Stress-assisted Dissolution Research Articles

Corrosion kinetics study of 316 N weldment under cyclic loading in acidified chloride environment

Corrosion fatigue crack initiation (CFCI) life of AISI Type 316 N weldment in as-welded (AW) and heat treated (Solution annealed (SA), 550 °C/4h and 750 °C/1h) conditions was studied in high cycle fatigue regime (above 104 cycles) with a frequency (η) of 0.1 Hz and and R-ratio (σmin/σmaxwhere σmin and σmax signify minimum and maximum tensile stress in MPa, respectively) of 0.5 for different stress amplitude (σa) values in boiling acidified 5 M NaCl + 0.15 M Na2SO4 + 2.5 ml/l HCl solution under axial loading in load controlled mode. In-situ electrochemical measurements were employed during corrosion fatigue tests to study the passive film characteristics. SA weldment showed better corrosion fatigue (CF) resistance as compared to AW and heat treated conditions. Homogenization of microstructural and microchemical heterogeneities facilitates highest resistance to CF cracking process for SA weldment condition. Number of cycles to failure (Nf) increased with decrease in stress amplitude for all material conditions. CFCI mechanism was identified based on variation in the strain rate (ε°) due to combined effect of both emergence of persistant slip bands (PSBs) casued by rupture of passive film and initiation of microcrack caused by dissolution around depassivation site. The failure of weld metal occurred by a combination of transgranular (TG) and stress-assisted dissolution (SAD) of delta-ferrite. Crack growth zone was minimum for AW and heat treated (550 °C/4h and 750 °C/1h) conditions as compared to SA weldment.

Lessons from the Lollipop: Biotribology, Tribocorrosion, and Irregular Surfaces

Biotribology and tribocorrosion are often not included in numerical or computational modeling efforts to predict wear because of the apparent complexity in the geometry, the variability in removal rates, and the challenge associated with mixing time-dependent removal processes such as corrosion with cyclic material removal from wear. The lollipop is an accessible bio-tribocorrosion problem that is well known but underexplored scientifically as a tribocorrosion process. Stress-assisted dissolution was found to be the dominant tribocorrosion process driving material removal in this system. A model of material removal was described and approached by lumping the intrinsically time-dependent process with a mechanically driven process into a single cyclic volumetric material removal rate. This required the collection of self-reported wear data from 58 participants that were used in conjunction with statistical analysis of actual lollipop cross-sectional information. Thousands of repeated numerical simulations of material removal and shape evolution were conducted using a simple Monte Carlo process that varied the input parameters and geometries to match the measured variability. The resulting computations were analyzed to calculate both the average number of licks required to reach the Tootsie Roll® center of a Tootsie Roll® pop, as well as the expected variation thereof.

Influence of roughness on surface instability of medical grade cobalt–chromium alloy (CoCrMo) during contact corrosion–fatigue

Surface roughness and contact load play a major role in contact corrosion–fatigue phenomena that accelerates corrosion pits on the stressed surface area. The local yielding and stress concentration will be produced when the rough surface is brought into cyclic contact. Subsequently, the selective electrochemical attacks on the stressed surface will lead to the roughness evolution during environmental corrosion. The continuous roughness evolution by the stress-assisted dissolution will ultimately nucleate microcracks on the surface. In this article, a new evaluation method was introduced to identify the thermodynamic driving forces responsible for the stress-assisted dissolution. In order for complete understanding of the mechanical and electrochemical response of materials surface during contact corrosion–fatigue, finite element calculations and contact corrosion–fatigue experiments were performed on textured medical grade cobalt–chromium–molybdenum (CoCrMo) specimen surfaces. Consequently, the quantitative model of roughness evolution was developed to predict contact corrosion–fatigue damage of CoCrMo surface.

Roughness evolution of metallic implant surfaces under contact loading and nanometer-scale chemical etching

Surface damage of metallic implant surface at taper lock and clamped interfaces may take place through synergistic interactions between repeated contact loading and corrosion. In the present research, we investigated the influence of surface roughness and contact loading on the mechanical and chemical damage phenomena. Cobalt–chromium (CoCrMo) specimens with two different roughness configurations created by milling and grinding process were subjected to normal and inclined contact loading. During repeated contact loading, amplitude of surface roughness reached a steady value after decreasing during the first few cycles. During the second phase, the alternating experiment of rough surface contact and micro-etching was conducted to characterize surface evolution behavior. As a result, surface roughness amplitude continuously evolved—decreasing during contact loading due to plastic deformation of contacting asperities and increasing on exposure to corrosive environment by the preferential corrosion attack on stressed area. Two different instabilities could be identified in the surface roughness evolution during etching of contact loaded surfaces: increase in the amplitude of dominant wavenumber and increase in amplitude of a small group of roughness modes. A damage mechanism that incorporates contact-induced residual stress development and stress-assisted dissolution is proposed to elucidate the measured instabilities in surface roughness evolution.

