Corrosion Resistance of Aluminium–Silicon Hypereutectic Alloy from Scrap Metal

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.

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.

In the present study, the corrosion behavior of aluminum Al-7075 tempered (T-6 condition) alloy was evaluated by immersion testing and electrochemical testing in 1.75% and 3.5% NaCl environment at acidic, neutral and basic pH. The data obtained by both immersion tests and electrochemical corrosion tests (potentiodynamic polarization and electrochemical impedance spectroscopy tests) present that the corrosion rate of the alloy specimens is minimum for the pH=7 condition of the solution due to the formation of dense and well adherent thin protective oxide layer. Whereas the solutions with acidic and alkaline pH cause shift in the corrosion behavior of aluminum alloy to more active domains aggravated by the constant flux of acidic and alkaline ions (Cl− and OH−) in the media which anodically dissolve the Al matrix in comparison to precipitated intermetallic phases (cathodic in nature) formed due to T6 treatment. Consequently, the pitting behavior of the alloy, as observed by cyclic polarization tests, shifts to more active regions when pH of the solutions changes from neutral to alkaline environment due to localized dissolution of the matrix in alkaline environment that ingress by diffusion through the pores in the oxide film. Microscopic analysis also strengthens the results obtained by immersion corrosion testing and electrochemical corrosion testing as the study examines the corrosion behavior of this alloy under a systematic evaluation in marine environment.

Magnesium alloys have gained considerable interest as a material for automotive and aerospace applications due to its low density, high specific strength and good castability. However, an issue is their corrosion properties. This restricts their practical applications. In this work, a comparative evaluation of corrosion behavior of friction stir welded AZ61A magnesium alloy weldments was investigated by immersion and salt spray corrosion tests. Extruded Mg alloy plates of 6 mm thick of AZ61A grade were butt welded using friction stir welding (FSW) process. The specimens were exposed to immersion and salt spray environments in order to characterize their corrosion rates. In addition, an attempt was made to develop an empirical relationship to predict the corrosion rate of the FSW joints in immersion tests and salt spray corrosion tests using response surface methodology. The corrosion morphology and pit morphology observation was carried out by optical microscopy. General corrosion was more prevalent on the immersion surface. However, the pits on the salt spray surface showed larger surface areas, larger volumes, and covered more area on the micrographs as compared to the pits on the immersion surfaces, due to the pit debris that trapped chloride ions within the pits. Finally, it concludes that the FSW AZ61A weldments are suitable for immersion conditions than salt spray environments.

A novel high entropy alloy (HEA), designated as HE6, is created by combining the features of HEAs and Stellite alloys in this research. The new alloy has the equiatomic Co-Cr-Fe-Ni composition (22 at.%) with a large amount of W (19 wt.%), small amounts of C (0.96 wt.%) and Mo (3 wt.%). The bulk specimens of HE6 alloy are fabricated from the alloy powder via spark plasma sintering (SPS) and plasma transferred arc (PTA) welding processes. The microstructures of the SPS and PTA specimens are studied using a scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDX) and using X-ray diffraction (XRD). A series of material characterization tests such as hardness, wear and corrosion are performed on the bulk HE6 specimens. As the benchmark of Stellite alloy family, Stellite 6 is investigated along with HE6 for comparison. It is found that HE6 alloy has a microstructure which is similar to that of Stellite alloys where various carbides and intermetallics are embedded in a solid solution matrix, but compared to Stellite 6, the FCC solid solution of HE6 consists of multi-element structures (Co, Cr, Fe and Ni), not single FCC Co structure, and also, the carbides and intermetallics in HE6 alloy are more diverse. The hardness and dry-sliding wear tests show that HE6 alloy does not perform as well as Stellite 6. In the electrochemical and immersion corrosion tests, similar to Stellite alloys, HE6 alloy displays passivation ability by forming protective Cr-rich oxide films in 3.5% NaCl, 2% HCl and 10% H2SO4 solutions, but localized corrosion (pitting) can occur when the oxide films are broken. HE6 alloy shows less resistance to corrosion under the electrochemical impedance spectroscopy (EIS) and cyclic polarization tests than Stellite 6, but has lower corrosion rates under immersion test in 5% HCl and 10% H2SO4 solutions for the longer testing duration (72 hours), also showing nearly stable corrosion rate with testing time, which indicates better repairing ability of the oxide films.

Corrosion behavior of 2205 duplex stainless steel

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

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 .

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.

