Narrow Results

A system subject to internal defects and external shocks is analyzed in this paper. Internal defects initiate following a nonhomogeneous Poisson process (NHPP) and they grow according to deterioration processes modeled as gamma processes. A corrective replacement is performed when the deterioration of a defect exceeds a failure threshold. The system is subject to external shocks. After an external shock, the system is replaced with probability 1 – p and is minimally repaired with probability p. The system is preventively replaced at the age of T. Costs are associated with the maintenance actions. The value of T that minimizes the expected cost rate is obtained analytically. Numerical examples are showed to illustrate the theoretical results.

Corrosion behavior of the casting Al-Cu alloys with and without Sc addition was studied by both the immersion test and electrochemical measurement methods. The results show that pitting corrosion was found on the surfaces of these alloys in 3.5wt.% NaCl solution, while the corrosion resistance of the Al-Cu alloys with Sc is better than that of the Al-Cu alloy without Sc. With increasing Sc content, the corrosion resistance of the present Al-Cu alloys varies significantly, and the 0.1wt.% Sc alloy has the best corrosion resistance. Also, the corrosion potential of the present alloys with Sc is shifted positively compared with that of the alloy without Sc by electrochemical measurement, indicating that the alloys with Sc have better corrosion resistance. The microstructures of the Al-Cu alloys change from the coarse grains to the fine equiaxed grains due to Sc addition, which is responsible for the reduced susceptibility to corrosion and the enhanced corrosion resistance of the present alloys.

Thermal spray coating method has been known to be an attractive technique due to its relatively high coating speed. However, a high corrosion resistance of the coating film deposited by thermal spray method should be improved to prolong its lifetime. In this study, four types of coated films (DFT: 400 μm), that is, pure zinc, pure aluminum and two Al – Zn alloy (Al:Zn = 85:15 and Al:Zn = 95:5) films were coated onto a carbon steel (SS401) with arc spraying, and the corrosion behaviors of these samples were investigated using the electrochemical method. The pure aluminum sample had the best corrosion resistance in seawater solution and alloy (Al:Zn = 85:15) film, so called galvalume followed the pure aluminum sample, moreover, the alloy (Al:Zn = 95:5) sample exhibited the worst corrosion resistance.

Electrochemical noise (EN) data is commonly used to monitor corrosion of metals in various environments. In this work we use recurrence quantification analysis (RQA) to study EN time series of stainless steel AISI 316 samples immersed in a mildly corrosive electrolyte. It is found that RQA of current and potential time series reveal different information: current time series provides detailed information on the kinetics of the pitting corrosion process, while the potential time series identifies the transitions from one thermodynamic state to another in the pitting corrosion process.

Ni-Al nanoparticles coating (NAN) was manufactured via atmospheric plasma spraying (APS) and thermal treated under hydrogen atmosphere at 1300${}^{\circ }$C (TNAN) remained 1 h, and NiAl microparticles coating (NAM) was manufactured as a reference. Nanoscale particles were observed in NAN by TEM, and these nanoscale particles disappeared in TNAN. Many pores and cracks were observed in NAM. Few pores and cracks were observed in NAN, and no pores and cracks were found in TNAN with SEM. A scanning electrochemical microscopy (SECM) testing in 3.5% (wt.) NaCl solution for 3 h revealed that NAM underwent several pitting corrosion, NAN pitting corrosion was relatively minor, and TNAN had no pitting corrosion.

The present study was focussed on investigating the corrosion properties of plasma-oxidized and -nitrided CoCrMo alloys under different conditions. The structural properties of untreated and treated samples were examined by using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Corrosion behavior of samples was mainly investigated by potentiodynamic polarization and electrochemical impedance spectroscopy in simulated body fluid solution. The results showed that corrosion resistance of the oxidized layers was better than that of the nitrided ones. The corrosion resistance of the alloys increased as the plasma oxidation process temperature and time increased. However, the corrosion resistance of the alloys reduced with increase in the process temperature and time after plasma nitriding process.

