Effect of mechanical stress on stainless steel pitting behavior

With the implementation of the "Made in China 2025" and "Belt and Road" strategies, China's marine engineering equipment, power equipment and large-scale basic engineering facilities are facing the challenge of harsh or even extreme service environment. It is very important to strengthen the basic research of equipment reliability engineering and ensure the reliable and effective service of the full life cycle of major equipment. Corrosion is the main cause of the destruction and scrapping of various infrastructure and industrial equipment, and china's annual losses due to corrosion amount to 5% of GDP

. According to the failure accident data caused by corrosion in the project, the failure ratio caused by pure pitting is 25% and the proportion of stress corrosion and corrosion fatigue fracture is 38%.
, pitting is also one of the main causes of material stress corrosion cracking and corrosion fatigue crack germination, cracks often crack at the point pit where the maximum stress concentration is concentrated. In order to accurately predict the location of critical crack germination, it is necessary to understand the environment and stress state around and inside the pitting pit. Stress concentration at pitting pits is related to the depth, size and shape of pitting, and is closely related to the stress state of the material. The failure of key parts and structures caused by this electro-chemical interaction will pose a great threat to the safety of human life and property. It is of great significance to study the pitting behavior of materials under mechanical stress and to understand the pitting mechanism and to establish a scientific prediction model of stress corrosion and corrosion fatigue life.
stainless steel, as an important engineering structural material, is widely used in the nuclear industry, aviation, aerospace, marine, petroleum and chemical industries because of its good mechanical properties and high corrosion resistance, but contains aggressive ions such as
In the
-
, B
-
, SCN-
-
, etc.), the surface passivation film is prone to local damage and induced pitting.
, the study on the effects of composition and microstructure, agative
and environmental factors on stainless steel pitting behavior is more mature.
recent years, more and more scholars have focused on the effects of mechanical stress on pitting.
This paper summarizes the influence of one-way stress, cross-strain and surface residual stress on the various stages of stainless steel pitting, discusses and analyzes the relevant pitting theory and research methods, and looks forward to the development trend of the germination and growth of pitting under mechanical stress and the key scientific issues to be explored in depth.
1 Effect of mechanical stress on stainless steel pitting germination
1.1 pitting germination mechanism (passivation membrane rupture)
It is well known that the corrosion resistance of the metal or alloy itself is mainly due to the thickness of 0.5 to 20 nm dense oxide film (passivation film) formed by the surface. For blunt materials such as stainless steel, the formation of pitting experiences a series of continuous steps: the
passivation membrane ruptures the pitting nuclei, and then enters the substation growth stage; There are three main mechanisms for the germination of pitting, namely, penetration mechanism, membrane rupture mechanism and adsorption mechanism. The penetration mechanism considers that the aggressive anions (e.g. Cl ) migrate to the metal surface through passivation film under the action of high electric field, so that the metal/film interface is empty, and the metal ions migrate to the solution and dot erosion germinates. The membrane rupture mechanism states that when the membrane stress exceeds the critical rupture strength of the membrane, the membrane is destroyed and the pitting occurs immediately. The adsorption mechanism holds that the aggressive anion is preferred to adsorption in the defective position of the membrane surface and the cation formation complex, the ion conductivity of the local membrane and the dissolution speed of the membrane is much greater than that of the surface without Cl2012
adsorption, forming a randomly occurring active point, called the hole core. The passivation film at the hole core can be re-passivated and repaired, the hole core disappears, or the hole core continues to grow beyond the critical radius, the pH in the hole decreases, and anion concentration leads to pitting formation. The passivation membrane breaks very quickly, and the dissolution scale is usually at the nanoscale. Due to the accuracy of current monitoring technology, it is not possible to directly observe the process of passivation membrane destruction, including how to determine the dominant mechanism of pitting germination. Therefore, most studies have speculated and corrobored the membrane rupture mechanism by using surface composition subtitization techniques such as pitting potential and pitting pronation period, pitting stabilization, and X-ray photoelectronic energy spectrum (XPS) and secondary ion mass spectrometry (SIMS).
For stainless steel, pitting tends to take precedence over the physical and chemically uneven position of the surface, i.e. the weakest area of the passivation film, especially oxide mixing, alloy carbide and sulphide (e.g. MnS) mixing. The unevenness of the composition and structure leads to the unevenness of the passivation film, which makes the germination law of pitting more complicated.
