Influence of Microstructure on the Corrosion Resistance of Low Carbon Uns S41003 Stainless Steel

Abstract

The UNS S41003 (ASTM A1010) is a lean stainless steel with a chromium content of 10.5-11.0%. Its relatively low cost and enhanced corrosion resistance make it a promising alternative to carbon steels in various applications. Depending on the processing route the UNS S41003 with low carbon (<0.02%) and small Ni addition (~0.30%) may present ferritic, martensitic or ferritic-martensitic microstructures. In this work the corrosion resistance was evaluated in specimens processed by three different ways: hot rolling (HR), batch annealing (ANN), and water quenching from 1000ºC (Q). The microstructure of the HR is majoritarily martensitic, but also contains delta (δ) ferrite and fine dispersed carbides. The ANN specimen has equiaxial α ferritic grains with intergranular Cr carbides, and the quenched (Q) was martensitic. Polarization curves were carried out in 0.1M HCl solution. DL-EPR tests were also performed to evaluate the intergranular corrosion susceptibilities. It was observed that the martensitic microstructures have better corrosion than the annealed steel in polarization tests. which was highly susceptible to intergranular attack due to sensitization caused by intergranular Cr carbides.

Keywords:
Ferritic-martensitic stainless steel; Pitting corrosion; UNS S41003; Microstructure

1. Introduction

The UNS S41003 steel, is a lean stainless steel with 10,5-12.5% (wt) chromium, and maximum carbon 0,03% (wt). This alloy is manufactured to promote greater insertion in the civil construction market, which rarely uses stainless steel. This steel grade corresponds to ASTM A709 GR50CR¹, formerly referred to as ASTM A1010². With a low cost, this steel can be used in applications replacing low carbon steels where better corrosion resistance is required, such as in bridges and railroad cars³-⁵. The initial cost of UNS S41003 steel for bridge construction is higher than a painted carbon steel, but its significantly lower maintenance costs over time justify its application as a sustainable structural solution⁴,⁵. Fletcher⁵ conducted corrosion tests on various structural steels with different Cr, Si, and Al additions and reported that ASTM A1010 steel, containing 11% Cr, exhibited the lowest corrosion rate in 5% and 3% NaCl solutions. Furthermore, their findings confirmed the superior corrosion resistance of ASTM A1010 steel compared to ASTM A588 weathering steel in atmospheric corrosion environments.

This alloy has expanded its applications as it can exhibit different properties, depending on the microstructure present. The UNS S41003 stainless steel, depending on the process conditions, can present martensitic, ferritic, or ferritic-martensitic microstructures⁶. The difference between the microstructures can cause a variation in the corrosion resistance and mechanical properties. Pimenta et al.⁶ investigated the microstructural changes in UNS S41003 stainless steel by comparing hot-rolled with batch annealed steels. Scanning electron microscopy (SEM) revealed that the microstructure of the hot-rolled samples was predominantly martensitic with the presence of δ-ferrite. In contrast, the annealed samples exhibited a microstructure composed of equiaxial ferritic grains and intergranular carbides. The microstructural transformation was correlated with variations in the mechanical properties of the steel.

Ferritic low-Cr stainless steels exhibit high susceptibility to corrosive attacks, such as pitting and intergranular corrosion (IGC). Intergranular corrosion may occur due to sensitization, a phenomenon in which, at elevated temperatures, the precipitation of Cr-rich carbides along grain boundaries leads to Cr-depleted zones adjacent to these boundaries, making these regions more vulnerable to corrosion⁶-⁹.

Several authors have studied the corrosion resistance of these alloys, such as Kim et al.⁹ investigated the occurrence of intergranular corrosion in ferritic stainless steel 409L under free exposure conditions. The results of the double loop electrochemical potentiokinetic reactivation (DL-EPR) tests demonstrated the occurrence of IGC in samples aged at 400 °C, 500 °C, and 600 °C.

Microstructural changes in the steel and precipitation of chromium carbides can occur when the stainless steel is heat treated and welded¹⁰,¹¹. Li et al.¹⁰ studied the influence of annealing temperature on the microstructure and corrosion resistance properties of ferritic stainless steel X2CrNi12. The specimens were annealed at 700 °C, 740 °C, 770 °C, 850 °C, and 950 °C, at a rate of 15 °C/min and held for 60 min, followed by the air cooling. The study revealed that the increase in the martensitic phase effectively prevented the precipitation of chromium carbides, thereby enhancing corrosion resistance.

