Nuclear grade cast and welded austenitic stainless steels are subjected to thermal aging during long-term service in light water nuclear reactors (LWRs), primarily due to the existence of certain amount of ferrite in the steels. The thermal aging results in degradation of the mechanical and corrosion properties of the steels, which leads to a potential concern to the structural integrity of the relevant LWR components. This paper reviewed the recent research progress of the effect of thermal aging on microstructures and properties of the nuclear grade cast and welded stainless steels, as well as the kinetics, assessment and prediction methods for thermal aging. The mechanism of thermal aging induced embrittlement was discussed. Challenges and trends for the research of thermal aging in the future were also briefly addressed.
Three different low alloy steels 1Cr-0.5Al-0.3Mo, 5Cr-0.4Ni-0.3Cu and 10Cr-0.3Ni were designed, then taking steel 20MnSi as a reference, their corrosion behavior was comparatively investigated in simulated marine atmosphere by means of periodic immersion accelerated corrosion test and electrochemical test, as well as XRD, SEM and EDS etc. In addition, electrochemical performance of the formed rust layer was assessed. The results showed that the corrosion rate of the test steels was decreased with the increasing Cr content, the formed rusts of the steels with 1% and 5%Cr composed of two layers with an inner layer of alternating band-like structure with Cr enriched sub-layers and Cr depleted sub-layers. While the steel with 10%Cr was passivated during the test. Besides, with the extending corrosion time, the corrosion rate of the steel with 5%Cr firstly increased and then stabilized, but it kept very low level for the steel with 10%Cr, and it decreased for the steel 20MnSi. Cr enhanced the free-corrosion potential and charge transfer resistance of the steels, thereby improved the compactness of the formed rusts.
The conversion period for the corrosion data of steels acquired respectively by the outerdoor exposure test and the indoor neutral salt spray test, which aims to simulate the corrosivity of the marine atmospheric environment at Wanning, was calculated by means of the equivalent conversion method, while taking the real marine atmospheric environmental data over the past three years at Wanning area into account. Then the corrosion behavior of an extra deep drawing cold rolled sheet DC06 and a carbon steel Q235 was comparatively studied through neutral salt spray test for desired conversion periods. In the meanwhile, the correlation between the results of the indoor neutral salt spray accelerated test and the outdoor air exposure test at Wanning is quantitatively studied by the grey correlation degree method. The results show that the corrosion kinetics curves of DC06 and Q235 by neutral salt spray test are in accordance with the power function law. For DC06 and Q235, their relevant results of neutral salt spray test for the conversion period (528 ha-1) and the common period (480 ha-1) respectively showed gray correlation degrees greater than 0.6 with those of the outdoor atmospheric exposure test at Wanning,indicating that the corrosion dynamics results measured indoor and outdoor respectively are of consistency. It is noted that the grey correlation degrees for the outdoor test results with those of indoor test for the conver sion period are higher than those for the common period.
Atmospheric corrosion products of pure aluminum alloy 8A06 exposed for 20 a in coastal- and industrial-atmosphere respectively was investigated by electron probe microanalysis (EPMA), wavelength-dispersive X-ray spectroscopy (WDS) and electrochemical impedance spectroscopy (EIS). Results show that the corrosion rate of the specimen exposed in industrial atmosphere is approximately 1.5 times higher than that exposed in coastal atmosphere, though the corrosion products in both atmospheres all mainly contained elements O, Al and S. In 0.6 mol/L Na2SO4 solution, Rp could be arranged in a decreasing order as follows: withdrawn specimens exposed for 20 a in coastal atmosphere>withdrawn specimens exposed for 20 a in industrial atmosphere>bare substrate of pure aluminum 8A06. This indicates that the protectiveness of corrosion products on the specimens exposed in coastal environment is better than that exposed in industrial environment.
Weld joints of Mg-alloy AZ31 were prepared by friction stir welding, and then their corrosion behavior was assessed in NaCl solution. The results show that the free corrosion potential of the friction stir weld joint of Mg-alloy AZ31 is like that of the base metal; however, the corrosion current density of the base metal was 0.45 mAcm-2, while it was 1.63 mAcm-2 for the joint. Moreover, the corrosion resistance in weld nugget zone was the worst because of the effect of the grain size and the distribution of β phase, and the corrosion initialed in this region. In the initial corrosion stage, the corrosion resistance of the base metal was superior to that of the weld joint, but later the base metal exhibited faster corrosion rate rather than the weld joint, which may be ascribed to the occurrence of passivation of the weld joint.
