Pitted Specimens Research Articles

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.

Symbiotic embedment structures in Silurian Caryocrinites (Echinodermata, Rhombifera, Hemicosmitida)

AbstractA variety of pits representing symbiotic embedments, sometimes associated with pathological deformation in the host, are known from the skeletons of Paleozoic stalked echinoderms. These structures are well known from multiple genera of crinoids and a limited number of blastozoans but have not previously been described in detail from the skeletons of rhombiferans. This is surprising given the abundance of rhombiferans in certain deposits, the co-occurrence of rhombiferans with frequently infested taxa, including diploporitans, in multiple assemblages, and the morphological similarity between certain rhombiferan taxa and coeval infested crinoids. The common hemicosmitid rhombiferan Caryocrinites Say, 1825 is widespread throughout the middle Silurian of eastern North America and is herein reported to contain symbiotic (potentially parasitic) embedment structures. Specimens were collected from the lower portion of the mudstone lithofacies of the Massie Formation (Wenlock, Sheinwoodian) at the Napoleon quarry of southeastern Indiana, USA. Strong host specificity is indicated by the absence of pits in C. ornatus Say, 1825 and exclusive infestation of a smaller co-occurring species of Caryocrinites. Thecae with embedment structures are consistently smaller than thecae without such structures, with pitted specimens being restricted to a narrow range of thecal heights (20–24 mm). All embedment structures are present only on the proximal portion of thecae, with individual specimens containing between one and 30 pits. No elevated rims or significant swelling were observed on any specimens, and all pits are relatively small (~1 mm in diameter). The presence of symbiotic embedment structures represents an additional example of a crinoid-like aspect to the ecology of Caryocrinites.

Microscopical and elemental FESEM and Phenom ProX-SEM-EDS analysis of osteocyte- and blood vessel-like microstructures obtained from fossil vertebrates of the Eocene Messel Pit, Germany

The Eocene (∾48 Ma) Messel Pit in Germany is a UNESCO World Heritage Site because of its exceptionally preserved fossils, including vertebrates, invertebrates, and plants. Messel fossil vertebrates are typically characterized by their articulated state, and in some cases the skin, hair, feathers, scales and stomach contents are also preserved. Despite the exceptional macroscopic preservation of Messel fossil vertebrates, the microstructural aspect of these fossils has been poorly explored. In particular, soft tissue structures such as hair or feathers have not been chemically analyzed, nor have bone microstructures. I report here the preservation and recovery of osteocyte-like and blood vessel-like microstructures from the bone of Messel Pit specimens, including the turtles Allaeochelys crassesculpta and Neochelys franzeni, the crocodile Diplocynodon darwini, and the pangolin Eomanis krebsi. I used a Field Emission Scanning Electron Microscope (FESEM) and a Phenom ProX desktop scanning electron microscope (LOT-QuantumDesign) equipped with a thermionic CeB6 source and a high sensitivity multi-mode backscatter electron (BSE) for microscopical and elemental characterization of these bone microstructures. Osteocyte-like and blood vessel-like microstructures are constituted by a thin layer (∾50 nm thickness), external and internal mottled texture with slightly marked striations. Circular to linear marks are common on the external surface of the osteocyte-like microstructures and are interpreted as microbial troughs. Iron (Fe) is the most abundant element found in the osteocyte-like and blood vessel-like microstructures, but not in the bone matrix or collagen fibril-like microstructures. The occurrence of well-preserved soft-tissue elements (at least their physical form) establishes a promising background for future studies on preservation of biomolecules (proteins or DNA) in Messel Pit fossils.

Short‐crack modelling of the effect of corrosion pits on the fatigue limit of 12% Cr steel

AbstractComprehensive data on the fatigue limit of smooth uncorroded specimens and specimens with single corrosion pits of a 12% Cr steel for steam‐turbine blades have been generated by means of ultrasonic fatigue testing. The threshold for fatigue‐crack growth has also been determined. Available (short‐)crack models due to El Haddad and Härkegård and Smith and Miller generally yield safe predictions for the fatigue limit of the pitted specimens, whereas Murakami's √area model yields inconsistent predictions in the present case.

