Diffusion Of Corrosion Products Research Articles

Insight into the corrosion behaviors and mechanism of the self-healing Si/N-DLC films in oilfield environment

The Si/N-DLC films with different Si and N contents were deposited and the corrosion behaviors in the simulated CO2-saturated oilfield produced water was mainly investigated. Results reveal that Si/N-DLC films improves dramatically the corrosion resistance of the substrate. This attributes to not only the channel blockage caused by diffusion of corrosion products, being composed of silicon oxide (SiOx) and metal oxide, but also the stronger support of increased roughness induced by N to the aggregated corrosion products, implying that the Si/N-DLC film are a self-healing protective film. These results provide theoretical guidance to develop the DLC films for anti-corrosive applications.

In situ and dynamic observation of coating failure behavior

In this study, the failure behavior of epoxy coating in 3.5% (mass fraction) NaCl solution was investigated by electronic speckle pattern interferometry (ESPI) and electrochemical impedance spectroscopy (EIS). We found that ESPI images changed with the diffusion of corrosion products and the appearance of defects such as blistering. Direct information can be obtained from the images taken at different times. The relation between impedance characteristics and ESPI images was investigated. Experimental results showed that ESPI is an effective technique to detect coating degradation, and it provides a comprehensive approach to studying the process of coating degradation and under-film corrosion.

Corrosion evaluation of microarc oxidation coatings formed on 2024 aluminium alloy

Abstract Dense alumina ceramic coatings of 7 μm thickness were fabricated on 2024 aluminium alloy by microarc oxidation (MAO). The corrosion behaviour of the MAO coated alloys was evaluated using potentiodynamic polarisation and EIS measurements. The results show that the corrosion process of the coated alloy can be divided into three stages: (1) the initial stage (the first 2–6 h of immersion): penetration of corrosion medium into the aluminium alloy was inhibited by coating; (2) the second stage (after 24 h of immersion), corrosion medium penetrated to attack the interface between the substrate and the coating; (3) the final stage (after about 96 h): corrosion process was controlled by the diffusion of corrosion products.

Growth of corrosion products on thermally sprayed coatings with FeAl intermetallic phases in aggressive environments

The cyclic corrosion behavior of coatings with FeAl intermetallic matrix was investigated in agressive gases for exposure times of up to 500 h. The composite coatings strengthened by a fine dispersive Al 2O 3 were thermally sprayed by HVOF method in Jet Kote 2 system. One portion of intermetallic coatings was sealed with an inorganic phosphate seal. The surfaces of HVOF sprayed coatings after a corrosion test were subjected to observation. A kinetics test was carried out by periodic method. Mass changes of the studied coatings during the corrosion test are presented. The surfaces of the composite coatings after the corrosion test in aggressive environments were analyzed by scanning microscopy observation. The results show that an adherent alumina scale and iron oxide layers were formed on all studied coatings. A stable α-Al 2O 3 phase on the surface of the studied coatings ensure high oxidation resistance. The corrosion rate is controlled by diffusion of corrosion products in the channels of oxide layer. An analysis of the corrosion products’ phase composition was conducted by an X-ray diffraction method. The growth of corrosion products on the thermally sprayed coatings during the corrosion test and the chemical reactions between the corrosion products and aggressive environments are presented. All the results confirm good heat proofness of HVOF sprayed coatings with an intermetallic FeAl matrix.

Chemical Interactions in the Near-Field of a Spent Fuel Repository - Preliminary Results from a Long-Term Laboratory Experiment

ABSTRACTA ten-year-long laboratory experiment was recently started in order to study interactions between the groundwater, the bentonite, and the canister in the near-field of the planned Finnish repository for spent nuclear waste. The experimental set-up consists of a number of samples containing sodium MX-80 bentonite and cast iron cylinders placed in copper vessels. The bentonite stays in contact with an external solution, either distilled water or a 0.5 M NaCl solution, via metal sinters. The experiments are performed under anoxic conditions (N2 atmosphere) at ambient room temperature (∼30 °C). The experimental measurements focus mainly on the development of (1) the water chemistry, (2) the cast iron corrosion, and (3) the diffusion of corrosion products inside the bentonite, since these processes are the most likely ones to produce detectable changes during the ten-year-long programme.The first analyses, after 9 months, showed that the iron corrosion had started and that the corrosion products had migrated into the bentonite. The corrosion products could not be identified by XRD, which either meant that the material was amorphous or present in concentrations too low to be detectable. The analyses of the gas phase indicated the presence of hydrogen, which most probably was the result of the iron corrosion. The compositions of the external water and the porewater were determined with regard to the major species of interest. A closer evaluation of the results will not be given until more data are at hand.

