Effect of pH and Concentration on Electrochemical Corrosion Behavior of Aluminum Al-7075 T6 Alloy in NaCl Aqueous Environment

This study was carried out to investigate the corrosion resistance of Al–Si hypereutectic alloy in 5.0% NaCl solution using BMW 5 series piston head. The corrosion behaviours were investigated through immersion and electrochemical corrosion tests. The immersion test was carried out for 7–28 days, whereas for electrochemical test the exposure time was only about 15 min. Hardness test and microstructure study were carried out on samples before and after corrosion test. It was found that the dark phase is silicon and the white area is aluminium. The silicon dispersed in aluminium alloy. It was also found that corrosion rate of the Al–Si hypereutectic alloys decreases as exposure time increases in immersion corrosion test. This was due to the formation of thin and protective corrosion product on the metal surface that acted as barrier between the metal and the environment. The corrosion rates of immersion test were more than electrochemical test due to high concentration of oxygen ions in the solution. SEM/EDS analysis confirmed that all metals suffer from uniform and localized corrosion, namely pitting. The localized corrosion is mainly due to breaking of oxide layer. The hardness was also confirmed to decrease as the time immersion increased.

This report deals with the impact of fabrication processes on the localized corrosion behavior of Alloy 22 (N06022). The four fabrication processes that were analyzed are: (1) Surface stress mitigation of final closure weld, (2) Manufacturing of the mockup container, (3) Black annealing of the container and (4) Use of different heats of Alloy 22 for container fabrication. Immersion and Electrochemical tests performed in the laboratory are generally aggressive and do not represent actual repository environments in Yucca Mountain. For example, to determine the intergranular attack in the heat affected zone of a weldment, tests are conducted in boiling acidic and oxidizing solutions according to ASTM standards. These solutions are used to compare the behavior of differently treated metallic coupons. Similarly for electrochemical tests many times pure sodium chloride or calcium chloride solutions are used. Pure chloride solutions are not representative of the repository environment. (1) Surface Stress Mitigation--When metallic plates are welded, for example using the Gas Tungsten Arc Welding (GTAW) method, residual tensile stresses may develop in the vicinity of the weld seam. Processes such as Low Plasticity Burnishing (LPB) and Laser Shock Peening (LSP) could be applied locally to eliminate the residual stresses produced by welding. In this study, Alloy 22 plates were welded and then the above-mentioned surface treatments were applied to eliminate the residual tensile stresses. The aim of the current study was to comparatively test the corrosion behavior of as-welded (ASW) plates with the corrosion behavior of plates with stress mitigated surfaces. Immersion and electrochemical tests were performed. Results from both immersion and electrochemical corrosion tests show that the corrosion resistance of the mitigated plates was not affected by the surface treatments applied. (2) Behavior of Specimens from a Mockup container--Alloy 22 has been extensively tested for general and localized corrosion behavior both in the wrought and annealed condition and in the as-welded condition. The specimens for testing were mostly prepared from flat plates of material. It was important to determine if the process of fabricating a full diameter Alloy 22 container will affect the corrosion performance of this alloy. Specimens were prepared directly from a fabricated container and tested for corrosion resistance. Results show that both the anodic corrosion behavior and the localized corrosion resistance of specimens prepared from a welded fabricated container were the same as from flat welded plates. That is, rolling and welding plates using industrial practices do not hinder the corrosion resistant of Alloy 22. (3) Effect of Black Annealing Oxide Scale--The resistance of Alloy 22 to localized corrosion, mainly crevice corrosion, has been extensively investigated in the last few years. This was done mostly using freshly polished specimens. At this time it was important to address the effect an oxide film or scale that forms during the high temperature annealing process or solution heat treatment (SHT) and its subsequent water quenching. Electrochemical tests such as cyclic potentiodynamic polarization (CPP) have been carried out to determine the repassivation potential for localized corrosion and to assess the mode of attack on the specimens. Tests have been carried out in parallel using mill annealed (MA) specimens free from oxide on the surface. The comparative testing was carried out in six different electrolyte solutions at temperatures ranging from 60 to 100 C. Results show that the repassivation potential of the specimens containing the black anneal oxide film on the surface was practically the same as the repassivation potential for oxide-free specimens. (4) Heat-to-Heat Variability--Testing of Ni-Cr-Mo Plates with varying heat chemistry: The ASTM standard B 575 provides the range of the chemical composition of Nickel-Chromium-Molybdenum (Ni-Cr-Mo) alloys such as Alloy 22 (N06022) and Alloy 686 (N06686). For example, the content of Mo is specified from 12.5 to 14.5 weight percent for Alloy 22 and from 15.0 to 17.0 weight percent for Alloy 686. It was important to determine how the corrosion rate of welded plates of Alloy 22 using Alloy 686 weld filler metal would change if heats of these alloys were prepared using several variations in the composition of the elements even though still in the range specified in B 575. All the material used in this report were especially prepared at Allegheny Ludlum Co. Seven heats of plate were welded with seven heats of wire. Immersion corrosion tests were conducted in a boiling solution of sulfuric acid plus ferric sulfate (ASTM G 28 A) using both as-welded (ASW) coupons and solution heat-treated (SHT) coupons.