With the increasing import of high-sulfur oils, the storage tank bottom plates suffer severe corrosion from sediment water, which often cause the leakage of crude oil and result in serious economic loss and environmental pollution. The corrosion behavior of Q235B carbon steel in sediment water from crude oil was investigated by means of electrochemical impedance spectroscopy, wire beam electrode, electrochemical measurements and SEM/XRD techniques. Results showed that during the initial immersion period of 21 d the carbon steel suffered from uniform corrosion and its corrosion rate decreased with time. Afterwards, the corrosion was transformed to localized corrosion and its corrosion rate increased with time during the period of 21~35 d of immersion. The fact can be explained that the carbon steel surface was covered by scale of CaCO3, and the corrosion process can be hindered. However, corrosion pits were initiated and accelerated with increasing of time due to loss of corrosion protection from CaCO3 deposits after 21 d.
Effect of temperature on pitting behavior and corrosion morphology of 304 stainless steel in 3.5%NaCl solution was studied by means of potentiostatic polarization, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and three-dimensional video microscope. The results showed that as the temperature increased from 20 ℃ to 40 ℃, the initiation period of metastable pits shortened, the number the nuclei formed per unit time increased, the average peak-current and the average peak-width (which could represent the lifetime of metastable pits) increased, therefore, the increasing number of metastable pits led to higher probability for the occurrence of stable pits. The growth rate of pitting current, pit volume, pit mouth diameter and pit depth of single pit increased with the increasing temperature by applied potential of 0.15 V(vs SCE). The growth rate of pit mouth diameter and pit depth gradully decreased with the extension of time at the same temperature. The corrosion pit was incompletely covered by lace-like corrosion products, and of which the integrity decreased with the increase of temperature.
Four oleic acid-based imidazoline derivates with different hydrophilic groups were synthesized in this work. The performance of the synthesized products, such as the hydrophobicity and hydrophily, the adsorption and corrosion inhibition on the 20# carbon steel in flow CO2/H2S environment were assessed by means of measurements of contact angle, AFM force curve and mass loss, as well as molecular dynamics simulation. The results showed that under static conditions the imidazoline derivate with two amino ethylene units processes the best inhibition efficiency of 86.8% for the dosage of 100 mg/L, while under high flow rate (5.5 m/s), the inhibition efficiency of imidazoline with three amino ethylene units was the highest, i.e. 73.6% for the dosage of 100 mg/L. The hydrophobicity, adhesion force and adsorption energy of imidazolines were enhanced gradually with the increase of the number of amino ethylene unit.
The influence of magnetic field on the corrosion of X80 pipeline steel in meadow soil at Shenyang area was investigated by means of mass loss method, X-ray diffractometer (XRD), scanning electron microscope (SEM), and energy dispersive spectrometer (EDS). The results indicated that the magnetic field accelerated the corrosion rate of X80 pipeline steel in the soil. In contrast to the circumstance without magnetic field, porous FeO(OH) emerged on the X80 steel electrode surface under magnetic field which weakened the protectiveness of the formed scales. With the increase of the magnetic field strength, Fe3O4 content decreased and FeO(OH) content increased in the corrosion products, which led to the increase of the corrosion rate of X80 steel. However, the increment of the corrosion rate slowed down during the process. As the magnetic field intensity increases to a certain extent, the formation of an integrated corrosion product may be suppressed. As the magnetic field intensity reached to 20 mT, the integrity of the formed scale became poor with cracks, thereby the corrosion rate of X80 steel increased to 73.799 μm/a. These phenomena may be ascribed to that the magnetic field facilitates the preferred oriented growth of oxide grains.
The corrosion behavior of automotive cold rolled steels DC06 and DP600 in 0.02 mol/L NaHSO3 solution was studied by full immersion test, mass loss method and electrochemical impedance spectroscopy, as well as SEM and EDS. The results showed that the corrosion product continuously grew, while the mass loss value increased with the extending test time. The corrosion product presents agglomerate- and uneven fluctuant-morphology, which was composed of iron oxides and sulfate. The electrochemical impedance spectroscopy results revealed that the corrosion rate of low carbon steel DC06 and dual phase steel DP600 increased first and then decreased, and then gradually stabilized.
The electrochemical characteristics of the passivation film formed on 316L stainless steel in borate buffer solutions with pH of 4, 7 and 11, respectively were characterized by means of potentiodynamic polarization, electrochemical impedance spectroscopy and Mott-Schottky technology. The composition of passive film was analysized by X-ray photoelectron spectroscopy (XPS). The results showed that the stable passivation film could form on the steel surface in all the three borate buffer solutions. The passivation potential range decreased and the transpassive potential dropped significantly with the increasing pH value. The transpassive current of 316L stainless steel in alkaline borate buffer solution increased significantly. The integrity of the passive film was the best in the neutral solution, while it was the worst in the acid solution. The Mott-Schottky results showed that the semiconductor type of the passive film transferred from n-type to p-type with the increasing potential in acid solution. It was n-type and p-type semiconductor in the neutral and alkaline solution, respectively. It was attributed to that the chromium hydroxide dropped down and the formed iron compound was transformed from FeO(OH) to Fe3O4. The content of Cr2O3 was decreased in alkaline solution, which resulted in lower corrosion resistance.