Short-crack modelling of the effect of corrosion pits on the fatigue limit of a 12% Cr steel

EPRI organised an international research project [1], where ultrasonic testing was used to determine the fatigue limit of plain specimens of 12% Cr steel at R > 0 with artificially produced, sub-mm size corrosion pits. Moreover, the plain fatigue limit of uncorroded specimens was determined at R > 0, and fatigue-crack-growth testing, including the threshold, was carried out at R-values ranging from −1 to 0.9. This comprehensive set of fatigue data constitutes an ideal basis for investigating the capability of a variety of methods to predict the fatigue limit of pitted specimens. A pre-pitting procedure was used to produce single, nearly spheroidal corrosion pits as shown in Fig. 1. The semi-width-to-depth ratio r/d was typically around 0.5. It was therefore decided to model two extremes: a semi-spherical pit with r/d = 1 and a cylindrical “pit” with r/d = 0. To simplify the analysis, the fatigue behaviour of the semi-spherical surface pit was approximated by that of a spherical pore in a wide body. Consider an annular crack of depth a at the equator of a spherical pore. For a shallow crack, a a∗, the asymptotic stress intensity factor is given by

Localized Corrosion of Stainless Steel in a Nuclear Waste Cooling Water System—Part 5: Inhibition Studies Using “All-Metal” Artificial Pits

Parts of a nuclear waste cooling water system, constructed from an austenitic stainless steel, are known to be susceptible to localized corrosion. To enable the investigation of corrosion inhibitors for potential application to that system, an artificial corrosion pit cell made from stainless steel was developed to simulate corrosion of large pits. A number of inhibition treatments were tested using these pit specimens, including substitution of chloride-containing water with demineralized water, sodium hydroxide dosing, and molybdate dosing. Corrosion was monitored by measuring the current flowing between the pit specimens and a large piece of non-corroding stainless steel wire mesh using a “zero-resistance” ammeter. The corrosion persisted despite long-term purging with high-quality demineralized water or sodium hydroxide dosing to pH 11. Sodium molybdate dosing to a molybdate concentration of 50 mg/L was ineffective, though it did have a strong effect of catalytically decomposing hydrogen peroxide (inc...

Experimental Study on Fatigue Strength of Corroded Bridge Wires

Fatigue tests were conducted for corroded galvanized steel wires on three corrosion levels, showing that fatigue strength of corroded wires lowers as corrosion progresses. Corrosion pits were measured on the corroded specimens, showing severer corrosion produced deeper pits in more condensed areas. Fatigue tests were then conducted for wire specimens with artificial pits whose sizes were decided by the measured corrosion pit data. Three different pit shapes were assumed: round, triangle, and triangle with a notch. The wire specimens with round pits did not break until 1 million cycles in the stress range of 400 MPa. The fatigue strength of wires with the triangular pit was lower than that with a round shape. Triangular pit specimens broke at fewer cycles for shorter pit length. The fatigue strength of wires with a notched triangle further decreased, and critical cycles did not depend on pit length. As the S-N relation of the wire specimens with triangular pits and notched triangular pits has a similar tendency as those of the actually corroded wires, the pit shape seems to be a dominant factor in lowering fatigue strength. The stress concentration factor at the sharp edge of the pits were obtained by strain gauge measurement and FEM analysis. Both methods showed that the stress concentration is larger for sharper pit shapes, indicating that this is the major cause for the decrease of fatigue strength.

Influence of pitting corrosion on strength of steel ships and offshore structures

The present paper deals with the influence of pitting corrosion on mechanical properties of mild and low alloy steels and strength of steel structures under static and quasi-static loads. Pitting corrosion is a very important phenomenon that influence local strength of ship hull members. The present paper is based mainly on Japanese publications. Analyzing the standard tensile diagrams one can see that that pitting corrosion reduces the load corresponding to yield stress (YS) and, even more markedly, tensile strength UTS, reduces almost to zero plastic flow strains at YS load level and dramatically reduces the total elongation to fracture. Nominal UTS load for uniformly corroded specimens is higher than that for pitted specimens at the same average thickness loss. The strength related to the true fracture surface area (i.e. reduced by pits, taking into account real path of the crack through the pits) is almost independent of the thickness loss and is almost the same as for uniform thickness loss. For wide specimens the tensile strength depends mainly on the local deformation ability, and the maximum loading ability for large members, predicted on the base of the total true fracture surface area, can be overestimated. Concept of equivalent thickness loss for plates under bending and compression and for more complex structural models as well as relation between the average and the equivalent thickness losses has been presented. Approach based on degree of pitting (ratio of pitted area to total plate surface area) has been shown as a real and more convenient than the approach based on the thickness loss.