This report deals with the impact of fabrication processes on the localized corrosion behavior of Alloy 22 (N06022). The four fabrication processes that were analyzed are: (1) Surface stress mitigation of final closure weld, (2) Manufacturing of the mockup container, (3) Black annealing of the container and (4) Use of different heats of Alloy 22 for container fabrication. Immersion and Electrochemical tests performed in the laboratory are generally aggressive and do not represent actual repository environments in Yucca Mountain. For example, to determine the intergranular attack in the heat affected zone of a weldment, tests are conducted in boiling acidic and oxidizing solutions according to ASTM standards. These solutions are used to compare the behavior of differently treated metallic coupons. Similarly for electrochemical tests many times pure sodium chloride or calcium chloride solutions are used. Pure chloride solutions are not representative of the repository environment. (1) Surface Stress Mitigation: When metallic plates are welded, for example using the Gas Tungsten Arc Welding (GTAW) method, residual tensile stresses may develop in the vicinity of the weld seam. Processes such as Low Plasticity Burnishing (LPB) and Laser Shock Peening (LSP) could be applied locally to eliminate the residual stresses produced by welding. In this study, Alloy 22 plates were welded and then the above-mentioned surface treatments were applied to eliminate the residual tensile stresses. The aim of the current study was to comparatively test the corrosion behavior of as-welded (ASW) plates with the corrosion behavior of plates with stress mitigated surfaces. Immersion and electrochemical tests were performed. Results from both immersion and electrochemical corrosion tests show that the corrosion resistance of the mitigated plates was not affected by the surface treatments applied. (2) Behavior of Specimens from a Mockup container: Alloy 22 has been extensively tested for general and localized corrosion behavior both in the wrought and annealed condition and in the as-welded condition. The specimens for testing were mostly prepared from flat plates of material. It was important to determine if the process of fabricating a full diameter Alloy 22 container will affect the corrosion performance of this alloy. Specimens were prepared directly from a fabricated container and tested for corrosion resistance. Results show that both the anodic corrosion behavior and the localized corrosion resistance of specimens prepared from a welded fabricated container were the same as from flat welded plates. That is, rolling and welding plates using industrial practices do not hinder the corrosion resistant of Alloy 22. (3) Effect of Black Annealing Oxide Scale: The resistance of Alloy 22 to localized corrosion, mainly crevice corrosion, has been extensively investigated in the last few years. This was done mostly using freshly polished specimens. At this time it was important to address the effect an oxide film or scale that forms during the high temperature annealing process or solution heat treatment (SHT) and its subsequent water quenching. Electrochemical tests such as cyclic potentiodynamic polarization (CPP) have been carried out to determine the repassivation potential for localized corrosion and to assess the mode of attack on the specimens. Tests have been carried out in parallel using mill annealed (MA) specimens free from oxide on the surface. The comparative testing was carried out in six different electrolyte solutions at temperatures ranging from 60 to 100 C. Results show that the repassivation potential of the specimens containing the black anneal oxide film on the surface was practically the same as the repassivation potential for oxide-free specimens. (4) Heat-to-Heat Variability--Testing of Ni-Cr-Mo Plates with varying heat chemistry: The ASTM standard B 575 provides the range of the chemical composition of Nickel-Chromium-Molybdenum (Ni-Cr-Mo) alloys such as Alloy 22 (N06022) and Alloy 686 (N06686). For example, the content of Mo is specified from 12.5 to 14.5 weight percent for Alloy 22 and from 15.0 to 17.0 weight percent for Alloy 686. It was important to determine how the corrosion rate of welded plates of Alloy 22 using Alloy 686 weld filler metal would change if heats of these alloys were prepared using several variations in the composition of the elements even though still in the range specified in B 575. All the material used in this report were especially prepared at Allegheny Ludlum Co. Seven heats of plate were welded with seven heats of wire. Immersion corrosion tests were conducted in a boiling solution of sulfuric acid plus ferric sulfate (ASTM G 28 A) using both as-welded (ASW) coupons and solution heat-treated (SHT) coupons. Results show that the corrosion rate was not affected by the chemistry of the materials within the range of the standards.

