1 Introduction

Metallic corrosion is the result of random deterioration of metals and alloys mostly beginning at the surface due to electrochemical interaction with their environment [1]. Stainless steel belongs to the group of metallic alloys known as ferrous metals. They are extensively used as the material of construction for applications in petrochemical plants, chemical processing industries, energy generation, boilers, automobiles and desalination plants due to their higher corrosion resistance compared to carbon steels, and excellent mechanical and physical properties tailored to meet the specific requirements of application requirements, conditions and specifications. The corrosion resistance of stainless steels is due to the presence of an impenetrable, self-healing and transparent protective oxide film on their surface. The film is characteristically passive; however, in the presence of highly reactive anions such as chlorides, sulphates, thiosulphates etc. weakening and localized breakdown of the film occurs especially at regions or sites with defects and non-metallic inclusions resulting in accelerated dissolution of the underlying metal [2]. This results in limited but yet destructive corrosion reaction mechanisms which in many cases are deep with major impact on the practical application of steels and the national economy [3]. One of the most common localized corrosion damaging stainless steels is pitting corrosion. Pitting corrosion occurs in limited dimensions across the surface of stainless steels in the form of cavities or ‘holes’ [4, 5]. In addition to chlorides, low dissolved oxygen concentrations and poor application of protective coatings also lead to pitting corrosion. The most significant dangers of pitting corrosion are its insidious nature and short initiation time without any warning. Metallic passivation and its collapse leading to pit initiation has been one of the major focuses of corrosion research [6,7,8,9,10,11,12,13,14]. The extent of pitting damage differs with respect to concentration of corrosive anions in aqueous industrial environments and the degree to which it propagates autocatalytically [15]. Pitting corrosion penetrates the mass of the metal from the initiation point with limited diffusion of metallic ions. Chlorides and sulphates are the most important ions when considering the possibility of pitting corrosion of stainless steels. 904L steel is a super austenitic stainless steel designed for moderate to high corrosion resistance in a wide range of process environments where 316L application is limited. Its molybdenum and copper content enhances its resistance to localized attack by chlorides and reducing acids. The nickel composition of the steel gives it greater resistance to hydrogen embrittlement. Its low carbon content makes it resistant to sensitization and intergranular corrosion. 904L stainless steel is applied as bleaching equipment in pulp and paper processing industries, process equipment in chemical industry, oil refineries, sea water cooling devices, hydrometallurgy, washers and fans in organic acid treatment systems, storage and transportation equipment for sulfuric acid, acid production, power plant flue gas desulfurization device etc. Previous research has shown that 904L undergoes pitting corrosion even though the steel has high molybdenum content [16, 17]. According to Abdullah et al. [18], the size and aspect ratios of pits depend on the aggressiveness of environmental condition to which 904L is exposed. 904L has been shown to have a higher pitting resistance than 304L, 316L and 317L stainless steels [19]. Chen et al. [20] studied the influences of chloride anions on corrosion electrochemical characteristics of 904L stainless steel in blast furnace gas/water system and observed a direct relationship between self-passivation characteristic of 904L chloride concentration. 904L has shown considerable thickness loss in atmospheric distillation units, but has been used successfully to resist fatty acid corrosion in residence time distribution (RTD) reactor which operates at high temperatures and pressures [21, 22]. 439L steel is titanium stabilized ferritic stainless steel specifically produced to withstand corrosion attack in oxidizing conditions and boiling acids where Type 304 and 409 steel are considered inadequate due to chloride stress corrosion cracking and higher thermal conductivity. The steel exhibits enhanced formability capabilities and has extensive application in the automotive industry for use as tubular manifolds and other exhaust system components, residential furnace primary heat exchangers and nuclear applications. Type 439L stainless steel has being indicated as a substitute tube material for moisture separator/reheater in the steam cycle of pressurized water reactor nuclear power plants due to improved component reliability and steam system chemistry [23]. Tiburcio et al. [24] showed that type 439 steel has weaker resistance to general corrosion compared to 409 steel in H2SO4, however, it exhibits higher pitting resistance in chloride solutions. Loto and Loto [25, 26] studied the corrosion resistance of 439L ferritic steel in chloride/sulphate solution and observed that it outperforms 420 ferritic steel, 2101 duplex steel and 301 austenitic steel in terms of pitting and general corrosion resistance. 439L was also observed to exhibit sufficient corrosion resistance in hot chloride/sulphate solution. Research has also shown that 439L is more prone to general and pitting corrosion in highly oxidizing environments compared to NO7718, NO7208 nickel alloys [27]. This research studies and compares the pitting corrosion resistance and passivation characteristics of 439L ferritic and 904L austenitic stainless steels in sulphate–chloride environments.