Krell et al.¹² studied the effects of HCl concentration on the degradation of AISI 410 stainless steel in different temperatures. At low temperature (38°C), the occurrence of pitting corrosion was observed within the pH range of 2.25 to 4.25, and E_pit increases within decreasing pH.

The lean UNS S41003 as candidate to replace carbon and low alloys steels, has been rarely investigated till now. The influence of multiple processing routes and resulting microstructures of this steel grade must be investigated more. In this study, the influence of microstructures generated by different processing methods on the corrosion resistance of UNS S41003 stainless steel was investigated. The tests were conducted in 0.1M HCl solution using a three-electrode cell. Intergranular corrosion was also evaluated through double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests. The microstructural features were then correlated to corrosion test results.

2. Materials and Methods

This study used two different plates of UNS S41003 stainless steel, one in a hot-rolled (HR) condition and the other in an annealed (ANN) condition. Both plates were supplied in those conditions by the steel industry. The steelmaker produces the HR material in a Scteckel hot rolling mill with a low coiling temperature (< 400^oC). To produce the ANN material, the batch annealing is carried out in a reducing atmosphere (H₂) with a long duration thermal cycle (∼24h) in temperatures between 800^oC and 650^oC. The chemical compositions of the plates were determined using the combustion method for C, S, and N and optical emission spectroscopy for the other elements. Table 1 shows these results, making it possible to observe that the compositions of the two plates, HR and ANN, are very similar. Based on this chemical compositions the M_S temperature of both steels can be estimated at ∼400^oC, according to Equation 1¹³.

Table 1
Chemical Composition.

Material	C	Cr	Ni	Si	Mn	S	P	N
HR	0.015	10.86	0.33	0.59	0.61	0.001	0.022	0.017
ANN	0.016	10.78	0.33	0.61	0.53	0.002	0.026	0.012

$$M_S=540-\left[\begin{array}{l}497\left(\%C\right)+6.3\left(\%Mn\right)+36.3\left(\%Ni\right)+\\ 10.8\left(\%Cr\right)+46.6\left(\%Mo\right)\end{array}\right]$$ (1)

According to the steelmaker, the plate HR is a martensitic steel, and the ANN is a ferritic steel. Nonetheless, as it will be presented, the microstructural investigation indicated the presence of delta ferrite in HR steel. The other two microstructures were produced by quenching HR and ANN specimens, as described in Table 2.

Table 2
Samples Processing Conditions.

Processing condition	Description	Code
Hot Rolled	As received (thickness 3.0 mm)	HR
Annealed	Annealed in box furnace for 24 hours	ANN
Hot Rolled and Quenched	HR+Heating to 1000°C for 30 min, water quenching	HR-WQ
Annealed and Quenched	ANN+Heating to 1000°C for 30 min, water quenching	ANN-WQ

To evaluate the microstructural differences between the samples, optical microscopy was performed using a Opton Trinocular Metallographic Microscope, model TNM07YPL. Scanning electron microscopy (SEM) was conducted in a MEV-FEG JEOL JSM 7100F. For microscopy observation, the samples were prepared by grinding using 220 to 1200 grinding papers, polishing with diamond paste (6 to 1 µm), and chemical etching with Vilella’s reagent (5mL HCl + 2g picric acid + 100 mL ethyl alcohol). For detection and quantification of δ-ferrite the samples were electrolitically etching with 10% NaOH solution (30 s, 3 V).

Electrochemical corrosion tests were performed using a three-electrode electrochemical cell, with the samples as the working electrode, a saturated calomel electrode as a reference electrode (RE), and a platinum sheet as the counter electrode (CE). Data were collected using a µAutolab Type III potentiostat/galvanostat, and data analysis was conducted using the GPEs software. Two electrochemical corrosion tests were performed: the anodic polarization tests using a 0,1 mol⁻¹ HCl solution and the double loop electrochemical potentiodynamic reactivation (DL-EPR) tests with 0.05M H₂SO₄ + 0.0005M KSCN solution. This solution was based on the previous work of Hu et al.¹⁴. The electrochemical tests were made in triplicate.

To expose only the face of the sample in the electrochemical corrosion tests, the worked electrodes were prepared with the following procedure: a copper wire was connected to the bottom of the samples, which was stripped only in the region in contact with the sample; the samples were embedded in cold-curing resin ensuring that only the test face of the sample remained uncovered. The test surfaces of the electrodes were ground (220 to 1200 grinding papers) and polished using 1µm diamond paste, cleaned in an ultrasonic bath for 3 minutes, and then dried.

