Showing papers on "Corrosion published in 1985"

Modification of the Electrochemical and Corrosion Behavior of Stainless Steels with an Electroactive Coating

Abstract: Electroactive polyaniline coatings have been deposited on ferritic stainless steels. The coatings appear to be deposited over the passive metal oxide film but can undergo electron transfer with the metal. Polyaniline immobilized on the alloys imparts a form of anodic protection which stabilizes the materials in mineral acids. Electrochemical and SEM characterization results are presented, and effects of coating application techniques are discussed. Oscillations in the open‐circuit voltage occur in solutions containing a high enough concentration of chloride ion to initiate pitting at potentials dictated by the coating. Inhibition of localized corrosion may also be obtainable for favorable systems.

Structure and Corrosion Resistance of Plasma Nitrided Stainless Steel

Abstract: AISI316 stainless steel has been plasma nitrided at 570°C over a range of processing conditions, and the resultant corrosion properties have been investigated by a potentiodynamic polarization technique. X-ray diffraction and transmission electron microscopy studies have been used to map the process parameters under which a duplex surface compound layer of γ' phase and austenite is formed. This surface compound layer has better corrosion resistance than a plasma nitrided stainless steel surface, where the normal hardened layer consists of austenite' and chromium nitride precipitates. It has been found that the improvement in corrosion resistance is related to the presence of the γ' nitride. Furthermore; low temperature plasma nitriding at 400° C produces a nitrided layer which has a corrosion resistance equivalent to that of the original material.

Surface Composition of Stainless Steels during Anodic Dissolution and Passivation Studied by ESCA

Abstract: A molybdenum containing austenitic steel was exposed in hydrochloric acid at various potentials in the active and passive ranges of the alloys. The surface compositions were analyzed by the ESCA technique. The influence of pretreatment by ion bombardment and mechanical polishing on the passivation behavior was investigated. The passive film formed on the surface consists mainly of a mixed Fe‐Cr‐Mo oxide. The average content of Cr3+ in the oxide is about 70%. The inner layers of the oxide product consist mainly of Cr oxide. The Ni content in the oxide is low. The concentration and the chemical state of Mo is potential dependent. At low potentials in the passive range, the four‐valency state is predominant, while at high potentials Mo exists mainly in its six‐valency state. On the surface of the oxide, a layer of hydroxide is present. Chloride ions are incorporated into the passive film. The thickness of the passive film increases with the potential in the passive range from 10 to 15A. The composition of the metal phase changes during active dissolution. Thus, the alloying elements are enriched on the surface and thereby control the dissolution rate, control overpotentials, and provoke passivation of the alloy.

Titanium: The mystery metal of implant dentistry. Dental materials aspects

Abstract: A number of important points concerning titanium and its alloys have been discussed. They are summarized as follows. Ti and its alloys, particularly the alpha-beta alloys, possess mechanical properties that make them ideal implant materials. Ti and its alloys oxidize readily in air. This surface oxide is extremely stable in the physiologic environment of the body. The stability and inertness of this surface oxide layer acts to protect Ti from corrosive breakdown when used in the body. The elimination of surface irregularities and contaminants is important when preparing a metal for implantation. Titanium can be coupled with equally passive metals in the body without causing galvanic corrosion.

Effect of Reinforcement on the Pitting Behavior of Aluminum‐Base Metal Matrix Composites

Abstract: This paper examines the effect of graphite and silicon carbide reinforcements on the pitting behavior of graphite/aluminum (Gr/Al) and silicon carbide/aluminum (SiC/Al) metal matrix composites. Electrochemical corrosion tests were performed on both Gr/Al and SiC/Al composite specimens. Identical tests were completed on powder metallurgy processed aluminum and wrought aluminum of the same composition. The electrochemical behavio of the SiC/Al composites was essentially identical to that of the powder processed and wrought aluminum alloys; however, the pitting attack on the SiC/Al composites was distributed more uniformly across the surface, and the pits penetrated to significantly less depths. The presence of graphite in the Gr/Al composites did not cause an electropositive shift in corrosion potential as anticipated, but caused a substantial decrease in resistance to passive film breakdown. This effect is the predominant reason for the poor performance of Gr/Al composites in marine environments.

Zinc alkaline battery

Abstract: This invention uses as the anode active material a zinc alloy containing Ni, at least one element selected from In, Pb, Ga and Cd and, optionally further, one element selected from Al, Mg, Ca, Ba and Sr for the anode of a conventional zinc-alkaline battery which employs zinc as the anode active material, aqueous alkaline solution as the electrolyte, and manganese dioxide, silver oxide, oxygen and so forth as the cathode active material. The use of such zinc alloy permits the reduction of the amount of mercury to be used for amalgamation of the anode zinc surface which is made for the purpose of corrosion inhibition, thereby enabling the provision of a low-pollution zinc-alkaline battery.