X70 steels with different microstructure were obtained by heat treating at 1050 ℃ for 3 h and subsequently air cooling, water cooling and furnace cooling respectively. Corrosion behavior of the X70 steels with different microstructure in 3.5%(mass fraction)NaCl solution with sulfate reducing bacteria (SRB) was studied by means of potentiodynamic polarization measurement and electrochemical impedance spectroscopy (EIS) as well as SEM and EDS. The results showed that the as received X70 steel shows a microstructure composed of ferrite and pearlite; the air cooling steel composed of globular pearlite distributed along the ferrite grain boundaries; furnace cooling steel composed of lamellar pearlite and proeutectoid ferrite and water cooling one composed mainly of lath martensite and a small amount of blocky ferrite. In the initial stage of corrosion in 3.5%NaCl solution with sulfate reducing bacteria (SRB), the test steels were all covered with a compact biofilm, which played a roll in protection to the substrate. However fractures occurred for the biofilms formed on the water cooling- and air cooling-steels after immersion for 8 and 10 d respectively, while the biofilms kept integrity on the furnace cooling- and as received-steels. The corrosion resistance of the furnace cooling- steel was best, and the water cooling-steel was worst in 3.5%NaCl solution with SRB.
The effect of moistened electrical insulation on galvanic corrosion behavior of dissimilar metals was investigated with a simulated galvanic corrosion setup, which consisted of a cell containing 3%NaCl solution with hull steel and cupronickel as electrodes and then by series connecting resistance elements or moistened insulation materials into the circuit. The results showed that through moistening the insulation material can be transformed into electrical conducting material. The resistance of moistened insulation material (Rm) may involve ionic resistance (Ri) and electrical resistance (Re), which are produced respectively by ionic conducting and electrical conducting. The corrosion behavior of the hull steel for the couple steel/cupronickel beneath a moistened insulation material may relate to the term Re. When Re is not higher than 1 kΩ, the galvanic current of the couple connected with a resistance element of a moistened insulation material may higher than that of the short-circuited couple, and the corrosion rates of hull steel of the couples connected with resistance elements may higher than that the short-circuited couple. When Re is higher than 1 kΩ, the galvanic currents of the couples connected with resistance elements decreased with the increase of Re, whilst the corrosion rate of the hull steel of the couples connected with resistance elements decreased with the increase of Re. The conduction of the moistened insulation material related to the nature of insulation material, the key to prevent galvanic corrosion of couple hull steel/cupronickel is the appropriate selection of insulation material.
The effect of hydrostatic pressure and its alternative variation on the corrosion behavior of 10CrNi3MoV steel in 3.5%(mass fraction)NaCl solution was studied by means of high-pressure static immersion test, potential measurement and electrochemical impedance spectra (EIS). The results show that the dissolved oxygen of solution and the ratio of Fe2O3 in the corrosion products all decrease with the enhancement of hydrostatic pressure, which result in the decline of corrosion potential and the decrease of corrosion rate. Alternative hydrostatic pressures led to the changes of the interaction between corrosion products and the steel, and the rust structures, which then enhanced the corrosion rate as a result, the corrosion rate increased with the increasing hydrostatic pressure. Under the specific conditions in this paper, the corrosion rate of 10CrNi3MoV steel in 3.5%NaCl solution under alternative hydrostatic pressures between atmospheric pressure and 4 MPa is 3.5 times of that under atmospheric pressure.
Conductive anticorrosion coatings were prepared with high viscosity acrylic resin as matrix, Ni- powder and defoamer etc. as additives, and then applied on 20# carbon steel. The electrical conductivity and the anti-corrosion performance in simulated seawater of the coatings with different Ni-powder content were characterized by means of surface contact resistance measurement, electrochemical impedance spectroscopy, and scanning electron microscopy respectively. Results show that with the increasing amount of Ni-powder, the electrical conductivity of the coatings increased. While, as the longer of the immersion time and the higher amount of the Ni-powder are, the lower of the capacitive reactance arc, the impedance, the phase angle and |Z |0.05 of the coatings are. For a desired immersion time, the higher amount of the Ni-powder is, the faster decline of the capacitive reactance arc, the impedance, the phase angle and the |Z |0.05 of the coatings is. According to Tafel polarization result, the higher amount of the Ni-powder is, the greater of the corrosion current of coatings is. It is concluded that with the higher amount of Ni-powder, the coatings exhibit better electrical conductivity, but the lower of the viscosity and worse corrosion resistance. A coating with about 20% Ni-powder possesses a comprehensive performance with better electrical conductivity and corrosion resistance.
