Artificial Corrosion Pits Research Articles

Quantitative evaluation of surface cleanliness of weathering steel treated by continuous wave laser

A 3 kW continuous-wave (CW) laser was used for the laser cleaning of weathering steel with artificial corrosion pits. Specimens with different degrees of corrosion were obtained by NaCl deposition in artificial pits of 10 mg/m2, 100 mg/m2 and 1000 mg/m2 through constant dew point dry and wet cycles for one year. To develop quantitative evaluation methods that can be applied on-site, the cleanliness of the laser-cleaned surface was evaluated through visual observation, turning time, and photographic analysis. The results reveal that the photometric analysis based on the CIE L*a*b* color space can be used to evaluate the surface cleanliness of surfaces treated with a CW laser, which is consistent with the results of the turning time test. The color difference of the surface before and after the laser treatment can be used as a quantitative index to evaluate cleanliness. The proposed laser surface treatment procedures can provide a reference for cleaning corroded weathering steel structures and achieve cleanliness of more than 80 % and a longer turning time.

EXPERIMENTAL STUDY OF THE INTERACTION OF CORROSION DAMAGE AND OUT-OF-CIRCULARITY IN THE COLLAPSE OF SUBMARINE PRESSURE HULLS


 
 
 The effect of corrosion damage on overall collapse strength of submarine pressure hulls was studied experimentally. Ring-stiffened cylinders were machined from aluminium tubing and loaded to collapse under external pressure. In selected specimens, some of the outer shell material was machined away in large single patches, representing general corrosion. Other specimens had many smaller patches, representing corrosion pitting from the outside of the hull, followed by grinding. Large-amplitude out-of-circularity (OOC) was introduced by mechanically deforming selected cylinders. Clusters of artificial corrosion pits were found to have approximately the same effect on collapse pressure as equal-depth general corrosion covering the same region of plating. General corrosion was found to be most severe when it was “in-phase” with OOC, since, during pressure loading, high compressive stresses resulting from corrosion were compounded by compressive bending stresses associated with OOC, and furthermore, the corrosion tended to increase the geometric imperfection itself. On the other hand, out-of-phase corrosion reduced the effect of OOC, while at the same time the thinning-associated compressive stresses were counteracted by local tensile bending stresses associated with OOC, so that strength reductions were correspondingly smaller. Overall collapse pressures for corroded specimens were reduced by, on average, 0.85% for each 1% of shell thinning. That result is based on a linear approximation of the nonlinear relationship between thinning and collapse pressure. The linear trend-line, which was used to account for the experimental scatter, is based on specimens with 13 to 27% shell thinning, and with a variety of corrosion areas and OOC amplitudes.
 
 

Statistical constitutive model of steel in randomly pitted structures

Pitting corrosion degrades mechanical properties of steel by inducing thickness reduction and stress concentration on the rough surface of a structural member in ship and marine structures. This paper presents a scheme for ultimate strength assessment of randomly pitted structures to treat such an adverse effect using an equivalent steel of degraded mechanical properties. Tensile tests of steel specimens with/without artificial corrosion pits were carried out to validate their numerical models. Numerical analyses were performed to explore the effects of pitting parameters (angularity, spacing, coalescence, and ratio of diameter, d, to depth, h, of pits) on tensile strength of pitted specimens. Numerous stochastic simulations indicate that random natures of pitting parameters induce a great variation of mechanical properties of a randomly pitted specimen obeying a normal distribution. Meanwhile, tensile strength of randomly pitted specimens is highly depended on equivalent d/h (the ratio of mean diameter to mean depth of all pits) besides degree of volume loss (DOV) of corroded material. A constitutive model of randomly pitted steel was established to be bilinear, whose material parameters were statistically described by the equivalent d/h and DOV. It was verified to be sufficiently precise in predicting the ultimate strength of pitted structures.

On the effect of pit shape on pitted plates, Part I: Tensile behavior due to artificial corrosion pits

Pitting corrosion threatens the safety of steel structures by degrading material properties. In this paper, the effect of pitting damage on tensile behavior of Q345 and Q235 steels is studied experimentally and numerically. Material tensile tests were carried out on small steel specimens with artificially introduced corrosion pits of a cylindrical, semi-ellipsoidal, or conical shape, and used to validate their numerical models. Numerous numerical analyses were carried out to study the tensile properties and behavior of different steels due to varying pitting features in the pit shape and degree of volume loss (DOV) of corroded material. Results showed that pit shape influences the tensile properties of corroded steel significantly, depending on the ratio of pit diameter to pit depth. In the worst case, variation of pit shape could result in a difference of 7.58% in strength deterioration, and it could become worse if combined with the random nature of pit size, pit depth, and pitting distribution. An equivalent material model was proposed to correlate the reduction ratios of tensile properties with the DOV and pit shape. The ultimate strength of pitted plates can be predicted properly, using the equivalent material model instead of modelling the detailed corrosion pits.

