Atmospheric corrosion mechanism of 316 stainless steel in simulated marine atmosphere

Abstract

The evolution of atmospheric corrosion of 316 stainless steel subjected to a simulated marine atmosphere was investigated by scanning electron microscopy, optical microscopy, X-ray photoelectron spectroscopy, and electrochemical measurements. The results indicate that the initiation of pits is associated with the dissolution of MnS inclusion; the maximum pit depth of 316 stainless steel increased in linear relationship with the extension of corrosion time; the corrosion products possess more hydroxide; the ratio of [Cr]/{[Cr]+[Fe]} in the corrosion products gradually increases with increasing time. The protective ability of corrosion products formed on 316 stainless steel has also been discussed.

Keywords

Atmospheric corrosion Stainless steel Pitting corrosion XPS

Introduction

Austenitic stainless steels have been widely applied in various fields, such as electrical engineering, transportation, aerospace, petrochemical industry, buildings, etc., because of their high degree of corrosion resistance. In these service environments, stainless steel would inevitably have to be in contact with the atmosphere that contains water and contaminants, resulting in the possibility of corrosion. Especially in marine environment, chloride ions can disrupt the passive film on the stainless steel surface, leading to severe localised corrosion that significantly affects the service life and safety. Therefore, the atmospheric corrosion of stainless steel has become a subject of great interest. Although some studies have been carried out to investigate the corrosion mechanism of stainless steel in atmospheric environments and meaningful results have been obtained,^1–9 chloride ions are the most corrosive in humid marine atmosphere;³ the dust particles landing on the surface would accelerate the pitting corrosion of stainless steel;⁴ the surface roughness would greatly influence the corrosion resistance of stainless steel;⁵ the content of Cr element in the passive film of the stainless steel had a big relationship with atmospheric corrosion resistance,⁶ and there is still a lack of full understanding about the evolution of atmospheric corrosion, including the chemical composition and the protective ability of corrosion products formed on stainless steel that could influence the corrosion behaviour of stainless steel.

Real-field exposure testing, which reflects the comprehensive influence of the environment, is the most common and direct test method in atmospheric corrosion research, and it can provide the most reliable information about the atmospheric corrosion of metals. However, it usually takes at least several years for one evaluation test, which cannot meet the timely demand of research into corrosion control and the engineering science of materials. By contrast, laboratory accelerated tests can produce useful data in a short time and can be used to assess the durability of metals in certain specific atmospheres. Therefore, in the past decades, several simulated indoor atmospheric corrosion tests enabling the acceleration of corrosion have been developed.^10–15 Wet/dry cyclic accelerated test, which can reflect the wet–dry cycle characteristics of a metal surface in the real atmosphere, is considered as the foundation for developing more accurate simulation of the atmospheric corrosion testing methods and is very widely used.

In present work, with the intention of clarifying the evolution of atmospheric corrosion of 316 stainless steel, the initiation and development of pits, the chemical composition and the electrochemical characteristics of corrosion products formed on 316 stainless steel subjected to a simulated marine atmosphere (wet/dry cyclic test) have been investigated.

Experimental

Sample preparation

The material used in this study is 316 stainless steel, and its composition (in mass%) is given as follows: Fe–10.7Ni–16.8Cr–2.2Mo–1.28Mn–0.04N–0.3Si–0.04C–0.026P–0.002S. 316 stainless steel plates with a size of 50 mm × 25 mm × 2 mm were used as specimens. The specimens were ultrasonically cleaned in acetone, dried, weighed and stored in a moisture-free desiccator before use. The electrochemical specimens were cut into 10 mm × 10 mm × 2 mm coupons and then were embedded in epoxy resin, leaving an exposed working area of 1 cm². The electrochemical specimens were ground to 2000 grit SiC paper and then degreased by acetone, dehydrated with alcohol and finally dried for 24 h.

