The sensitization process of austenitic stainless steel in, indeed, the one during which carbide (Fe, Cr)_(23)C_6 precipitates along grain boundaries from the oversaturated austenite, and meanwhile, solute atoms, such as Cr, Ni and C, redistribute in the matrices.This process is simulated by computer calculation as reported in this paper.It is considered that the precipitation of the carbide is controlled by the diffusion of substitutional solute atoms, and the interface between the austenite and carbide can reach a thermodynamic equilibrium at a time.Through a dynamic supply of phase equilibrium information in Fe-Cr-Ni-G system, the boundary conditions of the diffusion process are determined; the distribution curves of solutes in the matrices are solved; and still further, the Temperature-Time-Sensitization (TTS) curves are predicted.The practical application of the above simulation has been conducted in austenitic stainless steels.A quantitative theoretical basis has been provided for the Cr-depleted theory on intergranular corrosion.

The effect of Cl~- ion on the features of impedance of mild steel and iron electrodes with passive film on their surfaces is studied.In NaNO_2 solution, the polarization resistance and time constant of mild steel electrode are estimated to be 6.35 megohm/cm~2 and 87.5 sec respectively, which are very close to the published data for iron in borax-boric acid solution.As Cl~- ion is introduced into various passivators solutions, the polarization resistance and time constant of the passive-film-covered mild steel and iron electrodes reduce by ca.4 orders of magnitude during pit incubation period,i.e., before the passive film is perforated.In addition, an inductive component is observed on impedance plane in low frequency range for all systems studied.This inductive component is fading away with time gradually.However, even if the inductive are disappears at last, the shrinkage of the real part of the impedance at very low frequencies can still be observed before pits initiate.An equation describing the impedance of passive-film-covered electrodes is proposed, based on which, three cases of the impedance diagrams of passive-film-covered electrodes can be demonstrated.If R_f, which denotes the resistance for the passive-film-forming current density passing through the passive film, is approximately equal to R_s, which denotes the resistance for the passive film dissolution process, a single capacitive arc on the impedance diagram is obtained and the estimated polarization resistance R_p equals R_f, whose magnitude is abont 6 megohm·cm~2 as usually observed for passive electrodes.When R_s>R_f, the faradaic impedance is equivalent to a serial connection of a resistance and a capacitance as observed for valve metals electrodes.If R_s References | Related Articles | Metrics

Features of AC impedance of passivated mild steel and stainless steel electrodes suffering from pitting corrosion are studied.For the electrodes during pits propagation, the Sluyters diagrams are composed of two capacitive loops.Based on the assumption that the impedance for a pitting corroded electrode is composed by parallel connection of the impedance within pits and the impedance of the passive surface outside of pits, an equivalent circuit to interprete the features of the impedance of pitting corroded electrodes during pits propagation has been proposed.By the IFP technique developed in this laboratory, the parameters in the equivalent circuit can be estimated.Variations of these parameters with the conditions of electrode surface are studied both for real pitting corroded electrodes and composite electrodes simulating pitting corroded surface.The latter are made of Ni-plated iron electrode with predetermined number of holes (φ0.3mm) drilled through the Ni layer.Results are discussed.

The antitarnish effects of various antitarnish agents of copper have been compared in this paper.The results showed that the l-pheny1-5-mercaptotetrazole (PMTA) is a more excellent antitarnish agent than the Benzotriazole (BTA), Hydroxybenzoiri azole(HBT), 2-mercapto-benzothiazole (MBT).The optimum conditions of treating copper with PMTA are PMTA 1～1.2g/L, pH5, 20～40℃, 4min. The results of XPS and AES analyses showed that the coordination compound film of PMTA on copper surface was formed on Cu_2O, the valence state of copper in this film was+1.The N (1s) and S (2p) in PMTA molecule have been coordinated onto Cu_2O.The relative atomic percent contents (A.C.%) of the composition elements on the film were determined from the constant composition region of the depth profile curves as: Cu 40.9, C 32.3, N 16.2,O 7.9, S 2.9.

A random simplex method, which could be used to evaluate the value of components in equivalent circuit from impedance data, is proposed.It is a combination of Monte Carlo method and simplex method.The flowchart for fitting operation is given.The first estimates are arbitrarily assigned to each component, and their final solutions could be obtained through consecutive optimization and approximations.As a result, it is possible to find out simultaneously a number of unknown parameters, especially in case of complicated equivalent circuit, and so it is suitable for processing impedance data when dispersion phenomenon appears.The effect of errors in impedance value on fitting results is examined by using computergenerated impedance data contaminated deliberately by random error.Starting from experimentally measured impedance spectra, the processing is demonstrated for practical systems.The significance of objective function, which is the criterion of fitting, is discussed.