Diffusion Behavior of Iron Corrosion Products In Buffer Materials

ABSTRACTCarbon steel is one of the candidate overpack materials for high-level waste disposal and is expected to assure complete containment of vitrified waste glass during an initial period of 1000 years in Japan. The lifetime of the carbon steel overpack will depend on its corrosion rate. The corrosion rate of carbon steel is reduced by the presence of buffer material such as bentonite. Buffer material will delay the supply of corrosive materials and discharge of corrosion products through it. Carbon steel overpack will be corroded by consuming oxygen introduced by repository construction after closure of repository and then will keep the reducing environment in the vicinity of repository. The reducing condition will be expected to retard the migration of redox-sensitive radionuclides by lowering their solubilities. Therefore, the diffusion of corrosion products of iron in buffer material is important to discuss the corrosion rate of overpack, migration of redox-sensitive radionuclides and properties of buffer material. The purpose of this paper is to study diffusion behavior of a corrosion product of iron in compacted bentonites under a reducing condition with a carbon steel. The diffusion mechanism of iron in the compacted bentonites were discussed by estimation of iron species in the bentonite pore water. There were two diffusion paths of iron in the compacted bentonites used in this study; the fast path has low capacity of iron, ca. 1wt%, and large apparent diffusion coefficient, ca. 10−12m2/s and the slow path has high capacity of iron, ca. 10wt%, and small apparent diffusion coefficient, ca. 10−14m2/s.

The corrosion behaviour of a plasma spraying Al 2O 3 ceramic coating in dilute HC1 solution

Al 2O 3 ceramic coatings plasma sprayed on the surface of metals change greatly the corrosion law of metals in strong acid solutions and enhance effectively their corrosion resistance property. In this paper, the corrosion behaviour of a Q235 steel with plasma sprayed Al 2O 3 coatings in a boiling 5% HCl solution is investigated. The corrosion rate of the Al 2O 3 coating sprayed on the surface of Q235 steel is only 1/30 that of the 18-8 austenitic stainless steel under the same corrosion condition. However, the substrate and the Al 2O 3 ceramic coating itself are also subjected to some corrosion because of pores and nonequilibrium γ-Al 2O 3 existing in the ceramic coating. The experimental results demonstrate that at the early corrosion stage the corrosion mechanism is channel corrosion with a large length to diameter ratio for the channels; and the corrosion rate is controlled by the diffusion of corrosion products in the channels; while at the late corrosion stage electrochemical corrosion occurs between the Ni-base alloy bond and the substrate. It was shown that channel corrosion can be restrained and the substrate can be significantly protected by choosing a suitable alloying base layer, increasing appropriately the thickness of the coating, and adopting a porous sealing treatment.

Investigations of the pitting corrosion behavior of passive film on REBAR electrodes containing ‘artificial pit’ in simulated cement solutions

In this paper, in-situ photoelectrochemical microscopic method has been used to characterize the spatial variations of the passive film on ‘artificial pit’-containing REBAR electrodes in simulated cement pore solution. It is clearly shown that the photocurrent at the artificial pit region was larger than that at other normal regions under suitable conditions. This was attributed to a thicker and more porous passive film formed potentiostatically at the artificial pit. With the help of ‘line-scan’ technique, we have provided a first example of in-situ monitoring the changes of local photocurrent in the artificial pit region on REBAR electrode surface. It was found that the photocurrent at the pit reduced gradually during the incubation time of pitting corrosion. It is thought that this phenomenon was correlated to the dissolution of the passive film in the artificial pit region which caused by the penetration of chloride ions, and the changes of photocurrent may reflect the accumulation and diffusion of corrosion products.

Simple passivation and depassivation model for pitting corrosion.

A simple two-dimensional passivation and depassivation model for pitting corrosion has been developed. This model describes corrosion processes that are limited by the diffusion of either a corrosive agent or the diffusion of corrosion products that inhibit the corrosion process. Despite its simplicity, this model exhibits both stable and unstable pit growth, along with distinctive current fluctuations. For the case of a single corrosion pit, the corrosion current I(t) has the form I(t)=f(t/${\mathit{t}}^{\mathrm{*}}$)/ln(t) if there is no depassivation. The function f(x) has the form f(x)=const for x\ensuremath{\ll}1 and f(x) decays faster than any power of x for x\ensuremath{\gg}1. The characteristic time ${\mathit{t}}^{\mathrm{*}}$ is given by ${\mathit{t}}^{\mathrm{*}}$ln(${\mathit{t}}^{\mathrm{*}}$)\ensuremath{\simeq}${\mathit{k}}_{\mathit{p}}^{\mathrm{\ensuremath{-}}2}$, where ${\mathit{k}}_{\mathit{p}}$ is the passivation rate constant.

The Anodic Dissolution of a Ni-Base Superalloy

Abstract A single phase nickel base superalloy, HASTELLOY Alloy C-276, was anodically polarized in 0.5 N NaCl at 70 C, and the concentrations of metal-ion complexes in solution were determined. Relative dissolution rates of alloying elements were obtained and selectivity coefficients were calculated. The corrosion products that formed at high anodic potentials were identified as FeCl2+, Ni2+, HCrO4−, and MoO42−. The pH was observed to decrease and the chloride ion concentration to increase in the electrolyte when diffusion of corrosion products out from an occluded region at the alloy surface was restricted. The increased hydrogen ion concentration in the occluded region was explained by the formation of molybdate and chromate ions as corrosion products. The high corrosion resistance of Alloy C-276 in chloride may be related to the formation of corrosion-inhibiting molybdate ion.