Reducing Pitting Corrosion Trend of Cast GCr15 Steel by Inoculation: An In Situ Corrosion Morphology Study

Abstract

The corrosion resistance of bearing materials is crucial for the long-term service and high precision of modern equipment, and has aroused widespread research interest. Inoculation treatment is beneficial for improving the mechanical properties of GCr15 bearing steel, but its impact on corrosion behavior remains to be investigated. In the present work, the influence of inoculation treatment on the corrosion morphology, open circuit potential, impedance spectrum, and polarization curve evolution of GCr15 steel was studied through in situ corrosion morphology analysis and electrochemical testing. The results showed that the samples treated after inoculation showed a reduced tendency for pitting corrosion and an obviously improved corrosion resistance. This improvement is related to the transformation of columnar grains into equiaxed grains during the inoculation process, which reduces the amount and distribution of inclusions and pores, thereby delaying the rapid development of pitting corrosion. This study provides new insights into the corrosion mechanism of gestational steel.

1. Introduction

Bearings, as common steel components in equipment, generally have poor working environments. In addition to bearing high cycle loads, they are often subject to harsh conditions, such as impact stress, corrosive chemical media, and temperature fluctuations. In order to adapt to these complex working environments, bearings should have good comprehensive performance, including high mechanical properties and high corrosion resistance, which means higher requirements for bearing steel [1]. For example, the control of the chemical composition in the manufacturing process is very strict to reduce the content of impurities, such as carbides and non-metallic inclusions, to a very low level. Therefore, bearing steel belongs to the category of high-grade and high-quality steel, which is one of the steel grades with the highest requirements for metallurgical quality [2].

The microstructure of GCr15 bearing steel in service is mainly tempered martensite (M), which contains a large number of carbide particles and a small amount of retained austenite (A’). Due to the difference in electrode potentials, the Fe element and carbides in tempered M can form small primary batteries, dominating the corrosion behavior, which is characterized by typical pitting corrosion [3]. The corrosion reaction generally starts near the carbides, then extends to the surrounding grains, forming larger corrosion pores [4]. Moreover, pitting also originates at inclusions on the surface, due to the dissolution of some special impurities, such as MnS particles [5].

The improvement in corrosion resistance of GCr15 steel is mainly based on preventing or delaying the pitting process, generally by surface modification [6], heat treatment [7], alloying elements [8], etc. Recently, the addition of rare earth (RE) elements has been proven to have a significant grain refinement effect on GCr15 steel [9], effectively improving the mechanical properties [10] and corrosion resistance of GCr15 [11]. The addition of RE element reduced the surface charge of GCr15 steel, leading to a decrease in the adsorption trend of Cl ions on the metal surface, thereby reducing the surface electrochemical activity [11]. Additionally, RE elements can adsorb oxygen to hinder the electronic transmission on the metal surface [12], leading to an increase in resistance in electrochemical reactions and reducing the corrosion rate [13]. However, the RE element was prone to oxidation, so only under low oxygen conditions could RE exert effective modification effects and reduce the number and size of inclusions, playing an important role in improving the corrosion resistance of RE steel [14].

In addition to RE alloying, inoculation treatment is also an effective method for refining the microstructure of bearing steel [15], but its impact on the corrosion resistance of GCr15 steel still needs to be verified. In this work, the in situ corrosion behavior of GCr15 ingots treated with inoculation was investigated in detail through optical microscopy, and the influence mechanism of inoculation treatment on the corrosion behavior of as cast GCr15 steel was studied through electrochemical experiments.

2. Experimental

2.1. Preparation of Samples

Commercial GCr15 steel (China Baowu Steel Group Corporation Ltd., Shanghai, China) raw material pieces were put into a high-purity Al₂O₃ crucible, which was placed in a graphite crucible, preventing the mixing of carbon atoms into the melt. The graphite crucible was placed into a quartz protective sleeve in a water-cooled copper induction heating coil of the vacuum induction heating furnace (Shanghai MTINST, MZG, Shanghai, China). Then, the GCr15 steel raw material was melted under Ar protection, and an NbC-type inoculant developed by our laboratory was added at an amount of 1.5% (in weight). A stirrer made of W was used to mix the liquid and the inoculant evenly for about 5 min, and the steel liquid was poured into an iron mold with a cylindrical inner cavity of Ф14 mm × 200 mm. When the mold was cooled, the GCr15 steel ingot was taken out to obtain the inoculated sample. An ingot without inoculant was also prepared using the same process as for the control experiment, termed the uninoculated sample.