2 Experimental Methods

2.1 Materials

904L super austenitic stainless steel (ST904L) and 439L ferritic stainless steel (ST439L) plates were sourced from Vienna University of Technology, Austria and McMaster University Canada. Both steels were examined with PhenomWorld scanning electron microscope (Model No. MVE0224651193) at the Materials Characterization Laboratory in Department of Mechanical Engineering, Covenant University, Ota, Ogun State, Nigeria. The nominal (wt%) content of the steels are shown in Table 1. ST904L and ST439L were machined to average surface dimension of 1 cm2 prior to mounting in acrylic resin. The exposed surface of the steel was smoothened with emery grinding papers (80, 320, 600, 800 and 1000 grits) and polished with 6 µm diamond polishing paste before cleaning with distilled water and acetone. 200 mL each of 3.5 M H2SO4 at 0, 1, 2, 3, 4, 5 and 6% NaCl concentration were prepared from analar grade reagent of the acid-recrystallized NaCl. Neutral chloride solutions at similar NaCl concentration were also prepared.

Table 1 Composition (wt%) of ST904L and ST439L
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2.2 Electrochemical Analysis

Electrochemical analysis through potentiodynamic polarization, open circuit potential measurement and current–time measurement was done at 30 °C with a ternary multicomponent electrode system (acrylic mounted steel working electrodes with exposed surface area of 1 cm2, Ag/AgCl reference electrode and platinum counter electrode) within a transparent glass cell containing 200 mL of the electrolyte solution interfaced with Digi-Ivy 2311 potentiostat and computer. Potentiodynamic polarization plots were produced at scan rate of 0.0015 V/s between potentials of − 1.5 V and + 1.5 V. Corrosion current density CD (A/cm2) and corrosion potential CP (V) were determined from the Tafel extrapolation method.

Corrosion rate CR (mm/year) was calculated from the formula below;

$$ C_{\text{R}} \, = \,\frac{{0.00327 \times C_{\text{D}} \times E_{\text{QV}} }}{D} $$
(1)

EQV is the equivalent weight (g) of stainless steel, 0.00327 is a corrosion rate constant and D is the density (g). Open circuit potential measurements were performed at a step potential of 0.1 V/s for 6300 s to obtain information on the thermodynamic stability of the steels in H2SO4 and neutral chloride solution with three-electrode electrochemical cell consisting of Ag/AgCl reference electrode and resin-mounted 1060AL/SiC working electrode (exposed surface of 1 cm2) submerged in 200 mL of the test electrolytes and linked to Digi-Ivy 2311 potentiostat. Current–time measurements at initial voltage of 0.15 V and step potential of 0.1 V were performed for 3250 s with the same electrochemical system. The electrochemical system was checked for possible causes of systematic errors. The uncertainty of single measurement is limited by the precision and accuracy of the measuring instrument. As a result, calibration of the instrument and hardware test was performed with the results shown in Table 2. Test for reproducibility of consistent results was also performed.

Table 2 Results of calibration and hardware test
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3 Results and Discussion

