Research Progress on the Nucleation Process of Pitting in Metal Materials

Thread Content

1 Theory of pitting nucleation process: Pitting undergoes a incubation period of several months or even years before nucleation occurs. The incubation period is the time that begins from the point where the metal comes into contact with the solution and pitting starts. The incubation stage is a metastable stage that includes the processes of metastable pore nucleation, growth, and the transformation of metastable pores into stable erosion pores. The initiation of any stable etch hole must go through a metastable stage; the emergence of a stable etch hole is the result of a metastable pore growing to a certain size【3】. Burstein C T et al. [4] believed that the surface nucleation of metastable pores mainly depends on the geometry of active sites on the surface: narrower and deeper active sites can give rise to metastable pores at lower potentials or in solutions with low Cl− concentrations, whereas shallower and more open active sites require higher potentials for the formation of metastable pores. On the surface of amorphous alloys free from inclusions, metastable pores can also nucleate, and the nucleation rate increases as the potential rises. In their exploration and research of the formation of pitting, predecessors have proposed many theories and models, including the adsorption theory, the anion penetration and migration theory, mechanical-chemical models, the point defect model for pitting initiation, the local acidification theory, the chemical dissolution theory, thermodynamic theories, and the depassivation-repassivation theory. These theories can be roughly divided into 2 categories. 1) Theory of passive film breakdown. This theory suggests that when corrosive ions adsorb onto the passivation film of stainless steel, the small radius of C1- allows it to penetrate the passivation film. Once Cl- enters the film, it \"contaminates\" the oxide layer, resulting in strong induced ionic conductivity. As a result, the film becomes capable of sustaining high current densities at certain points, and this enables cations to move around more freely. When the electric field at the film-solution interface reaches a certain critical value, pitting occurs. 2) Adsorption theory. This theory suggests that pitting is caused by the competitive adsorption of Cl- and oxygen. When the oxygen adsorption sites on the metal are replaced by Cl-, soluble metal hydroxo-chloro complexes are formed, and pitting occurs as a result of the breakdown of the passivation film. As shown in Figure 1, M represents metal; on the metal surface in solution, it are not oxygen molecules that adsorb, but stable oxide ions formed from water. ZX- is a complex ion of chlorine; once this complex ion replaces the stable oxide ions, the adsorbed film at that location is destroyed, leading to pitting corrosion. This theory suggests that the pitting breakthrough potential Eb is the potential at which corrosive anions can reversibly displace the adsorbed layer on the metal surface. At a certain point where the value exceeds Eb, the competitive adsorption of Cl- is strong, leading to pitting corrosion. Hoar【5,6】first proposed the surface complex model: 3 or 4 Cl- ions adsorb simultaneously around the cations in the oxide lattice, forming high-energy surface transition complexes. Such complexes can easily enter the solution, leading to the thinning and destruction of the passivation film. Chao C Y, Lin L F【7-9】 and Maedonald D D【10-11】 proposed the pnint defect model (PDM) for the occurrence of pitting. According to this model, the migration rate of oxygen ions or oxygen vacancies is key to the growth of the passivation film ； On the other hand, the diffusion of metal ions or metal vacancies plays a major role in metal dissolution. The breakdown of the passive film and the nucleation of pitting are the result of the accumulation of cationic vacancies at the metal/passive film interface, which is catalyzed by anions. Erosive anions (Cl-) first adsorb at the cationic sites on the surface of the passivation film; through a Schottky electron pair reaction, they cause the cations to leave the surface of the passivation film and enter the solution, thereby triggering metastable pitting. The adsorption of anions leads to the formation of cationic vacancies at the passivation film/liquid interface; ultimately, these cationic vacancies penetrate the passivation film and accumulate at the passivation film/metal interface. If the diffusion rate of vacancies in the passivation film is greater than the generation rate of cations at the passivation film/metal interface, then cation vacancies will accumulate there, resulting in local thinning of the passivation film or even its detachment from the metal surface. —When the radius of a void exceeds the critical size, stable pitting forms rapidly. Meanwhile, Chao C Y et al. analyzed the impedance spectra of the passivation system using a point defect model. It is believed that at high overpotentials, the membrane/solution interface reaction is the dominant factor, and multiple semicircles can be observed on the complex impedance plane ； At low frequencies, the transport of point defects in the passive film is the rate-determining factor, and a Warburg diffusion impedance can be observed. These predictions have been well confirmed through experiments