Pickling Behavior of Duplex Stainless Steel 2205 in Hydrochloric Acid Solution

,e oxide-scale structure and pickling behavior of oxided 2205 duplex stainless steel in the electrolytes containing hydrochloric acid were investigated. ,e oxide scales mainly consist of two layers: the outer layer is dense Fe2O3, and the inner granular is FeCr2O4 spinel. During the pickling process, pittings form around the boundaries of FeCr2O4 particles or interfaces of two kinds of oxides, which results in that the electrolyte can directly react with the chromium-depleted layer along the pittings to produce an “undercut” effect so that the pickling efficiency is improved markedly. ,e pickling mechanism was discussed, and the model was established.


Introduction
Duplex stainless steel 2205 is one of the most common kinds of DSSs with the volume fraction of each phase above 30%.Due to the proper austenite-ferrite balance, 2205 exhibits exceptional corrosion resistance properties except for excellent strength and impact toughness [1,2] and thus has been widely used in oil and gas exploration, shipping preparation, flue gas desulfurization, desalination, and other industrial fields [3][4][5][6].
Pickling is one of the most important steps in the manufacture of 2205 and can become the limiting factor of production efficiency.Pickling of 2205 is very difficult for four reasons.Firstly, the oxide scales on 2205 are dense and adherent strongly to the underlying metal.Secondly, the removal of the chromium-depleted layer beneath the oxide scales is imperative due to its low corrosion resistance [7][8][9][10].irdly, the alloying element contained in 2205, such as molybdenum and nitrogen, can improve the stability of the oxide scales remarkably [11].Fourthly, the composition, thickness, and protectiveness of the oxide scales formed on the austenitic phase and ferritic phase are not the same due to the different chromium contents in them [12][13][14].
Researches [15] have shown that electrolytes containing hydrochloric acid can efficiently remove the chromiumdepleted layer for hot-rolled 304 due to the anodic brightening mechanism [16].But whether the hydrochloric acid can improve the pickling efficiency of 2205 is still not clear.Moreover, the researches on the pickling of 2205 mainly focus on the electrochemical pickling [17][18][19], and little work has been done on the chemical pickling.is paper mainly discussed the pickling behavior of 2205 in electrolytes containing hydrochloric acid and the evolution of the oxide scale by chemical pickling.Finally, a hydrochloric acid pickling model was built on these results.

Experiment Procedure
2205 duplex phase stainless steel (with a chemical composition of 0.018 wt.% C, 1.2 wt.% Mn, 22.6 wt.% Cr, 5.3 wt.% Ni, and balanced Fe) was hot rolled into a plate, following annealing and blasting treatment (called as oxidized 2205).Specimens (30 mm × 30 mm) were machined from the plater.e phase components of the oxide scale were investigated by a Japan Rigaku D/Max-IIIB X-ray diffractometer with Cu Kα1 radiation (λ �1.5405 Å).
e accelerating voltage, emission current, and scanning speed were 40 kV, 40 mA, and 0.2 °/s, respectively.e morphologies and microstructures of the specimens were observed using a UK Leica Cambridge S360 scanning electron microscope (SEM).Analytical grade chemicals and distilled water were used to prepare the electrolyte containing 110 g/L HCl.And a little oxidant was added to the electrolyte to advertise overcorrosion.e tests were carried out at 80 °C under the unstirred condition.
Corrosion potential during the pickling process was measured by an electrochemical workstation (PARSTAT ® 2273, USA), and a saturated calomel electrode was used as the reference electrode.When the electrodes were introduced into the test electrolyte, the corrosion potential measurement started.Furthermore, some specimens were immersed into the same electrolyte and taken out after the following time intervals: 30 s, 60 s, and 90 s.Afterwards, the specimens were rinsed with distilled water to remove the residual electrolyte and dried to analyze the evolution process of the oxide scale in the pickling electrolyte by SEM. e specimen rinsed for 90 s was then slightly brushed to remove the residual oxides to observe the micromorphology of the matrix.

Composition of Surface Oxide Scale.
Figure 1 presents the X-ray diffraction pattern of the 2205 surface oxide scale.It clearly shows the typical diffraction peaks of the matrix indicating that the X-rays completely penetrated the oxide layer so that the possibility of undetected oxide phases was minimized.Moreover, the pattern reveals that the oxide scale consists of Fe 2 O 3 , FeCr 2 O 4 spinel, and SiO 2 , which is in accordance with the research of Li et al. [12].

Cross-sectional Morphology and Elements Distribution of
Surface Oxide Scale. Figure 2 shows the SEM image of the cross-sectional morphology and the EDS maps showing the distribution of the main elements, which combine with oxygen to form the surface oxide scale.e thickness of the oxide scale is approximately 10 μm.And the scale can be divided into two layers: the outer layer of iron-rich oxidation and the inner layer of chromium-rich oxidation.And also some silicon oxides are mainly enriched at the interface of the chromium oxide and matrix.Combining this result with the X-ray spectra, it can be inferred that the outer layer is Fe 2 O 3 and the inner layer is FeCr 2 O 4 (a kind of spinel).

Corrosion Potential of Pickling Process.
e corrosion potential of 2205 pickling in the electrolyte containing hydrochloric acid shows a typical characteristic of hydrochloric acid pickling [16] (Figure 3).In the initial stage, the corrosion potential decreases sharply as the electrolyte permeates the interface of the oxide scale and the chromiumdepleted layer.e corrosion potential stays first at a low level after decreasing down and then abruptly increases up to a relatively high value after duration because of the dissolution of the chromium-depleted layer. is reflects an active-topassive transition rather than an anodic brightening [16].
In addition, as shown in Figure 3, the whole pickling process lasted 60 s.However, the pickling process was kept for 90 s to ensure the uniformity of pickling in the immersion test.

