Hydrogen effect on the passivation and crevice corrosion initiation of AISI 304L using Scanning Kelvin Probe

Highlights

• Inhibition of AISI 304L steel passivation in air after cathodic chargings in 0.1 M NaOH.

• Decrease of oxide layer thickness and surface enrichement with Fe (II) after cathodic charging.

• Decrease of surface oxidation rate as a function of the charging time and cathodic current density.

• Crevice corrosion initiation after prolonged cathodic chargings in 0.58 M NaCl.

• Anodic location in the crevice corresponds to an increased sub-surface hydrogen concentration.

Abstract

Scanning Kelvin Probe was applied to study passivation of AISI 304L stainless steel after cathodic polarisation. The rate of passivation in air decreased as a function of duration and current density. X-ray Photoelectron Spectroscopy showed enrichment of the surface film by hydroxides of Fe (II) that was the result of hydrogen effusion from the bulk. SKP measured a decreased potential drop in the passive film. Pre-polarisation accelerates the crevice corrosion of steel in presence of chlorides. Using SKP mapping, increased hydrogen sub-surface concentration and lower level of passivity was observed in anodic zones of the crevice.

Introduction

Different criteria such as machinability, weldability, mechanical performance and cost are important for final material selection in the oil and gas industry. Corrosion resistance is also one of the key parameters for application of materials in seawater. For these reasons, stainless steels are the conventional materials used for critical components of the marine offshore platforms. Cathodic protection is widely used for carbon steel offshore constructions and when the individual components of the structures are made from stainless steel they are also protected in the same way due to the electric continuity [1]. Briefly, cathodic protection is a method to prevent the corrosion of a steel by transforming it to the cathode of an electrochemical cell. A shift of the steel potential to more negative values could be realized by coupling of construction with more electrochemically "active" metal like zinc, so-called galvanic or “sacrificial” protection. The potential can be also shifted by applying of cathodic impressed current (ICCP) [2]. The shift of the corrosion potential lower than the critical potential of pitting/crevice formation decreases the ability of aggressive ions to break down the steel passivity and prevents the formation of active anodic locations such as pits, crevices and cracks [2]. However, the cathodic protection can be switched off during maintenance operations or due to the replacement of galvanic anodes. Thus, the impact of cathodic polarisation on the materials must be understood both on a time scale and in term of other accidental conditions, such as over-polarisation, for example.

During the cathodic polarisation, hydrogen as product of the cathodic water reduction diffuses inside the alloy affecting its properties. For example, a phase transformation of austenite to martensite for AISI 304 or cracks formation can happen [3] on the surface of AISI 310 after prolonged cathodic charging [4]. Several reaction steps take place upon a metal surface during the Hydrogen Evolution Reaction (HER), including hydrogen adsorption, recombination and absorption of hydrogen into metal lattice [5]. The hydrogen solubility in the lattice of carbon steel is reported between 0.001−0.01 ppm below 100 °C [6], while the solubility of hydrogen in various stainless steels could reach the range of 30–60 ppm [7,8]. On the other hand, the hydrogen diffusion is relatively high in iron and carbon steel, about 5 orders of magnitude higher than stainless steels, which showed usually a diffusion coefficient around 10⁻¹⁵ m²/s below 50 °C. [9]. Thus, after prolonged cathodic polarization stainless steel can accumulate significant amount of sub-surface hydrogen that can slowly release later [9]. Undoubtedly, the passive layer will be more destabilised due to hydrogen atoms release. The structure of stainless steel passive films is often reported as bi-layered [10]. The inner layer providing passivity is enriched with chromium (III), while the outer layer is a mixture of iron (II, III) oxides and hydroxides [10,11]. The surface oxide film plays a role of an effective barrier for hydrogen with diffusivity near in 10¹² times lower comparing to metal lattice [12,13]. On the other hand, hydrogen is a strong reducing agent and can alter the chemical composition of the passive film reducing metal oxides [14]. Oxides reduction is accompanied by oxidation of atomic hydrogen and therefore the enrichment of the oxide film by hydrogen ions. Hydronium ions interacting with oxygen in the oxides-hydroxides (O₂⁻ and OH⁻) dissolves the film [15]. The dissolution of oxide film due to protonation can be the main reason of local corrosion that could further leads to stress corrosion cracking of stainless steels according to Okamoto’s theory of bound water [16].

