The effect of long-term exposure in mixed sulfuric acid and copper sulfate solution on corrosion behavior of austenitic stainless steels

Corrosion resistance of austenitic stainless steels in sulfuric acid solutions depends mainly on concentration and temperature of the sulfuric acid, the presence of contaminants and velocity flow. Current studies aim mostly at evaluation of electrochemical parameters in sulfuric acid solutions. Research focused on long-term exposure is lacking. This article deals with the corrosion behavior of AISI 304 and AISI 316L stainless steels in mixed 10 wt. % sulfuric acid and 10 wt. % copper sulfate solution. Sensitized (650 °C/40 hours) and solution annealed (1050 °C/15 min) specimens together with the untreated (as received) ones are evaluated by long-term (22-month) exposure immersion test at the temperature of 22 ± 3 °C on the bases of optical microscopy, SEM and mass losses. Both tested stainless steels in as received and solution annealed state proved high corrosion resistance in the given corrosive environment. The cross-sections edges of sensitized specimens revealed a close relationship between local corrosion sites and weakened grain boundaries, which indicates incipient intergranular corrosion.


Introduction
In addition to common domestic, industrial, transport and architectural applications, austenitic stainless steels are also acceptable construction material across the chemical-processing and petrochemical industries.It is due their ability to withstand attack from highly corrosive substances when they are in the passive state, as well as their effective mechanical characteristics -good creep resistance, high strength, ductility and malleability, weldability and toughness [1][2][3].However, it is necessary to take into account, that stainless steels have some important limits in their corrosion resistance.In addition to the susceptibility to the local pitting corrosion in chloride containing media they may also suffer from the intergranular corrosion [1][2][3][4][5][6].This corrosion form usually takes place in aggressive environments after sensitization by thermal exposure in critical temperatures with consequent slow cooling in the air.Under these conditions, M23C6 chromium-rich carbides precipitate on the grain boundaries.It causes a drop of the chromium content near the grain boundaries under the passivity limit (11.5 wt.%) and a narrow band along the grain boundaries becomes anodic to the unaffected grains [5][6][7].Chromium depleted zones often become the preferential paths for pitting corrosion attack and in this way can be both pitting and intergranular corrosion closely related [8,9].
Stainless steels generally offer good resistance to the corrosion in sulfuric acid, which is one of the most important industrial chemicals.The level of their resistance varies depending on the grade of stainless steel and the concentration and temperature of the sulfuric acid [10][11][12].The presence of oxidizing or reducing contaminants, velocity effects and solids in suspension also affect the aggressiveness of this acid toward the stainless steels [12].At ambient temperatures austenitic stainless steels exhibit stabile passivity state in highly concentrated sulfuric acid (above 93 %) and they are frequently used for piping and tankage where product purity is desirable.The upper temperature limit for stable passivity for 304 and 316 in 93% solution is around 40 °C [12].In dilute sulfuric acid solutions, austenitic stainless steels can be used but molybdenum containing grades exhibit higher corrosion resistance than Cr-Ni steels [12].The authors [1] studied corrosion resistance of citric acid passivated AISI 304 stainless steel in 1 wt.% sulfuric acid by potentiodynamic polarization and revealed a passive behavior in temperature range 25 -49 °C.Sun et al. [13] documented a positive effect of alloying tin which through tin oxides improves the density and uniformity of the passive film, and ensures higher corrosion resistance in boiling sulfuric acid solutions.
However, diluted sulfuric acid solutions are considered the environments that can induce intergranular corrosion of austenitic stainless steel [14][15][16].The aggressiveness of these solutions in relation to the intergranular corrosion can be increased if mixed with copper sulfate due to the catalytic effect of copper ions and promoting the precipitation of copper-rich phases at the grain boundaries [15,17].
The objective of the presented study is evaluation of corrosion resistance of AISI 304 and AISI 316L stainless steel in mixed 10 wt.% sulfuric acid and 10 wt.% copper sulfate solution.Sensitized (650 °C /40 hours) and solution annealed (1050 °C /15 min) specimens together with untreated (as received) ones are evaluated by long-term (22-month) exposure immersion test at the temperature of 22 ± 3 °C on the bases of optical microscopy (OM), SEM and mass losses (corrosion rates).

Materials and methods
AISI 304 and AISI 316L austenitic stainless steels with chemical compositions in table 1 were used as the experimental material.They were purchased in sheets (1000 x 2000 mm) of 1.5 mm thickness.Their production processes were based on continuous casting in electric arc furnace.Then they were annealed (AISI 304 at 1040 -1100 °C, AISI 316L at 1050 °C).The IIB surface finish (smooth and matte metallic glossy surface) was prepared by pickling after slightly smoothing rolling.Table 1.Chemical compositions of experimental stainless steels (wt.%).The rectangular specimens (15 mm × 40 mm × 1.5 mm) were prepared for the corrosion resistance testing.A part of the specimens was left in the as received state.The additional specimens were used for heat exposure carried out before the corrosion test.The heat exposure for sensitization (i.e. in critical temperatures) was performed in furnace at 650 °C for 40 hours with consequent slow cooling in the air to create suitable diffusion conditions for chromium carbides precipitation (conditions were adjusted according to the diagram of carbon solubility in austenite [18]).Sensitization of specimens was confirmed by the oxalic acid etching test performed according to A practice of ASTM A262 standard method [17] under the conditions listed in table 2. Before the etching test the specimen surface was prepared metallographically, rinsed with ethanol and air-dried.During electrochemical etching, the specimen was connected to the positive pole as the anode (+), the cathode (-) was stainless steel block.The cathode and anode were mutually parallel (distance 5 mm).The etched surface was evaluated by OM [17].The heat exposure for solution annealing was performed in furnace at 1050 °C for 15 minutes with cooling in the air.Microstructures of solution-annealed specimens were observed by OM.