Shear strength reduction of trapezoidal corrugated steel plates with artificial corrosion pits

To investigate the ultimate shear strength reduction of trapezoidal corrugated steel plates (TCSPs) under corrosive condition, TCSPs with artificial corrosion pits (ACP) were taken as the research objects, and nonlinear shear buckling analysis was conducted by numerical simulation and experimental verification based on finite element method (FEM). The formula of the ultimate shear strength reduction coefficient η is proposed using the typical parameters influencing ACP, including ACP range Rr, ACP depth Rd, and equivalent corrosion rate γ. Fifteen specimens were designed with different ACP parameters, including Rr, Rd, and ACP diameter dp, and the shear buckling load test of TCSPs was performed. Results show that the distribution pattern of ACP has no effect on the shear strength of TCSPs, and the error between random distribution and array distribution is less than 1.5%. The ultimate shear strength reduction of TCSPs is related to Rr, Rd, and γ. The influence of ACP parameters on the shear strength of TCSPs is similar in different initial geometric defect modes. The proposed formula for calculating the strength reduction of TCSPs is in good agreement with the FEM and experiment results.

Experimental evaluation of the shear buckling behaviors of corrugated webs with artificial corrosion pits

Fifteen corrugated steel web I–beams were designed to investigate the shear buckling behavior and shear strength of corrugated steel webs with artificial corrosion pits. Different corrosion height, depth, and pit diameter, were considered in the shear loading tests. Critical buckling load and ultimate strength were compared with those calculated by existing formulas. In addition, Failure modes were discussed. Test results showed that corrosion volume decreased by 8% and bearing capacity decreased by nearly half compared with no corrosion. All corrugated steel webs were evenly sheared before the critical load was reached. Before reaching ultimate strength, the curve of vertical displacements and load is linear and the slope was affected by the corrosion. Perforation affected the failure modes of corrugated steel webs. Existing formulas yielded accurate and reliable results for uncorroded corrugated steel webs but cannot be used in the evaluation of the bearing capacity of corroded specimens. Corrosion volume, which was nonlinearly related to bearing capacity, was identified as an important factor that affects bearing capacity. Thus, corrosion volume must be considered in accordance with the actual working situation of corrugated steel webs. Future research should focus on the effect of perforation location and corrosion volume on ultimate strength to develop reliable designs for corrugated steel webs.

In Situ Synchrotron X-Ray Diffraction Characterization of Corrosion Products of a Ti-Based Metallic Glass for Implant Applications.

Ti-based bulk metallic glasses are under consideration for implants due to their high yield strength and biocompatibility. In this work, in situ synchrotron X-ray diffraction (XRD) is used to investigate the corrosion products formed from corrosion of Ti40 Zr10 Cu34 Pd14 Sn2 bulk metallic glass in artificial corrosion pits in physiological saline (NaCl). It is found that Pd nanoparticles form in the interior of the pits during electrochemical dissolution. At a low pit growth potential, the change in lattice parameter of the Pd nanoparticles is consistent with the formation of palladium hydride. In addition, a salt layer very close to the dissolving interface is found to contain CuCl, PdCl2 , ZrOCl2 ∙8H2 O, Cu, Cu2 O, and several unidentified phases. The formation of Pd nanoparticles (16 ± 10 nm at 0.7 V vs Ag/AgCl) containing small amounts of the other alloying elements is confirmed by transmission electron microscopy. The addition of albumin and/or H2 O2 does not significantly influence the nature of the corrosion products. When considering the biological compatibility of the alloy, the biological reactivity of the corrosion products identified should be explored.