Local surface damage and material dissolution in 45S5 bioactive glass: Effect of the contact deformation

Bioactive glasses react with the human physiological solution in control of their biofunctionality. The stress state in bioactive glasses determines the chemomechanical reaction and their biofunctionality. Using the microindentation technique, the effect of the indentation deformation on the surface damage and material dissolution of 45S5 bioglass was investigated. The indentation-induced local surface damage, including ring cracks and radial cracks, was revealed before and after the immersion tests in phosphate buffer solution (PBS). There existed a critical load for the formation of the radial cracks, which emanated from the periphery of the outmost ring crack. The growth of the radial cracks in the PBS solution displayed the stress-corrosion behavior with the crack-growth speed being a linear function of the indentation load. Fast dissolution occurred at the edges of the surface-damaged zone due to stress-assisted dissolution under the action of local tensile stress, which was different from the dissolution behavior of stress-free 45S5 bioglass.

Mechanical load-assisted dissolution of metallic implant surfaces: Influence of contact loads and surface stress state

Mechanical load-assisted dissolution is identified as one of the key mechanisms governing material removal in fretting and crevice corrosion of biomedical implants. In the current study, material removal on a stressed surface of cobalt–chromium–molybdenum (CoCrMo) subjected to single asperity contact is investigated in order to identify the influence of contact loads and in-plane stress state on surface damage mechanisms. The tip of an atomic force microscope is used as a well-characterized “asperity” to apply controlled contact forces and mechanically stimulate the loaded specimen surface in different aqueous environments from passivating to corroding. The volume of the material removed is measured to determine the influence of contact loads, in-plane stresses and the environment on the material dissolution rate. Experimental results indicate that surface damage is initiated at all the contact loads studied and as expected in a wear situation, removal rate increases with increase in contact loads. Removal rates display a complex dependence on residual stresses and the environment. In a passivating environment, the material removal rate is linearly dependent on the stress state such that surface damage is accelerated under compressive stresses and suppressed under tensile stresses. In a corrosive environment, the dissolution rate demonstrates a quadratic dependence on stress, with both compressive and tensile stresses accelerating material dissolution. A surface damage mechanism based on stress-assisted dissolution is proposed to elucidate the experimental observations.

Onset of Surface Damage in Modular Orthopedic Implants: Influence of Normal Contact Loading and Stress-assisted Dissolution

Surface damage at interfaces of modular implants results from repeated fretting contacts between metallic surfaces in a corrosive environment. As a first step in understanding this complex process, multi-asperity contact experiments were conducted to characterize roughness evolution due to action of contact loads and exposure to a reactive environment. Cobalt–chromium specimens with surface roughness similar to modular implant were first subjected to only contact loading and subsequently, to alternating contact loads and exposure to reactive environment. During repeated normal contact loading, amplitude of surface roughness reached a steady value after decreasing during the first few cycles. However during the second phase surface roughness amplitude continuously evolved—decreasing during contact loading and increasing on exposure to corrosive environment. The increase in roughness amplitude during surface reaction depended on the magnitude of applied contact loads. A damage mechanism that incorporates contact-induced residual stress development and stress-assisted dissolution is proposed to elucidate the measured surface roughness evolution.

Single Asperity Wear and Stress-Assisted Dissolution of Copper

AbstractThe focus of this paper is to investigate the synergistic influence of contact loads, surface stress state and chemical environment on material removal during mechanical stimulation of copper surface. A unique setup is used to generate well characterized stress states on a polished copper specimen. Stressed surface of copper specimen is stimulated using tip of the atomic force microscope (AFM) and material removed during stimulation is measured as a function of contact loads, surface stress state and chemical environment. Measured material removal rates display a complex dependence on contact pressures and in-plane stress state which changes as the pH of chemical environment changes from acidic to basic. A surface material removal mechanism based on a single asperity wear and stress-assisted dissolution is proposed to explain the experimental observations

On the evolution of surface morphology of polysilicon MEMS structures during fatigue

This paper presents the results of a combined experimental and computational study of surface topology evolution preceding fatigue crack nucleation in polysilicon MEMS structures. The evolution in surface topology observed during the crack nucleation stage is related to the underlying notch-tip stress distributions calculated using the finite element method. Measured changes in surface topography due to the stress-assisted dissolution of silica are shown to be predicted by a linear stability analysis. The implications of the results are discussed for modeling of fatigue in polysilicon MEMS structures.