It has been reported several times, that the cathodically induced hydrogen decreases the corrosion resistance of austenitic [12,19], martensitic [24] and duplex [[25], [26], [27]] stainless steels. In works [17,28], authors reported, that AISI 304 became remarkably less resistant to pitting after cathodic hydrogen charging. Hua et al. [18] tried to clarify the hydrogen local transport behaviour and highlighted a heterogeneous hydrogen distribution in thermally-hydrogen-charged AISI 304 using Scanning Kelvin Probe Force Microscopy (SKPFM) and Electron Backscattered Diffraction (EBSD). X-ray Photoelectron Spectroscopy (XPS) in [20], highlighted weaker intensities of all Fe compounds (Fe³⁺, Fe²⁺ oxides and hydroxides) and stronger intensity of metallic iron indicating a thickness decrease of passive film [29]. Albeit showed by X- ray Photoelectron Spectroscopy (XPS) analysis, no indication of film thinning but oxide film was found to be less enriched with chromium [17]. Ningshen et al. stated that the breakdown of passive film was promoted by hydrogen atoms due to proton de-insertion reaction (electro-oxidation) and reduce of Fe³⁺ to Fe²⁺ [22]. It has been also found by nanoindentation, that hydrogen decreased mechanical properties of passive film on AISI 316, such as the Young’s modulus, fracture stress and breaking load [21]. Similar observations of hydrogen pre-charging causing instability of passive film and increase of pitting susceptibility have also been reported for duplex 2507 [25]. Otherwise, using SKPFM it was found that austenite grains are more sensitive to pit nucleation than ferrite ones [26]. In other study on duplex stainless steel, EBSD showed that hydrogen induces the formation of stacking faults in austenite and increases the dislocations density in ferrite phase [27].

To resume, a lot of studies were dedicated to the different aspects of hydrogen-facilitated localised corrosion of stainless steels using electrochemical methods and ex-situ surface analysis. Despite these numerous studies, hydrogen-assisted evolution of passive films has still not been completely understood. To the best of our knowledge, there are no study to date regarding the influence of different cathodic charging parameters (current density, charging duration and charging solution composition) on the passivation behaviour of stainless steels.

In present work, to clarify the effect of sub-surface hydrogen on the passivity of the AISI 304L stainless steel Scanning Kelvin Probe (SKP) has been used. Atomic hydrogen was introduced into AISI 304L by controlled cathodic charging. The effect of hydrogen on the passive film was investigated as a function of current density and charging duration in 0.1 M NaOH and 0.58 M NaCl. XPS was used as a complementary technique to study the surface oxide-hydroxide layer composition after the hydrogen effusion. The parameters of cathodic polarisation that causes a limited corrosion stability and leads to crevice corrosion were also discussed. The impact of sub-surface hydrogen on the initiation of crevice corrosion was investigated, for the first time using SKP and the distribution of the locations of hydrogen absorption in crevice zone was also highlighted.

SKP is a non-contact technique and extremely sensitive to the changes in surface oxide films. SKP measures the contact potential difference between two metallic electrodes (the working and the probe) separated by an air gap. The potential of the probe is calibrated, which makes it possible to determine the electrochemical potential of the working electrode versus any reversible electrode (e.g. SHE) [30]. The electrochemical potential consists of two primary factors: the chemical potential of an electron inside the metal (μ_e) and the potential drop (X_w) across the metal/air interfaces [31]. Stainless steels are covered by surface oxides and the potential drop $X_w$ is mainly concentrated in oxide film (Eq. 1), where the first contribution is the contact potential difference metal/metal oxide, which is proportional to the difference in the chemical potentials of the electron in the metal (μ_e) and in the oxide (μ_ox) [32].

$$X_w=\frac{{\mu }_{ox}-{\mu }_e}{e}+F_b+{\beta }_{ox/air}$$

The potential drop F_b (Eq. 1) locates in the oxide film is a result of adsorption environmental components (e.g. oxygen, water). This drop is compensated by β_ox/air, located in adsorbed species at the oxide/air interface. In the air, the surface oxides of alloys can change the oxidation state, which will influence the potential measured by SKP (Eq. 2). For iron surface, the potential of the electrons in the oxide (μ_ox, Eq. 1) is a function of the activities of Fe species with different degrees of oxidation (Eq. 2) that relates to the Nernst equation:

$${\chi }_w=const+\frac{RT}{F\ln (a_{F{e}^{3+}}/a_{F{e}^{2+}})}$$

where $a_{F{e}^{3+}}$ and $a_{F{e}^{2+}}$ are the activities of the corresponding iron species in the surface oxide film, T is the temperature, and R and F are constants [33]. Thus, according to Eq 1 and Eq 2, a modification of the surface oxide film will change the potential drop X_w detecting by SKP.

It was reported, that SKP and SKP combined with Force Microscopy (SKPFM) are well-suited methods to investigate the hydrogen-induced changes on the electron work function (Ф_W) of iron [25,26,34,35], palladium [26], carbon steels [30,31,36] and duplex stainless steels [26,[37], [38], [39], [40], [41]]. It has been shown that through the change of work function on a metal surface, SKP could be used to detect absorbed hydrogen concentrations, as low as 0.01 atomic ppm [42]. The present work aims to increase a knowledge on impact of sub-surface hydrogen on passivation rate and crevice corrosion initiation of AISI 304L using SKP.