Corrosion Behavior of 17-4PH Martensite Stainless Steel Sprayed with Sodium Hypochlorite Disinfectant

Sodium hypochlorite (NaClO) disinfectant is widely applied in the industrial and medical fields to achieve disinfection, especially during the COVID-19 pandemic.¹  In this study, the NaClO disinfectant was used for aircraft engines and other components. As aircraft engines and other components are composed of complex curved surfaces,^2-3  NaClO disinfectant will flow freely after spraying on the surface of the parts, resulting in inhomogeneous corrosion. In addition, NaClO disinfectants are a very common type of disinfectant used in various aspects of human life. Therefore, it is necessary to research NaClO disinfectants to reduce the adverse effects of NaClO on steel corrosion.

For example, Su, et al.,⁴  found that carbon steel exhibits a higher corrosion rate in sterile water with NaClO than without NaClO because of its strong oxidizing ability. Giovanardi, et al.,⁵  explored the effect of disinfection treatment on the corrosion of copper pipes in hot water distribution systems; that is, the transformation of the corrosion behavior from uniform to local pitting with the environmental temperature increase was mainly attributed to the appearance of corrosion product, namely malachite. Li, et al.,⁶  found that NaClO disinfectant promoted the growth of oxide film on the surface of Ti6Al4V alloy, providing better corrosion resistance and tribological properties. Grgur, et al.,⁷  found that the presence of Ni in 316Ti stainless steels (SSs) (UNS S31635⁽¹⁾) accelerates the formation of metal oxides and hydroxides, which restrains the transition from hypochlorite ions (ClO⁻) into chlorine (Cl⁻) and oxygen (O₂), avoiding local pitting appearance in the NaClO solution. To some extent, the 316L SSs will exhibit good corrosion behavior as the corrosion time in disinfectant increases. On the contrary, Tranchida, et al.,⁸  found that an increased concentration of hydrated Fe in the 316L SSs corrosion product layer accelerates the corrosion process. Meanwhile, as the exposure to the corrosive solution increased, the Fe/Cr ratio of the sample passivation film composition decreased.

Cl⁻ is easily adsorbed in the weak points of the corrosion product layer and interacts with the corrosion product layer to form soluble intermediate complexes, causing the dissolution of the film and pit nucleation.⁹  When the concentration of Cl⁻ increases to a certain degree in the weak zones due to the constant absorption effect, the development of pits is accelerated until active pits formation; meanwhile, the pH of the solution in the pits decreases with the anodic reaction processes auto-catalytically.⁷  In addition, the relationship between the pitting potential of metals and Cl⁻ concentration is given as E_b = A − B log C_Cl−, which shows that an increase in the concentration of Cl⁻ can decrease the pitting potential and promote pitting when the other conditions remain invariant.¹⁰  Romanovski, et al.,¹¹  found that trapped chlorinated compounds will likely influence steel corrosion after surface disinfection treatment. Moser, et al.,¹²  reported that the corrosion resistance of martensitic stainless steel was enhanced with a decrease in Cl⁻ concentration and an increase in the stability of the passivation film. Fan, et al.,¹³  found that trivalent Fe ions (Fe³⁺) can accelerate the uniform corrosion of the alloy in the Cl⁻ free solution and the pit growth mechanism of alloys is different in the solution containing Fe³⁺ and Cl⁻. Zhang, et al.,¹⁴  mentioned that Fe³⁺ could decrease the pH of the solution because of its high hydrolysis ability, which increases the corrosion rate and helps pit growth by a reduction reaction. Shinozaki, et al.,¹⁵  reported that the effect of Fe³⁺ on the corrosion behavior of high-purity Al was caused by the formation of a less protective passivation film consisting of Al(OH)₃ and Fe(OH)₃ during the impregnation of high-purity Al. Xiao, et al.,¹⁶  studied the effect of Fe ions concentration on the electrochemical behavior of carbon steel in an aqueous environment of Cl⁻ and sulfate ion (SO₄^2–) and found that the formation of Fe³⁺ by redox reaction intensifies the substrate corrosion behavior and promotes the formation of the product FeOOH, which reacts with the anode product of Fe with two valences (Fe²⁺) easily.

Maraging hardening stainless steels are widely applied in the pharmaceutical, biological, and many other manufacturing industries owing to their outstanding atmospheric corrosion resistance and excellent mechanical properties.¹⁷  This study researched the corrosion behavior of 17-4PH martenistic stainless steels (MSs) influenced by the NaClO disinfectant. The disinfectant is generally sprayed to reach the component surface to reduce disinfection costs and provide more even coverage of the sample surface. The surface of structural parts covered with the NaClO disinfectant cannot be stable in the same location due to gravity effects, leading to inhomogeneous corrosion on the sample surface. Therefore, the disinfectant was sprayed onto the surface of 17-4PH MSs to study the formation and evolution of the corrosion product layer under the circumstances close to the practical disinfection conditions in this paper.