Evolution Process of Oxide Scales in Pickling Electrolyte.
e SEM images of the oxide scales after immersion in the pickling electrolyte for different times and the matrix after pickling are shown in Figure 4. e EDS results show that the outer layer of Fe 2 O 3 is dense and the FeCr 2 O 4 spinel stacking beneath the outer layer is granular.In the whole pickling process, the lumpy Fe 2 O 3 had little changes, but the amount of FeCr 2 O 4 decreased gradually.ere were some cavities at the boundaries of the FeCr 2 O 4 particles or the interfaces of two kinds of oxides (as indicated by the arrows).As the immersion time increased, the number and size of the cavities increased constantly.Up to 90 s, the surface oxide scale detached completely from the matrix, and the residual oxide could be easily removed by a nylon brush.e surface of the matrix after pickling was smooth without local pitting corrosion or other obvious corrosions, which can satisfy the requirements of cold rolling.

Discussion
Based on the XRD and EDS analysis results, it is known that the oxide scale is mainly composed of Fe 2 O 3 and FeCr 2 O 4 .
e reaction in the electrolyte containing hydrochloric acid is as follows: (1) e variations of the standard Gibbs free energy ΔG θ for chemical reactions (1) and (2) at 80 °C are 19.128kJ and −63.122 kJ [20], respectively, suggesting that FeCr 2 O 4 could be dissolved prior to Fe 2 O 3 when immersed in the same reducing acid liquor.is accounts for why Fe 2 O 3 shows little variation with the increase of time during the whole pickling process, while the FeCr 2 O 4 spinel particles reduce with the increase of time.
e electrolyte contains a large amount of Cl − , which can be preferentially adsorbed at the regions with higher energy, such as the boundary of the FeCr 2 O 4 spinel and the interface of the two oxides.
e adsorption of Cl − promotes the dissolution of the oxides and the formation of cavities.e volume and depth of the cavities increase constantly with the increase of time till reaching the chromium-depleted layer.
en, the elements in the chromium-depleted layer react with the electrolyte as follows (taking Fe and Cr for example): (3) e variations of the standard Gibbs free energy ΔG θ at 80 °C are −91.283kJ and −197.861kJ, [20] respectively, 2 Advances in Materials Science and Engineering meaning that the chromium-depleted layer will dissolve prior to the oxides.ese reactions will produce an "undercut" e ect on the oxide scale, and the reaction product H 2 can also degrade the integrality and adhesiveness of the oxide scale.erefore, the oxide can be removed easily at the end of pickling.
According to the results above, a pickling mechanism model is built for the oxidized 2205 in hydrochloric acid solution, as shown in Figure 5.After hot rolling and high temperature annealing, the black-oxide scales on the surface of 2205 are integrated and compact (Figure 5(a)).e oxide scales consist of two layers: the outer layer is dense Fe 2 O 3 , and the inner is FeCr 2 O 4 .A thin chromium-depleted layer is formed between the inner oxide layer and matrix because of the formation of the oxide scales.After blasting, the outer Fe 2 O 3 is mechanically ruptured and partially falls o from   According to the thermodynamic calculation, the electrolyte will preferentially react with FeCr 2 O 4 .erefore, the FeCr 2 O 4 crystals around the pit nucleus continuously dissolve, and the pits propagate along the grain boundaries to the matrix (Figure 5(d)).e electrolyte replenishes into the pits to maintain the continuous dissolution of the FeCr 2 O 4 crystals.When the pits penetrate the oxide layer, the chromium-depleted layer is exposed to the electrolyte and preferentially reacts to dissolve.At this time, dissolution of the chromium-depleted layer becomes the main reaction of pickling and causes the "undercut" effect (Figure 5(e)).In addition, the hydrogen bubbles (not marked) generated can promote the fluidity of the electrolyte and mechanically damage the oxide scales.With prolongation of the pickling, the chromium-depleted layer is continuously dissolved until the whole oxide scale breaks away from the matrix, which indicates the end of the pickling process.

Conclusions
In conclusion, the oxide scales formed on the 2205 hotrolled plate after annealing is mainly divided into two layers: the outer layer is the dense Fe 2 O 3 crystal, and the inner is the granular FeCr 2 O 4 spinel.e outer layer is broken after shot blasting treatment.When put in the electrolyte, the potential of the oxided 2205 decreases rapidly to the minimum value for some time and then gradually increases, showing obvious characteristics of hydrochloric acid pickling.During the pickling process, pittings are firstly formed around the boundaries of the FeCr 2 O 4 particles or interfaces of the two kinds of oxides and then the electrolyte penetrates the oxide scales along the pittings to react with the chromiumdepleted layer directly.is reaction produces an "undercut" effect so that the oxide scales are effectively removed.

Figure 1 :
Figure 1: XRD pattern of the 2205 matrix and the surface oxide scale.

Figure 2 :Figure 3 :
Figure 2: SEM images of the cross-sectional morphology (a) and EDS maps of Fe (b), Cr (c), and Si (d) for oxidized 2205.

Figure 4 :
Figure 4: SEM images of the oxide scales after immersion in the pickling electrolyte for 30 s (a), 60 s (b), and 90 s (c), respectively.e matrix after pickling (d) and EDS spectrums corresponding to points A (e) and B (f ) in (a), respectively.