STUDY OF THE X70 PIPELINE STEEL CORRODING IN 3.0wt% NaCl SOLUTION USING ELECTROCHEMICAL IMPEDANCESPECTROSCOPY TECHNIQUE

The corrosion process of the X70 pipeline steel in 3.0wt% NaCI solution were studied using polarization method, and the chronological characteristics during the entire polarization plot were investigated in detail using EIS technique. In the active region of X70 steel, such as 20mV potential bias applied on open circuit potential (OCP), the impedance spectra was comprised of three parts: a high-frequency capacitive loop, a middle-frequency capacitive loop and a low-frequency inductive component. When positive polarization potential increased, the capacitive loops at high and middle frequency range merged, and the inductive component at low frequency shrunk. At high positive polarization potential bias (500-800mV vs. OCP), the high-frequency capacitive loop and the low-frequency inductive loop exhibited as disheveled points due to the synergism of the inhomogeneity of the corroding material and the localized corrosion. The results were fitted utilizing the equivalent circuits to simulate the impedance spectra and to interpret the electrochemical features shown during the experiments.     

The Effect of Silicon on the Oxidation of a Ni-6 at.% Al Alloy in 1 atm of pure O2 at 900°C

The oxidation behavior of a binary Ni–6Al alloy and of three ternary Ni–xSi–6Al alloys containing 2, 4 and 6 at.% Si has been studied at 900°C under 1 atm of pure O₂. The addition of 2 at.% Si to Ni–6Al increases the short-time oxidation rate of Ni–6Al, which is subsequently reduced and becomes similar to that of the binary alloy. However, the presence of this silicon level is already able to stop after some time the coupled internal oxidation of Al+Si by forming a healing oxide layer rich of alumina at the front of internal oxidation. The addition of 4 at.% Si to the same alloy permits a more rapid inhibition of the internal oxidation and the formation of a steady-state, inner alumina-rich scale. Finally, the addition of 6 at.% Si prevents the internal oxidation completely and leads to an earlier growth of a protective oxide layer in contact with the alloy as well as to a further reduction in the scaling rate. The role of Si in promoting the formation of protective scales in comparison with the binary alloy is examined on the basis of an extension to ternary alloys of a criterion proposed by Wagner for the transition between the internal and external oxidation of the most reactive component in binary alloys.     

Effects of NaCl and SO2 on the initial atmospheric corrosion of zinc

The influence of NaCl deposition on the corrosion of zinc in atmospheres with and without SO₂ was studied via quartz crystal microbalance. Regularity of the initial corrosion of zinc under these conditions was analyzed. The results show that NaCl can accelerate the corrosion of zinc. Mass gain of zinc increases with the exposure time, which can be correlated by using exponential decay function. The relationship between mass gain and amount of NaCl deposition is well linear at any time in air containing 1 ppm SO₂, but follows quadratic function in air without SO₂. More amount of NaCl deposition will slow down the corrosion to some extent after exposure for certain time in the presence of SO₂. The combined effect of NaCl and SO₂ on the corrosion of zinc is greater than that caused by each single component. Fourier transform infrared spectroscopy and X-ray diffraction were used to characterize the corrosion products of zinc. In the absence of SO₂, simonkolleite, Zn₅(OH)₈Cl₂·H₂O and zincite, ZnO are the dominant corrosion products, while zinc hydroxysulfate (Zn₄SO₄(OH)₆·3H₂O), zinc chloride sulfate hydroxide hydrate (Zn₁₂(SO₄)₃Cl₃·(OH)₁₅·5H₂O) and simonkolleite dominate in the presence of SO₂. Brief discussion on the mechanisms of atmospheric corrosion under these conditions was introduced.     

高温高压水环境下传热管失效形式及防腐措施研究进展

分析了在高温高压水环境下，压水堆(PWR)蒸汽发生 器传热管的失效形式及原因，总结了应力腐蚀开裂(SCC)的相关机理，并针对奥氏体不锈钢的应力腐蚀开裂—一种严重的环境促进腐蚀开裂(EAC)形式，分析了相应的解决措施，特别是缓蚀剂解决措施.     

Growth process of plasma electrolytic oxidation films formed on magnesium alloy AZ91D in silicate solution

In order to get a clear picture for describing the growth process of the oxide film formed on magnesium alloy AZ91D under plasma electrolytic oxidation (PEO) in alkaline silicate solution, the characteristics of PEO films formed at different reaction stages were systemically investigated. The results of morphologies, compositions and electronic properties indicated that the PEO films had a different growth behavior as the PEO treatment proceeding. At the initial stage (before the occurrence of sparking), the growth rate of PEO films was low, the elements (O, Mg, Al and Si) contents were varied obviously and the donor concentration in the film was kept at a high level. After sparking occurred, the PEO films showed a higher growth rate due to the high transfer rate of ions and electrons and the existence of plasma reactions; simultaneously, the films formed on α- and β-phase exhibited different growth rate. With treated time increased, the thickness of PEO films and transfer resistance to ions and electrons were also increased; thereby, the growth rate of the PEO films was decreased gently.     

