Model-Based Expert System for Design and Simulation of APS Coatings

This article aims at presenting an expert system to assist the design and the simulation of 2-D shapes of alumina-titania (i.e., Al₂O₃-13 wt.% TiO₂) Atmospherically Plasma Sprayed (APS) coatings. Indeed, the expert system derives from a spray deposition mathematic model resulting from experiments. The varied processing parameters were the geometric and the kinematics parameters, mainly, such as: the relative speed gun-substrate, the spray distance, the spray angle, the relative positioning powder injector-spray gun trajectory, the number of passes and the powder feed rate. The variations of the geometry and some of the structural parameters were analyzed relatively to the aforementioned varied parameters. Thus, a large set of spray pattern parameters was designed. This set considers mostly the spray pattern geometry. All the relationships between the processing parameters and the spray pattern parameters were hence grouped in a spray deposition model. The second step of this work consisted in optimizing the robotic (i.e., spray gun) trajectory using a robotic code, which permits a realistic simulation of the spray gun speed and its inertia. Using this simulation software, a trajectory file was built. In the third step of the work, an expert system was developed by combining the spray deposition model with the trajectory. The tasks of the expert system are: (1) to assist the user in designing the coatings by selecting the processing parameters and (2) to simulate the coating shapes by integrating the gun trajectory.

Experimental investigations of mechanical and tribological behaviours for organic and inorganic coatings deposited on mild steel substrate

ABSTRACT

Surface treatment technologies are constantly evolving. Increasingly, surface functionalisation is of major concern to lighten structures, improve product performance and reduce costs. To fulfil its decorative and protective functions, it is essential that a coating adheres properly to its support. During the last few decades, many new deposition techniques have been developed and more and more tribological and corrosion resistant coatings have been made available. In this context, the present research has focused on two pneumatically sprayed epoxy/polyamide and inorganic silicate coatings which are deposited on the mild steel substrate. First, the main interest in this study is devoted to the analysis of the coating/substrate adhesion strength by using the squaring and pull-off tests. Second, the study has also focused on the analysis of friction and wear behaviours for the two systems of paint coatings under consideration by using a linear reciprocating tribometer under dry sliding condition. Experimental findings have shown that after appropriate surface preparation and degree of cleaning according to the standard ISO 8501-1, selected paint systems composed of organic or inorganic coatings effectively improve the service life of coated steel. Meanwhile, under the same sliding conditions, the wear results have revealed that the organic coatings have a higher anti-wear ability compared to the inorganic coatings. Thus, both organic and inorganic coatings play an important role in protecting the substrate against severe industrial environment. From the squaring and pull-off analyses, it has been deduced that scratch and pull-off behaviours of both coatings have been severely affected by the surface preparation, the number of protective layers and the thickness of coatings. In fact, the best adhesion, friction and wear properties are obtained for the paint system containing organic coatings where the third body is formed on the entire sliding strip in the test.     

Residual stresses in thick, nonhomogeneous coatings

Residual macrostresses in thick, nonhomogeneous planar coatings are investigated theoretically using the methods of the isotropic theory of elasticity. The dependence of the elastic properties and of the sources of residual stresses on the coordinate perpendicular to the interface is considered. The results can be applied to thick graded or sandwich coatings. A simplification of the results for the cases of homogeneous and thin coatings is shown. Some differences for coatings on cylindrical and spherical surfaces are also mentioned.     

Failure of aluminium metal spray/organic duplex coating systems on structural steel

Abstract

Individually, aluminium metal spray (AMS) and organic paints are well established as effective protective coatings for steel substrates. These coatings are also frequently used together as duplex systems where their combination should produce a synergistic effect. However in certain, mainly marine, environments premature failure of such coatings, involving early blistering of the paint, has been observed in service after 3–5 years. This work aims to understand the mechanisms associated with the early failure of the AMS+paint duplex system. Various panels were initially prepared for accelerated exposure using combinations of coatings on blast cleaned steel. Visual inspection after salt spray exposure revealed the presence of white corrosion product, which was confirmed afterwards as Al(OH)₃. Observation of the coating microstructures before and after salt spray exposure confirmed significant local corrosion of aluminium underneath the paint layer. It is suggested that failure of the duplex AMS/paint system initiates by corrosion of the porous AMS underneath the organic layer then progresses by the combined effect of volume expansion due to formation of Al(OH)₃ and cathodic blistering of the paint from the AMS surface.     

Metal Coatings on Steel at Lighthouse Beach,Lagos

Abstract

Trials are reported of zinc, lead and aluminium coatings exposed for 8·7 years at Lighthouse Beach, Lagos. Sprayed aluminium coatings gave good results. Zinc coatings applied by hot-dipping or electrodeposition were also successful but, after 8·7 years, sprayed zinc coatings were beginning to fail. Electrodeposited lead coatingscould not be recommended. The thickness of coating applied to the steel specimens was only 50–80 μm.     