1.2 Point erosion germination under one-way and cross-strain stress
the study focused on changes in pitting lead or other macro-test results to indirectly reflect the passivation membrane rupture mechanism under stress. Such as Yang indirectly reflects the germination of stainless steel pitting when the plus stress is less than half the yield stress by measuring changes in the pitting level. Studies have shown that increasing
stress
significantly reduces the pitting level at a sufficiently high Cl-to-1000 concentration. Combined with the adsorption mechanism and point defect model, it is concluded that pull stress increases the concentration of defects in the passivation film and the diffusion rate of cation voids in the passivation film, and promotes the adsorption of aggressive Cl
-
, so pitting is more likely to occur. Results based on thermodynamic calculations such as Tanahashi prove that pull stress increases the diffusion coefficient of the empty position and pressure stress is the opposite, thus supporting the view of Yang et al. With the development of electrochemical noise technology, the breeding and formation process of pitting can be monitored in-place real-time. Shi Zhiming and others use electrochemically inductively inductive level noise to signal the cracking behavior of the membrane under pull stress. The results show that when the pull stress is higher than the elastic limit, with the increase of the stress, the signal of the momentum noise of the epitomized membrane cracking increases correspondingly, and decreases exponentially with the load time, while when the pull stress is below the elastic limit of the metal, the passivation film hardly cracks, at which time the position noise is comparable to the background noise. Because the pull stress is higher than the elastic limit, the sample produces plastic deformation, with the internal misality constantly moving out of the surface, passivation membrane rupture, the level of the likeness noise increases. According to the membrane rupture mechanism, it can be predicted that passivation membrane cracking will inevitably increase the chance of the active point there to form pitting, that is, to improve the nucleation rate of pitting.
corrosion fatigue is a form of failure of metal materials under the joint action of circulating or random cross-strain and corrosion environment. Similar to stress corrosion cracks, corrosion fatigue cracks are mostly micro-cracks that begin to form at the point of pitting damage on the metal surface. Some researchers have suggested that there is a critical point erosion depth when pitting pits are converted into crack sources. The critical point pit depth decreases with the stress amplitude and is related to the stress ratio. The pitting germination and evolution process controlled by the interaction between cross-strain and corrosion environment has a significant effect on corrosion fatigue life. From a microscopic point of view, high density misalmout is formed inside the material under interstite stress, and the short-range interaction of the misalmed in the process of back-and-forth motion produces a higher density point defect cluster (≈1015 cm
-3
), which becomes the source of pitting germination. Xie Jianhui and others believe that cross-strain stress on the rolling state of 316L stainless steel in hank's solution pitting nuclei and growth are promoting. Dynamic stress leads to misalmutation and plane misplaceration, thus increasing the active points and active channels produced by pitting.
cross-over stress is more complex than one-way static stress, in which changes in parameters such as stress amplitude, stress ratio, load frequency, cycle cycle times, etc. affect the stability and re-passivation tendency of the passivation membrane. Huang and others studied the germination and growth characteristics of 304 stainless steel pitting under proportional and non-proportional cycle loading. The experimental results show that 304 stainless steel pit depths in 6 wt.% FeCl3 solution are subject to the normal distribution of the number before and after the cross-strain is applied. Compared with non-proportional loading, the pitting density observed under proportional loading conditions is significantly higher, and the number of pits under the action of two cross-strain stresses is significantly higher than that of unloaded conditions. Because the proportion load maximum main stress (382.5MPa) is constant and the value is higher, the passivation film is more prone to rupture, the inside of the larger pit is always stress concentrated, and the pitting is easier to germinate and grow.
1.3 Pitting germination under the residual stress of the surface
stainless steel tends to produce residual stress on the surface during production, processing and manufacturing. Residual pull stress or residual pressure stress may occur on the surface due to differences in the processing process. According to the relationship model of corrosion current increment with misplaced density and residual stress, when away from the equilibrium state, the anode current density increment is:
of which n is the number of misplaceds, Δτ is the shear stress increment caused by misalming, α is the ratio coefficient of the misplaced density equivalent to plastic strain (value 109 to 1011cm
-2
), T is temperature,
R
kNmax (k is the Boltzmann constant and Nmax is the maximum misplaced density). Thus, high error density and residual stress increase the anode dissolution rate.
