Pitting Resistance Research Articles

Effect of retrogression and reaging (RRA) on pitting and stress corrosion cracking (SCC) resistance of stir zone of high strength AA7075-T651 alloy joined by friction stir welding

The main purpose of this study is to investigate the influence of retrogression and re-aging post weld heat treatment (RRA-PWHT) on potentiodynamic corrosion (PC) and stress corrosion cracking resistance (SCC) of stir zone (SZ) of AA7075-T651 alloy butt joints developed using friction stir welding (FSW). FSW was employed to overcome the problems in fusion welding of AA7075-T651 alloy such as solidification cracking, heat affected zone (HAZ) softening, grain coarsening, dissolution of strengthening precipitates and inferior mechanical performance. The SZ of joints were subjected to potentiodynamic corrosion test in 3.56 wt% NaCl solution. The circumferential notch tensile (CNT) specimens were subjected to SCC test at various conditions of axial stress level in 3.56 wt% NaCl solution. The microstructural features of butt joints SZ were characterized using optical (OM), scanning electron (SEM) and transmission electron microscope (TEM). The results disclosed that RRA-PWHT joints exhibited greater PC and SCC resistance compared to as-welded joints. The threshold stress level for the failure of CNT specimens of parent metal, as-welded joints and RRA-PWHT joints was 242 MPa, 175 MPa and 198 MPa, respectively. The threshold stress level of RRA-PWHT joints and as-welded joints is 81.81% and 72.31% of parent metal threshold stress level. It is correlated to increase in vol% and coarsening of grain boundary precipitates with enrichment of copper at the grain boundaries.

Structure and properties of laser cladding CoCrNi multicomponent alloy coating used in rain gauge

Rain gauge is an important measuring instrument, which is widely used for rainfall measurement under various conditions. However, since the rain gauge is used in outdoor exposed environment, its shell is easily corroded by rain, especially by acid rainwater, which will affect the service life. In order to improve the corrosion resistance and prolong its service life, CoCrNi medium entropy alloy (MEA) coating was prepared on the surface of rain gauge's shell by laser cladding. The microstructure and properties of the CoCrNi coating were studied comprehensively. The results show that the coating and the substrate form a metallurgical bond. The microstructure of CoCrNi MEA coating is mainly composed of two kinds of microstructures, and mainly containing face centered cubic (FCC) structure. The coatings have excellent corrosion resistance in acid rainwater with different pH. The corrosion current density of the laser cladding CoCrNi MEA coatings are lower than that of 304 stainless steel (SS), reduced by 2 orders of magnitude, the corrosion potential increased in varying degrees, with an increase range of 0.03∼0.47 V. The pitting corrosion resistance of the coating is excellent, with the increase of pH, the pitting resistance is enhanced. The corrosion resistance of the coating is related to the phase structure, uniformity and compactness of the microstructure as well as the corrosion resistant elements in the coating. The microhardness of the coating is 703.6 HV, the specific wear rate of the alloy is 1.4×10−4 mm3/Nm, and the friction coefficient is 0.47. Experiments show that the coating have excellent corrosion resistance and mechanical properties for the rain gauge's shell, which provides a guarantee for prolonging its service life.

Mathematical Modeling of the Corrosion Behavior of Austenitic Steels in Chloride-Containing Media During the Operation of Plate-Like Heat Exchangers

Mathematical models that describe the dependences of the critical temperatures of pitting formation of AISI 304, 08Kh18N10, AISI 321, 12Kh18N10T steels in model circulating waters with pH 4…8 and chloride concentrations from 350 to 600 mg/l on their chemical composition and structure have been developed. They are based on linear squares regressions and a feed-forward neural network for reduced feature numbers. Using the developed mathematical models, it was found that the critical pitting temperatures of these steels increase with an increase in the pH of the circulating water, the number of oxides up to 3.95 μm in size, the average distance between titanium nitrides, the Cr content and a decrease in the concentration of chlorides in the circulating waters, the average distance between oxides and average austenite grain diameter. At the same time, it was found that the geometric dimensions of the steel structure most intensively affect their pitting resistance in circulating waters, and the effect of their chemical composition is minimal and is determined by the amount of Cr, which contributes to an increase in the pitting resistance of steels, probably increasing the solubility of nitrogen in the austenite solid solution. It is proposed to use the developed mathematical models to select the optimal heats of these steels for the production of heat exchangers and predict their pitting resistance during their operation in circulating waters.