Fatigue life prediction of welded joints with artificial corrosion pits based on continuum damage mechanics

An approach based on continuum damage mechanics (CDM) is applied to predict the fatigue life of welded joints with artificial corrosion pits. Full penetration load-carrying fillet cruciform welded joints with a 45° inclined angle were constructed, and artificial corrosion tests and fatigue tests of the welded joints were carried out. A new damage variable based on the crack size was defined to assess the stiffness degradation. Material parameters in the damage evolution equations were obtained from the fatigue experimental data. The CDM model combined with numerical simulations was used to describe the fatigue damage evolution process. A comparison between the fatigue life predicted results and the test results was made. The results show that fatigue life decreases with the increase of pit depth, decreasing by approximately 50% from d = 0 mm to 2 mm at the same stress range. The fatigue damage curves can be divided into three stages: the crack initial growth stage (D < 0.3), the crack slow growth stage (0.3 ≤ D ≤ 0.8), and the crack rapid fracture stage (D > 0.8). The fatigue damage curves for different stress ranges are nearly the same under the same pit depth. In addition to the material and loading conditions, the corrosive environment also has an effect on the material parameters in the fatigue damage evolution process. The fatigue life predicted results agree well with the test results, and the maximum relative error is 10.6%. The crack size can be used to describe the fatigue damage evolution of welded joints with artificial corrosion pits.

Mechanical Integrity of Steel Discs with Corrosion Pits

Currently, prediction of crack initiation by corrosion pits is only possible by assuming regular geometrical shapes, such as semi-spheres or semi-ellipsoids. Moreover, typical fatigue life diagrams associate the crack initiation life with geometrical features, such as pit depth or aspect ratio, often leading to unsatisfactory correlations due to high pit shape variability and data scatter. In the context of blade-disc fixation in aero engine turbines, this limitation translates into highly conservative life estimations. Therefore, a new crack initiation predictor is formulated based on experimental testing and numerical analysis of 28 artificial corrosion pits. A low-cycle fatigue test campaign is conducted using three-point bending test specimens to simulate maximum takeoff operation conditions of the aero engine and the associated loading of the blade root designed as firtree. An artificial pit is located at the critical point of each test specimen, respectively. The prediction criterion is based on finite element analysis and is formulated as the lowest plastic strain of a plastic region with a certain volume in the corrosion pit. This reference volume is varied until an optimum correlation with experimental crack initiation life is obtained. The criterion shows a superior correlation with crack initiation life compared to pure geometrical parameters such as pit depth.

A study of fatigue crack growth from artificial corrosion pits at welded joints under complex stress fields

AbstractThe paper studies the effects of artificial corrosion pits and complex stress fields on the fatigue crack growth of full penetration load‐carrying fillet cruciform welded joints with 45° inclined angle. Parameters of fatigue crack growth rate of welded joints are obtained from S‐N curves under different levels of corrosion. A numerical method is used to simulate fatigue crack growth using different mixed mode fatigue crack growth criteria. Using polynomial regression, the crack shape correction factor of welded joints is fitted as a function of crack depth ratios. Because the maximum circumferential stress criterion is simple and easy to use in practice, fatigue crack growth rate is modified using this criterion. The relationship of effective stress intensity factor, crack growth angle and crack depth is studied under different corrosion levels. The simulated crack growth path obtained from the numerical method is compared with the actual crack growth path observed by fatigue tests. The results show that fatigue cracks do not initiate at the edge or bottom of pits but at the weld toes where the maximum stress occurs. The artificial corrosion pits have little effect on the effective stress intensity factor ranges and crack growth angle. The fatigue crack growth rates of welded joints with pits 1 and 2 are 1.15 times and 1.40 times larger than that of the welded joint with no pit, respectively. The simulated crack growth path agrees well with the actual one. The fatigue life prediction accuracy using the modified formulation is improved by about 18%. The crack shape correction factor obtained using the maximum circumferential stress criterion is recommended being used to calculate fatigue life.

In-Situ Synchrotron Studies of the Effect of Nitrate on Iron Artificial Pits in Chloride Solutions: II. On the Effect of Carbon

The effect of nitrate on the iron dissolution in artificial corrosion pits in chloride-containing solutions has been studied with in-situ synchrotron X-ray diffraction and ex-situ Raman spectroscopy. A black layer containing Fe3C and carbon was found in the pit in pure 1 M HCl at 0.1 V(Ag/AgCl), but at higher voltages up to 1 V(Ag/AgCl), the black layer was not observed and Fe3C was not detected. The dissolution of Fe3C at low potentials may be inhibited by a carbon layer. In the presence of nitrate, the black layer containing Fe3C was present at all the voltages tested, and current fluctuations were observed and found to be potential-dependent, with greater fluctuations at higher voltages. After dissolution in 1 M HCl, the metal surface appeared slightly roughened with some small cavities, but the surface became crystallographically etched after dissolution in 1 M HCl and 50 mM NaNO3.