Although zinc (Zn) is one of the elements with the greatest potential for biodegradable uses, pure Zn does not have the ideal mechanical or degrading properties for orthopaedic applications. The current research aims at studying the microstructure and corrosion behaviour of pure Zn (used as a reference material) and Zn alloyed with 1.89 wt.% magnesium (Mg), both in their extruded states as well as after being coated with polymethyl methacrylate (PMMA). The grafting-from approach was used to create a PMMA covering. The “grafting-from” method entails three steps: the alkali activation of the alloys, their functionalization with an initiator of polymerization through a phosphonate-attaching group, and the surface-initiated atom transfer radical polymerisation (SI-ATRP) to grow PMMA chains. Electrochemical and immersion corrosion tests were carried out in a simulated body fluid (SBF), and both confirmed the enhanced corrosion behaviour obtained after coating. The electrochemical test revealed a decrease in the degradation rate of the alloy from 0.37 ± 0.14 mm/y to 0.22 ± 0.01 mm/y. The immersion test showed the ability of complete protection for 240 h. After 720 h of immersion, the coated alloy displays minute crevice corrosion with very trivial pitting compared to the severe localized (galvanic and pitting) corrosion type that was detected in the bare alloy.

A study on corrosion behaviour of various modes of ATIG welded plain carbon steel

Coronary artery disease (CAD) affects every fifth person in the world. The gold-standard treatment for CAD is stent implantation, however, the existing therapy is not sufficient. In recent years, titanium oxynitride (TiOxNy) coatings on bare metal stents (BMSs) attracted the attention of many researchers around the world due to their promising results and improved surface properties. However, good coating adhesion and coverage of the inner surface in stent applications is still a challenging task. Moreover, enhanced corrosion resistance and durability over a longer period under the influence of an aggressive biological environment is one of the main requirements while developing novel coatings for bare-metal stents. In this work, the titanium oxynitride (TiOxNy) coated stainless steel stents were fabricated by magnetron sputtering and the corrosion behavior of coated and uncoated stents has been studied using immersion, fluid dynamic, and electrochemical corrosion tests. For the first time, the entire stent surface has been used for quantitative corrosion tests on stents. We discuss and compare the in vitro biostability and corrosion behavior of bare stainless steel (316L) and titanium oxynitride films (TiOxNy) coated stents.TiOxNy coatings provide valuable stability to the BMS against harsh environments or conditions. The coated stents are remarkably more stable when compared with the reference uncoated stents (316L BMS), regardless of the ratio of O2 and N2. The variation of stent coating parameters is still possible to get more anticorrosive and biostable behavior; therefore, the results could provide the basis for further research.

Three kinds of ultra-low carbon IF steel with different grain sizes,and same chemical composition were prepared by different rolling and heat treat process.The relationship between grain size and atmospheric corrosion resistance of IF steel was investigated by immersion corrosion test,cyclic immersion corrosion test,AFM/SEM micro-analysis and electrochemical test. The results show that the local corrosion in grain boundary increases after immersion corrosion test,the depth of crack in grain boundary becomes deeper and the width of crack becomes wider with grain sizes of IF steel increase from 15μm to 220μm.The crack and cavity in the rust after cycle immersion corrosion test are increased and the atmospheric corrosion resistance is decreased with IF steel grain size coarsing from 15μm to 46μm.As grain size increase from 15μm to 220μm,the whole compactness of rust are increased,the rust resistance and the atmospheric corrosion resistance are increased.The effect of grain size on the corrosion current density of local grain boundary was analysed and the mechanics of corrosion was discussed.The total quantity of corrosion surface defect is decreased due to the decrease of grain boundary energy with the increase of grain size and the atmospheric corrosion resistance is increased.Meanwhile,the local corrosion near the grain boundary is increased duo to the increase of local corrosion current density with the increase of grain size and the atmospheric corrosion resistance is decreased.The atmospheric corrosion resistance is influenced by the two factors simultaneously.

The goal of the current investigation was to analyze the influence of friction stir processing (FSP) on the corrosion behavior of a magnesium alloy, AE42. The alloy was friction stir processed under a wide range of FSP conditions. The corrosion behavior of the as-cast as well as friction stir processed AE42 alloy was investigated using immersion corrosion and electrochemical corrosion tests in 3.5% sodium chloride (NaCl) solution. Electrochemical corrosion testing comprised potentiodynamic polarization, polarization resistance and corrosion potential trend (Rp/Ec), and electrochemical impedance spectroscopy (EIS). The electrochemical as well as immersion corrosion tests revealed reduced a corrosion rate for the friction stir processed AE42 alloy. The reduced corrosion rate of the FSP AE42 alloy may be attributed to the microstructural refinement resulting in grain size refinement and presence of fine in situ precipitates.

The hypoeutectic Al-10Mg2Si (wt%) composites, also denoted asAl-6.3Mg-3.7Si (wt%) alloy, were cast in a steel step mould, in which differentsolidification rates were obtained. The microstructural characterizations weremade by optical microscope and XRD. Corrosion performances were measured byimmersion and electrochemical corrosion tests in 3.5% NaCl solution. Microstructureanalysis showed that all the alloys consisted of a-Al, Chinese script type Mg2Si andneedle-like Al5FeSi phases. Increasing solidification rate resultedin a remarkable refinement of Chinese script-like eutectic Mg2Siphases. Corrosion tests revealed that increasing solidification rate improvedthe corrosion resistance of Al-10Mg2Si composites due to the moreuniform distribution of Mg2Si phases and the stabilization ofprotective oxide films on the sample surface.