Introduction Due to excellent mechanical property and corrosion resistance, aluminum alloys are widely used in airplanes, automobiles, and kitchen utensils. The corrosion resistance of aluminum alloys relies on highly protective and reformative passive films. However, the passive films are destroyed in presence of chloride ions, and the corrosion rate of aluminum alloys depends on the concentration of chloride ions 1 - 3. A few studies have shown the corrosion behavior of aluminum alloys in freshwater; however, the corrosion rates of aluminum alloys were changed in freshwater which sampled different place 4. Because freshwater also contains some metal cations in dilute concentration. The authors expected that the metal cations changed the corrosion rate of aluminum alloy in freshwater. The authors have been investigated the corrosion of A3003 aluminum alloy in freshwater focused on the effects of metal cations5 - 8. The corrosion rate of A3003 aluminum alloy is changed with metal cations in model freshwater despites the concentration of chloride ions, dissolved oxygen and pH are the same. The researches also clarified that Zn2+ is the best metal cation to inhibit the freshwater corrosion of A3003 among the metal cations used6, 7. An Auger spectroscope was used to investigate the corrosion morphology and composition of corrosion products of A3003 formed in model freshwater with different Zn2+ concentration8. From the cross-sectional AES point analysis, it has been clarified that the corrosion products have multi-layer structure.However, it is not fully elucidated the role of the metal cations on the corrosion behavior of aluminum alloys. The purpose of this study is to investigate the effects of metal cations on corrosion behavior of aluminum alloys in aqueous environment. Experimental Sheets of aluminum alloys (A7075, A6061and A2024) were used as the specimens. The sheets were cut into 7 7 mm for immersion corrosion tests, and 10 10 mm for electrochemical measurements. The specimens were grounded with SiC abrasive paper. Before the tests, specimens were cleaned in highly purified water and in ethanol using ultrasonic bath.Model solutions of 1 molm-3 NaCl (NaS), 0.5 molm-3 MgCl2 (MgS), and 0.5 molm-3 ZnCl2 (ZnS) were used in this experiment. The pH of the model solutions was maintained at around neutral.Electrochemical tests were carried out in a three-electrode cell using a potentiostat. An 4 cm2 Pt plate and an Ag/AgCl-saturated KCl electrode were used as the counter and the reference electrodes. Before the measurements, the specimens were immersed in the solutions for 3.6 ks (1 h) at 298 K. Electrochemical impedance spectroscopy (EIS) tests were carried out in the frequency range from 1 mHz to 10 kHz, and the modulation amplitude of the applied sinusoidal wave was 10 mV.Specimens were immersed in the model freshwater for 2.59 Ms (30 d) and 0.6 Ms (7 d) at 25 ºC. The mass of a specimen was measured before and after the test. Surface analysis were carried out by scanning electron microscope (SEM) and X-ray photoelectron spectroscope (XPS). Results From the immersion tests, the mass of specimens increased after immersion in the solutions and corrosion behavior of aluminum alloys was changed with the metal cations present in the solutions. Independent of used alloy, SEM images clearly showed the deposited corrosion products on the surface. Uniform deposition of corrosion products was observed on the specimen immersed in ZnS as compared to the NaS and MgS. EDS results demonstrated the presence of zinc-related products on the specimen immersed in ZnS XPS results suggested that zinc ions were incorporated in the oxide films of aluminum alloy and increased the corrosion resistance property of the oxide films. EIS results showed the highest impedance in ZnS as compared to the other solutions. References 1) T. H. Nguyen and R. T. Foley, Journal of the Electrochemical Society, 127, 2563-2566 (1980).2) R. T. Foley, Corrosion, 42, 277-288 (1986).3) B. Zaid, D. Saidi, A. Benzaid, and S. Hadji, Corrosion Science, 50, 1841-1847 (2008).4) Editorial board of Japan Aluminium Association: "Aluminum Handbook. Tokyo", Japan Aluminium Association, (2007), p 73.5) K. Otani, M. Sakairi, T. Kikuchi, and A. Kaneko, Zairyo-to-Kankyo, 59, 330-331(2010).6) M. Sakairi, K. Otani, A. Kaneko, Y. Seki, and D. Nagasawa, Surface and Interface Analysis, 45, 1517-1521 (2013).7) K. Otani, M. Sakairi, R. Sasaki, A. Kaneko, Y. Seki and D. Nagasawa, Journal of Solid State Electrochemistry, 18, 325-332 (2014).8) K. Otani, Md. Saiful Islam, M. Sakairi, and A. Kaneko, Surface and Interface Analysis, 51, 1207-1213 (2019).