An electric discharge CNC wire cutting machine (Jiangsu DRAGON, DK7732, Taizhou, China) was used to cut the ingot to prepare the metallographic sample with a size of Ф14 mm × 15 mm. The cut samples were sequentially polished with sandpaper, mechanically polished, and corroded using 4% nitric acid alcohol solution. An X-ray fluorescence spectrometer (XRF, Rigaku, ZSX Primus 2, Tokyo, Japan) was used to examine the chemical composition of the samples, as shown in

Table 1. Chemical composition of cast GCr15 steel before and after inoculation treatment.

Sample	Elements/wt.%
	Fe	Cr	Mn	Si	Al	Mg	Nb	S	P	C
uninoculated	97.8	1.54	0.252	0.200	0.096	0.0451	—	0.0219	0.0103	—
inoculated	97.5	1.64	0.267	0.195	0.139	0.0171	0.168	0.0225	0.0105	—

. Apart from a slight increase in Nb and Al content (from the inoculant), the chemical composition of the samples before and after inoculation remained essentially the same.

2.2. In Situ Corrosion Morphology Investigation

Standard metallographic samples were also used for in situ corrosion morphology analysis. A micro-Vickers hardness tester (Shimadzu, HMV-2T, Kyoto, Japan) was used to make identification markers for in situ observation, with a load of 19.614 N and a loading time of 15 s. Then, the marked samples were placed in 30 mL of 3.5 wt.% NaCl aqueous solution (pH = 7.23, Yueping, PHS-3CU, Shanghai, China) at 25 °C, soaking for 2.5 min, 5 min, 10 min, 30 min, and 50 min, respectively. A metallographic microscope (OM, Zeiss, Axio Imager M2m, Jena, Germany) was used to observe the samples after corrosion. To observe the morphology of the corrosion pits, the sample, soaked for 2 h, was sectioned, and its cross-sectional microstructure was analyzed using the microscope.

2.3. Electrochemical Testing

The electrochemical corrosion samples, with a size of Ф14 mm × 2 mm, were ground with 2000CW sandpaper to achieve a bright finished surface. An electrochemical workstation (Chenhua, CHI660E, Shanghai, China) was used to examine the open circuit potential (OCP) curve, polarization curve, and impedance spectrum, sequentially. The sample was encapsulated with epoxy resin, leaving only the testing surface exposed, to prevent corrosive media from reaching the non-test area. The encapsulated sample functioned as the working electrode, a saturated calomel electrode as the reference electrode, a graphite electrode as the counter electrode, with a 3.5% NaCl aqueous solution as the corrosive medium. The OCP was initially determined to serve as the reference for the polarization curve testing range. The potential range for polarization curve testing was set to ±400 mV around the OCP, with a scan rate of 0.5 mVs⁻¹. The impedance spectrum was measured within a frequency range of 0.01 to 100,000 Hz.

3. Results

3.1. Microstructure of the As Cast GCr15 Steel

Figure 1 shows the optical metallographic photographs of different cast GCr15 samples. Figure 1a presents a metallographic image of the uninoculated sample at 25× magnification, exhibiting columnar-dendritic-grain features. The columnar grains extend over mm-level lengths, interspersed with numerous casting pores and inclusions. In the 1000× metallographic image shown in Figure 1b, coarse plates are clearly intersecting the secondary dendrites, indicating the presence of larger martensite (M) in the uninoculated sample. However, for the inoculated sample shown in Figure 1c, the metallographic morphology is obviously different from the uninoculated, featuring typical equiaxed grains with an average size of ~80 μm. In the equiaxed grains, sheet-like M, nearly 80 μm in length, is visibly penetrating the interior of the grains, as shown in Figure 1d. In addition, the inoculated samples exhibit fewer pores and inclusions (Figure 1c), which is beneficial for reducing the corrosion caused by inclusions.

3.2. In Situ Corrosion Morphology

Figure 2 presents the in situ corrosion morphology observations of the inoculated and uninoculated GCr15 steel samples in a 3.5% NaCl solution at 25 °C, respectively. The Vickers hardness indentation (white arrows) in the figures serves as a marker to aid in distinguishing the corroded areas. For the uninoculated sample, after soaking for 2.5 min, some ~10 μm corrosion black spots appeared on the sample surface, as shown in Figure 2a. Interestingly, these early black corrosion spots did not grow larger over the course of the experiment. After soaking for 5 min, the number of corrosion black spots gradually increased, yet their size did not change significantly (Figure 2b). After 10 min of soaking, the sample surface displayed multiple new sources of corrosion, with some areas exhibiting a more extensive corrosion range (Figure 2c). It is suggested that the initial corrosion sites do not necessarily correspond to the most rapidly advancing corrosion areas. However, certain corrosion sites exhibit an obvious increase, as highlighted by the red dashed circles in Figure 2a–f. After 50 min of soaking, the largest corrosion area (pitting) on the sample measured reaches approximately ~1000 μm (Figure 2e).