Wet–dry cyclic accelerated test

The wet/dry cyclic test was conducted using a cyclic salt spray testing machine produced by American Q-Fog Corporation. The corrosion test was conducted for a total of 60 days. The corrosion test consists of the following steps within 1 day: (i) spray salt solution (5%NaCl) for 40 min at 35°C; (ii) drying the specimens in a chamber maintained at 35°C for 20 min; (iii) wetting the specimens in a chamber maintained at 35°C and 100% relative humidity (RH) for 60 min; (iv) redrying the specimens in a chamber maintained at 35°C for 112 min; (v) repeating the above steps from (iii) to (iv) seven times. The test specimens were retrieved for analysis according to the exposure programme, i.e. after 10, 20, 30, 40, 50, 60 days. Four replicate plate specimens were collected in each period. Three of them were used to observe micro-morphologies of pits and measure the maximum pit depth after removal of corrosion products, and the other one was used to identify the nature of the corrosion products. In addition, three replicate electrochemical specimens were retrieved for the following electrochemical test. The corrosion products of the three retrieved specimens were removed chemically by immersion in a specific solution (100 mL of nitric acid+900 mL of distilled water) that was vigorously stirred at 60°C according to ISO 8407. After corrosion products were removed, the specimens were rinsed with distilled water, dried with warm air, and then stored in a moisture-free desiccator. In addition, statistical analysis on the depth of the largest pits observed on stainless steel specimens has been performed.

Analysis of corrosion products

X-ray photoelectron spectroscopy (XPS) measurements were adopted to analyse the corrosion products formed on stainless steel surfaces after 30, 40, 50, and 60 days of corrosion. The core level spectra of the main elements in the alloy and the O2p, C1s core level spectra were recorded using a ESCALAB250 X-ray photoelectron spectrometer with monochromatic Al K _α (1486.6 eV) radiation. The measured sample current during the depth profiling experiment was 2 μA, and the bombardment area was 2 mm × 2 mm. Peak identification was performed with reference to an XPS database.

The morphologies of pits formed on 316 stainless steel were observed using a scanning electron microscope (SEM) (XL30FEG). The pit depths were measured by applying the fine focus technique of photomicrography, where the distances required to shift the optical objective between the focal points on the original surface of the sample and on the bottom of the pit are compared.¹⁶

Electrochemical measurements

The corroded samples served as the working electrode. Electrochemical measurements were performed using a PARSTAT 2273 potentiostat/galvanostat in a conventional glass cell at ∼20°C, with a large platinum plate as the counter electrode and a saturated calomel electrode as the reference electrode. All potential values reported in the present paper are with reference to saturated calomel electrode in saturated KCl solution whose potential value versus standard hydrogen electrode (SHE) is 0.2438 V. The test solution used in all experiments was 0.5%NaCl air-saturated solution prepared from reagent grade chemicals and distilled water.

The specimens were kept in the test solution until a stable corrosion potential was attained. The scanning rate of the potentiodynamic polarisation measurements was 20 mV min^{− 1}. The electrochemical measurements have been repeated three times, and the average results were presented here.

Results and discussion

Macro-morphologies of corrosion products

Figure 1 displays the evolution of macro-morphologies of 316 stainless steel with corrosion products during its rust process in the simulated marine atmosphere. Owing to the presence of chloride ions in the atmosphere, the stainless steel suffered from corrosion. As is shown in Fig. 1, only several small pits are scattered on the sample surface after 10 days of corrosion; with the extension of corrosion time, several adjacent corrosion regions connected with each to form the initial corrosion scale; a large area of corrosion products appeared on the 316 stainless steel surface until 60 days of corrosion.

Initiation of pitting corrosion

Pit initiation has been reported to proceed via initial dissolution of MnS in the presence of salt water.^17–20 In order to identify the pit initiation of 316 stainless steel in atmospheric corrosion, energy dispersive spectroscopy (EDS) was employed to analyse the chemical composition of the corresponding sites (on and around the corrosion products formed on 316 stainless steel after 20 days of corrosion). Figure 2 shows the micro-morphology and the corresponding EDS results of the initial corrosion products. It is clear that there are bulges that appeared on the specimen surface, which are the initial corrosion products formed on the 316 stainless steel specimen. The pit initiation sites (at the bulges) are within the white area. The grey area (around the bulges) is composed mainly of alloy elements, such as Fe, Cr, Ni, Mo and Mn (as shown in spectrum A). However, the composition of the grey area (at the bulges) includes not only alloy elements but also S (as shown in spectrum B). Replicate experiments were carried out on several specimens, and the same results were obtained. Since the chemical element sulphur is present in the 316 stainless steel matrix as MnS inclusions, pit initiation in stainless steel subjected to simulated marine atmosphere is also associated with MnS dissolution, corresponding to the results in the literature. The dissolution of MnS inclusions can be described by the following reaction²¹ (1) Reaction (1) would decrease the pH at the vicinity of the MnS inclusions, leading to the dissolution of alloy elements (Fe, Cr, Ni) and then the occurrence of pitting corrosion.²¹ A part of the dissolved metal ions (Fe²⁺, Cr³⁺, Ni²⁺) further lower the pH due to the following hydrolysis reactions (2)–(4), resulting in a self-catalytic action, and thus the pits gradually became deeper and larger (2)