Corrosion behaviour of PEO coating sealed by water based preservative containing corrosion inhibitors

Effect of laser shock peening on the dissolution of precipitates and pitting corrosion of AA6061-T6 with different original surface roughness

In this study, the corrosion behaviour of several aluminium alloys in ethanol fuels was investigated by immersion and polarization tests. The corrosion properties of cast aluminium alloys (Al–17wt%Si–4wt%Cu–Mg, Al–8wt%Si–3wt%Cu, Al–7wt%Si–Mg and Al–17wt%Si–4wt%Cu–Mg with a chemically deposited nickel layer) in ethanol blended gasoline fuels were examined at various ethanol and water contents and various temperatures. Electrochemical and gravimetric measurements revealed a pronounced acceleration of the corrosion process above the boiling point. Additions of water restrain the corrosion. Increasing the ethanol content and the temperature leads to a higher corrosion sensitivity of the aluminium alloys. Furthermore, the nickel layer is very protective in all tested fuels. For aluminium alloys, a theory of the corrosion process in ethanol blended gasoline fuels is proposed.

Pollution control measures have resulted in replacement of chlorine by peroxide as bleaching chemical. Change of chemical affects corrosion aspects, the suitability of existing plant metallurgy and materials of construction of bleach plants. Accordingly long term immersion and electrochemical corrosion tests were conducted on stainless steel 304L, 316L, 2205 and 6% Mo and mild steel in peroxide solutions of pH 10. The materials were tested for uniform corrosion, pitting and crevice corrosion and attack around the weld area. Corrosion attack estimated from long term immersion tests is found in agreement, by and large, with that analyzed from electrochemical test. E-pH diagrams drawn for water-peroxide system have been used to understand the corrosivity of the peroxide media. An attempt has been made to suggest a suitable material of construction for handling the test media on the basis of degree of corrosion attack on them and their cost and the mechanical properties .

Sintered low carbon steels are developed using prealloyed and elemental powders to improve the mechanical properties of powder metallurgy and powder forged parts. The research focuses on the mechanism of workability and corrosion studies on sintered preforms of Alloy 1 (ATOMET4601 + 0.35%C) and Alloy 2 (ATOMET4601‐0.35%C‐0.25%Mn‐0.1%Si‐0.9%Cr). Sintered preforms of relative densities of 81%, 84%, and 90% were used for the present work. The preforms with 84% relative density have been used to study the formability parameters. It is observed from the experimental study that the Alloy 2 preforms with the addition of alloying elements have undergone lesser densification and deformation due to the work hardening mechanism. Corrosion studies have been carried out by conducting aqueous immersion and electrochemical corrosion tests on these two alloys using 18% HCl solution at different timings. It is found that the Alloy 2 has exhibited a better corrosion resistance than the Alloy 1 due to the addition of various alloying elements. It is also observed that the corrosion rate has decreased with an increase in densification irrespective of the alloys. The microstructures, scanning electron microscopy, and X‐ray diffraction of corroded surfaces have been corroborated with densification and the corrosion behavior of alloys.

SUMMARYThe corrosion protection performance of selective area electroplated cadmium on mild steel has been assessed as a function of the depositing anode speed and current density, using salt spray, constant immersion, and electrochemical corrosion tests. The results indicated that the cadmium coatings provided very good corrosion protection performance without the standard Chromate conversion coating, providing the anode speed was maintained below 0.2 m s1. but not at very low speeds and extremes in the permitted deposition current density range. A good inverse correlation was found between values of corrosion current obtained from the electrochemical tests, and constant immersion corrosion rates for the cadmium coatings, with high currents corresponding to low dissolution rates. This effect is thought to be associated with the porosity of the coatings. A mechanism of corrosion protection by the cadmium based on the formation of a bi-polar precipitate of corrosion product consisting of cadmium chloride hydroxide and cadmium carbonate has been proposed.

In the present work the corrosion behavior of both aircraft aluminum alloy 2024 and 6061 were studied by cyclic polarization test in Rainwater, before and after heat treatment at room temperature (25 °C). The corrosion resistance of both alloys decreases after these alloys were solution treatment at a temperature of 495 °C for 2hr for AA2024 alloy and artificial aging at temperature (150, 200, 250, and 300 °C) for 1 and 2 hours this decreasing was from 1.767× 10−3 to 1.031× 10−3 mm/y, and for solution treatment at a temperature of 530 °C for 2hr for AA6061 alloy and artificial aging at temperature (150, 200, 250 and 300 °C) for 1 and 2 hours this decreasing 6.279× 10−3 to 1.204× 10−3 mm/y.