For the inoculated GCr15 sample under the same in situ corrosion conditions, the corrosion patterns are similar to that of the uninoculated sample, characterized by pitting corrosion. However, during the initial soaking stage, the inoculated samples exhibited more corrosion initiation points. For example, by comparing the corrosion morphology of the two samples shown in Figure 2b, it can be seen that the inoculated sample shows more corrosion black spots. After soaking for 10 to 30 min, the corrosion rate significantly accelerates, leading to widespread corrosion. Similarly, the inoculated samples showed some areas with less significant pitting growth and other areas with significant pitting growth. Finally, after soaking for 50 min, the maximum pitting area on the inoculated sample is approximately 600 μm, smaller than that of the uninoculated samples. This indicates that the GCr15 sample treated with inoculation exhibits better long-term corrosion resistance.

3.3. Electrochemical Analysis

Figure 3 shows the potentio-dynamic polarization curve of GCr15 steel in a 3.5% NaCl solution at 25 °C. A passivation phenomenon appears in the anode curves of the two samples, and as the voltage increases, the passivation current density i_p remains almost unchanged, forming a plateau region with a width of ΔE, as shown in Figure 3a, which is indicative of the stability of the passivation film. The inoculated sample exhibited a significantly wider ΔE compared to the uninoculated sample, suggesting a higher stability of its passivation film. The polarization curve of the uninoculated GCr15 steel shows a significant fluctuation in the current value in the passivation zone, which may be caused by the appearance of cast pores or inclusions on the surface of the sample. The current fluctuation value in the passivation zone of the inoculated GCr15 steel is relatively small, indicating that the size of cast pores or inclusions on the surface is fewer and smaller.

Table 2. Electrochemical parameters of both inoculated and uninoculated samples as measured from the passive potential polarization curves.

Sample	OCP /V	Ecorr /V	Eb /V	icorr /Log(μA/cm2)	ip /Log(μA/cm2)
uninoculated	−0.45	−0.65	−0.33	0.63	1.10
inoculated	−0.43	−0.57	−0.23	0.28	0.78

shows the parameters measured based on the Tafel curve in Figure 3. It can be observed that the corrosion potential E_corr of GCr15 steel after inoculation treatment increases, the corrosion current density i_corr slightly increases, and the i_p value decreases. When the voltage increases to a certain extent, the corresponding current value on the Tafel curve suddenly increases sharply, indicating that the passivation film is broken down, and the corresponding potential is called the breakdown potential E_b. Generally, the breakdown potential is related to pitting, and a higher E_b value indicates a lower tendency for pitting in the sample. From Table 2, it can be seen that the E_b value of GCr15 steel after inoculation is higher than that of GCr15 steel without inoculation, which also indicates that inoculation treatment improves the corrosion resistance of GCr15 steel.

Figure 3b,c displays the Nyquist and Bode plots for two cast GCr15 samples. The Nyquist plots for both samples exhibit unclosed arc-shaped patterns. Samples treated with an inoculant exhibit a larger impedance arc, reflecting poorer conductivity and a reduced propensity for charge transfer required for pitting reactions, consequently leading to a lower pitting corrosion tendency. Conversely, samples without inoculation display columnar crystals with superior conductivity, characterized by a smaller impedance arc radius, which facilitates the development of pitting corrosion. Typically, pitting corrosion is most prevalent at inclusions and grain boundaries. The microstructure of the inoculated samples is refined into equiaxed crystals, increasing the number of grain boundaries and, consequently, pitting sites; however, the reduced conductivity is unfavorable for pitting corrosion. Moreover, the equiaxed crystal structure of the samples post-incubation treatment leads to the differentiation and concealment of inclusions, markedly reducing their count compared to columnar crystals, which also contributes to a decreased propensity for pitting corrosion. Figure 3c presents the Bode plots for the various samples, showing a phase angle peak at approximately 10 Hz, correlating with the capacitance value of the passivation film, indicating that both samples are capable of forming a passivation film. In addition, we have drawn the equivalent circuit diagrams of the two samples based on Figure 3a, as shown in Figure 3d. Obviously, the total resistance mainly depends on the double layer resistance R_a and the passive film resistance R_f. The high stability of the passivation film means that its conductivity is poor, so it has higher R_a and R_f values, leading to an increase in the total resistance value. The incubation treatment sample has a fine equiaxed crystal structure, which is conducive to the formation of a stable passivation film and can effectively improve corrosion resistance.

