Edward Ghali

General and localized corrosion of magnesium alloys: A critical review

Magnesium (Mg) alloys as well as experimental alloys are emerging as light structural materials for current, new, and innovative applications. This paper describes the influence of the alloying elements and the different casting processes on the microstructure and performance of these alloys and corrosion. It gives a comprehensible approach for the resistance of these alloys to general, localized and metallurgically influenced corrosion, which are the main challenges for their use. Exposure to humid air with ∼65% relative humidity during 4 days gives 100–150 nm thickness. The film is amorphous and has an oxidation rate less than 0.01 µm/y. The pH values between 8.5 and 11.5 correspond to a relatively protective oxide or hydroxide film; however above 11.5 a passive stable layer is observed. The poor corrosion resistance of many Mg alloys can be due to the internal galvanic corrosion caused by second phases or impurities. Agitation or any other means of destroying or preventing the formation of a protective film leads to increasing corrosion kinetics. The pH changes during pitting corrosion can come from two different reduction reactions: reduction of dissolved oxygen (O) and that of hydrogen (H) ions. Filiform corrosion was observed in the uncoated AZ31, while general corrosion mainly occurred in some deposition coated alloys. Crevice corrosion can probably be initiated due to the hydrolysis reaction. Exfoliation can be considered as a type of intergranular attack, and this is observed in unalloyed Mg above a critical chloride concentration.

Corrosion and protection of magnesium alloys

The oxide film on magnesium offers considerable surface protection in rural and some industrial environments and the corrosion rate lies between that of aluminum and low carbon steels. Galvanic coupling of magnesium alloys, high impurity content such as Ni, Fe, Cu and surface contamination are detrimental for corrosion resistance of magnesium alloys. Alloying elements can form secondary particles which are noble to the Mg matrix, thereby facilitating corrosion, or enrich the corrosion product thereby possibly inhibiting the corrosion rate. Bimetallic corrosion resistance can be increased by fluxless melt protection, choice of compatible alloys, insulating materials, and new high-purity alloys. Magnesium is relatively insensible to oxygen concentration.

Corrosion prevention and protection : practical solutions

Preface. Acknowledgments. PART I. 1 Introduction and Principles of Corrosion. 1.1 Impact of Corrosion. 1.2 Preliminary Aspects of Thermodynamics and Kinetics. 1.3 Nature of Corrosion Reactions. 1.4 Oxidation and High-temperature Corrosion. 1.4.1 Oxidation of Alloys. 1.5 Corrosion Prevention. 1.6 Design Factors. 1.7 Life Prediction Analysis of Materials. 1.8 Corrosion Protection. References. 2 Corrosion Testing, Detection, Monitoring and Failure Analysis. 2.1 Corrosion Testing. 2.2 Corrosion Detection and Monitoring. 2.3 Failure Analysis. References. 3 Regulations, Specifications and Safety. 3.1 Regulations and Specifications. 3.2 Safety Considerations. References. 4 Materials: Metals, Alloys, Steels and Plastics. 4.1 Cast Irons. 4.2 Carbon and Low-alloy Steels. 4.3 Stainless Steels. 4.4 Aluminum and Aluminum Alloys. 4.5 Copper and Copper Alloys. 4.6 Nickel and its Alloys. 4.7 Titanium and its Alloys. 4.8 Cobalt Alloys. 4.9 Lead and Lead Alloys. 4.10 Magnesium and Magnesium Alloys. 4.11 Zinc and Zinc Alloys. 4.12 Zirconium and its Alloys. 4.13 Tin and Tin Plate. 4.15 Polymeric Materials. References. 5 Corrosion Economics and Corrosion Management. 5.1 Corrosion Economics. 5.2 Corrosion Management. 5.3 Computer Applications. References. PART II. 6 The Forms of Corrosion. 6.1 Corrosion Reactions. 6.2 Corrosion Media. 6.3 Active and Active-Passive Corrosion Behavior. 6.4 Forms of Corrosion. 6.5 Types and Modes of Corrosion. 6.6 The Morphology of Corroded Materials. 6.7 Published Corrosion Data. References. Bibliography. 7 Practical Solutions. 7.1 Cathodic Protection of Water Mains. Bibliography. 7.2 Internal Corrosion of Aluminum Compressed Air Cylinders. Bibliography. 7.3 Some Common Failure Modes in Aircraft Structures. 7.4 Premature Failure of Tie Rods of a Suspension Bridge. 7.5 Corrosion and Lead Leaching of Domestic Hot and Cold Water Loops in a Building. References. 7.6 Cathodic Protection of Steel in Concrete. Bibliography. 7.7 Corrosion of Aluminum Components in the Glass Curtain Wall of a Building. References. 7.8 Corrosion in a Water Cooling System. 7.9 Pitting Corrosion of 90/10 Cupronickel Chiller Tubes. Bibliography. 7.10 Weld Metal Overlay: a Cost-effective Solution to High-temperature Corrosion and Wear Problems. Bibliography. 7.11 Equipment Cracking Failure Case Studies. 7.12 Failure of a Conveyor Drive Shaft. 7.13 Failure Analysis of Copper Pipe in a Sprinkler System. 7.14 Failure of Rock Bolts. References. 7.15 Failure Analysis of 316L Stainless Steel Tubing of a High-pressure Still Condenser. References. 7.16 Failure of a Landing Gear Steel Pin. Reference. 7.17 Hydrogen-induced Cracking. References. 7.18 Micromechanisms of Liquid and Solid Metal-induced Embrittlement. References. 7.19 Nitrate SCC of Carbon Steel in the Heat Recovery Steam Generators of a Co-generation Plant. References. 7.20 Performance of Stainless Steel Rebar in Concrete. Bibliography. 7.21 Corrosion of an Oil Storage Tank. References. 7.22 Corrosion of a Carbon Steel Tank in a Phosphatizing Process. 7.23 Underground Corrosion of Water Pipes in Cities. 7.24 Corrosion in Drilling and Well Stimulation. References. Index.