Friction stir welding (FSW) is being considered as a potential repair welding technique for nuclear spent fuel dry storage canisters. The heat affected zones (HAZ) of the fusion welded regions of the canisters become prone to stress corrosion cracking (SCC) due to chlorides in the environment. Pitting corrosion is usually observed in the welded regions which is the precursor for SCC. The goal of the present work is to understand the effect of chloride on the atmospheric corrosion behavior of FSW 304L stainless steel. A simulated crack was introduced into hot rolled and annealed 304L plates by electric discharge machining, and FSW was performed at two different temperatures, 725 and 825 °C in order to repair the simulated crack. Figure 1(a)-(c) shows the optical images of the 825 °C FSWed 304L alloy. Electrochemical corrosion tests are being performed on FSWed 304L alloy in 3.5 wt% NaCl aqueous solution at 20 OC as a thin-layer electrolyte. A comparison will be made between the corrosion behaviors of the base metal, HAZ region and the weld nugget region for both FSWed specimen types. The electrochemical tests are carried out in an in-house designed electrochemical cell with less than 10 ml of electrolyte and variable electrolyte thicknesses in the range of 0.5 – 2 mm. Open circuit potential, cyclic polarization and potentiostatic tests at different passivation potentials are being conducted to understand the pit initiation and growth events as a function of FSW microstructures. Electrochemical impedance spectroscopy and Mott-Schottky analysis are carried out to understand the semiconducting properties of the passive film and their correlation with the passive film breakdown. This presentation will discuss the effect of FSW microstructures on the passivity breakdown in thin electrolytes containing chloride ions, and the results will be compared with that of tests conducted in the conventional bulk electrolyte arrangement. Figure 1

Spent nuclear fuels are stored in stainless steel canisters. The containers are cooled naturally by a convective airflow through the annulus between the overpack and the canister. This air flow draws dust into the overpacks, and the surfaces of containers are covered with dust. Salts carried by the dust will deliquesce as heat generated by radioactive decay declines over time. The deliquescence of salt deposit could induce various forms of corrosion attack such as pitting and chloride-induced stress corrosion cracking (SCC). In this study, the corrosion behavior of 304L SS - a typical dry storage container material, is evaluated by simulating the deliquescence of salt deposit using a thin electrolyte test condition. Electrochemical corrosion test was performed on 304L Stainless Steel in 3.5 wt% NaCl solution at 20 OC. The tests were carried out in a newly fabricated electrochemical cell with exposed surface area of test specimen being 1 cm2. The cell has a movable PTFE wall which houses a Pt foil as a counter electrode. The adjustable wall enables the thickness of electrolyte to be varied between the SS specimen and the counter electrode. The thickness of the electrolyte was maintained at 1 mm in all the tests. This resulted in the volume of the electrolyte to be 10 ml. Open circuit potential, cyclic polarization and potentiostatic tests at different passivation potential were carried out. The potentiostatic passivation tests were followed by electrochemical impedance spectroscopy and Mott-Schotky analysis in order to understand the initiation of localized corrosion. For comparison, the electrochemical tests are being performed by using bulk electrolytes. The electrochemical polarization behavior in the thin film electrolyte was more or less similar to that in the bulk electrolyte. However, the pitting protection was shifted in the negative direction by about 80 mV in the thin electrolyte as compared to that of bulk electrolyte. Potentiostatic tests conducted at potentials below the pitting protection potential did not show pit initiation. Relatively a quick pit initiation event was observed when the applied potential was 60 mV anodic to the pitting protection potential. Bulk electrolytes showed no passivity breakdown at this potential. The results will be analyzed based on the EIS data, semiconducting properties of the passive film, and UV-Vis absorption of the test electrolytes.

A gradient layer of Al/Cr diamond-like carbon film has been prepared by magnetron sputtering on 2024 Al-alloy. Emphasis has been placed on the anti-corrosion and tribological properties of the Cr-doped diamond-like film. Experimental research: scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectrometry, nano-indentation testing, potentiodynamic polarization, electrochemical impedance spectroscopy and tribological tests were undertaken to investigate the surface morphology, surface structures, chemical composition, anti-corrosion and mechanical properties of the as-prepared gradient diamond-like carbon film. The results indicated that the Cr-doped diamond-like carbon film with intermediate transition chromium layer showed improvements with regards to hardness and corrosion prevention due to the dense surface structure and decreased electrical conductivity. Cr-doped diamond-like carbon film also displayed excellent tribological properties such as a low friction coefficient and wear rate during tribological testing.

Influence of Sn addition on microstructure and corrosion resistance of AS21 magnesium alloy