Influence of Cold Work on Pitting Corrosion Behavior of a High Nitrogen Stainless Steel

The effects of cold work on the pitting corrosion resistance of a nickel-free high-nitrogen stainless steel in chloride solution have been investigated by electrochemical tests, surface chemical analysis, immersion tests, and microscopic observations. Potentiodynamic polarization revealed that pitting resistance was degraded by cold work as convinced by the decreased critical pitting potential. This could be due to a less compact and protective anodic passive film based on the results of electrochemical impedance spectroscopy, Mott–Schottky measurement, and X-ray photoelectron spectroscopy analysis. The growth of such an imperfect passive film could be attributed to a high density of deformation bands and other defects introduced by cold work. Scanning electron microscopy observation of the pitted specimens after polarization tests showed no obvious change in size and density of corrosion pits with increasing cold work level, whereas immersion tests showed varied pit density with cold work although the average size of pits did not increase linearly as a function of cold work level. The effects of cold work on the characteristics of passive films are discussed.

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.

On the pitting corrosion resistance of nitrogen alloyed cold worked austenitic stainless steels

Pitting corrosion studies were carried out on cold worked (5%, 10%, 15%, 20%, 30% and 40%) nitrogen-bearing (0.05%, 0.1% and 0.22% N) type 316L austenitic stainless steels in neutral chloride medium. Potentiodynamic anodic polarisation study revealed that cold working up to 20% enhanced the pitting resistance, and thereafter a sudden decrease in pitting resistance was noticed at 30% and 40% cold working. Increase in nitrogen content was beneficial up to 20% cold work in improving the pitting corrosion resistance, beyond which it had a detrimental effect at 30% and 40% cold working. The role of nitrogen in influencing the deformation band width and dislocation configuration is explained based on the results of transmission electron microscopy investigations. Scanning electron microscopy observation of the pitted specimens indicated decreasing size and increasing density of pits, along the deformation bands with increasing nitrogen for 40% cold worked specimens. The macrohardness values increased as the cold working increased from 0% to 40%. X-ray diffraction studies revealed the increased peak broadening of austenite peak {0 2 2} with increase in cold working. The relationship between pitting corrosion and deformation structures with respect to nitrogen addition and cold working is discussed.

Role of prior cathodic polarization in the pitting corrosion of pure aluminium in acidic chloride solution

Role of prior cathodic polarization in the pitting corrosion of pure aluminium (Al) has been investigated in aqueous 1 M HCl solution (pH 0) using a potentiostatic current transient technique combined with an electrochemical quartz crystal microbalance technique, impedance spectroscopy and scanning electron microscopy (SEM). From the analysis of the first period of combined cathodic current transient and electrogravimetric curves, it is suggested that the hydrogen evolution current starts obeying the Tafel relation via proton absorption, formation of such hydrogen-containing species as OH − ions and/or H 2 O molecules within the native oxide film and proton reduction at the metal/oxide interface. During the second period appearing at potentials −1.5 and −1.7 V SCE (SCE, saturated calomel electrode) hydrogen evolution may occur at an enhanced rate at the bottom of micro-pits, formed due to local breakdown of the oxide film. This breakdown was ensured by the more abrupt decrease in oxide film resistivity ρ ox values with increasing prior cathodic polarization, calculated at −1.5 and −1.7 V SCE , from the analysis of the measured impedance spectra as compared to that reduction in ρ ox calculated at −1.0 V SCE . The surface area of the pitted specimens was quantitatively estimated from the measured capacitance values to increase with rising prior cathodic polarization, as evidenced by SEM. This means that the exposed metal surface at the bottom of micro-pits formed during prior cathodic polarization serves as a preferential site for Cl − ion attack during the following anodic polarization. Hence, a sufficient prior cathodic polarization for the formation of the micro-pits is a necessary condition for the promotion of pitting corrosion of pure Al.

Effects of Laser Surface Melting on the Pitting Resistance of Sensitized Nitrogen-Bearing Type 316L Stainless Steel

Laser surface melting of sensitized nitrogen-bearing type 316L austenitic stainless steel was carried out using a pulsed ruby laser. The sensitization heat treatment was carried out at 923 K for 50, 200, 1000, and 2500 h, and the sensitized microstructure was classified according to ASTM A 262 practice A. The degree of sensitization was assessed by the electrochemical potentiokinetic reactivation (EPR) test. The critical pitting potentials of as-sensitized as well as sensitized-laser melted specimens were determined by potentiodynamic anodic polarization method in a medium containing 0.5 M NaCl and 0.5 M H2SO4 at room temperature. Results indicated that upon laser melting the pitting resistance increased significantly. This increase was attributed to the elimination of the sensitized heterogeneous microstructure by laser melting. The microscopic examination of the pitted specimens showed only micropits that developed at the interfaces of oxide/sulfide inclusions of titanium and matrix.