(3)

(4)

Evolution of pitting corrosion

Figure 3 depicts a series of SEM images showing the evolution of micro-morphologies of the pits observed on stainless steel specimens after removal of corrosion products during its corrosion process. It is clear that the pits formed on the specimen just had a diameter of about 5 μm after 10 days of corrosion, and then the diameters of the pits formed during the corrosion process gradually increased with the prolongation of corrosion time, and finally, the diameter of the pit approached ∼50 μm after 60 days of corrosion. It is considered that the depth of the largest pit formed on the stainless steel surface is related closely with the use security of stainless steel under service. Thus, statistical analysis was performed in the present work so as to reveal the relationship between the depth of the largest pit and the corrosion time, to enable better prediction of the service life of stainless steel materials. Figure 4 displays the variation of the depth of the largest pit formed on 316 stainless steel with changing corrosion time. It can be seen that the depth of the largest pit increases linearly with increasing corrosion time, and the corresponding fitting curve is also shown in Fig. 4. This is consistent with the results in the literature.²

Analysis of corrosion products

As is shown in Fig. 1, the corrosion of stainless steel is not homogeneous. In our opinion, the passive film without corrosion products is more protective than the corrosion products; it is thus considered that the protection of the corrosion products would significantly influence the subsequent corrosion behaviour of 316 stainless steel. Therefore, it is necessary to gain information about the composition evolution of the corrosion products formed on the specimens. However, in present work, it is impossible to gain the composition of corrosion products using X-ray diffraction as a result of the low amount of corrosion products formed on 316 stainless steel. In order to reveal the composition evolution of corrosion products during its corrosion process, XPS analysis was thus undertaken to provide more information about the composition of the corrosion products formed on the 316 stainless steel surface after 30, 40, 50 and 60 days of corrosion time.

Figure 5 shows the XPS spectra observed for the 316 stainless steel specimen after 60 days of corrosion in Fe 2p2/3, Cr2p2/3, Ni 2p2/3 and O 1s. In the Fe 2p2/3 region, the peaks at 706.5, 710.8 and 711.8 eV were assigned to Fe(II) oxide, Fe(III) oxide and Fe hydroxide respectively (Fig. 5a). In the Cr 2p2/3 region, the peaks at 576.4 and 577.4 eV assigned to Cr(III) oxide and Cr hydroxide were clearly observed (Fig. 5b). The Ni compounds were also observed, which are attributed to Ni(II) oxide at 855.6 eV and Ni hydroxide at 856.5 eV (Fig. 5c). Oxygen species are mainly O^{2 −} (531.1 eV) and OH⁻ (529.6 eV), which play the role of connecting metal ions (as shown in Fig. 5d). It is obvious that OH⁻ accounts for the majority. It is reported that bound water including aquo and hydroxyl that existed in the passive film could be replaced easily by chloride ions. However, this kind of bound water also acts as an effective species to capture the dissolved metal ions and forms a new film that resists a further attack by surroundings.^22,23 From this viewpoint, the ratios of [hydroxide]/{[hydroxide]+[oxide]} of the main alloy elements in the corrosion products have been estimated by computing the ratio of peak area of each hydroxide/oxide to overall area, and the results are shown in Fig. 6. Figure 6a and b present the ratio of [Fe_hyd]/{[Fe_hyd]+[Fe_ox]} and [Fe²⁺ _ox]/{[Fe²⁺ _ox]+[Fe³⁺ _ox]} in the corrosion products formed on the 316 stainless steel specimen as a function of corrosion time respectively. It can be seen that the content of Fe compounds in the corrosion products is in the order of Fe hydroxide>Fe(III) oxide>Fe(II) oxide. Furthermore, the ratio of [Fe_hyd]/{[Fe_hyd]+[Fe_ox]} in the corrosion products gradually increases as a function of corrosion time (Fig. 6a), but the ratio of [Fe²⁺ _ox]/{[Fe²⁺ _ox]+[Fe³⁺ _ox]} slightly decreases (Fig. 6b). The situation of Ni hydroxide differs from that of Fe hydroxide, which shows an irregular trend (Fig. 6c). This may be related to the low amount of Ni element in the corrosion products. The ratio of [Cr_hyd]/{[Cr_hyd]+[Cr_ox]} increases more significantly compared with the situation of Fe. The content of Cr_hyd is about 35% after 30 days of corrosion time, whereas it arrives at approximately 70% when the corrosion test lasts for 60 days (as shown in Fig. 6d). This indicates that Cr_hyd was easily formed on the 316 stainless steel specimen. In addition, it is considered that the high content of Cr in the passive film is beneficial for improved corrosion resistance of the material.⁶ However, the changes in Cr content in the corrosion products with increasing time, which may influence the corrosion resistance of 316 stainless steel, are still blurred. Therefore, in order to identify the changes in Cr content in the corrosion products formed on the 316 stainless steel specimen, the evolution of the ratios of [Cr_ox]/{[Cr_ox]+[Fe_ox]} and [Cr_hyd]/{[Cr_hyd]+[Fe_hyd]} in the corrosion products as a function of corrosion time has also been calculated in the present work, and the corresponding results have been plotted in Fig. 7. It can be seen that the ratio of [Cr_ox]/{[Cr_ox]+[Fe_ox]} increases gradually with increasing corrosion time. The trend of ratio of [Cr_hyd]/{[Cr_hyd]+[Fe_hyd]} is the same as that of [Cr_ox]/{[Cr_ox]+[Fe_ox]} but more pronounced. That is to say, the ratio of [Cr]/{[Cr]+[Fe]} in the corrosion products gradually increased as corrosion continued. Higher content of Cr in the corrosion products may improve the corrosion resistance of stainless steel.