on Ni and 304 stainless steel in buffered solutions. Okada T【12】conducts theoretical calculations from the perspective of statistical mechanics regarding the possibility of pitting on the surface of such halogen-containing transition metal complexes, and predicts the occurrence of transient noise at the stage when pitting takes place, based on the competitive adsorption of halide ions and OH- around the cations in the oxide lattice. There is currently no direct evidence to prove the existence of halogen complexes, but electron microscopy probe analysis has revealed significant C1ˉ aggregation on the metal surface, with Cl- playing a direct role in the metal dissolution process【13】. Marcus P et al. [14], by observing at the nanoscale the changes in the internal grains that occur when the passivation film is damaged and pitting takes place, proposed that there are three reasons for the breakdown of the grain boundaries in the oxide film during pitting: 1) The surface layer of the oxide film is weak ； 2) The presence of metal vacancies ； 3) Non-uniform growth on the surface of the metal oxide film. The role of Cl− in the mechanism of passive film breakdown was analyzed. It is believed that when Cl− is present in the electrolyte, it competes with OH− for adsorption at surface active sites; the resulting M–Cl− or M(OH)–Cl− complexes have weaker binding to the oxide, thereby reducing the activation energy required for their transfer into the electrolyte. Therefore, the local dissolution rate increases, membrane growth is hindered, which leads to a rapid thinning of the passivation film and an accelerated depassivation process at the local corrosion-sensitive sites. After partial depassivation, Cl- still competes with OH- for adsorption on the metal surface. If the OH- concentration is low, Cl- continuously replaces OH-, and the passivation process will be hindered. To prevent metal re-precipitation, a locally higher Clˉ concentration is required. This process leads to the selective nucleation of pits at locally corrosion-resistant inactive sites, as shown in Figure 2. Studies have shown that non-metallic inclusions in steel are the main cause of pitting corrosion. In the 1970s and 1980s, foreign research mainly focused on the mechanism of pitting corrosion induced by sulfides. Wrang1en C [16] was the first to propose that sulfides in the same piece of steel can be classified as active and inactive. Regarding the phenomenon where pitting corrosion originates from the matrix at the boundaries of inclusions, it is believed that there exists a sulfur-contaminated zone surrounding active sulfides. This zone acts as an anode relative to both the sulfides and the surrounding steel matrix, thus undergoing dissolution prior to the sulfides and the surrounding matrix. Smi-alowska S【17】 believes that the oxide film at the metal matrix/sulfide interface is defective; at higher potentials, the weak points in this passivation film break down first due to the action of Cl- in the solution, thereby triggering pitting. The dissolution of the sulfides themselves releases corrosive S2-, which prevents the surrounding metal matrix surface from being passivated anymore. Eklund G【18】believes that sulfides dissolve first, releasing S2-, which disrupts the passivation layer on the surface of the surrounding metal and induces pitting. Lin Changjian et al. [19] used an electrochemical scanning tunneling microscope to study the early stages of pitting corrosion in stainless steel. The results show that in the early stage of pitting corrosion in stainless steel, unstable pitting corrosion occurs first, and its surface morphology changes as this unstable pitting corrosion develops and subsides. The geometric size of unstable pitting corrosion is approximately 10 nm. The more positive the polarization potential, the greater the tendency for local dissolution and destruction of the stainless steel passivation film as well as the occurrence of pitting corrosion. Under certain conditions, unstable pitting corrosion can develop into stable pitting corrosion. The chemical reactions at local surface areas (including the hydrolysis, deposition, acidification, and other reactions of metal ions in those areas) intensify. When pitting corrosion progresses to a certain extent, that is, when certain geometric and chemical microenvironment conditions are met, it can develop into macroscopic pitting failure. Chen Xuequn, Zhang Chunya, and others [20] compared the susceptibility of four representative low-carbon steels to pitting initiation, and found that inclusions are the main sources of pitting initiation in steel. The passivation film at the steel matrix adjacent to these inclusions has the weakest protective effect, and pitting initiates from this area of the matrix. Research suggests that the common mechanism by which inclusions induce pitting is as follows: under anodic polarization, a passivation film forms on the steel surface, and the continuity and integrity of this passivation film are disrupted by the exposure of non-metallic inclusions on the surface. At the interface between the inclusions and the steel matrix, the arrangement of iron atoms is disordered; they are in a high-energy state with poor thermodynamic stability and a strong tendency to ionize. This area also has the thinnest passivation layer and the weakest protective effect. Erosive anions