3. Results and Discussion

The microstructures of the four conditions examined are presented in Figure 1a-d. Those optical micrographics were revealed by Vilella’s etching. The ANN sample (Figure 1a) exhibits a ferritic microstructure. The equiaxial grains are decorated, denoting an intergranular precipitation. These precipitates are shown in more detail in the SEM image of Figure 2a. They are identified as Cr carbides⁶.

Figure 1
OM images of microstructures of specimens: (a) ANN; (b) HR; (c) ANN-WQ; (d) HR-WQ. (etched with Villela’s solution).

Figure 2
SEM images of the microstructure of specimens: (a) ANN; (b) HR; (c) ANN-WQ; (d) HR-WQ. (etched with Villela’s solution). In (a), the black arrow indicates the Cr carbide; In (b), the black arrow indicates the δ-ferrite; In (c) and (d), the black circles indicate the fine and spherical Cr carbides.

In contrast to the annealed steel, the hot rolled (HR) sample (Figure 1b) displays a predominantly martensitic microstructure. The microstructure is fine and elongated in the rolling direction. The observation of this sample in the SEM (Figure 2b) reveals elongated islands of δ-ferrite and fine spherical particles, probably Cr carbides.

After quenching, the ANN-WQ (Figure 1c) and HR-WQ (Figure 1d) samples exhibited a martensitic microstructure with coarser martensite packets than specimen HR. The SEM images (Figure 2c and Figure 2d) of these samples also show fine and spherical Cr carbides. Although the material has not been heat treated by tempering, the presence of such carbides in the as quenched may be associated to a self tempering process, since the M_S temperature of the steel is high (∼400^oC).

Etching with 10%NaOH solution (30s, 3V) was used to reveal the δ-ferrite in specimens HR, HR-WQ and ANN-WQ, Figure 3. The Image J software¹⁵ was used to quantify de δ-ferrite, and the results for HR, HR-WQ and ANN-WQ were 23.3±0.5%, 6.6±2.7% and 0.9±0.3% respectively. Table 3 summarises the microstructural features observed in the four samples.

Figure 3
Microstructures of the HR (a), HR-WQ (b), and ANN-WQ (c) specimens. MO with 10% NaOH solution (30s, 3V), showing δ-ferrite (black arrows).

Table 3
Microstructural Characteristics of ANN, HR, HR-WQ and ANN-WQ.

Specimen	Microstructures
ANN	Ferrite + intergranular Cr carbides
HR	Martensite + 23.3% delta ferrite + fine Cr carbides
ANN-WQ	Martensite + fine Cr carbides (<1.0% δ ferrite)
HR-WQ	Martensite + 6.6%δ ferrite + fine Cr carbides

Typical polarization curves in 0.1M HCl solution are shown in Figure 4. Table 4 presents the average values of corrosion potential (E_corr.) and the pitting potential (E_Pit). The corrosion potential (E_corr​) represents the threshold below which the metal's dissolution rate is minimized due to the predominance of cathodic reactions. E_corr corresponds to the open circuit potential (OCP) after stabilization for 1h. Above this potential, a current inversion occurs, marking the beginning of the anodic region of the polarization curve¹⁶,¹⁷.

Figure 4
Polarization curves of specimens HR, ANN, HR-WQ, and ANN-WQ.

Table 4
E_corr and E_pit of specimens HR, ANN, HR-WQ, and ANN-WQ.

Material	Ecorr (VSCE)	EPit (VSCE)
HR	- 0.555	- 0.155
ANN	- 0.559	- 0.255
ANN-WQ	- 0.557	- 0.091
HR-WQ	- 0.557	- 0.080

In the initial anodic region, characterized as active, anodic dissolution of the metal takes place alongside the oxidation of compounds in the solution. Some degree of passivation is observed, as the current density does not increase linearly with potential. However, the passive region is not well-defined, and localized corrosion can occur at relatively low potentials, as indicated by the E_Pit values. This suggests that the studied steels exhibit limited passivation compared to other stainless steels, showing higher susceptibility to localized attack¹⁶,¹⁷.

After E_corr, an increase in current density is observed in the active region. At higher applied potentials, localized corrosion may occur, as indicated by the E_Pit values, suggesting that the material is prone to pitting corrosion rather than exhibiting well-defined transpassivation behavior.