Mechanisms of Fatigue in Polysilicon Mems Structures

AbstractFatigue crack initiation is shown to be associated with the stress-assisted evolution of a surface silica layer that forms during the normal exposure of unpassivated polysilicon surfaces to lab air In-situ atomic force microscopy (AFM) techniques are used to reveal the evolution of overall surface topology during incremental cyclic deformation to failure. Linear perturbation analysis of stress-assisted dissolution is then utilized to predict the evolution of the surface morphology. The predictions from the perturbation analysis are shown to be consistent with measured surface morphologies obtained using AFM techniques.

The effect of chromate addition to a chloride solution on crack growth in pre-pitted samples of Al–Zn–Mg alloy

Abstract The effect of chromate addition to a chloride solution on crack growth in pre-pitted samples of Al–Zn–Mg alloy AA-7039 has been investigated using potentiodynamic polarization experiment, open circuit potential (OCP) transient during slow strain rate test (SSRT), scanning electron microscopy (SEM), abrading electrode technique and ac-impedance spectroscopy. The addition of CrO 4 2− ion to NaCl solution raised the OCP in value on the potentiodynamic polarization curve. The OCP transients and SEM fractographs revealed that the initiation of stress corrosion (SC) crack from pits occurs in CrO 4 2− ion-free chloride solution during SSRT, but mechanical crack develops in CrO 4 2− ion-containing chloride solution. From the analyses of data obtained by using abrading experiments and ac-impedance spectroscopy, it was found that the repassivation of newly exposed bare surface is enhanced by adding CrO 4 2− ion to chloride solution, which is ascribed to the formation of protective chromium oxide film on the surface. This is responsible for the hindrance to pit-to-SC crack transition of AA-7039 in CrO 4 2− ion-containing chloride solution. Based upon the experimental results, it is suggested that the occurrence of a stress-assisted dissolution just after the mechanical exposure of the pit bottom at the transition time t tr is a necessary condition for proposed pit-to-SC crack transition mechanism.

Crack propagation behavior under low-cycle corrosion fatigue of A7003-T6 aluminum alloy.

Crack propagation behavior has been observed in A7003-T6 aluminum alloy under low cyclic loading with hold time, over the frequency range 4×10-3∼1 Hz, in 3.0% saline solution. From the experimental results, the effect of cyclic frequency on crack propagation rates was found to be exemplified by two different regimes ; one has a positive dependency on the frequency below a critical frequency, fcrit, at which point maximum environmental attack occurs in terms of da/dN, and the other is negative above fcrit. The behavior of crack propagation below fcrit was explained by the concept of stress-assisted dissolution which tends to inhibit mechanical failure by crack blunting and microbranching. The number and depth of secondary cracks occurring under the fracture surface are measured by metallographic examination, and the actual crack tip stress intensity factor was estimated.

Corrosion fatigue crack growth characteristics of high tensile strength steel HT 60 in synthetic sea water.

Corrosion fatigue crack growth and near-threshold characteristics of high tensile strength steel HT 60 (σB = 669 MPa) were investigated. Compact specimens were exposed to synthetic sea water under ΔK-decreasing and ΔK-increasing conditions at two stress ratios (R = 0.1 and 0.7) and at a cyclic frequency of 5 Hz. The results obtained were compared with that of high strength steel AISI 4340 (σB = 1 209 and 1 695 MPa) and are summarized as follows : (1) A da/dN-ΔKeff relation in air can give a conservative estimation of da/dN-ΔK relations in sea water. (2) In the case of high R, however, crack growth acceleration at high ΔK regions appears to be controlled by the stress-assisted dissolution. (3) The crack opening stress intensity factor Kop detected by the back-face-strain method is the result of crack surface contact with the corrosion products and gives an overestimation of Kop at the crack tip.

Corrosion Fatigue Crack Growth of a Martensitic Stainless Steel in NaCl Solution

Corrosion fatigue crack growth rate of a martensitic stainless steel has been measured at various stress cycle frequencies and stress ratios to elucidate crack growth and crack closure behaviors. Crack closure behavior is affected by plasticity-induced crack closure (PCC), corrosion products-induced crack closure as solid properties (ECC) and as viscoelastic properties (VCC). The load sharing capacity of the wedge effect as VCC is negligibly small, though crack closure behavior is varied by VCC. Meanwhile the wedge effect of ECC is exclusively effective resulting in a raised stress intensity to close the crack, thereby reducing growth rate. The wedge effect of ECC is much affected by stress cycle frequency, stress ratio and cathodic protection. The influence of stress ratios is diminished in da/dN-ΔKeff diagram, and a single curve is obtained at both frequencies. da/dN at f=0.1 Hz is considerably greater than that at f=10Hz. da/dN under cathodic protection is less great than that under free corrosion at low ΔKeff. da/dN is enhanced through stress-assisted dissolution and hydrogen embrittlement.