3.1 Polarization Studies

Potentiodynamic polarization plots of ST439L and ST904L in 3.5 M H2SO4 and H2O solution at 0–6% NaCl concentration are shown from Figs. 1, 2, 3, and 4. Tables 3 and 4 show the data obtained for ST439L and ST904L polarization. Comparison of the corrosion rate values in Table 3 shows ST439L to be relatively more resistant than ST904L in 3.5 M H2SO4 solution at 1–5% NaCl concentration. Their corrosion rate values are comparable at 0% NaCl (0.130 mm/year and 0.189 mm/year). However, addition of Cl− ions into the acid solution significantly increased the corrosion rate values of ST904L compared to ST439L. The corrosion rate is proportional to the corrosion current density obtained, signifying higher redox electrochemical processes occurring on ST904L. This observation shows ST904L has a slightly higher tendency to oxidize in Cl− ion containing sulphate environments. Beyond 5% Cl− ion concentration, the corrosion rate of ST439L is 2.355 mm/year which is significantly higher than 0.778 mm/year for ST904L. The results show ST439L is more resistant to general corrosion in the presence of Cl− ions at concentration (1–5% NaCl). However, at higher Cl− ion concentration 904L is more corrosion resistant. Significant cathodic shift on the corrosion polarization plots of ST439L beyond 0% NaCl shows hydrogen evolution and oxygen reduction reactions dominated the redox electrochemical mechanisms on 439L. This is further proven from the higher anodic Tafel slope of ST439L signifying lower anodic exchange current density. This observation suggests ST439L is prone to localized corrosion reactions under certain conditions. The polarization plot of ST904L varies between cathodic and anodic potential shifts. The Tafel slopes values of ST904L agree with this assertion where the redox electrochemical reactions associated with corrosion counter balances each other. The polarization plots in Figs. 3 and 4 significantly contrast the plots earlier discussed (Figs. 1, 2). The presence of Cl− ions (without SO42−) caused unstable passivation of both steels resulting in additional anodic–cathodic peaks at − 0.764 V, − 0.349 V, − 0.784 V and − 0.309 V (4% and 6% Cl− ion concentration) for 439L, and − 0.563 V, − 0.243 V, − 0.494 V and − 0.438 V (3% and 4% Cl− ion concentration) for 904L. This phenomenon is due to the re-initiation of active redox electrochemical reactions on the alloy surface. The presence of chlorides breaks down the passive film that formed after the first anodic polarization. The corrosion rate results of both steels in the chloride solution are comparable without any significant variation though they are significantly lower than the values obtained in the presence of SO42− ions. However, the anodic passivation of ST904L tends to be more stable before trans-passivation. In the presence of Cl− ions the variation of the polarization plots coupled with the values of the anodic–cathodic Tafel slope shows oxidation and redox electrochemical processes counterbalances on the alloys surface. Nevertheless, the anodic Tafel slopes for both steels are generally lower than the values obtained for the cathodic Tafel slope. This signifies the oxidation and passivation characteristics of the steels have strong influence on the corrosion reaction processes due to the action of chlorides.

Fig. 1
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Potentiodynamic polarization plots of 439L corrosion in 3.5 M H2SO4 at 0–6% NaCl concentration

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Fig. 2
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Potentiodynamic polarization plots of 904L corrosion in 3.5 M H2SO4 at 0–6% NaCl concentration

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Fig. 3
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Potentiodynamic polarization plots of 439L corrosion in H2O at 0–6% NaCl concentration

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Fig. 4
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Potentiodynamic polarization plots of 904L corrosion in H2O at 0–6% NaCl concentration

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Table 3 Potentiodynamic polarization plots of ST439L and ST904L corrosion in 3.5 M H2SO4 at 0–6% NaCl concentration (n = 1)
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Table 4 Potentiodynamic polarization data of ST439L and ST904L corrosion in H2O at 0–6% NaCl concentration (n = 1)
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3.2 Pitting, Metastable Pitting and Passivation Studies