Electrochemical measurements

The atmospheric corrosion of metal materials is usually an electrochemical process between the steel and its surrounding environment. As an efficient method to evaluate the protective ability of corrosion products formed on the matrix, electrochemical measurement has been increasingly used in the field of corrosion.^24–26 Therefore, investigating the atmospheric corrosion by electrochemical measurements is conducive to understanding the corrosion mechanism of 316 stainless steel subjected to a simulated marine atmosphere. In order to reveal the effect of corrosion products on the corrosion behaviour of 316 stainless steel, a potentiodynamic polarisation test was performed on both unexposed and corroded specimens.

Figure 8 shows the evolution of the polarisation curves of unexposed and corroded 316 stainless steel specimens in 0.5%NaCl solution as a function of corrosion time. From the variation of polarisation curves, it can be seen that all the samples exhibited passivation ability; both cathodic and anodic current densities of corroded specimens are smaller than that of unexposed samples; the cathodic and anodic current densities decrease slightly with changing corrosion time. This variation demonstrates that the corrosion products formed on the stainless steel specimen could inhibit both anodic and cathodic processes. In addition, the longer the corrosion continues, the more obvious the inhibition is. That is to say, the corrosion products could provide protection for the substrate against the attack of corrosive ions. In our opinion, this protective ability must be associated with the structure and composition of the corrosion products. From the viewpoint of structure, the corrosion products formed on the specimen surface could act as a barrier to physically block the diffusion of corrosive ions, thus inhibiting the occurrence of electrochemical reactions. As corrosion time continued, the area of corrosion products gradually increased (as shown in Fig. 1), and the physical barrier effect became more and more apparent. From the viewpoint of composition, high content of Cr in the corrosion products may have a big relationship with atmospheric corrosion resistance. The increasing ratio of [Cr]/{[Cr]+[Fe]} in the corrosion products improves the protectiveness of corrosion products as corrosion continues. In summary, the enhanced protectiveness of corrosion products formed on 316 stainless steel is due to the physical barrier effect of corrosion products and the increasing ratio of [Cr]/{[Cr]+[Fe]} in the corrosion products.

However, it can also be seen that the apparent breakdown potential of corroded specimens decreases as a function of corrosion time as shown in Fig. 9. This may be attributed to the fact that the corrosion products did not cover the whole sample surface. With the extension of corrosion time, the passive film in the area where there was no corrosion products had also been destroyed gradually, and the protectiveness of the passive film in these areas decreased significantly, contributing to the decrease in the apparent breakdown potential.

Conclusion

The corrosion evolution of 316 stainless steel subjected to a simulated marine atmosphere has been investigated. The main corrosion type of 316 stainless steel is pitting corrosion; the initiation of pits is often associated with the dissolution of MnS inclusions; the maximum pit depth of 316 stainless steel increased in linear relationship with extension of corrosion time. The protective ability of corrosion products formed on the 316 stainless steel surface gradually increased as the corrosion time continued, which could be attributed to the physical barrier effect of corrosion products and the increasing ratio of [Cr]/{[Cr]+[Fe]} in the corrosion products.

Acknowledgements

The present work is supported by the Technology projects from China Southern Power Grid (K-GD2014-0532) and by the National Natural Science Foundation of China (nos. 51131007 and 51401222).

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