Hot Corrosion of Sintered α‐Sic at 1000°C

Abstract: The hot corrosion of sintered alpha-SiC by thin films of Na2SO4 and Na2CO3 was studied at 1000 C in controlled gas atmospheres. Under all conditions, corrosion led to 10 to 20 times the amount of SiO2 formed in pure oxidation after a 48-h exposure. In addition, small amounts of sodium silicate formed. Melts of Na2SO4/SO3 caused uniform pitting of the SiC substrate; Na2CO3/CO2 melts caused localized pitting and grain-boundary attack. In all cases, the protective SiO2 layer dissolved to form silicate, leading to corrosion. In the sulfate case, free carbon in the SiC promotes this process. In all cases the presence of liquid films is responsible for rapid transport rates and the subsequent rapid reaction.

Cathodic Properties of Different Stainless Steels in Natural Seawater

Abstract: The cathodic properties of a number of stainless steels, which were exposed to natural seawater flowing at 0 to 2.5 m/s and polarized to potentials from −300 to −950 mV SCE, have been studied. The current density development at constant potential and the free corrosion potential during the exposure time were recorded continuously. At the end of the exposure period, after approximately 28 to 36 days of exposure, polarization curves were determined. After one to three weeks of exposure, depending on the water velocity, microbiological activity on the surface caused an increase in the current density requirement of the specimen. An explanation for the mechanism behind the current density increase caused by slime production from marine bacteria may be increased exchange current density, i0. There was no measurable calcareous deposit on the stainless steel surfaces at the end of the exposure periods.

Engineering the Surface with Boron Based Materials

Abstract: Boron, boron carbide, boron nitride, and transition metal borides have many attractive properties, including high melting point and hardness, good wear and corrosion resistance, excellent electrical conductivity, and resistance to attack by molten metals. There therefore appears to be significant scope for enhancing surface properties of metals and ceramics by applying coatings constituted from these materials. Currently, the only commercial coating processes practised have been the thermochemical diffusion techniques termed boronizing or boriding, whereby boron is diffused into, and combines with, the substrate material forming a single or double phase metal boride layer at, and adjacent to, the surface. This review not only describes the various media used for such treatments and their limitations, but attention is also given to the lesser known processes, not yet commercially practised, which provide for the deposition of a wide variety of boron based materials. Such processes, like PVD and CVD...

Progress toward Understanding the Stress Corrosion Problem

Abstract: This paper describes current understanding of the propagation of transgranular stress corrosion cracking (TGSCC) in engineering alloys. In contrast to intergranular stress corrosion cracking (IGSCC), which proceeds by preferential anodic dissolution at the crack tip, TGSCC is considered to propagate by discontinuous brittle fracture (cleavage). First, the propagation process is described, outlining current knowledge of the crystallography of the cleavage surfaces and reviewing the evidence for the discontinuous nature of cracking. An evaluation follows of the models that are currently proposed to explain how interaction with the chemical environment induces brittle fracture in normally ductile alloys, focusing, in particular, on a new approach, the film-induced cleavage model. Finally, attention is directed to step formation between the parallel, but displaced, primary cleavage facets. It is suggested that step formation, which in face-centered cubic (fcc)alloys occurs by highly localized plastic...

Corrosion of stainless steels in chloride solution: An XPS investigation of passive films

Abstract: Five commercial steels ranging from the martensitic stainless steel containing 12% chromium to the superferrite containing 29% chromium, 4% molybdenum, and 2% nickel have been studied by XPS. In addition, a pure iron-chromium alloy containing 7% chromium has been investigated. Armco iron and pure chromium (99.99%) were included as references. The formation of the passive films (or corrosion) occurred in deoxygenated 0.1 M NaCl solution (pH=5.6), from which the samples were transferred directly to the XPS chamber under controlled atmosphere (Ar). Concentration profiles (at.-%) of the alloy constituents in their oxidized and metallic states have been determined separately from the measured XPS depth profiles. Forc≳= 12% chromium the passive films have the following structure: there is a depletion of Cr in the inner region, followed by an enrichment (concentration maximum) in the central region of the films. The height of this maximum increases, and its position shifts towards the surface with increasing chromium content in the alloy. The outermost monolayers are rich in water and hydroxyl groups. Various significant properties of the films change drastically at the critical chromium concentration of about 12%. This behaviour is rather independent of the other components (Mo, Ni, Cu) present in the alloys and is discussed in terms of a phase transition in the films which is controlled by the chromium concentration.

A unified mechanism of stress corrosion and corrosion fatigue cracking

Abstract: A mechanism of stress corrosion cracking (SCC) is outlined in which anodic dissolution at film rupture sites relieves strain hardening and reduces the fracture stress at the crack tip. Experimental evidence is cited to suggest that relief of strain hardening occurs by interaction of subsurface dislocations with divacancies generated by the anodic dissolution. A transgranular crack propagates by accumulation of divacancies on prismatic planes which then separate by cleavage under plane strain conditions at the crack tip. At appropriate metallurgical and chemical conditions, anodic dissolution and/or divacancy migration may be enhanced at grain boundaries, leading to an intergranular failure mode. Evidence is also available to indicate that cyclic loading relieves strain hardening. Relief of strain hardening by combined cyclic loading and corrosion accounts for the higher incidence of corrosion fatigue cracking (CFC) without the requirement of any critical dissolved species. Data on fatigue of stainless steel at elevated temperature in both vacuum and air provide additional support for the proposed mechanism.