In-situ Synchrotron Studies of the Effect of Nitrate on Iron Artificial Pits in Chloride Solutions: I. On the Structures of Salt Layers

The effect of nitrate on the salt layers in iron artificial corrosion pits in acidic chloride solutions has been studied using in-situ synchrotron X-ray diffraction. During dissolution in 1 M HCl, there is a salt layer of FeCl2.4H2O on the electrode surface, which is isotropic. With addition of trace nitrate, the salt layer remains FeCl2.4H2O and no nitrate phase is observed, but the diffraction pattern becomes anisotropic, consistent with the formation of platelets with (1 2 0) planes settling horizontally. In nitrate solution containing trace of chloride (0.1 M HNO3 + 10 mM HCl), a salt layer is formed that is isostructural with Co(NO3)2.6H2O, and therefore assumed to be Fe(NO3)2.6H2O. This is the first reported crystal structure of ferrous nitrate. The salt layer is also found to give an anisotropic diffraction pattern, consistent formation of platelets with (0 2 0) planes settling horizontally.

ショットピーニング後に人工腐食ピット加工を施した懸架ばね用鋼の疲労特性に及ぼす水素の影響

In order to reduce the weight of the suspension springs by increasing the steel strength, it is necessary to improve the corrosion fatigue property. However, the dominant factor governing the corrosion fatigue property of the suspension spring has not been satisfactorily revealed yet. Therefore, the investigation into elucidating the mechanism has been carried out to independently evaluate the influences of corrosion pits, shot peening, hydrogen, prior-austenite grain size, and so forth on the corrosion fatigue property. In this study, the influence of hydrogen on rotary bending fatigue test was investigated using specimens with artificial corrosion pits after shot peening. The dominant factors governing the corrosion fatigue property of the suspension spring were discussed based on the result of the present study in conjunction with the results of the previous study on the influences of shot peening and corrosion pit depth. The conclusions obtained were as follows: (1) In the case of the specimen without corrosion pit or with only shallow corrosion pits having still large compressive residual stress at the bottom of the pits, the fatigue property was fairly good but declined with the increment of charged hydrogen due to the diffusible hydrogen. On the other hand, in the case of the specimen with deep corrosion pits having only small compressive residual stress, the fatigue property was poor regardless of the charged hydrogen content. We inferred from this result that the deep corrosion pit releases the compressive residual stress at the bottom of pits. (2) The effect of prior-austenite grain size on fatigue properties of hydrogen charged specimens turned out to be almost negligible.

Experimental Study Of The Interaction Of Corrosion Damage And Out-Of-Circularity In The Collapse Of Submarine Pressure Hulls - Technical Note

The effect of corrosion damage on overall collapse strength of submarine pressure hulls was studied experimentally. Ring-stiffened cylinders were machined from aluminium tubing and loaded to collapse under external pressure. In selected specimens, some of the outer shell material was machined away in large single patches, representing general corrosion. Other specimens had many smaller patches, representing corrosion pitting from the outside of the hull, followed by grinding. Large-amplitude out-of-circularity (OOC) was introduced by mechanically deforming selected cylinders. Clusters of artificial corrosion pits were found to have approximately the same effect on collapse pressure as equal-depth general corrosion covering the same region of plating. General corrosion was found to be most severe when it was “in-phase” with OOC, since, during pressure loading, high compressive stresses resulting from corrosion were compounded by compressive bending stresses associated with OOC, and furthermore, the corrosion tended to increase the geometric imperfection itself. On the other hand, out-of-phase corrosion reduced the effect of OOC, while at the same time the thinning-associated compressive stresses were counteracted by local tensile bending stresses associated with OOC, so that strength reductions were correspondingly smaller. Overall collapse pressures for corroded specimens were reduced by, on average, 0.85% for each 1% of shell thinning. That result is based on a linear approximation of the nonlinear relationship between thinning and collapse pressure. The linear trend-line, which was used to account for the experimental scatter, is based on specimens with 13 to 27% shell thinning, and with a variety of corrosion areas and OOC amplitudes.

XANES Study of the Chemistry of Molybdenum in Artificial Corrosion Pits in 316L Stainless Steel

The chemistry of molybdenum species in artificial corrosion pits on 316L stainless steel was investigated using X-ray absorption spectroscopy (XANES). It was found that the K-edge spectra are consistent with the presence of an Mo(III) species. The presence of Mo(VI) polymolybdates as been proposed to explain the well-known beneficial effects of Mo in decreasing the susceptibility of stainless steel to localised corrosion, but no evidence of these species was found. High resolution measurements (with a spatial resolution of 3.5 μm in the vertical direction) through the dissolving metal interface did not detect the presence of any different Mo-containing species in the vicinity of the salt film. However, the spatial resolution was insufficient to detect the presence of submonolayer species on the dissolving metal surface that have been proposed to block active dissolution.