As shown in Table 4, the annealed steel (ANN) exhibited the lowest E_pit compared to the other samples. Additional information is obtained observing the specimens after the corrosion tests in the microscope. Figure 5a show the pits in the ANN specimen after the test. The intergranular attack is observed inside the pits, in agreement with the microstructure composed of ferrite and intergranular Cr carbides. The presence of Cr carbides in the grain boundaries is responsible for the poor corrosion performance of the ANN sample, (Figure 1a and 2a), since they created a region around them with low chromium content in a process named sensitization. This chromium-poor region impairs the corrosion resistance of the alloy.

Figure 5
Pits on the Surface of the ANN (a-b)(intergranular attack) and HR (c-d) specimens after anodic polarization.

The HR specimens presented higher E_pit values than ANN, but lower than the specimens submitted to quenching (HR-WQ and ANN-WQ). In HR, the δ-ferrite islands are nobler because contain more Cr than the martensitic matrix. Figure 5b shows the SEM images taken inside the pits of a HR specimen. Some δ-ferrite islands can be observed, proving its better corrosion resistance than the matrix. The water quenching treatments promoted a substantial reduction of the δ-ferrite in the HR-WQ specimen. The ANN-WQ contains even less δ islands. The higher pitting potential in these specimens can be associated with the reduction in δ in comparison to HR specimens. As described, δ-ferrite is rich in chromium, therefore, the greater amount of this phase in the HR sample compared to the quenched samples resulted in a martensitic matrix poorer in chromium, with lower corrosion resistance.

Figure 6 shows a DL-EPR curve of a HR specimen. The degree of sensitization (DOS) in the DL-EPR tests corresponds to the I_r/I_a ratio. Table 5 presents the average results. There is not a significant difference between the DOS of the four conditions tested. When the samples are examined in the SEM after the tests (Figures 7a and 7b), two behaviors are observed. The ANN specimen shows classical intergranular attack due to Cr carbides, while the specimens with martensite and δ plus martensite suffered a more uniform corrosion. It was expected that the ANN sample would have a worse performance in the DL-EPR test due to Cr carbides presence, but this was not observed. This fact can be attributed to the healing process, where the chromium present in the matrix undergoes diffusion, recovering the depression zones formed around the carbides. The healing process occurred during the manufacturing of the ANN sample, where it was placed for a long period of time (24h) in the furnace at temperatures between 800 and 650 ºC.

Figure 6
DL-EPR curve of specimen HR.

Table 5
Degree of sensitization in the DL-EPR tests.

Material	Degree of sensitization (Ir/Ia)	Specimen observation after the test
HR-CP1	0.404	Uniform attack
ANN-CP1	0.386	Intergranular attack
ANN-WQ-CP1	0.401	Uniform attack
HR-WQ-CP1	0.320	Uniform attack

Figure 7
SEM images of (a) ANN and (b) HR specimens.

4. Conclusions

The UNS S41003 annealed presented a microstructure of ferrite and intergranular Cr carbides susceptible to intergranular attack, as observed in anodic polarization tests in 0.1M HCl solution and in DL-EPR tests. This specimen exhibited the lowest E_pit. This behavior can be attributed to the sensitization process that occurs during the formation of Cr carbides.

The as received HR steel, with a microstructure of martensite and 23.3% δ-ferrite, showed slightly better corrosion resistance in the anodic polarization tests. The quenching treatment (1000°C, water cooling) promoted a further increase of the pitting potential, due to the dissolution of δ-ferrite islands. The smaller volume fraction of δ-ferrite in the microstructures of quenched samples increases the amount of Cr in the martensitic matrix, improving corrosion resistance.

Comparing the two conditions as forced by the manufacturer, HR, and ANN, the HR conditions showed better corrosion resistance and should be chosen for applications in more severe corrosive environments. However, it has a martensitic microstructure, which exhibits lower ductility and toughness, a fact that may compromise its use in some structural applications. On the other hand, it has greater mechanical resistance, a requirement for structural applications.