, pitting is more likely to occur when residual pull stresses and residual stresses are unevenly distributed on the surface of the material. At present, there have been a lot of research on the effect of plastic deformation on stainless steel pitting position. In general, for stainless steel, a large amount of plastic deformation will lead to a decrease in pitting capacity. With the development of surface reinforcement processes such as blasts, the introduction of residual pressure stress reduces the chance of part failure to a certain extent. Laser blasting (LSP) as a new material surface strengthening technology, the use of strong laser beam plasma shock wave, so that the material produces yield and plastic deformation, in the plastic deformation area to produce residual pressure stress, metal parts due to mechanical processing, heat treatment, welding and other processing of residual pull stress into residual pressure stress, change the microstructure of the near surface, the schematic diagram shows in Figure 1.
According to literature, laser blasts can improve the pitting performance of Austral stainless steel, mainly reflected in the reduction of passivation current density, pitting potion increase (Figure 2), the number of steady-state pits decreased, the size decreased. Peyre and other analysis from the perspective of passivation film optimization, it is believed that the existence of pressure stress makes the surface film more dense, so when affected by external forces, the membrane is more difficult to break, the performance of a higher pitting potential;
1.4 The effect of microstructure on pitting germination under mechanical stress
Lu and others believe that for stainless steel substrates that are not sensitive, the defect concentration of the passivation film is not actually related to the stress level, and the pitting cause does not change due to the change of the stress level. It is obvious that the change of microstructure before and after the sensitivity treatment is the main cause of stainless steel pitting germination. Suter and others have studied the pitting behavior of different forms of MnS when applying pull stress using micro capillary electrolyte cell technology. When MnS is the only germ source of pitting, only pulling stress on areas containing MnS induces pitting germination. For Austella stainless steel, plastic deformation can easily induce the transformation of the marmatic body of the substation Austella. The marsome is generated in the austial maternal phase by misplaced reaction. Compared with one-way stress, under the effect of cross-variable stress, the local misplaced density is higher and the content of marsome is increased, which makes the passivation film more prone to rupture in the phase change process, and there is a difference in the position of austral and marmatics, and the pitting is very easy to germinate at the two-phase interface of the marzir or austral and marmatic bodies induced by deformation, resulting in an increase in the pitting nucleus.
mechanical residual stress and microstruct structure are often accompanied by, it is difficult to distinguish between the two in the course of research. After cold deformation (no stress plate surface bending, cold rolling, stretching, etc.), residual pull stress is produced, and significant changes in microstructure structure occur. Among them, in addition to the occurrence of Mars phase change also includes the following 3 aspects: 1, grain deformation, misalisation and twin crystal substructure, increased the number of defects; In addition to the residual pressure stress produced after laser blasting treatment, it also caused changes in the microstructure of the surface, as follows: 1, ultra-high strain rate (10
7
s
-1
) makes the coarse crystal refinement, as the number of laser impacts increases the misplaced density, the surface layer can form nanocrystals. Surface grain refinement can change the composition structure and film-forming mechanism of the passivation film, nanocrystals are conducive to the rapid collection of passivation elements in the passivation film, forming a more dense passivation film, so that pitting is not easy to germinate. However, the high density misalised outcrumb destroys the integrity of the passivation film and increases the substation pitting nucleation rate. 2, by adjusting the parameters of the laser shot can avoid phase-change Mars body production, thereby reducing the potential source of pitting germination. 3, the material surface spray treatment after the size of the debris reduced. After laser impact, the number and size of the second phase of induced pitting will inevitably change. The synergy between the change of microstructure after laser blasting and the residual pressure stress produced by the surface reduces the germination of pitting to some extent. In addition, in studies on the effects of hydrostational pressure on stainless steel corrosion and pitting sensitivity in different Cr, Ni, and Mo contents, Beccaria and others concluded that increased hydrostational pressure alters the composition of the passivation membrane. Because of this, Cr-Ni stainless steel that does not contain Mo exhibits better pitting resistance than normal pressure at high pressure.
The above study proves that stainless steel pitting germination under stress is not only a single effect controlled by stress, it is necessary to consider the effects of stress on the microscopic composition and organizational structure of induced pitting germination.
2 The effect of mechanical stress on the growth of stainless steel substation pitting
points of erosion immediately after germination into the sub-stable growth stage, to achieve the critical conditions formed by steady-state pitting, the sub-stabilized point erosion to steady-state pitting will occur, otherwise the sub-stabilized pitting pit will be re-passivated. Sub-stabilization pitting can be formed below the pitting potation, and sub-stabilization pits are smaller than steady-state pits, mostly micro-scales. The number, size and growth dynamics of substation pitting are closely related to the defects such as dissolved active points and clamping, and directly determine whether stainless steel can have steady-state pitting damage. The transient current signal is usually measured using the constant power polarization of the passivation interval to indicate sub-