Role of Cl/F Ions Concentration, pH and Temperature on Pitting Corrosion Behavior of 2507 Duplex Stainless Steel

In the chemical industry, 2507 duplex stainless steel is usually used for the pipes of absorption tower. The residual F− and Cl− in the tail gas could aggregate in the clean solution of ammonium hydroxide, which causes the risk of pitting corrosion. In this study, the role of Cl/F ions concentration, pH and temperature on the pitting corrosion of 2507 stainless steel is investigated by exposing to actual industrial ammonia desulfurization solution. The experimental results reveal that the 2507 stainless steel exposed the serious corrosion solution appears pitting corrosion problem. However, the 2507 stainless steel is free of pitting corrosion at the tested condition of pH value 3.59/5, temperature 80 °C/100 °C, and ion concentration 46375 ppm/80000 ppm. This study provides a fundamental data for the safety service of stainless steel used in chemical industry.


Introduction
In the field of energy, petroleum, and metallurgy, sulfur dioxide is the main tail gas, which could pollute environment and damage human health.The technology of wet ammonia desulfurization is traditionally used to adsorb the sulfur dioxide.The industrial tail gas is usually injected into the ammonium hydroxide to form the ammonia sulfate via chemical reaction.Many corrosive ions such as F and Cl ions appears in the industrial tail gas.The chemical reaction takes place in the pipe made of stainless steel.These corrosion ions could aggregate in the ammonium hydroxide and erode the stainless steel.Stainless steel containing plenty of Cr and Mo is an available metal owing excellent corrosion resistance via the passivation behavior for the pressure vessel [1,2].However, the concentrated corrosion ions of F and Cl could injury the passivation film on the steel surface and decrease corrosion resistance of stainless steel.Thus, pitting corrosion is the main failure case for stainless steel in the corrosion environment containing F and Cl ions.The serious pitting corrosion could degrade the mechanical property and cause failure events.
Many studies show that the pitting corrosion behavior of stainless steel is affected by many factors such as chemical composition, microstructure morphology, and corrosion environment.Pits is usually initiated at the locations where surface inclusions exist due to the effect of expressing morphological diversity [3].316 stainless steel suffers from more serious corrosion attack in salts vapor than that in NaNO3-NaCl-NaF molten salt regardless of the experimental condition.The oxides on the specimen surface could mitigate the corrosion attacks of corrosion ions and improve corrosion resistance to pitting [4].Grain boundary is the corrosion-sensitized zone.No significant segregation of alloying elements is observed around the grain boundaries.The σ phase in the 904 austenitic stainless steel is precipitated along the grain boundary and causes discontinuity in the passivation film near the grain boundary [5].At the pitting corrosion initiation position, a metastable pit was found and a S signal was detected from the residual.The pitting corrosion of Type 430 stainless steels is triggered by Cl -.The metastable pitting sites at the position of sulfide inclusion is the aggregation position of Cl - [6].The dissolution rate of metastable pitting sites increased with corrosion time.The average lifetime of metastable pitting sites also decreases with potential.Metastable pitting sites changes from cone shape to dish shape with the continuous growth.Metal cations diffusion is the main growth controlling factor [7].The cavitation erosion strengthens the surface plastic deformation and microcracks appears at grain boundaries.The 3%NaCl solution helps to suppress cavitation erosion and the suppression effect attenuates with increasing corrosion time [8].When high-nitrogen stainless steel is exposed to Cl-containing environment, the stability of the passive film decreases with improving applied potential.The constituents of the passive films on the surface are iron oxides, manganese oxides and iron oxides at the initial stage.Cl -is not aggregated in the oxide layer, but N enrichment appeared in the passive film [9].The σ phase and α phase in the stainless steel could decrease the corrosion resistance.The stability of passive film formed on the stainless steel surface is affected by the microstructure.The alternating current can decrease the passivation property and damage the passivation film formed on the sample surface.The corrosion rate and pitting sensitivity also increase with the increasing alternating current density [10].
Duplex stainless steel such as 2507 comprising of a mixture of ferrite and austenite is a candidate for corrosion-resistance materials.It has been increasingly used in chemical industrial due to its high strength and excellent corrosion resistance [11][12].As mentioned above, the process of ammonia desulfurization could promote the continuous concentration of corrosion ions of F -and Cl -, which could damage the passive film and cause pitting corrosion.Many studies studied the role of F -and Cl -on pitting corrosion behavior of stainless steel.These studies focus on the corrosion environment of NaCl solution or F containing solution.However, there are few studies about the pitting corrosion behavior of 2507 duplex stainless steel exposed to actual ammonia desulfurization solution.Especially, the role of ions concentration, pH value and temperature on pitting corrosion behavior is also seldom found.The 2507 stainless steel may suffer from different corrosion conditions consisting of different service temperature, pH value and ions concentration.Thus, this study investigates the pitting corrosion behavior of 2507 stainless steel exposed to industrial ammonia desulfurization solution and the role of ions concentration and temperature on corrosion behavior via immersion corrosion experiment.This paper may provide a fundamental data for the safe service of 2507 stainless steel used in chemical industry.

Materials and Corrosion Solution
The materials used in this experiment is 2507 duplex stainless steel.The corrosion solution is obtained from the industrial application site.The concentration of Cl -and F -ions is detected by using ICP spectrograph.