The composition of the 17-4PH MSs used in this study is 4.0Cu, 0.3Nb, 4.0Ni, 16.0Cr, and balance Fe (in wt%). The experimental samples were isothermally treated by solid solution in a chamber electric furnace under an ambient atmosphere of 1,038°C for 30 min, followed by air quenching. The samples were then aged at 552°C for 4 h in a resistance furnace, followed by cooling to room temperature in air. The commercial NaClO disinfectant, with an effective chlorine content of 34.0 g/L to 46.0 g/L (PH = 11.5), was used for the corrosion solution in this experiment. The main content of NaClO disinfectant used in this article is NaClO and water. To ensure the repeatability of the experiment and the popularization of research results (because in actual processing, such as the disinfection of aircraft engines, the sodium hypochlorite disinfectant used in this experiment is used), the model and manufacturer of the sodium hypochlorite disinfectant used in the article are Aitefu 84 disinfectant and Jiangsu Aitefu Group.

The studied samples were corroded with sodium hypochlorite solution for different times (0 h, 4 h, and 8 h) as above; these studied samples were conducted for electrochemical testing to investigate the influence of hypochlorite ions. EC-lab three-electrode cell equipment without deoxygenation was used to analyze the changes in the corrosion resistance of the sample after the corrosion process. The cell configuration consisted of the working electrode (17-4PH MSs), counter electrode (platinum), reference electrode (saturated calomel), and testing solutions (NaClO disinfectant). The working electrode was in contact with the electrolyte, as shown in Figure 1(c), with the actual contact area being a circle of 20 mm diameter of the exposed area. At the beginning of the electrochemical measurement period, the open-circuit potential (OCP) is measured for 30 min. Electrochemical impedance measurements were recorded by applying a sinusoidal signal of 20 mV using a frequency range between 0.01 Hz and 200 kHz. The dynamic potential polarization measurements were performed for potential between −1 V_SCE and 1 V_SCE at a scan rate of 0.1 mV/s.

Field-emission scanning electron microscopy (SEM, GAIA3^† GMU Model 2016) with a focused ion beam time of flight secondary ion mass spectrometry (TOF-SIMS) with Ga+ ion source (voltage: 30 kV) and transmission electron microscope (TEM, JEM 2100F^†) was used to observe the microstructure morphology and corresponding distribution of element in the experimental steel. The element valence state of the corrosion product layer and the corresponding distribution of the elements were characterized at the x-ray photoemission spectroscopy (XPS) by AXIS^† UltraDLD with a monochromated Al Kα line (anode voltage:15 kV, emission current:7 mA, spot step: 100 meV) and ION TOF SIMS 5-100^† tested the film component and morphology (Cs, 2 keV), respectively. The element valence state inside the pits was tested by XPS (Thermo Scientific Nexsa^†, anode voltage: 12 kV, emission current: 6 mA, spot step: 400 meV). The XPS spectra were analyzed using CasaXPS^† software and database, and all of the spectra were corrected to the carbon 1s peak of about 284.6 eV.

Figure 4(c) shows the EIS plots of the 17-4PH MSs with various corrosion times in the disinfectant. In the Nyquist diagram, the diameter of the single impedance curves for the Disinfection (8h) sample is much larger than others (0 h and 4h), which indicates that the corrosion rate was reduced and the corrosion product layer is relatively complete and compact and has better corrosion resistance. A simple equivalent circuit (illustrated in Figure 4[d]) is used in this study to fit the EIS data of each sample by the EC-lab software to analyze the corrosion product layer performance of the studied samples more intuitively. In this equivalent circuit diagram, R_sol represents the solution resistance. R₁ and CPE₁ represent the resistance and capacitance of the corrosion layer outside the substrate, and R_ct and CPE₂ represent the charge transfer resistance and capacitance, α is a critical parameter (α = 0 pure resistance; α = 1 pure capacitor). The polarization resistance (R_p) is used to estimate the corrosion resistance of the stainless steel,¹⁹  which is calculated by adding R₁ and R_ct in this paper, and the corresponding fitting values are given in Table 2. The sequence of the R_p is Disinfection (8 h) > Disinfection (4 h) > Disinfection (0 h).