Characterisation of salt films on dissolving metal surfaces in artificial corrosion pits via in situ synchrotron X-ray diffraction

The salt films formed on metal surfaces dissolving inside artificial corrosion pits formed in 1 M HCl have been probed with synchrotron X-ray diffraction. NiCl 2 · 6H 2O is the main phase in the salt film on nickel, whereas salt films on both iron and 316 L stainless steel are predominantly FeCl 2 · 4H 2O. However, the salt film on iron has a very fine homogeneous crystallite size whereas that on stainless steel is much coarser. The potential-dependence of the film formed on iron has been determined.

Small crack property of austenitic stainless steel with artificial corrosion pit in long life regime of fatigue

In order to investigate the fatigue strength in a long life regime of austenitic stainless steel with respect to the stress concentration, rotating bending fatigue tests were performed with pitted specimens (pit specimens). The Type 316NG and Type 304TP steel specimens had some artificial corrosion pits in its center. Further, in order to investigate the existence of a non-propagating crack in austenitic stainless steel, surface observations of the specimens that were not broken at 10 8 cycles were performed using a replication technique. The results of the observation verified that the fatigue strength of smooth (non-pit) specimens at 10 8 cycles is the fatigue crack initiation limit for both austenitic stainless steels because non-propagating cracks were not discovered in the surface of the specimens. On the other hand, only the pitted specimens of the Type 316NG steel showed non-propagating cracks; they were discovered at the bottom of the artificial corrosion pits. The initiation and propagation behaviours of the non-propagating crack were explained with the stress concentration factor of the artificial corrosion pits and the stress intensity factor range of the non-propagating crack tip.

オーステナイト系ステンレス鋼に付与した人工腐食ピットからの疲労き裂発生，進展挙動評価

In order to evaluate the effect of stress concentration on long life fatigue properties of SUS316NG and SUS304TP, rotating bending fatigue tests were carried out using the specimen with artificial corrosion pits. And then, to evaluate the fatigue crack initiation and propagation properties from the pits, the stress intensity factor range was calculated by the pit diameter at surface and the pit depth which were obtained by optical microscope observations, and also it was calculated by the pit area on fracture surface which obtained by SEM observations. As the results, the surface fatigue crack initiation and propagation properties of both austenitic stainless steels could be evaluated by the value of the stress intensity factor range which calculated using the pit diameter at surface and maximum pit depth by the optical microscope observation.

Fatigue Strength Evaluation of Austenitic Stainless Steel with Some Artificial Corrosion Pits

In order to investigate the effect of stress concentration on high cycle fatigue strength of austenitic stainless with corrosion pits, rotating bending fatigue tests were carried out. In this study, two types of austenitic stainless steels were used. One is Type 316 NG which is using for nuclear power generation plant and another one is Type 304 TP which is using as a pipe of industrial plants. The fatigue tests were carried out using the notch specimen which has some artificial corrosion pits on the center part of the specimens. The fatigue strength at 108 cycle of specimen having some artificial corrosion pits was much lower than that of specimen without pits and all fatigue cracks were generated from the pits. To evaluate the fatigue crack initiation property from the pits, stress concentration factor was estimated by its depth and its diameter which were measured using optical microscope. As that results, the values of stress concentration factor of pits were about estimated as values from 1.5 to 2.2. The fatigue crack initiation property from pits was explained by treating a stress concentration factor as fatigue strength reduction factor.

Automation of SQUlD nondestructive evaluation of steel plates by neural networks

This paper presents a method for automation of SQUID nondestructive evaluation (NDE) using neural networks, exempting the need for a trained technician, necessary to most of the usual NDE methods. An LTS-SQUID susceptometer, with a 16 mm diameter planar concentric gradiometer, was used to image flaws in steel samples from the bottom of an oil storage tank. Natural and artificial corrosion pits of various sizes were present in the samples, and a vertical magnetic field of 0.5 mT was applied by a superconducting magnet concentric with the gradiometer coils. A finite element model was used to simulate the magnetic signals due to the flaws, yielding training sets for the artificial neural networks. A neural system composed of two cascaded networks was developed to preprocess and analyze the magnetic signals. The first network removes a distortion that occurs in the experimental magnetic signal, and the second network detects the presence of flaws, and also assesses their severity. The trained neural networks were successfully tested with the experimental data obtained with the SQUID system.