Potentiostatic data showing metastable pitting potential (V), metastable pitting current (A), passivation potential (V), pitting potential (V), pitting current (A) and passivation range (V) for ST439L and ST904L are presented in Tables 5 and 6. In 3.5 M H2SO4 at 1–5% Cl− ion concentration, ST439L and ST904L exhibited significant passivation behaviour following anodic polarization and metastable pitting activity. This is due to the presence of Cr in both steels which reacts with dissolved O2 to form a passivating (chemically inactive) layer of Cr2O3 and Cr(OH)3. The passivation range in Table 5 shows the effect of Cl− ion adsorption on the steels. It is clearly evident that the presence of chlorides increases the localized corrosion resistance of ST439L and ST904L. At 0% NaCl, the passivation range of ST439L and ST904L are 0.931 V and 0.873 V in the presence of SO42− ions only. Beyond 0% NaCl, the passivation range value increases peaking at 1.117 V for ST439L and 1.213 V for 904L at 5% NaCl concentration. The passivation current value for ST904L increases with increase in Cl− ion concentration as a result the passive film of the alloy has a high tendency to reform after metastable pitting even under high electric field. ST439L displays this unusual property, however, after 2% Cl− ion concentration, the passivation current value decreases due to the effect of excess chloride ions hindering the reformation of the film. The presence of chlorides up to a certain concentration enhances the pitting resistance of the steels due to limited adsorption of Cl− ions in competition with O2 on the steel surface which reacts with Cr [28, 29]. The passive films on stainless steels has been known to be an adsorbed layer of O2 whereby formation of corrosion pits occurs due to displacement of O2 by Cl− ions. There is the possibility of the formation of metal ion–chloride complex enabling solution diffusion. Under this condition, the passive film probably weakens until complete film removal and active dissolution [30,31,32,33,34]. Hence, the chemical layer on ST439L and ST904L is basically a chemical complex consisting of adsorbed chlorides. ST904L generally exhibited greater passivation range values signifying higher pitting resistance in sulphate/chloride environment compared to ST439L. Beyond 5% NaCl, passivation behaviour was completely absent on the polarization plots in Figs. 1 and 2 due to collapse of the passivating film. Adsorption of chlorides beyond the threshold concentration for limited pitting resistance is responsible for this observation. The high electronegativity value of Cl− ions compared to the passive layer of absorbed O2 induces chemical instability to the passive layer. These steels are vulnerable at defect, flaw sites of the layer. The chloride reduces to a stable compound which breaks down the passive layer. Results for pitting potential of both steels contrast each other, while a decrease in value was observed for ST439L, the values for ST904L increased signifying enhanced pitting resistance. Nevertheless, the decrease in pitting potential values for ST439L where net out by early onset of passivation of the steel resulting in extended passivation range. Addition and increase of Cl− ions enhances the pitting resistance of both alloys before breakdown at the trans-passive region though this can be contested on the premise that the passive film formed consist of adsorbed Cl− ions complexed with O2 which give the steel a pseudo-passivating effect. At 0% NaCl, the pitting potential value of ST439L and ST904L are 0.976 V and 0.638 V, after which the values of ST439L decreased to ranges between 798 and 838 V (1–5% NaCl concentration), while the pitting potential values of ST904L increased between ranges of 741 V and 814 V (1–5% NaCl concentration). The metastable pitting portion of the polarization curves in Figs. 1 and 2 follows anodic polarization of ST439L and ST904L before stable passivation of both steels. In this portion of the polarization curve, collapse of the passive film and repassivation events occur at potentials well below the potential for stable pitting propagation [35,36,37,38]. It is observed that the stretch of the metastable pitting of the curves varies with respect to Cl− ion concentration in accordance with the adsorption mechanism of passivity breakdown. The adsorption of Cl− ions at the metal-solution interface stimulates the diffusion of oxidized metal ions into the acid [39,40,41]. At 1% NaCl, the stretch is at minimum while the largest stretch is at 5%. Observation of metastable pitting current in Table 4 for ST439Land ST904L shows a proportional increase in value with increase in Cl− ion concentration. However, metastable pitting events were absent at 6% NaCl for reasons earlier discussed.

