness of spring steel is caused by lower tempering temperatures.
(2) Corrosion product is grey with a thickness of about 200 μm.Cracks, spalling, and pitting are also observed in the corrosion product layer.
(3) The phenomenon that where there is higher chloride, there is higher carbon is observed.Chloride ion is adsorbed by carbides and then pitting preferentially nucleate at carbides.
However, the authors believe that the mechanism of corrosion caused by chloride ions needs further investigation in the future.
(b), A part and B part of the corroded are selected for further analysis.

Figure 2 .
Figure 2. Metallographic structure of the spring.

Figure 3 .
Figure 3. Vickers hardness of the spring.

Figure 5 .
Figure 5. Element distributions of corrosion product.