Search Results for chemical reactions

Curing is the irreversible change in the physical properties of a thermosetting resin brought about by a chemical reaction, condensation, ring closure, or addition. This article discusses the material types and functions of various components considered in the preparation for curing. It presents a discussion on the major elements of an autoclave system, namely, pressure vessel, gas stream heating and circulation sources, gas stream pressurizing systems, loading systems, and vacuum systems. The article describes a computerized approach to the simultaneous control of materials reaction behavior and consolidation dynamics, using an autoclave as the reaction vessel.

This article focuses on wet chemical methods that have stood the test of time in laboratories around the world. It begins with a description of the appropriateness of classical wet methods. This is followed by sections on sampling procedures, basic chemical equilibria, and wet analytical chemistry. Mechanical methods and nonoxidizing acids and/or acid mixtures for dissolving solid samples for wet chemical analysis are then reviewed. Qualitative methods that are used to identify materials by wet chemical reaction are also included. The article provides information on various methods for the separation of chemical mixtures and on the types of gravimetry and titrimetry. Strategies for removing inclusions are also included to aid in their compositional understanding. The article also briefly describes the processes involved in chemical surface studies and partitioning of oxidation states. It ends by presenting some examples of the applications of classical wet methods.

This article provides information on basic chemical equilibria, wet analytical chemistry, and the appropriateness of classical wet methods. It focuses on nonoxidizing acids and oxidizing acids. The article includes information on the qualitative methods used to identify materials by wet chemical reaction. Gravimetry, in which a chemical species is weighed; titrimetry, which involves volume measurement of a liquid reactant; and a host of separation techniques, which require diverse forms of laboratory manipulation, are discussed. The article briefly describes the partitioning of oxidation states as well as those applications in surface studies and rapid material identification in which chemical techniques have proved useful.

The atmosphere within a furnace chamber is a basic factor in achieving the desired chemical reactions with metals during heat treating. This article presents the fundamentals of heat treating atmospheres, and describes two groups of atmosphere control, namely, furnace atmosphere control and supply atmosphere control. The two basic types of atmospheric supply systems are generated atmospheres and nitrogen-base atmospheres. The article provides a brief overview of the gas reactions associated with oxidation and carbon control to ensure either carburization, or to prevent decarburization. It demonstrates how the carbon potential control is achieved by controlling water vapor concentration, carbon dioxide concentration, or oxygen partial pressure. The article also describes the various devices and analyzers used to monitor sampled gas from furnace atmospheres, namely, chromatographs, oxygen probes, Orsat analyzers, infrared analyzers, dewpoint analyzers, and hot-wire analyzers. Finally, it discusses the advantages, disadvantages, and limitations of these analyzers.

Crevice corrosion involves three fundamental types of processes such as electrochemical reactions, homogeneous chemical reactions, and mass transport. This article describes the critical factors of crevice corrosion, including crevice geometry, material, environment, crevice corrosion stifling, and pitting relationships. It explains the crevice corrosion of stainless steel, nickel alloys, aluminum alloys, and titanium alloys with examples. The article reviews the types of testing methods that have been developed for differentiating and ranking the resistance of alloys toward crevice corrosion. It also presents the strategies for the prevention of crevice corrosion or lessening its effects, such as design awareness, use of inhibitors, and electrochemical control methods.

Exothermic brazing is a process that utilizes the heat produced in a solid-state chemical reaction to melt a conventional filler metal or to produce molten filler metal as a product of the reaction. This article provides the pros and cons of exothermic brazing, describes procedure of the process, and illustrates a typical arrangement for the exothermic brazing of tube. It provides information on the exothermic compounds used for brazing refractory metals and aluminum alloys.

Ceramic-matrix composites (CMCs) have ability to withstand high temperatures and have superior damage tolerance over monolithic ceramics. This article describes important processing techniques for CMCs: cold pressing, sintering, hot pressing, reaction-bonding, directed oxidation, in situ chemical reaction techniques, sol-gel techniques, pyrolysis, polymer infiltration, self-propagating high-temperature synthesis, and electrophoretic deposition. The advantages and disadvantages of each technique are highlighted to provide a comprehensive understanding of the achievements and challenges that remain in this area.