such as Cl- adsorb at the interface between inclusions and the steel matrix, as shown in Figure 3a. When the potential is polarized to the pitting potential, the accumulated Cl- reacts with the oxide film to form soluble iron chlorides, causing the surface film to dissolve locally and activating the surface of the steel matrix in that area. At this potential, bare iron atoms still exhibit a tendency to be passivated, and they can repair the damaged oxide film. The destruction and repair of the passivation film give rise to alternating competition. As long as there is surface activation, some iron will dissolve and undergo hydrolysis and acidification, thereby increasing the acidity in localized micro-areas. This also causes a slight dissolution at the edges of inclusions, resulting in certain dissolution products, as indicated by “X” in Figure 3b. When the anodic polarization potential continues to rise and reaches the pitting potential, Cl- causes greater damage to the passive film and enhances the ionization tendency of iron; film repair becomes impossible, and pitting corrosion begins to occur. Grain boundaries are also locations sensitive to pitting corrosion. Studies on pitting of 3RE60 duplex stainless steel have shown [1] that when immersed in a FeC3 solution (with a mass fraction of 1%), pitting originates at the phase boundaries rather than on the austenite side. This is due to the uneven distribution of the alloying elements chromium and molybdenum between the two phases – these elements are more abundant in the ferrite phase, which gives the ferrite phase better passivation stability compared to austenite. Scratches or stress concentrations in the passivation film, as well as lattice defects (such as dislocations), can all be causes of pitting. Frankel C S et al. [21] proposed a model in which the growth of metastable pores is controlled by the voltage drop across the membrane cover, as shown in Figure 4. The metastable pore is covered with a membrane that contains numerous micropores; these micropores enable the exchange of substances between the inside and outside of the pore. When an electric current flows through these micropores, it encounters significant resistance, resulting in an ohmic voltage drop. The voltage drop across the membrane cover remains constant during pore growth, and this voltage drop keeps the current density for pore growth constant. In the later stage of metastable pore growth, the membrane ruptures due to stress or osmotic pressure, causing the voltage drop to disappear suddenly. At this point, an instantaneous increase in current density can be observed; however, as the solutions inside and outside the pore mix rapidly, a high enough concentration required for dissolution cannot be maintained within the pore, and the pore quickly tapers off. 2 The application of SECM/l in the study of pitting nucleation process. SECM is a new type of scanning probe microscopy technique that was developed in the late 1980s by Brad A. J.’s research team, drawing on the technical principles of the scanning tunneling microscope (STM) and taking into account the characteristics of ultra-fine electrodes used in electrochemical research. SECM can detect electric currents in solutions, and it can also apply an electric current between a microelectrode and the sample. The resolution of SECM lies between that of conventional optical microscopes and STM. It is a novel electrochemical method with high spatial resolution for in-situ observations; it exhibits unique chemical sensitivity. Its greatest advantage is its ability to enable real-time, in-situ, three-dimensional spatial observation of research systems within solution environments. SECM can be used to study the occurrence and development of pitting on the surfaces of various metal samples, as well as to describe the morphology of the pitting pits. Casillas N【23】 SECM studied the pitting initiation stage of Ti metal. Research shows that the early growth process of pitting corrosion consists of three stages. The first stage is the nucleation stage, which occurs very rapidly ； Stage 2 is the metastable stage. During this stage, the propagation of pitting corrosion has not reached a steady state; however, pitting corrosion either continues to grow or ceases to grow at this stage ； Stage 3 is the steady-state stage. ZhuY and Williams D E et al. [24-25] used SECM to study the initiation of pitting in stainless steel. Studies have shown that when stainless steel is polarized at high electrode potentials, metastable pitting corrosion occurs. Gonzalez-Garcia Y et al. [26-27] investigated the early stages of pitting corrosion in stainless steel using SECM under open-circuit conditions. The results of SECM studies show that when the SECM probe scans over pitting corrosion, metastable pitting corrosion can be observed by measuring the metal ions released from the corrosion pits. 3 Conclusion: Studying the nucleation process of pitting corrosion in metallic materials holds great theoretical significance and practical value for uncovering the fundamental causes of their corrosion. Therefore, scientists have been continuously working on the study of its mechanism and have proposed various models. The development of SECM is being increasingly applied in the study of pitting processes, providing a new and effective method for exploring the nucleation process of pitting.