Table 5 Potentiostatic data for ST439L and 904L in 3.5 M H2SO4/(0–6% NaCl concentrations) n = 1
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Table 6 Potentiostatic data for ST439L and 904L in neutral chloride solution at 0–6% NaCl concentrations (n = 1)
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The pitting resistance and passivation behaviour of ST439L and ST904L in neutral chloride solution at specific Cl− ion concentration significantly contrasts its behaviour in the presence of combined SO42−/Cl− ions (3.5 M H2SO4/NaCl concentrations). Metastable pitting activity was completely absent from the polarization plots in Figs. 4 and 5 after anodic polarization of the steels under study. However, dense current transients at the onset of passivation and the presence of additional anodic–cathodic peaks at some Cl− ion concentrations indicates the passive film formed on both steels are unstable. SO42− ions has been known to inhibit the onset of corrosion pits in the presence of chlorides in previous research due to competitive adsorption, hence the relatively stable passivation plots in Figs. 2 and 3 though some other research indicates increased vulnerability to pitting corrosion [42, 43]. The passivation range value of ST439L in Fig. 4 decreased from 1.633 V at 0% NaCl concentration to 1.276 V at 6% NaCl. The decrease was progressive with respect to NaCl concentration until 4% NaCl and is due to irreversible breakdown of the passive film at the pitting potential. The passivation current value varies with increase in Cl− ion concentration as a result of excess Cl− ions available localized electrochemical reactions on the steel surface. Observation of the pitting potential values in Table 5 shows a progressive decrease in value with respect to NaCl concentration from 1.336 V at 0% NaCl to 1.061 V at 6% NaCl. The pitting potential is the point at which stable passivation occurs following metastable pitting. The pitting potential values for ST904L decreased after 0% NaCl concentration signifying decreased resistance to corrosion. However, this assertion do no hold as a result of the corresponding passivation range values which do not necessarily imply decreased localized corrosion resistance with respect to NaCl concentration. In fact the pitting resistance of ST904L increased in the presence of Cl− ions at most concentrations studied.

Fig. 5
figure 5

Plot of corrosion potential versus exposure time for ST439L and ST904L in 3.5 M H2SO4 at 0, 1 and 6% NaCl concentration

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3.3 Open Circuit Potential Measurement (OCP)

Figures 5 and 6 shows the plots of open circuit potential versus exposure time for ST439L and ST904L in 3.5 M H2SO4 and H2O solution at predetermined concentrations of NaCl. The OCP plots of ST439L and ST904L at 0% NaCl (Fig. 5) were significantly more electropositive than the plots at higher NaCl concentration. The position of the two plots is due to the presence of the passive film (earlier discussed) on the steel surfaces. ST439L plot initiated at 0.054 VAg/AgCl (0 s) and progressively shifted to 0.141 VAg/AgCl at 1093 s due to the gradual formation of the impenetrable protective oxide on the steel. The oxide remained stable to the end of the exposure hours. The plot of ST904L initiated at − 0.182 VAg/AgCl and increased sharply over a wider margin to 0.033 VAg/AgCl after which it gradually increased, peaking at 0.152 VAg/AgCl (6300 s). It must be noted that thermodynamic instability was observed on the plot between 2118.82 s at 0.097 VAg/AgCl and 4292.23 s at 0.137 VAg/AgCl. While the OCP plots of ST439L and ST904L at 1% NaCl were comparatively similar throughout the exposure period, observation shows ST439L at 6% NaCl tends to be more corrosion resistant than ST904L though this could be due to the nature of the oxide/chloride complex formed on the steel with respect to the peculiarity of the metallurgical structure and surface properties of ST439L which differentiates it from ST904L. ST439L initiated at − 0.223 VAg/AgCl, attaining relative stability at 925.41 s. However, the presence of potential transients at some specific points signifies depassivation and repassivation of the steel. This occurrence was prominent before 1973.92 s. ST904L initiated at − 0.220 VAg/AgCl and peaked at − 0.207 VAg/AgCl (6300 s) signifying relative stability throughout the exposure hours. The plots in Fig. 5 shows Cl− ions significantly influence the passivation characteristic of both steels. Cl− ions increase the thermodynamic instability of the steels causing higher vulnerability to corrosion compared to SO42− ions without applied potential. This is the basis of the wide variation in the OCP plots with respect to Cl− ion concentration. The electronegative potential shows the protective passive film has thin out compared to the electropositive plots at 0% NaCl concentration. The chloride–oxide complex on the acidified steel surface prevents the formation and growth of the oxide film evident on the plot of ST904L at 0% NaCl concentration (Fig. 5).