Prominent role of reversed austenite on corrosion property of super 13Cr martensitic stainless steel

The microstructure and corrosion property of super 13Cr martensitic stainless steel (S13CrMSS) processed by quenching, one-step tempering and two-step tempering were investigated by XRD, SEM, EBSD and TEM-EDS analysis. It has been found that volume fraction of the reversed austenite in the sample treated by two-step tempering treatment was 13.5 vol. % up to the maximum. The reversed austenite almost always nucleated and distributed in martensite laths boundary during tempering and the diffusion of Ni, Mn and C promoted its growth. Enrichment of such elements as Ni, Mn, C and the ultrafine size of the reversed austenite were considered to be the main factor that accounted for the stability of the reversed austenite. At the same time, martensite and reversed austenite satisfy the (1 1¯ 1)γ // (011)α' and [011]γ // [1¯1¯ 1]α' Kurdjumov-Sachs (K–S) orientation relationship. Potentiodynamic polarization curves and electrochemical impedance spectroscopy indicated that two-step tempered specimen exhibited a higher pitting potential, lower corrosion current density, thicker passivation film and superior pitting resistance than the quenching specimen (without reversed austenite) as well as one-step tempering specimen (3.6 vol.%). It has been confirmed that the reversed austenite has a beneficial effect on the corrosion property of S13CrMSS.

Influence of the machined surface roughness on the pitting behavior of 304 stainless steel

AbstractThe relationship between specimens with different machined surface roughness and pitting resistance was explored using the critical pitting temperature (CPT), critical pitting potential (Ep), and FeCl3 pitting immersion tests. Compared with the arithmetic mean deviation of the surface contour (Ra), the depth–width ratio could characterize the mathematical relationship between the machined surface roughness and pitting parameters more simply. With an increase in the depth–width ratio, the CPT and Ep of 304 stainless steel decreased, the average corrosion rate increased, the number of pits decreased, and the pit sizes increased. A larger depth–width ratio led to a larger effective diffusion length of metal cations in the pits, thus, the critical pitting criteria could be reached easily, which resulted in the reduction of the pitting resistance of 304 stainless steel.

Effects of Heat Treatment on Microstructure and Mechanical Properties of a Transformation-Induced Plasticity-Aided Economical Duplex Stainless Steel

In order to obtain steel with high tensile strength and elongation and good pitting resistance, the economical duplex stainless steel with and without the addition of tungsten (W) was prepared in this paper. The comparison and distribution of the two phases were controlled by optimizing the heat treatment process. The highest product of tensile strength and elongation of Cr19 series duplex stainless steel (DSS) DSS were obtained after solution treatment at 1050 °C for 5 min, up to 58692 Mpa%(876 Mpa * 67%), when the area f.raction of α was about 50.7%. The addition of tungsten reduces tensile strength due to the increased area fraction of α at 1050–1150 °C. Finer grain size leads to higher tensile strength after solution treatment at 1050 °C. The microstructure is characterized by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). The steels exhibit an excellent transformation-induced plasticity (TRIP) effect after the solution treated is at 1050 °C, which may primarily be due to the existence of a more unstable austenite phase. The TRIP effect acts through the phase transformation of γ to α′-M during high deformation, and the orientation relationship of {111}γ || {110}α′-M & <110>γ || <001>α′-M belongs to the standard variants of Nishiyama–Wassermann relationship.