The mechanism of phosphating of steel

Abstract A schema of the mechanism involved in the different steps during phosphating of steel is proposed. The effect of electrochemical reactivity as a function of surface finishing, metal microstructure and quantity of inclusions is discussed. Certain aspects concerning crystallization and crystalline reorganization of phosphate layers are defined. The distribution of iron and zinc in the phosphate layers formed by the treatment of steel in zinc phosphating baths was investigated.

On the interpretation of cyclic voltammograms of iron electrode in alkaline solution at elevated temperatures

Abstract Cyclic voltammetry of iron electrode in alkaline solution (1 M NaOH) and at elevated temperatures (55–95°C) was performed. The increase in temperature simplifies the voltammograms and makes them invariant in the cyclization time. Three anodic processes were identified: active—passive transition of the metal, the oxidation of the initially formed passive layer and the field-assisted oxide growth controlled by the high-field mechanism. The active dissolution occurs via transport of ions through a spontaneously formed iron oxide (hydroxide) film. The concept of the thin layer voltammetry was employed to interpret the data obtained for the oxidation process of the passive film.

Leaching of gold from a chalcopyrite concentrate by thiourea

Abstract A gold containing chalcopyrite concentrate has been treated with thiourea using H 2 SO 4 . The effects of some leach parameters such as thiourea concentration, oxidizing agents, redox potential, pH, temperature, pulp density, acid pretreatment and SO 2 addition have been investigated. Maximum gold and silver extractions are respectively 95.5% and 85.4%. Hydrogen peroxide was found to be an appropriate oxidant and successive leaches were performed to handle a 60% S/L pulp. The consumption of thiourea is estimated to be 5–7 kg/t of concentrate. The value of the redox potential is an important factor that i influences thiourea consumption and gold extraction. Acid pretreatment and SO 2 addition were useful in lowering thiourea consumption. Pretreatment also reduces the SO 2 requirement of the system and improves silver extraction.

Testing of general and localized corrosion of magnesium alloys: A critical review

The degradation of materials generally occurs via corrosion, fatigue, and wear. Once a magnesium (Mg) alloy is chosen for a certain application, corrosion testing is generally required as a function of the expected service environment, the type of corrosion expected in service, and the type of surface protection, depending on the material and its use in the intended surface. In the absence of appropriate standards for the testing of magnesium alloys, a brief summary of the various procedures of accelerated electrochemical and corrosion testing of Mg alloys that have been adopted by different schools is given, accompanied by some critical comments for future work. Hydroxide, hydroxide-chloride, and corrosive water formulated according to American Society for Testing Materials (ASTM) standard 1384-96 are considered to evaluate general corrosion, localized corrosion, and corrosion influenced by metallurgical parameters. The influence of agitation, oxygenation, pH, and temperature are discussed. Surface cleaning, superficial microstructure, and surface preparation for testing are discussed. Appropriate electrochemical methods that can be applied to this relatively new and electrochemically active structural material are described. Corrosion potential measurements, polarization, impedance, noise electrochemistry, and surface reference electrode technique are recommended as valuable methods for evaluating the resistance of existing or experimental alloys to these types of corrosion. Corrosion kinetics and varying properties of the solution at the alloy/solution interface are examined. A critical description of the relevance and importance of these methods to corrosion testing of Mg alloys is given.

NANOCRYSTALLINE NI-MO ALLOYS AND THEIR APPLICATION IN ELECTROCATALYSIS

The structural and electrocatalytic properties of metastable Ni-Mo alloys have been investigated for the hydrogen evolution reaction in alkaline solutions. Amorphous and nanocrystalline phases have been prepared by mechanically alloying the elemental components under various milling conditions. Fcc nanocrystals are formed when the Mo concentration is smaller than 30 at. %. The nanocrystalline state becomes unstable with respect to the amorphous phase when the Mo content in the solid solution exceeds 30 at. %. The electroactive phase for the hydrogen evolution reaction in alkaline solutions is the nanocrystalline supersaturated solid solution. The presence of oxygen during the milling process improves the properties of the alloys.

Electrochemical aspects of zinc sulphide leaching by Thiobacillus ferrooxidans

Abstract It is proposed that the bioleachability of a zinc sulfide flotation concentrate is influenced by the electrochemical behavior of ZnS. The overall reaction of bio-oxidation: ZnS+ was found to involve a number of intermediate electrochemical reactions, as indicated by the cyclic voltammetric measurements using carbon paste-ZnS working electrodes. The solubilization of ZnS during leaching is solid-state diffusion controlled, as indicated by chronopotentiometric and chronoamperometric measurements. The activation energy of the solid-state-diffusion process was determined by Sands method to be 21.3 kJ.

The behaviour of iron electrode in CO2− saturated neutral electrolyte—I. Electrochemical study

Abstract The properties of iron electrode in CO2-saturated sodium chloride supporting electrolyte were investigated by cyclic voltammetry of static and rde-Ferrovac electrode in the potential region from −1.1 to −0.3 V (nhe). In addition to the reduction of H3O+ ions formed due to the protolysis of carbonic acid, the direct (activated) reduction of both H2CO3 molecules and HCO−3 ions was observed. It is proposed that catalytic, ErC′i mechanism accounts for the occurrence of the latter processes.