High-Temperature Corrosion in Coal Gasification Systems

Abstract: Extensive research has been performed over the past 10 years to evaluate the compatibility of engineering materials and model alloys in multicomponent gas environments that are relevant to various coal gasification schemes. This paper examines the available information to give insight into the development of protective scales on alloy surfaces and causes of scale breakdown. In addition, the available long-term kinetic data on sound-metal loss are examined to evaluate the effects of alloy and gas chemistry on corrosion of materials in coal gasification atmospheres.

Corrosion of metals by molten salts in heat-treatment processes

Abstract: 1. When choosing the composition of a salt for metal heat treatment, it is necessary to consider the corrosion activities of the molten salts in relation to the metal being heat treated. 2. Corrosion activity of the molten bath can be decreased by strictly following the rules required for their operation, i.e., storage and preparation (drying) of the salts and the rectifier prior to their loading. 3. Particular attention should be given to the selection of the bath (crucible) material: chemical activity of the crucible metal and of the heat-treated metal must be the same (or nearly the same), while the crucible lining material must be as chemically stable as possible in the molten salt used.

Thermal spray iron alloy powder containing molybdenum, copper and boron

Abstract: A novel iron based alloy is disclosed which is characterized by high resistance to wear and corrosion. The alloy consists essentially of 0 to 40% chromium, 1 to 40% molybdenum, 1 to 15% copper, 0.2 to 5% boron, and 0.01 to 2% carbon; the balance being incidental impurities and at least 30% iron, with the molydenum being at least 10% if the boron is greater than 2%. The alloy is preferably in the form of a powder for thermal spraying, and coatings produced thereby may have an amorphous structure.

Current and Potential Transients during Localized Corrosion of Stainless Steel

Abstract: The currents flowing from a localized corrosion site (LCS) on passive stainless steel in air saturated 0.25 M NaCl have been studied using a vibrating probe electrode. The potential behavior during applied currents was analyzed by assigning equivalent circuits to the passive surface and the localized site so that the paths of the current across the passive surface and the LCS could be calculated and compared with the measured values. The equivalent circuit of the passive surface without active localized corrosion was determined from potential transients and was found to be dependent on the prior history of the electrode. The LCS was equated to a resistance with an emf which were calculated from the LCS current and specimen potential. The variations of this resistance were attributable to changes in solution resistance adjacent to the LCS. It was found that the capacitance of the passive surface played a major role during potential transients and the initiation of pitting.

Wear improvement of surgical titanium alloys by ion implantation

Abstract: Titanium based alloys are being considered as orthopaedic implants for a variety of reasons such as excellent corrosion resistance, high fatigue strength, low modulus of elasticity, and most importantly, excellent biocompatibility. The only questionable property of these alloys is their wear resistance. While Ti–6Al–4V has recently been introduced for construction of the femoral component in knee‐joint systems, the questionable wear performance of the Ti–6Al–4V is a barrier for a more widespread acceptance of the alloy for this application. The ion implantation process as shown in this study is being considered as an effective method for improving the wear performance of titanium based alloys for orthopaedic implants. Ion beams of carbon and nitrogen have been used to implant the surface of Ti–6Al–4V hemispherical samples. The samples were tested in a pin‐on‐disk machine against ultrahigh molecular weight polyethylene (UHMWPE) with loads representative of total joint replacements. Deionized water and Ring...

Corrosion of high temprature alloys in the primary circuit helium of high temperature gas cooled reactors. – Part I: Theoretical background

Abstract: The interaction of Ni and Fe-Ni base alloys with the reactive impurities H2O, CO, H2 and CH4 in simulated cooling gas of the primary circuit of the High Temperature Gas Cooled Reactor (HTGR) causes corrosion effects that can significantly influence the mechanical properties. Apart from the formation of surface scales (oxides, carbides or mixed oxides/carbides), structural changes of the alloys are observed; depending on gas composition, gas supply rate and test temperature, carburization or decarburization can occur. In this report it is shown that an interpretation of the basic corrosion effects is possible on the basis of a modified stability diagram for chromium provided that - the kinetics of elementary gas metal reactions are incorporated in the expressions for carbon activity and oxygen partial pressure of the atmosphere and - the gradients of the potentials across the surface scales are taken into account. The interpretation allows the derivation of the corrosion behaviour of NiCr-base alloys in different HTGR helium compositions and enables the limits for the formation of protective chromia surface scales to be given. The influence of alloying elements other than chromium can be explained qualitatively. The results can be transferred to other reactive gas mixtures, which are characterized by an oxygen partial pressure near to the dissociation pressure of the scale forming oxides.

Cell corrosion reduction

Abstract: Corrosion is reduced in aqueous electrochemical cells having zinc anodes comprised of single crystal zinc particles by the addition of small amounts of a gas inhibiting surfactant, for example, an organic phosphate inhibitor such as RA600 from GAF Corp. to the cell. A synergistically lowered rate of corrosion and cell gassing is obtained even with reduction of mercury content.