The role of corrosion product layers in the pitting resistance of powder-processed Al–Cr–Co–Mn–Zr alloys containing icosahedral quasicrystalline dispersoids

The role of corrosion product layers in the pitting resistance of powder-processed Al–Cr–Co–Mn–Zr alloys containing icosahedral quasicrystalline dispersoids

The roles of Al addition and heat treatment temperature on chloride corrosion of 9Cr alloy steel

The effects of 0.7% Al and heat treatment temperatures on chloride corrosion of 9Cr alloy steel were investigated through potentiodynamic polarization measurement and salt spray test. Al promoted the formation of Al2O3-rich inclusions and AlN, thus reducing the pitting potential at initial stage. However, Al addition reduced weight loss after longer time by optimizing the α/γ * value, compactness and element distribution of rust layer. The mechanism of corrosion resistance changing with heat treatment temperature was understood from the decrease in the pitting resistance equivalent number caused by (Fe, Cr)23C6 precipitation and the microgalvanic corrosion between fresh martensite and tempered martensite/ferrite.

Effect of chloride threshold on pitting behavior of 316LN stainless steel in sour water solution by electrochemical analysis

AbstractThe effect of chloride threshold (CT = [Cl−]/[SO42−]) on the pitting behaviors of 316LN in the sour water was studied by using electrochemical tests. The result shows that the pitting resistance of 316LN decreases with the increase in CT and temperature. The critical chloride threshold (CCT) decreases from 3 at 40°C to 1.5 at 70°C and then remains unchanged when further increases the temperature. The EIS experiment manifests that the protectiveness of the passive film reduces with the increase of CT, and the variation of the charge transfer resistance (Rct) is in keeping with the pitting potential (Epit). Increasing the CT or raising the temperature from 40°C to 80°C enhances the donor (point defect) density, resulting in a thinning of the protective film. Methods to raise the value of the CCT, that is, to improve the chlorine corrosion resistance of the pipelines were proposed.

Exposure Aging of Passive Films on Multi-Principal Element Alloys and Ramifications Towards Local Corrosion Resistance

Both highly engineered legacy and emerging alloys lack information on the precise attributes of protective passive films especially after long-time exposures when it really matters. Relevant field exposure periods are nearly infinite in time but both accelerated testing and high-fidelity experiments are often very short term in duration. Recent studies have explored passivation and protection of a number of different MPEAs connecting oxide chemistry to corrosion electrochemistry but most reports are after reality brief exposure periods.The goals of this work are to (a) review the nature of passive films (i.e., solid solution oxides, phase separated single element oxides, and/or complex oxides) and (b) determine the effect of exposure aging on the evolution of oxide features such as composition, structure, phase separation and physical attributes such as thickness in MPEAs; and to correlate these features with both protectiveness in the passive range and local corrosion resistance. Furthermore, we aim to understand what factors limit beneficial alloying element enrichment in the oxide, and further insights on potential third element effects.The focus of the current work extends from the solid solution Ni-22Cr, Ni-22Cr-6Mo, and Ni-22Cr-6Mo-3W family to a variety of emerging MPEAs containing “d-block” elements. AC and DC electrochemistry during potential sweeps and potential step passivation studies were conducted in 0.1 M NaCl pH 4 (HCl) as well as other solutions in the passive range. Exposure and high-fidelity characterizations were investigated over 10 s to 10 days at select potentials.Improvements in passive films protectiveness were correlated with the enrichment of certain elements and depletion of others. Mn oxides were found to be detrimental when formed over the long term in the outer layer of oxides on MPEAs. The most corrosion-resistant alloys possess passivating films that are efficacious not only just after formation, but that self-heal as well as improve over long exposure times as evident from impedance spectroscopy and metastable pitting resistance. Such alloys were observed to be the most resistant when interrogated for local corrosion compared to inferior alloys whose impedance response degraded over time as the passive film evolved. The latter alloys eventually arrived at conditions susceptible to local corrosion. Time-potential-transformation diagrams based on long-term exposures in Cl- followed by high fidelity characterization is suggested to be an approach to improve scientific understanding of long-term passive-protection features relevant to chloride exposures.This work was supported as part of the Center of Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584; the Office of Naval Research under MURI ONR N00014-16-1-2280 and N00014-19-1-2420.