Fig. 6
figure 6

Plot of corrosion potential vs exposure time for ST439L and ST904L in neutral chloride solution at 0%, 1% and 6% NaCl concentration

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The OCP plots of ST439L in neutral chloride solution were significantly more electropositive than the plot obtained for ST904L at 0% NaCl concentration (Fig. 6). The oxide film on ST904L proves to be thinner than ST439L without applied potentials though it is thermodynamically stable. The OCP plots of ST439L at 1% and 6% NaCl concentrations attained thermodynamic stability at 521.80 s and 157.60 s (− 0.202 VAg/AgCl and − 0.406 VAg/AgCl) till 6300 s at − 0.192 VAg/AgCl and − 0.219 VAg/AgCl. The plot of ST904L at 1% and 6% NaCl concentration exhibited similar thermodynamic stability and active–passive transition behaviour during the exposure hours. However, the onset of OCP at these concentrations shows potential variation over a wide margin from electronegative potentials. ST904L (1% and 6% NaCl) initiated at − 0.788VAg/AgCl and − 0.867 VAg/AgCl (0 s), peaked at electropositive potentials of − 0.193 VAg/AgCl (3078.62 s) and − 0.161 VAg/AgCl (1210.21) due to formation of the protective oxide formation on the steel surface before attaining a final value of − 0.218 VAg/AgCl and − 0.253 VAg/AgCl at 6300 s. At 1% and 6% NaCl, the OCP plot of ST904L decreased due to stabilization of the passive film and more importantly inhibition of the growth of the protective oxide on the steel. However, the surface properties remain thermodynamically stable.

3.4 Current–Time Measurement

The active–passive behaviour of ST439L and ST904L corrosion in 3.5 M H2SO4 and neutral chloride solutions is shown in the current–time plots from Figs. 7a to 8b. Figure 7a and b shows the current time plot of ST439L and ST904L in 3.5 M H2SO4 at 0%, 1% and 6% NaCl while Figs. 8a and b shows the current time plot for ST439L and ST904L in neutral chloride solution at 0, 1 and 6% NaCl concentration. Comparison of the plots in Fig. 7a and b shows ST439L and ST904L at 0% and 1% NaCl concentration exhibited similar configuration compared to the plots at 6% NaCl. ST439L at 0% and 1% NaCl (Fig. 7a) initiated at − 2.63 × 10−5 A and − 3.42 × 10−6 A; this difference is due to the presence of chlorides at 1% NaCl, as a result the plot of ST439L at 1% NaCl progress from electronegative values before achieving relative stability at − 3.11 × 106 A. The plot at 0% and 1% NaCl alternated due to active passive transition between current values of − 6.07 × 10−7 A and 1.52 × 10−6 A. till 3250 s. ST904L plot at 0% and 1% NaCl attained relative stability at 92.80 s and shifted between current values of 1.75 × 10−6 A and − 6.07 × 10−7 A. Observation of the current transients shows a gradual decrease with respect to exposure time. These observations for ST439L and ST904L show low chloride concentration has minimal effect on the passivation behaviour and passive film characteristics of both steels. However, at 6% NaCl concentration significant shift to electronegative values was observed for both steels. The current–time plots for ST439L and ST904L initiated at − 2.02 × 10−5 A and − 1.18 × 10−4 A and achieved relative stability at 300 s (− 3.26 × 10−6) and 500 s (− 8.20 × 10−6). The current–time plots for ST904L (Fig. 7b) appears more thermodynamically stable at more electronegative potentials than the plot for ST439L with shorter current transients (amplitudes) due to the characteristic nature of its passive filmed formed and allows for rapid passivation of the steel surface. The transients are linked to variations to the rates of anodic and cathodic reactions as a result of breakdown and repassivation of passive film [44, 45]. The amplitudes of current fluctuations shown in Fig. 8a and b significantly exceeds the values in Fig. 7a and b. It must the noted that the excessive amplitudes for ST439L and ST904L at 0% NaCl in the neutral chloride solution the process does not involve active–passive film transition only but is also due to fluctuations in the O2 mass transport due to dominant reduction reactions [46]. The amplitude of current transients for ST439L at 1% NaCl from 0% NaCl concentration remained the same until 6% NaCl were a decrease in amplitude was observed. This is due to the counterbalancing action of anodic and cathodic reaction processes. The amplitude of current transients for ST904L at 1% NaCl and 6% also significantly decreased for reasons earlier mentioned.