Industrial News

Abstract

The use of computers to handle corrosion knowledge is beginning to attract much attention. Recent experience with a theoretically based mathematical model and an expert system has shown problems in using existing information that had not been anticipated. Both involve representing knowledge in a computer, but in different ways. The attempt to represent knowledge in a computer imposes special requirements, and information which is valid in its original context may not be adequate for the new purpose. Problems of a similar nature have been encountered in developing expert systems in other fields. These problems are discussed using a model of the relation between ‘information’, in the usual sense, and the ‘knowledge’ needed to address a problem, which helps to clarify the reasons for the problems encountered. Two factors are particularly important, the context of the type of problem being addressed and the nature of the software used.     

Towards novel ceramic base coatings for corrosive wear applications

Abstract

The present paper addresses the durability of thermally sprayed cermet coatings for application in aqueous environments with or without the influence of erosion. By reference to recent research on a wide range of thermal spray coatings, applied by oxyacetylene spraying and the high velocity oxyfuel process, it is argued that the corrosion behaviour in aqueous environments requires careful consideration when selecting a coating material. The key issues relating to the corrosion behaviour of thermal spray cermet coatings are summarised and potential methods for alleviating what are complex, and often rapidly propagating, corrosion mechanisms are discussed.     

Microstructure and wear behaviour of pulse electrodeposited alumina nanoparticle reinforced Co–W nanocomposite coatings

Abstract

The microstructure and wear behaviour of alumina nanoparticle reinforced Co–W alloy coatings have been investigated for potential replacement of hard chrome coatings. The composite coatings were pulse electrodeposited on steel substrates using a citrate bath. The effects of current density, in the range of 1–9 A dm^?2, on the particle reinforcement, phase/microstructure, microhardness, and wear properties of the coating have been studied. The coatings codeposited with current density of 5 A dm^?2 at 333 Hz pulse frequency and 33% duty cycle exhibited microhardness comparable to hard chromium coatings.     

Life extension of trivalent chromium passivation baths and enhanced product performance

Abstract

Although Cr (III) based passivation processes are mature, there is always pressure from end users to improve the performance of passivation films. The effect of incorporating ceramic nanoparticles into the film on the corrosion protection of steel components is reported for zinc coatings, with particular emphasis on cobalt-free processes. A novel technology based on selective ion-exchange to remove contaminants such as iron and copper as well as dissolved zinc from trivalent chromium baths is also described. Removal of these metals simultaneously without any appreciable reduction in chromium concentration rejuvenates the system and increases bath life significantly. This reduces not only the cost of frequent make-ups but also the load on the effluent treatment system, and imparts greater consistency to the life and appearance of the components processed. The concept can be extended to baths for chromating of aluminium and its alloys. Technical data generated from actual industrial installations are presented and discussed.     

Effect of prior conversion treatment on high temperature oxidation behaviour of ceramic coated 17Cr ferritic stainless steel

Abstract

Convers(on coatings modified by deposits of alumina or zirconia have been studied as a way of improving the resistance to thermal oxidation of a 17Cr ferritic stainless steel. Conversion coatings, characterised by high porosity, facilitate the electrochemical deposition of alumina or zirconia layers and enhance their adhesion to the substrate. The resulting coats, after heating, possess compositional gradients which are favourable to strong adhesion. Alumina coatings are more effective than zirconia coatings in suppressing thermal oxidation of the steel.     

Nanostructured coatings and nanomodified materials

Abstract

The research, development and application of nanostructured coatings and nanomodified materials are briefly introduced, including thermal sprayed ceramic and self-lubricating coatings, electroplated and electrophoresis deposited coatings, nanomodified hard metals and alloy cast irons. On the basis of these examples, it is believed that the industrial application of nanotechnology is not just a dream but becoming a reality.     

Effective conductivity measurements of silver coatings employing sapphire dielectric resonator technique

Abstract

A dielectric resonator method of measuring electrical sheet conductivity of electrolytic silver coatings is presented. The method is compared with standard methods normally used for conductivity characterisations of metal coatings. Three types of electrolytes to deposit silver layers have been used and sheet conductivity was measured for each type of coating. The results revealed different values for different types of coatings. The dielectric resonator method seems to be very useful for electrolytic metal coating characterisations.     

Tribological behavior and wear mechanisms of TiN/TiCN/TiN multilayer coatings

This work employs the PVD process to deposit coatings of single layer TiN, binary layer TiN/TiCN, multilayer TiN⇔⇔N, and sequenced TiN⇔CN⇔N multilayer coatings with variable individual TiN-layer and TiCN-layer thicknesses on tungsten carbide disks and inserts. Also investigated are the fracture mechanisms and the influence of sequence and thickness of these coatings on cylinder-on-disk, line-contact wear mode and ball-on-disk, point-contact wear mode through SRV reciprocating wear tests. Actual milling tests identify wear performance. Experimental results indicate that the coating with a total thickness of 7 Μm and layer sequence TiN/TiCN/TiN exhibits good wear resistance on SRV wear test and milling test. The thickest multilayer TiN/Ti/TiN coating, although having the highest hardness, has the worst wear resistance for all tests. No-tably zero-wear performance was observed for all coating disks under cutting fluid lubricated condition due to the transferred layers formed between the contact interface.     