Effect of Cu addition on phase fraction, mechanical properties and corrosion properties of Fe–Cr–Mn–Al duplex stainless steel

In recent years, Cu has been regarded as an important element to improve the corrosion resistance of stainless steels. For duplex stainless steels, especially those obtained by cold rolling and annealing, at present, little is known about the role mechanism of Cu addition. Therefore, on the basis of a new type of Fe–Cr–Mn–Al duplex stainless steel designed by the author, the effects of Cu on phase fraction, mechanical properties and corrosion resistance were studied in detail through tensile tests, electron backscattering diffraction (EBSD), polarization curve test and laser scanning confocal microscopy. The results show that the addition of 0.5% Cu to Fe–Cr–Mn–Al system increases the volume fraction of austenite and its stability, greatly improving the strength and ductility of steels due to the TRIP effect occurring in a wider strain range. In addition, the addition of Cu increases the content of Cr in austenite, improving the pitting resistance of duplex stainless steels. After annealing at 860 °C, the Cu-added duplex stainless steel exhibits superior mechanical and corrosive properties, i.e. the yield strength, tensile strength, elongation and pitting potential are 520 MPa, 760 MPa, 42.5% and 260 mV, respectively. In view of obvious advantages in mechanical properties and corrosion resistance, the present developed Cu-added Fe–Cr–Mn–Al duplex stainless steel is expected to replace the conventional 304 stainless steel for some applications.

Research on the pitting behavior of high nitrogen austenitic stainless steel 316LN in sodium chloride solution by using modified potentiostatic pulse test

• The modified potentiostatic pulse test is used to study the pitting behavior of 316LN. • The pitting size and number of 316LN are regulated. • The Si-enriched inclusions are initial sites of metastable pit on 316LN surface. • The pitting resistance of 316LN is enhanced after the potentiostatic pulse test. The pitting behavior of high nitrogen austenitic stainless steel 316LN was studied by using the modified potentiostatic pulse test (PPT). The results show that the duration of the high potential (E h ) and the low potential (E l ) significantly affect the pitting size and number of 316LN. However, the pitting behavior of 316LN only changes during the first five pulse cycles, because the metastable pitting initiates at the limited inclusions. The optimal PPT condition is regarded as E h = 0.5 V SCE for 1 s, E 1 = 0.2 V SCE for 2 s and ten pulse cycles. Metastable pits initiate at the susceptible sites of inclusions which are identified as SiO 2 and SiC. The metastable pits will further grow or restrain with the increasing time of E h or E l , respectively. After PPT modification, the pitting resistance of 316LN is enhanced due to the removal of the Si-enriched inclusions from the 316LN surface. Meanwhile, the fewer current transitions, the lower passivation current density and the higher impedance all suggest that the protectiveness of the oxidation film formed on 316LN surface is improved after PPT.

Pitting and General Corrosion Susceptibilities of Materials for High Level Radioactive Waste (HLW) Disposal.

The disposal of high-level radioactive waste (HLW) in deep stable geological formations is accepted at an international level to be the most promising option for its long-term management. The supercontainer concept is currently being considered as the Belgian reference design, wherein the waste will be stored in geological stable clay formations. The outer barrier of the supercontainer is the envelope, which should be made of a corrosion-resistant material as it will be in contact with the aggressive species leaching from the host rock (i.e., chloride) and diffusing through the cementitious barriers of the disposal system. Polarization measurements are carried out to study the pitting susceptibility and the uniform corrosion of possible candidate materials in chloride-rich concrete pore solutions, aerated by high-purity oxygen. The tests are carried out at a deep soil-representative temperature of 60 °C. All materials showed high pitting resistance in aerated concrete pore solutions and can withstand chloride concentrations up to 1 M. Regular 316L and LDX2304 stainless steel also showed good corrosion resistance and can serve as a more economical alternative. The pH of the used pore solutions did affect the measured corrosion rate irrespective of the alloying elements inside the steel grades.

