I.M. Allam

Solid lubricants for applications at elevated temperatures

Relative motion between mating surfaces at elevated temperatures often causes substantial material degradation due to friction and wear. Conventionally, solid lubricants have been used to reduce wear damage and friction drag under extreme conditions where liquid lubricants do not function properly. The recent trend towards higher operating temperatures in advanced power generating systems, i.e. turbomachinery, gas turbines, and hot adiabatic diesel engines, has imposed severe limitations on the currently available solid lubricants. The unusually aggressive conditions in these systems phased out most conventional solid lubricants and gave impetus to the search for more efficient materials. This paper discusses the lubricating characteristics of four different groups of materials known to provide lubricity under elevated temperature conditions. These groups are polymers, laminar solids, metal fluorides and metal oxides. Polymer lubricants are efficient lubricants within the range from room temperature to about 300 °C. Laminar solids extend that range to about 450 °C. Graphite, also a laminar solid, is an exception since it can offer excellent lubricity beyond 450 °C in the form of gaseous oxidation products. Stable fluorides and metal oxides are useful lubricants between 500 and 1000 °C, though their performance is rather poor at lower temperatures.

Initial stages of atmospheric corrosion of steel in the Arabian Gulf

Abstract Energy dispersive X-ray micro-analysis, X-ray diffraction and fluorescence, Auger, X-ray photo-electron spectroscopy and Fourier transform infrared spectroscopy have been used to characterise corrosion products on carbon steel after atmospheric exposure for periods up to 12 months to an industrial environment near the west coast of the Arabian Gulf. The results indicate that atmospheric corrosion starts by the formation of small blisters at discrete locations on the metal surface, presumably the anodic sites. The blister covers are very rich in iron chlorides and contain iron oxyhydroxides, oxides, sulphates and possibly hydroxide. The formation of iron chlorides as the primary corrosion product is only limited to the early stages of blister formation due to the aggressive nature of chloride ions. Chloride formation during later stages may be partially impaired since it requires the inward transport of fresh chloride ions through the then thick rust layer. In contrast, the formation of iron sulphates at the rust-metal interface continues by the acid regeneration mechanism (which leads to the electrochemical mechanism); therefore it is less dependent on the supply of fresh sulphate ions from the surface electrolyte through the growing rust layer.

Influence of atmospheric corrosion on the mechanical properties of reinforcing steel

Abstract This paper reports the results of an investigation carried out to evaluate the mechanisms of atmospheric corrosion of reinforcing steel in arid regions, and their influence on the weight loss, strength, elongation and bendability. The results indicated that atmospheric corrosion begins as a localized attack at discrete points on the metal surface. Upon extended exposure to the atmosphere, this localized attack gradually becomes a uniform form of attack covering the entire metal surface. Further, the atmospheric corrosion did not influence the strength and ductility of the steel.

BaSO4 scale deposition on stainless steel

An experimental study was conducted using a rotating cylinder electrode apparatus to establish the effect of fluid hydrodynamics on barium sulfate scale deposition. The results indicate that the deposition rate increases with increasing Reynolds number during agitation of the solution. The analysis of the data further demonstrates a reasonable agreement with theoretical prediction of a diffusion controlled process. In addition, the scanning electron microscopic examination revealed the nucleation and growth of crystals over the existing crystals of barium sulfate on SS-316 substrate.

Corrosion behavior of pre-rusted rebars after placement in concrete

This paper presents the results of a preliminary study in which the effect of the initial rusting on the corrosion behavior of rebars embedded in concrete has been investigated. Concrete specimens were made with pre-rusted and rust free rebars of different compositions. Two concrete mixes, one with a sodium chloride content of 2 kg/m3 of concrete and the other without any sodium chloride were used. The reinforced concrete specimens, immersed in potable water and then transferred to 5% NaCl solution, were subjected to corrosion monitoring for a period of 10 months. The test results indicate that the initial rusting does not have an adverse effect on the corrosion resistance of rebars embedded in concrete.