Immersion Experiment
The 2507 stainless steel is mechanically polished by using silicon abrasive paper of 80#, 240#, 400#, 600#, 1000#, 1200#, and 1500#.The tested temperature, pH value and ions concentration are listed in table 1.The tested temperature is 80 ºC and 100 ºC.The pH value is 3.59 and 5.The ion concentration of F -and Cl -is 46375 ppm and 80000 ppm.The value of environmental parameters used in this study is chosen according to the practical application of 2507 stainless steel to simulate the industrial application.The corrosion solution is injected into conical flask and, the tested stainless steel is immersed in corrosion solution.The immersion duration is 90 days.

Industrial Failure Analysis of 2507 Stainless Steel
The absorption tower is an important device to cool down and sorb industrial tail gas. Figure 1a is the actual absorption tower used in ammonia desulfurization process.It is comprised of plenty of pipes.Figure 1b is the magnified figure of the position circled by blue dot line.The inside diameter of these pipe is 38 mm and the wall thickness of 3 mm.After long corrosion service, these pipes suffer from pitting corrosion problem shown in figure 1c.Thus, Figure 1 indicates that the pipes made of 2507 stainless steel appear pitting corrosion, which is a potential threat for the safety.

Corrosion Behavior of 2507 Stainless Steel at Temperature of 80 ºC
During service process of stainless steel, the pH value could change due to the aggregation of acid ions.Thus, the usual pH value of 3.59 and 5 is use for the tested parameter.Figure 2 shows that the surface of stainless steel eroded at the condition of 80 ºC, Cl -and F -concentration of 46375 ppm is smooth.No pitting sites are observed.Thus, the 2507 stainless steel is free of pitting corrosion at the tested conditions.Figure 3 presents the morphology characteristics of 2507 stainless steel exposed to the condition of 80 ºC, Cl -and F -concentration of 80000 ppm.The corroded surface show rayless color, which indicates that some corrosion products are formed on the steel surface.Compared with the experimental result shown in figure 1, the increased ion concentration promotes the formation of corrosion products.Figure 3 also shows that the pitting sites are not observed on the steel surface.Thus, the improved ion concentration does not case pitting corrosion.

Corrosion Behavior of 2507 Stainless Steel at Temperature of 100 ºC
Temperature is a determining factor of chemical rate.Figure 4 shows the surface morphology characteristics of tested stainless steel immersed in the condition of 100 ºC, Cl -and F - concentration of 46375 ppm.The steel surface remains the plain state and no pitting sits are observed, which indicates that the role of pH on pitting corrosion of 2507 stainless steel is weak.Compared with the experimental results shown in figure 2, some spots with orange color are observed, which indicates that increased temperature accelerates chemical reaction.Pitting corrosion is a common failure mode for passive alloys.Generally, the pitting corrosion propagation is initiated at the localized areas where aggressive solution is concentrated.Non-metallic inclusions is identified as the most susceptible sites for pitting corrosion of stainless steel via various mechanism such as spontaneous dissolution of inclusions, active dissolution of Cr-depleted regions, and propagation of microcracks at the interface of inclusion and matrix [13].The number of non-metallic inclusions in the tested stainless steel is very low according to the supplement data of materials company.Thus, the tested 2507 stainless shows excellent corrosion resistance to pitting corrosion.All halides such as fluorides, chlorides, bromides, and iodides could cause the pitting corrosion of stainless steel due to these ions are not stable in industrial solution and could connect with ions in solution, which injury passive film of stainless steel [14,15].

Conclusions
In this study, the pitting corrosion behavior of 2507 stainless steel exposed to actual industrial ammonia desulfurization solution is investigated by immersion corrosion experiment.The role of temperature, pH value and ion concentration on pitting corrosion behavior is studied by characterizing surface morphology of corroded steel.The following conclusions are drawn based on experimental results.
(1) The pipes used for absorption tower is made of 2507 stainless steel and appears serious pitting corrosion problem after long service.
(2) The 2507 stainless steel does not show the pitting corrosion problem at the tested condition of pH value 3.59/5, temperature 80 ºC/100 ºC, and ion concentration 46375 ppm/80000 ppm.

Figure 1 .
Figure 1.The device morphology and pitting corrosion morphology: (a) the morphology of absorption tower; (b) plenty of pipes consisting of 2507 stainless steel; (c) pitting corrosion morphology of 2507 stainless steel.

Figure 2 .
Figure 2. The macro-surface morphology of tested stainless steel exposed to the same ion concentration of 46375 ppm and different pH value: (a) 3.59; (b) 5.

Figure 3 .
Figure 3.The macro-surface morphology of tested stainless steel exposed to the same ion concentration of 80000 ppm and different pH value: (a) 3.59; (b) 5.

Figure 4 .Figure 5
Figure 4.The macro-surface morphology of tested stainless steel exposed to the same ion concentration of 46375 ppm and different pH value: (a) 3.59; (b) 5.

Figure 5 .
Figure 5.The macro-surface morphology of tested stainless steel exposed to the same ion concentration of 80000 ppm and different pH value: (a) 3.59; (b) 5.

Table 1 .
The tested temperature and ion concentration used in immersion experiment.