Investigation on leakage cause of 13Cr pipe flange used for a Christmas tree in a high-pressure and high-temperature gas well

The martensitic stainless steel pipe flange used for the 138 MPa Christmas tree failed to leak during ultra-deep high-pressure, high-temperature gas well operations. In order to study the cause of leakage, non-destructive testing, chemical composition analysis, mechanical property analysis, metallographic structure testing, surface characterization, cracking characterization and finite element analysis were performed in the present research. The results show that the leakage failure is related to the crack in the ring groove. The cracks initiated from the edge of the corrosion pits and propagated along the delta ferrite/martensite interfaces. The Cr-rich carbides in the matrix reduced the pitting resistance, while delta ferrite deteriorated the crack resistance, which was the main factor contributing to the failure. In addition, the stress level in the sealing structure of the Type 6BX flange further influenced the crack propagation behavior.

Pitting resistance of reduced graphene oxide-layered double hydroxide reinforced aluminum composite coating deposited by cold spraying

Cold spraying aluminum-based coatings are widely used for corrosion protection of metals such as steel and magnesium alloy. However, poor pitting resistance and short protection life are the shackles of aluminum-based coating. In order to solve this problem, this paper innovatively utilized the synergistic effect of reduced graphene oxide (rGO) and layered double hydroxide (LDH) to modify the coating efficiently. Al-GLG coatings were prepared by cold spraying rGO and LDH-modified aluminum powders. The structure, composition and corrosion resistance of the Al-GLG coating were analyzed and compared with that of unmodified Al-based coating and graphene-modified Al-based (Al-G) coating. As a result, the Al-GLG coating presents a continuous network GLG surrounding the deformed aluminum powder, and this special coating exhibits pitting resistance and long life. The corrosion current density of the Al-GLG coating is only 0.01287 μA·cm−2, which is three orders of magnitude lower than that of the Al-based coating, and its pitting potential is about −0.24 VSCE, which is over 540 mV higher than that of the Al-based coating. During the full immersion text, the Al-GLG coating keeps the largest Rc (7.21×104 Ω·cm2) and low-frequency impedance modulus|Z|0.01Hz (3.66×104 Ω·cm2). The corrosion resistance mechanism of the Al-GLG coating was investigated via field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS), respectively. The excellent pitting corrosion resistance of the Al-GLG coating is attributed to the synergistic effect of the shielding effect of reduced graphene oxide and the ion exchange inhibition effect of LDH.

Deep learning framework for uncovering compositional and environmental contributions to pitting resistance in passivating alloys

We have developed a deep-learning-based framework for understanding the individual and mutually combined contributions of different alloying elements and environmental conditions towards the pitting resistance of corrosion-resistant alloys. A fully connected deep neural network (DNN) was trained on previously published datasets on corrosion-relevant electrochemical metrics, to predict the pitting potential of an alloy, given the chemical composition and environmental conditions. Mean absolute error of 170 mV in the predicted pitting potential, with an R-square coefficient of 0.61 was obtained after training. The trained DNN model was used for multi-dimensional gradient descent optimization to search for conditions maximizing the pitting potential. Among environmental variables, chloride-ion concentration was universally found to be detrimental. Increasing the amounts of dissolved nitrogen/carbon was found to have the strongest beneficial influence in many alloys. Supersaturating transition metal high entropy alloys with large amounts of interstitial nitrogen/carbon has emerged as a possible direction for corrosion-resistant alloy design.

Formation mechanism of MnS inclusion during heat treatments and its influence on the pitting behavior of 316L stainless steel fabricated by laser powder bed fusion

The formation mechanism of MnS inclusions during heat treatment and its effect on the pitting behavior of laser powder bed fusion (L-PBF)-fabricated 316L stainless steel (SS) were investigated. MnS inclusions precipitated at the grain boundaries of the samples, and the spherical oxide particles can become nucleation sites. The quantity and size of the MnS inclusions increased with prolonged heating. Compared with the as-received sample, the pitting of heat-treated L-PBF-fabricated 316L SS samples was mainly induced by the newly generated MnS inclusions rather than the fabricated gas pores and extended along the grain boundaries; this caused the dramatic pitting resistance deterioration.