Corrosion Protection of Steel Highway Bridges: I Full scale trials of paint systems

Abstract

Full scale trials of ten paint systems, applied on the plate girders of the Loudwater Viaduct, are described. The best performance during four years of exposure was given by a solvent less epoxy system, but its application requirements were stringent and may be difficult to meet in practice. For high corrosivity sites, a chlorinated rubber paint system should give a good performance with easy application, although transportation damage between fabrication shop and site may be a problem. A general conclusion is that aluminium pigment should be excluded from conventional micaceous iron oxide paints. In several instances aluminium has been leached out as aluminium salts leading to deterioration. of the paint system and rusting of the substrate.

The importance of the micro-climate to which sheltered bridge steelwork is exposed is discussed. Steel girders are not washed by rain and the combination of corrosive contaminants and long periods of wetness can be a particularly severe condition for paint coatings under a bridge deck. Further research on the characterisation of bridge sites and evaluation of priming paints has been initiated.     

Influence of heat treatment on surface morphology,hardness, wear and corrosion resistance of industrial phosphate coatings

Abstract

New phosphate black coatings for the improvement of mechanical properties on metallic objects have been developed to extend life time of machinery. The performance of phosphate black coatings was evaluated by weight gain studies, micro hardness studies, abrasive wear resistance and corrosion resistant measurement by electrochemical methods. The surface morphology of the coatings was assessed by XRD, SEM and XPS. The absorption coefficient of the coatings was evaluated by UV-visible spectrometer. Salt spray analysis was carried out to follow up the corrosion and get an idea about the performance of black coatings in automobile parts. The mechanical properties were very much improved after heat treatment of coatings at 200°C.     

A study of zinc phosphate coatings on 12·5Cr–21·0Ni stainless steel

Abstract

12·5Cr–21·0Ni stainless steel was chemically treated with zinc phosphate in order to find the most suitable phosphate solution and its operating parameters. The phosphate coatings were tested for their corrosion protection of stainless steel using three methods: the salt spray test, the humidity cabinet test and the brine immersion test. The phosphate coatings were also mechanically tested using a tensile test for determining their mechanical properties. Results clearly show that phosphate coatings with a uniform appearance and full coverage can give high corrosion protection to 12·5Cr–21·0Ni stainless steel by forming a physical barrier against the corrosive environment. The 12·5Cr–21·0Ni stainless steel after coating with zinc phosphate still retains reliable mechanical properties, thereby providing valuable applications in the engineering field.     

Statistical Process Control

Abstract

In order to improve the corrosion resistance provided by a micro-arc oxidation (MAO) coating on AZ31 magnesium alloy, a polypropylene film was prepared on its surface. Scanning electron microscopy, energy dispersive X-ray analysis and Fourier transform infrared spectroscopy were used to characterise the surfaces of the coatings. The corrosion protective performance of the coatings was evaluated by potentiodynamic polarisation curves, electrochemical impedance spectroscopy and immersion testing. The results show that the microdefects of the MAO coating can be filled by PP and the corrosion resistance of the AZ31 magnesium alloy is improved greatly.     

Anticorrosive effectiveness of coatings with reduced content of Zn pigments in comparison with zinc-rich primers

ABSTRACT

Typically, an anticorrosive coating system for long-term protection consists of a primer, one or several intermediate coats, and a topcoat. In such systems, zinc-rich primers are often used as they ensure good adhesion to the substrate and protect it from corrosion. Such coatings are very highly pigmented, which sometimes leads to losses in cohesive strength and deterioration of mechanical properties. Furthermore, Zn is known to be harmful to the environment. In this work we present Zn primers with reduced Zn content and better protective properties than traditional zinc-rich primers. The formulations of different Zn pigments in the epoxy matrix were developed with the concentration of zinc particles reduced to about 50% in the dry coating. To evaluate the distribution of pigments scanning electron microscopy has been employed. The anticorrosive properties were tested using the salt spray test and electrochemical impedance spectroscopy. The mechanical properties of the coatings were also tested.     

Protective Metal Coatings—a Review

Abstract

Metal coatings are used to combine corrosion resistance with other properties peculiarto the metal, and are not interchangeable to the extent of paints, for example. The decorative nickel plus chromium systems are omitted from this report, having being covered by Arrowsmith in 1966 and 1968.     

Alternative tribological coatings to electrodeposited hard chromium: a critical review

ABSTRACT

The traditional importance of hard chromium electroplating in surface engineering is recognised and the key features of this well-established technology are summarised. Despite the high hardness, corrosion protection and wear resistance of chromium electrodeposits, a number of alternative coating compositions and application techniques have been developed for specific applications in tribology. Environmental challenges associated with hard chromium electroplating are highlighted and the need to develop and evaluate alternative coatings is stressed. Key examples of the alternative coatings are described, including their method of application, microstructure and tribological performance in controlled service environments. Research needs requiring rapid development are highlighted. A summary is given of the most competitive coatings and those having the potential to match the performance of hard chromium in selected applications are identified.