Stress corrosion cracking (SCC) behavior and failure mechanism of GC-4 steel (40CrMnSiMoVA) with electroplated coatings in 3.5% NaCl solution were investigated and compared with those of bare steel using slow-strain rate tension and fracture mechanics methods. Both micro and macro-fractographic analyses were conducted using SEM and optical microscope. It was found that both anodic and cathodic electroplated coatings decreased K_(ISCC) and increased the crack propagation rate da/dt (Ⅱ ). The extent of influence on the cracking behavior by different electroplated coatings increased in the sequence of noncyanide Cd,Cd-Ti, and Cr. The slow tensile tests showed that both anodic and cathodic polarization reduced the ductility of steel. According to the fact that the crack propagated faster on the side surface than in subsurface layer and even than in the central part of BL-WOL specimens, and according to the influences of different platings as well as the plane strain conditions on the macro-fractographic morphology of crack growth, it was suggested that the SCC mechanism of GC-4 steel with and without plating in aqueous solutions involves combined action of local anodic dissolution at the crack tip and hydrogen assisted cracking rather than hydrogen embrittlement alone. In addition, the crack tip dissolution contributed to the crack propagation directly.

The hydrogen distribution in the welded joints and U-bend samples has been measured using the Laser Hydrogen Detector. WELDED JOINTS: 1. The highest hydrogen concentration is found along the fusion line. 2. The hydrogen distribution and content in welded joint depends on the original hydrogen content and the micro-structure of the welding metal and base metal. U-BEND SAMPLES: 1. In the plastic deformation region, the dislocation sets in motion of the hydrogen and the hydrogen is concentrated from the low-density region to the highdensity region of the dislocation. 2. The strain and residual stress has been calculated by means of the plastic mechanical nth-power law, shown as following: For the stainless steel 1Cr18Ni9Ti (cold rolled) F = 990 (MPa), n = 0.146.

Anodic protection is a useful method to prevent metals from corrosion. The anodic process protective parameters, properties of Pt reference electrode and cathodic material SNW-2 have been studied in 93 wt% and 98 wt% sulfuric acid solutions. An anodic protection cooler for sulfuric acid production has been designed and fabricated. Results of the pilot plant experiments show that the cooler works satisfactorily with stable protective parameters and high heat-exchange efficiency.

An electrochemical set-up was devised to measure the diffusion and permeation rates of hydrogen in cardon steel at elevated temperatures on the principle of Devanathan cell. The optimum parameters for the set-up were obtained as follows: The electrolyte in measuring room was 0.1N NaOH solution; The reference and appendix electrodes were made with pure nickel; The thickness of palladium plating on the measuring side was 0.2μm; The anodic polarized potential to oxidize permeated hydrogen atoms was +300mV (relative to the nickel reference electrode). The diffusion coefficients and diffusion rates of hydrogen in carbon steel at temperatures 100～200℃ were measured with the above mentioned set-up. The activation energy ot diffusion by means of diffusion coefficients was 8.61kJ/mol, The mechanism of H~+ reduction on carbon steel was discussed. The results showed that the mechanism was "fast charging,slow coupling electrochemically".

The adsorption behaviour of thiourea (TU) on pure iron was investigated in 0.5～2.0 mol·dm~(-3) H_2SO_4 over the temperature range of 20～60℃. The process was treated as a sabstitutional adsorption process. The degree of surface coverage increased with temperature and TU concentration. It was shown that at low TD concentrations the adsorption behaviour followed a Flory-Huggins isotherm with negligible lateral interaction. TU molecules were parallely adsorbed on the surface. At higher TU concentrations the molecules did not change their orientation, but the adsorption behaviour followed a Frumkin-type isotherm with strong lateral repulsion. The Gibbs energy, enthalpy and entropy of adsorption were calculated. The effect of temperature on the degree of surface coverage was at tributed to entropy, rather than enthalpy.

The pit corrosion, uniform corrosion, sea water corrosion, and marine atmosphere corrosion of rare earth elements added 6063 wrought aluminum alloy are studied by voltammetric and gravimetric methods. The experim ental results show that the addition of a suitable small amount of RE elements can appreciably improve the corrosion resistance of the alloy. The distribution and function of RE elements in the alloy is preliminarily discussed by metallography and electron probe analysis.