The Isolated Austenite Forming during High-Temperature Cooling and Its Influence on Pitting Corrosion Resistance in S32750 Duplex Stainless Steel

The formation mode of isolated austenite (IA) formed during high-temperature cooling and its effect on pitting corrosion resistance of S32750 DSS were investigated. As the structure formed during the high-temperature cooling process after hot rolling in S32750 duplex stainless steel (DSS), the precipitation of isolated austenite (IA) can be considered as the combined production of temperature and cooling rates. IA generated at the ferrite (α) grain boundaries or internal α and performed the K-S crystallographic relation with the α N element enriched in IA. The redistribution of elements not only increased the concentration gradient between α/IA, but also reduced the pitting resistance equivalent number (PREN) of original austenite (γ). Scanning Kelvin probe (SKP) and Gaussian fitting proved that with prolonged corrosion time, the potential difference enlarged and local corrosion aggravated. Measured by the weight loss method, pitting resistance decreased to one-third of the solid solution state due to the existence of IA; following, the quantities, sizes, and depth of corrosion pits increased significantly. In addition to α/γ boundaries and α/α boundaries, corrosion pits preferred to initiate around IA. The IA that appeared at α grain boundaries deteriorated the pitting corrosion resistance severely. Furthermore, original austenite with decreased PREN tended to be corroded. In general, the order of pitting occurrence is displayed as follows: α-γ-IA.

Effect of bias plasma on active screen nitrocarburising response of AISI 420 martensitic stainless steel

This study addresses the potential of active screen plasma nitrocarburising (ASPNC) treatment applying a carbon active screen for short-time low-temperature diffusion treatment of AISI 420 martensitic stainless steel in terms of varying the bias plasma power directly at the steel surface. Without the application of a bias plasma the nitrocarburising effect was reduced due to cementite formation near the surface. The application of bias plasma in ASPNC processes resulted in the formation of expanded α′-martensite and ε-carbonitride and inhibited cementite formation. Thickness and phase composition of nitrocarburised layers produced by means of ASPNC were comparable to those that can be generated by conventional plasma nitrocarburising (CPNC), but with reduced plasma impact on the steel surfaces implying considerably lower thermal stress. Surfaces treated by ASPNC with bias plasma showed a tendency towards improved pitting resistance in a 3.5 wt% sodium chloride electrolyte compared to those generated by CPNC.

Study on the Effects of Multiple Laser Shock Peening Treatments on the Electrochemical Corrosion Performance of Welded 316L Stainless Steel Joints

To study the influence of laser shock peening on the electrochemical corrosion resistance of welded 316L stainless steel joints, welded 316L stainless steel joints are treated with different laser shock peening treatments (i.e., one, two, and three times). Our analysis employs electron backscattering diffraction (EBSD), scanning electron microscopy (SEM), X-ray diffraction (XRD), an X-ray stress meter, and electrochemical corrosion tests to observe and analyze the microstructure, structural composition, residual stress, and corrosion resistance in different areas of the surface of 316L before and after the laser shock peening. The results show that the residual stress distribution of the welded joints is optimized after laser shock peening, with a maximum residual compressive stress near the matrix of 171 MPa. When the number of laser shock peening treatments is two, the corrosion current reaches a minimum of 9.684×10−7 A/cm2, and optimal pitting resistance is obtained. However, when the number of laser shock peening treatments is further increased to three, the corrosion current increase and the pitting resistance decreases. In summary, the electrochemical corrosion resistance of the welded joints effectively improves after laser shock peening, but its performance begins to decline after three repeated shocks, which is related to the combined effects of stress change and microstructure phase transformation.