Corrosion Fatigue Crack Growth of a High-tension Steel in NaCl Solution

The influences of specimen thickness, cyclic frequency and cathodic protection on the corrosion fatigue crack growth of a high-tension steel HT80 have been discussed in terms of ΔKeff, and the accelerating mechanism of crack growth rate was made clear. In the relation between crack growth rate and ΔK, environmental and cathodic protection effects were not clearly observed, because the reduction of growth rates due to the wedge effect cancelled the acceleration due to hydrogen embrittlement. When the wedge effect was eliminated by using ΔKeff, environmental effect, frequency effect and cathodic protection effect were clearly observed. Though there were observed no frequency effects at a low ΔKeff, the acceleration of growth rates continued to a higher ΔKeff at a lower frequency. The dominating factor below (ΔKeff)th in corrosion fatigue crack growth in HT80 was stress-assisted dissolution and the one above (ΔKeff)th was hydrogen embrittlement, and cathodic protection became effective below (ΔKeff)th.

高強度鋼の応力腐食割れき裂発生に及ぼす微小振動応力の影響

Stress corrosion Cracking (SCC) initiation behavior was examined on a high-strength low alloy steel sensitive to hydrogen embrittlement (HE) type SCC under a sustained load with vibratory stresses of small amplitude superimposed in an open circuit and cathodically charged conditions in 3.5%NaCl solution. In an open circuit condition, time to crack initiation under a sustained load with superimposed vibratory stress range greater than 74 MPa is shorter than that of static SCC under a sustained load, resulting in lower 600 ks strength than that of static SCC σSCC. At a stress range Δσ smaller than 74 MPa, however, 600 ks dynamic SCC strength hardly decreases from σSCC. In case of σmax>σSCC SCC cracks are always initiated at corrosion pits formed on specimen surfaces irrespective of superimposed vibratory stress range. On the other hand, at σmax<σSCC and Δσ>74 MPa, the superimposed vibratory stresses promote stress assisted crack-like dissolution preceded by a corrosion pit. Dynamic SCC crack is initiated at the bottom of crack-like portion, thereby reducing the dynamic SCC strength. K*ISCC calculated from the sizes of corrosion pits and crack-like portion is roughly equivalent to KISCC obtained from a long sharp one.When a cathodic potential is applied to specimens, corrosion pits or stress assisted crack-like dissolution preceded by corrosion pits, are no more generated, and no dependence of superimposed vibratory stresses of small amplitude on 600 ks SCC strength is shown. In this situation hydrogen occulusion is much more enhanced than that in an open circuit, and SCC crack is initiated at the interior of the specimen where hydrostatic pressure reaches the maximum.

Fractographic Study on Mode II and Mode III SCC Crack Growth in Al-Zn-Mg Alloy

A fractographic study on mode II and mode III static- and cyclic-SCC crack growth in 3.5% NaCl solution has been carried out for a high-strength Al-Zn-Mg alloy. Static- and cyclic-SCC crack growth was introduced under mode II and mode III loads. Unlike the mode I SCC, the fracture surface morphologies of mode II and mode III SCC were characterized by transgranular stress-assisted dissolution, piled-up trench-like dissolution along slip planes, slip planes and slip traces. Transgranular stress-assisted dissolution was dominant in static SCC, whilst piled-up trench-like dissolution along slip planes, slip planes and slip traces were dominant in cyclic SCC due to repeated stress. Mode II and mode III SCC crack grew at much smaller K values than the thresh-hold stress intensity factor for mode I SCC crack growth, KISCC.

Influence of Cl&lt;SUP&gt;-&lt;/SUP&gt; Concentration on Corrosion Fatigue Crack Growth of an Austenitic Stainless Steel

Corrosion fatigue crack growth rate of an austenitic stainless steel (SUS304) was measured in Cl- containing water. Acceleration mechanisms of crack growth in corrosive environment were discussed, and an acceleration of da/dN in SUS3O4 was compared with that in a high-tension steel (HT55). The first step acceleration of da/dN in water environment from that in dry air was due to the reverse slip suppressing effect induced by water molecule adsorption. The second step acceleration at a low frequency (f=0.1Hz) was due to hydrogen embrittlement of plasticity-induced martensite formed at crack tips. In 3% NaCl solution at f=0.1Hz, the acceleration by stress-assisted dissolution was also observed. The crack opening ratio (U) was independent of Cl- concentration as well as of cycle frequencies. The acceleration of da/dN in SUS304 amounted to 9.2 times that in HT55, and a water environment sensitive crack growth behavior under cyclic stresses was clear. However, there existed a threshold ΔK to commencing to accelerate da/dN from that in air, and the acceleration in HT55 exceeded that in SUS304 at f=5Hz near ΔKth.