Cr
The 22-month exposure immersion test (performed 31.07.2020-27.05.2022, i.e. 15980 hours) simulating aggressive sulfuric acid containing industrial environment was carried out in mixture 10 wt.% sulfuric acid and 10 wt.% copper sulfate solution at 22 ± 3 °C.Solution pH, measured before the long-term corrosion test was 0.49, specific conductivity 320 mS/cm.The group of three parallel specimens was tested for each type, i.e. as received, sensitized and solution annealed.The overview of the tested specimen types is shown in table 3. Sensitized and solution annealed specimens were left as heat tinted, i.e. with high temperature surface oxides.All specimens were degreased by ethanol and weighted out with accuracy ± 0.000 01 g before the test.After exposure, the specimens were brushed, washed by demineralized water, freely dried and weighted out again [19,20].The cross-sections observed by OM and SEM were used for evaluation of the corrosion resistance.Average corrosion rates after 22-month exposure (mm/y) were calculated from the specimen mass losses according Bell [21].

Results and discussion
The oxalic acid etching test performed according to ASTM A262 practice A standard on heat exposed specimens (650 °C/40 hours) confirmed their sensitization.Obtained etch microstructures (figure 2) can be considered the ditch ones because the grain boundaries are surrounded by ditches caused by carbide dissolution [6,17].The appearance of individual types of the specimens after the completed long-term corrosion test is shown in figure 4. The specimens of both AISI 316L and AISI 304 stainless steels in as received state remained without corrosion damage when viewed with the naked eye.The high corrosion resistance of these specimens was confirmed by almost intact edge of the cross sections observed by OM and SEM (figure 5 and figure 6) and also by very low average corrosion rates calculated from mass losses (table 4).The authors [1,16] also revealed stable passive behavior of austenitic stainless steels in diluted sulfuric acid solution by potentiodynamic polarization.According to figure 4, the surfaces of sensitized and solution annealed specimens were affected by colored high-temperature oxides which were partly removed during the long-term exposure and the uncovered surfaces were attacked.The high-temperature oxides arose during heat exposure by the oxidation of chromium.It led to a chromium depletion of the regions below the heat tint and consequently to a higher susceptibility to a local corrosion in the aggressive environment [22][23][24].
The chromium depletion could be the most intensive for solution-annealed specimens because of dependence of the high-temperature oxide film thickness on the temperature of heat treatment [24].The most attacked seems to be 304 SA surface (figure 4f).A difference between the appearance of 316L SA (figure 4c) and 304 SA specimen could be related to the protective effect of molybdenum contained in 316L stainless steel [3,5].Local corrosion damage of solution-annealed specimens can be observed in more detail in edges of cross sections taken from the middle parts of specimens (figure 7 and figure 8).In spite of the surface damage observed by naked eye (figure 4c, 4f) it is obvious that the corrosion attack is only superficial and the microstructure in deeper parts shows no signs of damage.The corrosion rates are on the order of 1000x higher than those in the case of as received specimens.It may be related to a partial dissolution of high temperature oxides during the long-term exposure.It should be remembered that corrosion rates below 0.01 mm/y are considered low and the high corrosion resistance of these samples in the used corrosive environment can be stated.Sadawi et al. [16] similarly detected the high corrosion resistance for solution annealed AISI 304 in 1 M H2SO4 (it approximately corresponds to 10 wt.% H2SO4).The improved corrosion resistance in diluted sulfuric acid solution was also recorded for post weld solution annealed austenitic stainless steel weldments by the authors [25].As can be seen in figure 9 and figure 10, the previous sensitization was reflected in the appearance of the specimen cross-sections after long-term corrosion test.The sites of local corrosion seem to be closely related to weakened chromium-depleted zones adjacent to grain boundaries and the damage extends deeper than in case of solution annealed specimens (figure 7 and figure 8).The weakened grain boundaries observed especially in optical microscopy micrographs (9a, 10a) point to incipient intergranular corrosion, which could progress deeper below the surface in the case of continued contact with the aggressive environment [14,16].The described phenomenon is more obvious in the case of AISI 304 stainless steel (10a).According to the average corrosion rates, the incipient intergranular corrosion had no effect on the mass losses and corrosion resistance.Still, based on observed microstructure a caution is necessary for the long-term use of sensitized steel in sulfuric acid.

Conclusions
Based on performed long-term 22-month exposure immersion test carried out in a mixture of 10 wt.% sulfuric acid and 10 wt.% copper sulfate solution can be concluded:  Both tested stainless steels in as received state proved high corrosion resistance in the given corrosive environment.It was confirmed by almost intact edge of the cross sections observed by OM and SEM, and by very low corrosion rates calculated from mass losses. The surfaces of sensitized and solution annealed specimens were negatively affected by colored high-temperature oxides which were partly removed during the long-term exposure and the uncovered surfaces were attacked. Detailed observation of solution annealed specimens in cross sections showed only shallow surface corrosion damage without affecting the deeper parts of the microstructure.Together with corrosion rates lower than 0.01 mm/y, this points to the high corrosion resistance of such treated material. Cross-sections edges of sensitized specimens revealed a close relationship between local corrosion seats and weakened grain boundaries.The observed damage indicates incipient intergranular corrosion; long-term exposure to sulfuric acid solutions is unsuitable.

Table 2 .
Conditions of the electrochemical etching (ASTM A262, A practice).

Table 3 .
The overview of the tested specimen types.

Table 4 .
Overview of average mass losses and average corrosion rates.