Pitting Corrosion Studies on Nitrogen-Bearing Austenitic Stainless Steels

AISI types 304, 316 and 317 stainless steels (SS) containing various levels of nitrogen were evaluated for their pitting corrosion resistance in an acidic chloride medium containing 0.5M NaCl and 0.5M H 2 S0 4 . Potentiodynamic anodic polarisation were conducted at room temperature to determine the critical pitting potentials of all the alloys, and the pit protection potentials of type 316 SS alloys. The pitted specimens were examined by optical microscopy and SEM. The results indicated that as the nitrogen content increased the critical pitting potentials increased, and the critical current density at the active to passive transition decreased indicating an improvement in the pitting resistance. Increase in nitrogen content in type 316 SS raised the pit protection potentials, and the potential range in which no new pits will initiate was also found to increase. The relationship between the critical pitting potentials and the nitrogen content was found using a nitrogen equivalent for molybdenum. Pitting attack was observed at triple points, grain boundaries, inclusions and at inclusion/matrix interfaces.

中・低炭素均質化組織の耐久限度における許容欠陥寸法の推定

A study has been made of the critical consideration on the possibility of the tolerant defect at the endurance limit of plain carbon steel specimen.There exists the critical size of pre-crack and micropit which would not reduce the level of original endurance limit of the fine homogeneous spherodized cementite structure, and these two sizes coincide well with each other in the range less than 100μm. This suggests the existence of tolerant defect which could be estimated by fatigue experiments on pre-crack and micropit specimens. Furthermore, disagreement of the above critical sizes of the fatigue pre-crack artificial pit specimens becomes distinct with increasing degradation of homogeneity and coarsening of microstructure.

３％食塩水中のスリーブ付軸材の疲労強度

Pump shafts are used being fitted with impellers and sleeves. Since they are often made from different metals and form crevices, galvanic corrosion or crevice corrosion occurs in a corrosive solution. Therefore, corrosion fatigue strength (CF) of these components may differ from that of bare metal specimen. In this paper, effects of galvanic couples and crevices on CF were investigated. Round bar materials tested are a 0.35%C steel (S35C) and a 13Cr stainless steel (SUS 403). Sleeves used are Cu, Al and S35C for S35C specimen and brass and SUS 403 for SUS 403 specimen. All tests were carried out at 1450 or 2770rpm in 3% NaCl solution. Test results are summarized as follows.(1) The corrosion fatigue strength σwc of a S35C at N=107 was 10kgf/mm2 (33% of that in air) and considerably decreased at higher cycles. For SUS 403 σwc was 19.6kgf/mm2 (59% of that in air) and decreased little at higher cycles.(2) The effect of sleeve materials on CF of S35C bar specimen was explained by considering the difference in corrosion potentials between the specimen and the sleeve. Namely, the highest σwc was obtained for Al sleeve (12.5kgf/mm2 at N=2×107), followed by S35C sleeve (10kgf/mm2: the same as that in bare specimen) and Cu sleeve (6.7kgf/mm2).(3) In the case of SUS 403 bar specimen, the same σwc was obtained for two different sleeve materials, while both SUS 403 and brass sleeve considerably decreased σwc because of the formation of corrosion pits at the ends of the sleeves. (10kgf/mm2 at N=5×107, 30% of that in air)(4) To clarify the effect of pit on CF, corrosion fatigue test of etch pitted round bar specimens was carried out, in which etch pits were induced by the potential method and the size of the pits was controlled to be as large as those found on fracture surface of the sleeve fitted specimens. It was shown that σwc of etch pitted specimens was roughly equal to that of the sleeve fitted specimen in the high cycle regime.

Effect Of Pits On Fatigue Strength Of Carbon SteelIn Oxygenated High Temperature Water

Most of corrosion fatigue tests have been carried out using polished specimens in high temperature water and these data have been used for the evaluation of actual component. Since degradations of surface such as corrosion or corrosion pit are induced during long period of operation, the surface of actual component may be different from that of polished specimen. Previous study showed that the fatigue strength of pre-corroded specimen of carbon steel was substantially reduced in oxygenated high temperature water [1]. This kind of effect should be considered for the accurate evaluation of fatigue strength of actual component. Strain-controlled fatigue tests on carbon steel were carried out in 290°C water with 8ppm dissolved oxygen. Test specimens were polished specimen and artificially pitted specimens with various shapes which simulated corrosion pit. Test results showed that the fatigue strength of the specimen with artificial pit decreased with an increase of the pit size and was influenced by the shape of pit. The relation between the fatigue strength and the size and shape of pit was discussed.