Fig. 7
figure 7

Plot of current versus time for in 3.5 M H2SO4 at 0%, 1% and 6% NaCl concentrations a ST439L and b ST904L

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Fig. 8
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Plot of current versus time for in neutral chloride solution at 0, 1 and 6% NaCl concentrations a ST439L and b ST904L

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3.5 Optical Microscopy Analysis

The optical microscopy images of ST439L and ST904L at mag. × 40 and × 100 are shown from Figs. 9a, 10, 11, 12 and 13c. Figure 9a and b shows the images before corrosion test. Figure 10a–c and 11a–c shows the optical images of ST439L and 904L after corrosion in 3.5 M H2SO4 at 0, 1 and 6% NaCl concentration. Figure 12a–c and 13a–c shows the optical images of ST439L and 904L after corrosion in neutral H2O at 0, 1 and 6% NaCl concentration. The effect of the electrochemical action of Cl−/SO42− ions on the morphology of ST439L is clearly visible in Fig. 10a–c. Corrosion pits and cracks representing the grain boundaries appeared on Fig. 10a due to surface deterioration. At 1% NaCl (Fig. 10b) the occurrence of corrosion pits increased coupled with fading of the grain boundaries due to wear resulting acidification of the steel surface. The grain boundaries have largely faded out at 6% NaCl due to the action of excess Cl− ion concentration compared to 0% and 1% concentration. However, the extent of surface deterioration of the steel significantly increased with newly formed and enlarged corrosion pits. The morphological characterization of ST904L (Fig. 11a, b) proves its more resistant to pitting corrosion than ST439L as discussed under the pitting corrosion section. The occurrence of pitting corrosorion on the steel surface is relatively scanty compared to ST439L. However, ST904L appears vulnerable to intergranular corrosions as the extent of grain boundary deterioration increased with increase in NaCl concentration. At 6% NaCl concentration (Fig. 11c), the extent of grain boundary deterioration increased with few, but significant corrosion pits. Figure 12b shows lower Cl− ion concentration has minimal effect on the localized corrosion resistance of ST439L due to the absence of corrosion pits. Variation in the surface morphology shows slight changes which appears to be superficial. However, at 6% NaCl concentration corrosion pits of various sizes appeared. ST904L gave similar observation at 0% NaCl; at 1% NaCl the extent of surface deterioration is comparatively mild with very few corrosion pits at specific regions of the steel signifying higher corrosion resistance. At 6% NaCl concentration (Fig. 13c), the extent of morphological damage is slightly significant due to the appearance of few shallow macro-pits and the metallurgical structure of the steel with visible equiaxed grain boundaries [47]. The shallow pits show ST904L is more resistant than ST439L because the autocatalytic mechanism responsible for pitting was arrested at the initial stages of pit propagation. ST439L at this same NaCl concentration (Fig. 11) exhibited corrosion pits at different stages of propagation.

Fig. 9
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Optical microscopy images before corrosion at mag. × 40 and × 100 a ST439L, and b ST904L

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Fig. 10
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Optical microscopy images of ST439L (mag. × 40 and × 100) after corrosion in 3.5 M H2SO4 a at 0% NaCl, b at 1% NaCl, and c at 6% NaCl concentration

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Fig. 11
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Optical microscopy images of ST904L (mag. × 40 and × 100) after corrosion in 3.5 M H2SO4 solution a at 0% NaCl, b at 1% NaCl, and c at 6% NaCl concentration

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Fig. 12
figure 12

Optical microscopy images of ST439L (mag. × 40 and × 100) after corrosion in neutral chloride solution a at 0% NaCl, b at 1% NaCl, and c at 6% NaCl concentration

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Fig. 13
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Optical microscopy images of ST904L (mag. × 40 and × 100) after corrosion in neutral chloride solution a at 0% NaCl, b at 1% NaCl, and c at 6% NaCl concentration

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4 Conclusion

904L austenitic steel exhibited enhanced resistance to pitting corrosion compared to 439L ferritic steel, though 439L steel was slightly more resistant to general corrosion under applied potential variations. The passive film formed on 904L steel hindered the formation of corrosion pits on the steel surface resulting in few macro indentations. This contrast the presence of numerous corrosion pits of different dimensions on 439L steel as a result of the nature of its passive film and chloride–oxide complex formed. Open circuit potential measure showed both steels are thermodynamically stable in the presence of chlorides and sulphates, however, the potentials of 904L were more electronegative.