This article provides a discussion on the corrosion of industrial refractory materials and technical ceramics. These materials, which are used to minimize heat losses and provide a barrier between the vessel and its contents, are utilized in the metallurgical, chemical process, power generation, automotive, and aerospace industries. The article covers the fundamental principles of chemical corrosion of refractories and ceramics, and the use of thermodynamic calculations and kinetic models to evaluate the probability of the occurrence of corrosion-causing chemical reactions. It describes the corrosion resistance characteristics of specific classes of refractories and structural ceramics. The article also examines the prevention strategies that minimize corrosion failures of both classes of materials.

Oxyfuel gas cutting (OFC) includes a group of cutting processes that use controlled chemical reactions to remove preheated metal by rapid oxidation in a stream of pure oxygen. This article discusses the operation principles and process capabilities of the OFC. It reviews the properties and compositions of fuel types such as acetylene, natural gas, propane, propylene, and methyl-acetylene-propadiene-stabilized gas. The article describes the effects of OFC on base metal, including carbon and low-alloy steels, cast irons, and stainless steels. It provides information on light cutting, medium cutting, heavy cutting, and stack cutting. The article informs that the basic oxyfuel method can be modified to allow gas cutting of metals, such as stainless steel and most nonferrous alloys, that resist continuous oxidation.

Electroslag welding (ESW) involves high energy input relative to other welding processes, resulting generally in inferior mechanical properties and specifically in lower toughness of the heat-affected zone. Electrogas welding (EGW) is a method of gas metal or flux cored arc welding, wherein an external gas is supplied to shield the arc, and molding shoes are used to confine the molten weld metal for vertical-position welding. This article describes the fundamentals, temperature relations, consumables, metallurgical and chemical reactions, and process development of ESW. The problems, quality control, and process applications of ESW and EGW are also discussed.

The effective integration of aluminum metal-matrix composites (Al-MMCs) into useful structures and devices often requires an understanding of the weldability of Al-MMCs that includes a thorough knowledge on the effects of various interactions between matrix and reinforcement. This article provides a detailed discussion on weldability and the effect of viscosity, chemical reactions, and solidification on weldability. It discusses different welding processes, namely, gas-tungsten arc welding, gas-metal arc welding, laser-beam welding, electron-beam welding, resistance welding, friction welding, transient liquid phase bonding, and capacitor discharge welding.

Electroslag welding (ESW) and electrogas welding (EGW) are two related procedures that are used to weld thick-section materials in the vertical or near-vertical position between retaining shoes. This article discusses the fundamentals of the electroslag process in terms of heat flow conditions and metal transfer and weld pool morphology. It presents constitutive equations for welding current, voltage, and travel rate for ESW. The article describes the metallurgical and chemical reactions in terms of fusion zone compositional effects, weld metal inclusions, solidification structure, and solid-state transformations. It describes the electroslag process development and the applications of electroslag and electrogas processes. The article concludes with a discussion on weld defects, such as temper embrittlement, hydrogen cracking, and weld distortion.

Oxyfuel gas cutting (OFC) includes a group of cutting processes that use controlled chemical reactions to remove preheated metal by rapid oxidation in a stream of pure oxygen. This article provides a detailed discussion on the principles of operation and the process capabilities of OFC. In addition to providing information on the equipment used, the article describes the properties of fuel gases (acetylene, natural gas). It also presents an overview of the effect of OFC on base metal and explains the application of OFC in cutting thin, medium, and thick sections, bars, and structural and close-tolerance shapes.

The electrode potential is one of the most important parameters in the thermodynamics and kinetics of corrosion. This article discusses the fundamentals of electrode potentials and illustrates the thermodynamics of chemical equilibria by using the hydrogen potential scale and the Nernst equation. It describes galvanic cell reactions and corrosion reactions in an aqueous solution in an electrochemical cell. The article explores the most common cathodic reactions encountered in metallic corrosion in aqueous systems. The reactions included are proton reduction, water reduction, reduction of dissolved oxygen, metal ion reduction, and metal deposition. The article also presents the standard equilibrium potentials measured at 25 deg C relative to a standard hydrogen electrode for various metal-ion electrodes in a tabular form.

Chemical vapor deposition (CVD) involves the formation of a coating by the reaction of the coating substance with the substrate. Serving as an introduction to CVD, the article provides information on metals, ceramics, and diamond films formed by the CVD process. It further discusses the characteristics of different pack cementation processes, including aluminizing, siliconizing, chromizing, boronizing, and multicomponent coating.