The composition and thickness of the corrosion product layer on the surface of stainless steel would change during the reaction with the external corrosive liquid. The elemental composition of the corrosion product layer upon the matrix is studied via XPS after the corrosion test. The atomic composition of Cr, Fe, Cl, and O in the corrosion product layers after the corrosion test (0 h, 4 h, and 8 h) was obtained by peak fitting, as shown in Table 3. The content of elements with different valence states can be obtained by narrow peak fitting (Cr2p3/2, Fe2p3/2, and O1s), as shown in Figures 6(a) through (c) and Supplemental Figure S2, the corresponding fitting peak position as shown in Table 4. Figure 5 shows the metal oxide and hydroxide proportions for Fe and Cr and the proportions of different oxygen states. As the corrosion time increases, the ratio between Fe oxide and Fe hydroxide hardly changes, but the ratio between Cr oxide and Cr hydroxide gradually increases. Meanwhile, the Fe/Cr in Disinfection (8 h) is lower than in Disinfection (0 h), as shown in Table 3. Previous studies reported that the formation of Cr₂O₃ was the main component of the corrosion product layer for stainless steels, and a higher Cr₂O₃ content⁷  and lower Fe/Cr ratio²⁰  could effectively increase the stability of the corrosion film layer. 

Based on previous research,⁸  it can be concluded that as the content of ClO⁻ decreases, the Fe/Cr ratio gradually decreases, similar to experimental results in this study, as shown in Table 3. The variation in Fe/Cr is caused by the increase in the content of ClO⁻. The content of ClO⁻ decreases as the corrosion time increases due to the degree of hydrolysis. As the corrosion phenomenon mentioned in Figure 3, the corrosion resistance of the centre of the A-face sprayed by the disinfectant increases, which is attributed to the decreased atomic ratio of Fe/Cr in the corrosion product layer.

The Cu-enriched domain is easy to pit erosion sprouting and speeding up corrosion, which is similar to metal carbides (MC)³⁴  and impurities introduced by smelting in stainless steel, owing to the difference in chemical potential between precipitation (except for Fe) and matrix (Fe). It is found that monomers Cu and Cu²⁺ in the active pits, as shown in Figures 7(d) and (e), make a difference to the elements in the corrosion product layer (A-face) and corrosion product (Figure 3[c]). Therefore, the Cu element is only involved in the reaction inside the active pit. In the substable pitting state, various ions from the disinfectant solution will pass through the corrosion film layer and come into contact with the substrate.

Hydroxyl ions (OH⁻) originate in the cathodic reaction, and pitting autocatalysis will enrich the pit solution with H⁺ and acidify the solution. According to the XPS result,³⁸  the role of Cu in the above corrosion reaction is mainly used as accelerated pitting autocatalysis. Based on element content change by the XPS peak-differentiating fitting (Figures 6[a] through [c] and Supplemental Figure S2) and distribution from SIMS results (Figure 7[e]).

In the splashing experiment, the edge of the A-face and the whole B-face of the sample were badly corroded with the breakdown of the passivation film. It is presumed that the composition of the corrosion solution changed and was enriched with offensive ions, e.g., Fe³⁺ and Cl⁻, after the precursor cathodic reaction. Besides, the solution flows into the fresh surface where there is no contact with disinfectant during the splashing process, causing the pitting corrosion in the B-face due to a breakdown in the passivation film. Meanwhile, Cu-enriched domain aggravates pits behavior and hinders the self-healing of pits. The Fe and Cl element distribution, according to the SIMS results, confirmed this assumption that Fe³⁺ and Cl⁻ would promote the occurrence of electrochemical corrosion. The interaction of the three elements causes violent corrosion of stainless steel in the disinfectant solution.

NaClO disinfectant was used for aircraft engines and other components, which are composed of complex curved surfaces, and NaClO disinfectant will flow freely after spraying on the surface of the parts, resulting in inhomogeneous corrosion. In addition, NaClO disinfectants are a very common type of disinfectant used in various aspects of human life. Therefore, it is necessary to conduct research on the corrosion behavior of NaClO disinfectants to reduce the adverse effects of NaClO on sample. The corrosion behavior of heat treatment 17-4 PH stainless steels by spraying NaClO disinfectants at ambient temperature was investigated, and the main results are presented as follows.

• Corrosion behavior of samples treated with different disinfection times (0 h, 4 h, and 8 h) were observed, and the samples treated with disinfection for 8 h had the best corrosion resistance, which may be due to disinfection for 8 h promoted the samples form a protective film layer, preventing further corrosion.

• Corrosion behavior of samples treated with different disinfection times (0 h, 4 h, and 8 h) was mainly influenced by the Fe/Cr ratio. Samples that have undergone longer disinfection treatments, making the Fe/Cr ratio decrease, become more corrosion-resistant.

• Transfer of the aggressive disinfectant solution containing Fe³⁺ and Cl⁻ ions to the normal passivation film where the Cu element-enriched area, due to the liquid flow in the substrate, will result in the pitting of the sample surface.