5. References

• ¹ ASTM: American Society for Testing and Materials. ASTM A709/A709M: standard specification for structural steel for bridges. West Conshohocken: ASTM International; 2021. http://dx.doi.org/10.1520/A0709_A0709M-21.
• ² ASTM: American Society for Testing and Materials. ASTM A1010/A1010M-13: standard specifcation for higherstrength martensitic stainless steel plate, sheet, and strip. West Conshohocken: ASTM International; 2018. http://dx.doi.org/10.1520/A1010_A1010M-13R18.
• ³ Russian O, Ocel J. Mechanical performance of welds using ASTM A709 grade 50CR steel as base metal. J Construct Steel Res. 2020;170:106121. http://doi.org/10.1016/j.jcsr.2020.106121
  » http://doi.org/10.1016/j.jcsr.2020.106121
• ⁴ Hebdon MH, Provines JT. The use of ASTM A1010 (A709 GR50CR) stainless steel for bridges in the United States. J Perform Constr Facil. 2020;34(6):1. http://doi.org/10.1061/(ASCE)CF.1943-5509.0001498
  » http://doi.org/10.1061/(ASCE)CF.1943-5509.0001498
• ⁵ Fletcher FB. Improved corrosion-resistant steel for highway bridge construction. McLean: U.S. Department of Transportation; 2011.
• ⁶ Pimenta AR, Loureiro RCP, Breves IMS, Perez G, Tavares SSM. Microstructural characterization of low carbon UNS S41003 stainless steel with diferent processing routes (hot rolling and annealing). Metallography, Microstructure, and Analysis. 2024;13(5):880-90. http://doi.org/10.1007/s13632-024-01092-z
  » http://doi.org/10.1007/s13632-024-01092-z
• ⁷ Park JH, Seo HS, Kim KY. Alloy design to prevent intergranular corrosion of low-Cr ferritic stainless steel with weak carbide formers. J Electrochem Soc. 2015;162(8):C412-8. http://doi.org/10.1149/2.1001508jes
  » http://doi.org/10.1149/2.1001508jes
• ⁸ Lippold JC, Kotechi DJ. Welding metallurgy and weldability of stainless steels. Hoboken: John Wiley & Sons; 2005.
• ⁹ Kim JK, Kim YH, Uhm SH, Lee JS, Kim KY. Intergranular corrosion of Ti-stabilized 11 Wt% Cr ferritic stainless steel for automotive exhaust systems. Corros Sci. 2009;51(11):2716-23. http://doi.org/10.1016/j.corsci.2009.07.008
  » http://doi.org/10.1016/j.corsci.2009.07.008
• ¹⁰ Li R, Fu B, Dong T, Li G, Li J, Zhao X, et al. Effect of annealing treatment on microstructure, mechanical property and anti-corrosion behavior of X2CrNi12 ferritic stainless steel. J Mater Res Technol. 2022;18:448-60. http://doi.org/10.1016/j.jmrt.2022.02.117
  » http://doi.org/10.1016/j.jmrt.2022.02.117
• ¹¹ Lacerda JC, Teixeira RLP, Rocha AG, Silva DFR, Sousa VSR. Effect of metal inert gas arc welding on mechanical properties of a UNS S41003 stainless steel. E-Tech. 2023;16(1):1-18. http://doi.org/10.18624/etech.v16i1.1276
  » http://doi.org/10.18624/etech.v16i1.1276
• ¹² Krell PD, Li S, Cong H. Synergistic effect of temperature and HCl concentration on the degradation of AISI 410 stainless steel. Corros Sci. 2017;122:41-52. http://doi.org/10.1016/j.corsci.2017.03.027
  » http://doi.org/10.1016/j.corsci.2017.03.027
• ¹³ Gooch TG, Woolin P, Haynes AG. Welding metallurgy of low carbon martensitic steels. In: Supermartensitic Stainless Steel; 1999; Ghent, Belgium. Proceedings. Ghent: Belgium Welding Institute; 1999. p. 188-95.
• ¹⁴ Hu S, Mao Y, Liu X, Han E-H, Hänninen H. Intergranular corrosion behavior of low-chromium ferritic stainless steel without Cr-carbide precipitation after aging. Corros Sci. 2020;166:108420. http://doi.org/10.1016/j.corsci.2019.108420
  » http://doi.org/10.1016/j.corsci.2019.108420
• ¹⁵ Schneider CA, Rasband WS, Eliceiri KW. NIH Image to ImageJ: 25 years of image analysis. Nat Methods. 2012;9(7):671-5. http://doi.org/10.1038/nmeth.2089 PMid:22930834.
  » http://doi.org/10.1038/nmeth.2089
• ¹⁶ Stansbury EE, Buchanan RA. Fundamentals of electrochemical corrosion. Materials Park: ASM International; 2000.. http://doi.org/10.31399/asm.tb.fec.9781627083027
  » http://doi.org/10.31399/asm.tb.fec.9781627083027
• ¹⁷ Brett CMA, Brett AMO. Electrochemistry: principles, methods, and applications. Oxford: Oxford University Press; 1993.

Publication Dates

• Publication in this collection
  23 May 2025
• Date of issue
  2025

History

• Received
  31 Jan 2025
• Reviewed
  30 Mar 2025
• Accepted
  11 Apr 2025