4. Discussion

When GCr15 steel contacts an NaCl solution, micro-scale galvanic cells comprising the metal matrix, impurities, and electrolyte form on the sample surface. Initially, the minuscule size of these cells leads to small pitting corrosion holes, but the dissolution of inclusions can rapidly enlarge them. Owing to the minor quantity of inoculant added, the inoculated sample has a composition almost the same as that of the uninoculated sample (see Table 1), both samples sharing the same corrosion mechanism, namely pitting corrosion. However, the significantly different microstructures between the two samples (see Figure 4), particularly in the distribution of inclusions, results in distinct pitting behaviors.

To observe the pitting morphology clearly, we immersed the samples before and after the incubation treatment in a 3.5% NaCl corrosion solution (pH 7.23, at 25 °C) for 2 h, then sectioned the samples and examined their cross-sectional microstructures, as shown in Figure 4a,b. Figure 4a shows the pit morphology of the uninoculated sample, which displays typical networked corrosion pore characteristics, suggesting that the corrosion solution permeated along the inter-dendritic spaces, forming this complex pattern. The large size and depth of these corrosion pits indicate the rapid progression of the corrosion process. Figure 4b shows the pit morphology of the inoculated sample. It is evident that the pitting pits predominantly extend inward perpendicular to the surface, with most remaining as single pits, exhibiting characteristics typical of corrosion infiltrating along grain boundaries. It is known that the uninoculated sample contains numerous impurities and casting defects within the dendrites, as shown in Figure 4c, which can readily initiate pitting corrosion and facilitate the growth of corrosion cavities. For the inoculated sample, it tends to adsorb impurities onto the nucleation substrate (NbC particle), creating an intragranular distribution of inclusions, as shown in Figure 4d. Therefore, it reduces impurity defects at grain boundaries, which is not conducive to the rapid growth of corrosion cavities.

As a result, for the uninoculated sample, the concentrated distribution of inclusions and pores (at the grain boundaries between dendrites) leads to rapid propagation of pitting and the intersection of pitting pits, thus exhibiting obvious dendritic interstitial induction characteristics. For the inoculated sample, pitting sites mainly occur at the grain boundaries of small equiaxed grains, resulting in a large number of dispersed potential pitting sites. Each pitting pit penetrates along the grain boundary at a slower rate. Additionally, the formation of pitting corrosion requires sufficient electron transfer to penetrate the passivation film, thus exhibiting obvious characteristics of passivation film breakdown. In other words, the pitting behavior in the inoculated samples is characterized by easy initiation but limited growth.

Bearing steel primarily operates in a lubricating medium, where fluctuations in Cl ion content and pH value impact the lifespan and precision of bearings. This study examines the impact of inoculation treatment on the corrosion resistance of bearing steel, offering significant theoretical insights into the corrosion mechanisms of cast steel.

5. Conclusions

In the present work, in situ corrosion morphology investigation and electrochemical experiments were carried out to investigate the effect of inoculation treatment on the corrosion behavior of GCr15 bearing steel in a 3.5% NaCl aqueous solution at 25 °C. The main conclusions obtained are as follows:

(1)

The NbC type inoculant can refine the as-cast structure of GCr15 steel from columnar grains to typical equiaxed grains, and effectively reduces the number of inclusions and casting pores on the grain boundary.

(2)

The in situ dynamic corrosion experiments show that the GCr15 steel after inoculation has more pitting sites but a lower tendency for pit growth, demonstrating that inoculation treatment can effectively reduce the pitting propagation tendency of cast steels.

(3)

By comparing to uninoculated samples, inoculated steel exhibits an increased corrosion potential, reduced passivation current density, expanded passivation zone width, and a larger impedance arc. It suggests that grain refinement enhances the stability of the passivation film, thereby increasing the corrosion resistance.

(4)

The cross-sectional morphology of corrosion pits in GCr15 steel samples before and after inoculation differs significantly. The columnar dendrites lead to networked pits, whereas the equiaxed grains tend to form isolated intergranular pits, suggesting that inoculation treatment can reduce the pitting trend of cast GCr15 steel. This study sheds lights on the development of corrosion-resistant high carbon cast steel and provides new insights into the design and manufacturing of long-life bearings.