Pitting corrosion of carbon steel in sodium chloride solution induced by Ce${}^{3+}$ and the synergistic inhibition effects of Ce${}^{3+}$ and Zn${}^{2+}$/SiO${}_3^{2-}$ were investigated using in-situ observation and electrochemical methods. The results showed that the presence of Ce${}^{3+}$ could result in severe pitting corrosion and a positive shift in the corrosion potential. It was found that individual Ce${}^{3+}$, Zn${}^{2+}$, or SiO${}_3^{2-}$ had low inhibition efficiencies, whereas the combination of Ce${}^{3+}$ and Zn${}^{2+}$ or SiO${}_3^{2-}$ proved to be highly effective in inhibiting the development of pits in two different ways and in enhancing the corrosion resistance. The pitting corrosion and inhibition mechanisms were discussed based on the results.

In the present work the onset of current oscillations at late stages of iron pitting corrosion in chloride containing sulfuric acid solutions is further studied. By increasing the chloride concentration and potential, repassivation of iron becomes gradually unlikely giving rise to the formation of salt layers in front of the iron electrode and electropolishing processes. Occurrence of qualitative changes in electrochemical current oscillations as the chloride concentration and potential are varied is shown and discussed. Analysis of the transient behavior of current vs. time curves occurring before the establishment of a specific oscillatory pattern, reflect changes in the composition of salt layers in front of the electrode. At a critical potential value (bifurcation potential, E_bif) the system exhibits a transition from one type of oscillations (beating, aperiodic bursting) to another (chaotic oscillations of small amplitude). The temporal behavior of the system at potentials E < E_bif is associated with a characteristic average time between bursts indicative of crisis-induced intermittency. In terms of physicochemical processes, the E_bif coincides with the repassivation potential used in steady-state pitting corrosion studies to indicate different states of dissolution due to changes in the pit chemistry. Since the present system exhibits a non-steady passive-active transition state we suggest that E_bif implies a transition from the electropolishing state dissolution to a passive-active state dissolution due to changes in the concentration of chloride and hydrogen ions inside pits.

This paper gives an overview of the use of Electrochemical Noise (EN) for corrosion studying and monitoring. Since the quality and reliability of noise data are affected by a number of acquisition parameters, such as sampling interval, sampling duration, D.C. trend and instrumental noise, some experimental and practical aspects were discussed. The use of statistical parameters such as standard deviation, Pit Index and/or Localization Index and Noise Resistance to analyze noise data of corroding systems were examined. Many experimental applications of Electrochemical Noise Measurements on different metals and alloys were given. EN data have been compared with traditional electrochemical techniques. EN allowed to characterize the corrosion behavior of samples giving in some cases good quantitative estimation.

The transposition of current and potential noise acquisition in the frequency domain (by Fast Fourier Transform and/or Maximum Entropy Method), gave further information on corrosion mechanism and in particular permitted to identify the type of corrosion. Finally the use of Discriminant Analysis permitted to deduce the best sampling frequency and sampling duration for EN acquisition, able to discriminate between two different situations.

Porous aluminum with high surface area for aluminum electrolytic capacitors was obtained by etching aluminum foils in hot hydrochloric acid solution under direct current (DC). Careful etching procedure produced parallel tunnels, in which numerous (100) faces was dissolved to form [001] etch tunnels. The porosity (ρ), average diameter (Ф) and average length (L) as well as the capacitance (C_p) of the etching tunnels were investigated in different etch conditions such as solution concentration, temperature and current density. Based on systematic study of the etching process, experimental parameters were optimized in obtaining porous aluminum with high specific capacitance. Growth mechanism of the porous aluminum was proposed based on the anodic polarization curves.

Anodes of aluminum electrolytic capacitor, which require large surface area for a high capacitance, are prepared in hot chloride solution to get numerous <100> etching tunnel. With the defect on the surface of the foils, the size and distribution of etch pits are not uniform. This paper compared the effect of five different pretreatment conditions on pitting corrosion of the anode aluminum foils by analyzing the morphology, the density and the distribution of the etching tunnels. It was shown that pretreatment with alkaline washing firstly to get rid of the oil stain and distorted oxidation film, and H₃PO₄ soaking sequentially to produce uniform oxide films, was beneficial for the uniform pitting corrosion.