Carburization behavior of 310 stainless steel in CH4/H2 gas mixture with trace amount of oxygen

The carburization behavior of 310 stainless steel has been studied after cyclic exposures to carburizing gas mixtures at elevated temperatures for 500 hr exposures. A thermodynamic analysis indicated that 1000 °C was an approximate critical temperature, below which the environment should result in mixed oxidizing/carburizing behavior while above this temperature, reducing-carburizing behavior should occur. The experimental results agree well with the thermodynamic analysis, at 800 °C in 2% CH4/H2 for 310SS which suffers both external oxidation, carburization, and internal carburization. At 1100 °C in 10% CH4/H2 external carburization occurs and internal carburization becomes less pronounced. Schematics are illustrated to show corrosion mechanisms in various exposure environments.

Wear Studies of Tungsten Carbide Cutting Tools Using Micro-Beam Techniques

Particle accelerator based μ-PIXE and μ-RBS techniques along with optical microscopy, SEM and EDS were employed to examine the worn faces of uncoated tungsten carbide inserts used for cutting free machining steel. The study is concentrated on the deposited manganese sulfide onto the rake and flank faces of the tool tips, and on the changes in the tool matrix composition arising from the prevailing wear mechanisms. The manganese and iron distributions on the worn faces of the tool tips showed a very good reproducibility for similar cutting conditions. Evidence of the decomposition of tungsten carbide in the crater and the diffusion and/or oxidation of the freed tungsten is presented. The strength of the μ-PIXE and μ-RBS versus conventional techniques is demonstrated by the ability of the former techniques to resolve the structure and the composition of deposited layers in three dimensions.

Performance of austenitic stainless steels in MSF desalination plant flash chambers in the arabian gulf

Abstract The corrosion behavior of AISI type 304L (304L (304L SS) and 316L (316L SS) stainless steels in a multi-stage flash (MSF) desalination plant located on the Arabian Gulf coast was investigated. The 304L and 316L stainless steel liner plates in the flash chambers of the plant developed severe localized corrosion within six years of operation. Liner plate samples retrieved from the floors of flash chambers were studied using optical and scanning electron microscopes. The study showed that both 304L and 316L SS liner plates suffered severe intergranular, pitting and crevice corrosion. Corrosion was more severe at welds and heat affected zones (HAZs). The main causes of corrosion were identified as (1) exposure of the liner plates to stagnant high chloride brine, and (2) grain growth and probably sensitization of the stainless steels at HAZ during welding and stress relieving heat treatment.

Dip process thermal barrier coatings for gas turbines

A new concept to apply zirconia‐based thermal barrier coatings on cobalt base alloys has been developed. Contrary to plasma spraying or electron beam vaporization, the new process produces a dense and highly adherent zirconia coating that resists thermal cycling and penetration by corrosive molten salts. The new method is based on thermally growing a ZrO2‐based layer from a Zr‐rich alloy, predeposited on a Y‐rich substrate by hot dipping. The coating consists of an outer ZrO2/Y2O3 layer and an inner oxide–metal composite layer next to the substrate surface. The outer oxide layer acts as a thermal barrier, while the inner layer acts as a graded seal that improves the adhesion of the coating to the substrate. Thermal cycling experiments showed that the coating has a good resistance to spallation between room temperature and 1100 °C.

A novel process for application of titanium nitride coatings

Abstract A novel coating process to apply a titanium nitride film on metallic substrates has been developed. The new process differs from all others in that the coating is thermally grown from a predeposited titanium-nickel alloy applied by hot dipping. The coating consists of an outer TiN thin film and an inner TiNi layer next to the original substrate surface. The interdiffusion of titanium from the predeposited layer and iron from the substrate across the interface provides excellent bonding between the coating and the substrate. Auger electron spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the coating morphology, composition and microstructure. The new technique is simple, capable of coating complex shapes and potentially applicable to other coatings. The coating produced is crack free, uniform in thickness, has excellent adhesion to the substrate and can be easily regrown when its life is used up by heating in pure nitrogen.