Electrochemical noise analysis of LY12-T3 in EXCO solution by discrete wavelet transform technique

Electrochemical noise technique coupled with SEM was used to study the corrosion process of LY12-T3 in the modulated EXCO test solution, and the EN data were analyzed by wavelet technique based on orthogonal db4 wavelet. The results show that, under the experimental conditions, the characteristics of energy distribution plot (EDP) obtained from wavelet analysis technique can be used as “fingerprints” of EN signal and can be used to differentiate the corrosion type. With the change of corrosion type of LY12-T3 from pitting corrosion through intergranular corrosion to exfoliation corrosion, the maximum relative energy defined in EDP will change from the region of crystal series coefficients with smaller scales through middle scales to larger scales.     

Preparation and oxidation behaviour of an AlSiY diffusion coating on a Ni-based single crystal superalloy

An AlSiY diffusion coating was developed by a combined approach of arc ion plating and subsequent diffusion treatment. Cyclic and isothermal oxidation behaviour of the coating specimens was tested in a muffle furnace at 1000 °C and 1100 °C. The results confirmed an enhanced performance of the diffusion coating due to the presence of β-NiAl enriched outer layer. The microstructure change and the oxidation behaviour of the diffusion coating are discussed.     

Anomalous Behavior of the Internal Oxidation of Dilute Cu�CNi Alloys Under 1?atm. O2 at 900?��C

The internal oxidation of Ni in dilute Cu?CNi alloys exposed to 1 atm. O₂ at 900 °C, and thus in the presence of external CuO_x scales, is absent for Ni levels below a critical value, extremely close to that required for the simultaneous equilibrium between the alloy and the oxides of the two components. For Ni contents not largely exceeding this limit the internal oxidation of Ni deviates from the classical behavior described by Wagner, approaching that predicted by Morral et al. for the precipitation of compounds with limited thermodynamic stability.     

Base material fatigue data for low alloy forged steels used in the subsea industry. Part 2: Effect of cathodic protection

In air S–N fatigue data for forged low alloy steels as used in the subsea industry are presented in Part 1 of this paper. The test scope in Part 1 included testing to quantify the effect of the surface roughness, mean stress and material strength on the high cycle fatigue strength of low alloy steels with a tensile strength in the range of 600–800 MPa. A method for estimating the in air S–N curve from the tensile strength (material grade), surface roughness (machining) and mean stress (such as residual stresses, pressure testing, pre-load and external loads) is presented in Part 1. In this Part 2, fatigue test results for low alloy steels and one carbon steel tested in seawater with cathodic protection with a potential of −1050 mV versus an Ag/AgCl reference electrode are presented. The fatigue testing has been performed using smooth specimens. The tested smooth specimens have (actual) tensile strengths in the range from 627 to 790 MPa. Penalty factors for the tested smooth specimens in seawater with cathodic protection with respect to in air performance (Part 1) are presented and compared with penalty factors used in fatigue design codes such as DNVGL-RP-0005 (former DNV-RP-C203) and BS 7608. The obtained environmental reduction factors are found to be in accordance with the penalty factors used in BS 7608 provided that the maximum stress in the cycle is less than 94% of the yield stress for the material. The penalty factors used for forged steels in DNVGL-RP-0005 are non-conservative compared to the test outcome for the steel tested in an artificial 3.5% NaCl seawater solution. For higher stress levels, larger penalty factors than used in BS 7608 are required. It is found that the obtained S–N based environmental reduction factors are of similar magnitude as BS 7910 fatigue crack growth based reduction factors for CP.     

Electrochemical corrosion behavior of electroless Ni–P coating in NaCl and H2SO4 solutions

Electrochemical corrosion behavior of electroless Ni–P coating in NaCl and H₂SO₄ solutions were studied by potentiodynamic polarization curves and electrochemical impedance spectra techniques, as well as the corrosion morphology was characterized. The results indicate that electroless Ni–P coating with about 25 µm is stable in 30 days immersion in NaCl solution. Although it was corroded with prolonged immersion days, the corrosive medium has not penetrated through the coating. During the H₂SO₄ concentration ranging from 5 to 10%, the corrosion current density of electroless Ni–P coating increased due to the intensified anodic dissolution process; in 15% H₂SO₄ solution, electroless Ni–P coating shows obvious anodic passivation effect.