The microstructure of arc sprayed stainless steel 316L coatings appears mainly in bright white matrix, deteriorated layers (grey), and black pores under optical microscopy. The black pores and the chromium-depleted areas in the deteriorated layers are known as the factors for decreasing the ability of protecting substrate under corrosive environments. Results of experiments in this paper suggests, in the condition of this study besides the factors mentioned above, Fe-Cr oxides should be another factor of dominating the corrosion resistance in the coatings. It also describes that the quantity and the distributions of such oxides are great influence on the corrosion behaviors. In this study, two kinds of coatings were used, one with thick deteriorated layers and another with thin deteriorated layers, which were sprayed on mild steel substrate by air atomization and nitrogen atomization respectively. Salt spray test and salt-water dip test were carried out to investigate corrosion behavior in macro and micro view. An effect of sealing treatment on the performance of the coatings was also examined. Results of metallographic examination and image processing analysis are well supported by a detailed investigation of corrosion behaviors of individual phases.

This paper presents the results an experimental study on ferroalloy-base flux-cored wire coatings. The work conducted shows that it is possible to improve the structure and properties of coatings by adding aluminum to the ferrochromium powder charge and rare-earth elements and calcium to ferroboron. This reduces the oxygen content and porosity of coatings by a factor of 1.5-2. It also reduces residual tensile stresses in outer coating layers and improves coating-substrate adhesion strength as well as abrasive wear resistance.

Wire arc spray is a process for producing coatings and forming structures through the deposition of metal droplets. Wire arc spray has the advantage of low material coast and low power requirements when compared to other thermal spray technologies. This article elaborates on the assumptions made for using single consumable wire geometry, discusses experiments performed to test these assumptions, presents droplet generation results, and addresses required future work. Experiments revealed a critical relationship between wire polarity, wire position, and droplet beam dispersion. The article identifies a critical relationship between wire polarity, wire position, and spray pattern divergence in a single wire arc spray device.

Arc spray systems are increasingly used in the overhaul of aircraft engine components and auxiliary power units. The increasing use of arc spray over plasma for metallic coatings has created a demand for new wire approvals. The chemistry is already established as a powder and it is a matter of conversion to a wire and the arc spray process. The increasing popularity of the arc spray process is due to its superior bond strength and microstructure that exceed those of plasma. In one case, there is a two and one-half percent porosity requirement for the arc spray and up to 15% is allowed for plasma. This density approaches HVOF quality requirements. This paper will discuss some historical background of the process, what is approved and then move on to the new materials that are submitted for approval. Microstructures and bond strengths will be presented and some information about a proprietary method to solve a coating problem in the aircraft overhaul industry of long standing. The paper will also discuss new advances in arc spray systems and materials, which makes these systems amenable to replacing plasma sprayed coatings.

Transferred-arc cleaning is being investigated as a solvent-free cleaning method for various metallic substrates. With the recent increase in attention given the hazards involved in the storage, use and disposal of organic solvents, cleaning methods which promise comparable cleaning effectiveness with reduced hazards are being sought. Transferred-arc cleaning of tungsten substrates has been studied to identify the effect of processing conditions on cleaning and roughening characteristics. A Box-Behnken response surface designed experiment varying the chamber pressure, substrate standoff distance and plasma torch arc current while observing the transferred-arc voltage, current, surface cleanliness and surface roughness was performed. The results of the analysis show the effect of the various independent variables on the measured responses. Particular difficulties in roughening tungsten are due to its exceptionally high arc voltage for metal arc attachment. The results presented here provide an enhanced understanding of the arcing properties of various cathode materials. Such information is useful in obtaining the desired cleaning and/or roughening of the substrate.

Wire arc spraying has traditionally filled metallic coating needs for low end users, while higher quality coatings required the use of higher cost systems. A new high velocity wire arc spray device has been developed through a NASA SBIR project whose high particle velocity capabilities could provide high quality coatings while keeping costs well below those associated with HVOF and plasma spray approaches. In addition, this technique achieves these high velocities in an extremely short acceleration path. This new device employs a pulsed plasma as the accelerative medium for the molten droplets. This pulsed plasma is capable of accelerating the droplets from the tips of the wires up to high velocities and atomizing them to very fine size. This results in a fine microstructure in the deposit. Recent experiments using a Control-Vision system measured velocities for aluminum droplets in the range of 950-1500 m/s and stainless steel droplets in the range of 850 m/s and 925 m/s. These velocities are achieved with an acceleration distance of only 3.2 cm, thus making this process an ideal candidate for coating the interior of automotive cylinder bores and other areas where only a short acceleration region is available.

The quality and durability of coatings produced by virtually all thermal spray techniques could be improved by increasing the velocity with which coating particles impact the substrate. Additionally, better control of the chemical and thermal environment seen by the particles during flight is crucial to the quality of the coating. A high velocity thermal spray device is under development through a BMDO SBIR project which provides significantly higher impact velocity for accelerated particles than is currently available with existing thermal spray devices. This device utilizes a pulsed plasma as the accelerative medium for powders introduced into the barrel. Recent experiments using a Control-Vision diagnostic system showed that the device can accelerate stainless steel and WC-Co powders to velocities ranging from 1500 to 2200 m/s. These high velocities are accomplished without the use of combustible gases, and without the need of a vacuum chamber, while maintaining an inert atmosphere for the particles during acceleration. The high velocities corresponded well to modeling predictions, and these same models suggest that velocities as high as 3000 m/s or higher are possible.

Dual-wire arc sprayed Al/Al2O3 MMC coatings have been successfully applied to shipboard and other steel structures. However, the feed wire used is difficult to produce, restricting the application potential of the method. To avoid this difficulty, arc sprayable cored wire was developed, with Al2O3 powder as the filling material and commercial aluminum strip as the retaining sheath. Arc sprayed coatings made from cored Al/Al2O3 wire have been characterized based on bond strength, Al2O3 content, microstructure, microhardness, and wear resistance, and the results are compared with those of pure aluminium coatings.

Among the different wires used in arc spraying, copper is a material of choice in some applications. Its malleability is used to allow an easy machining procedure after spraying. This article focuses on the limitations of the oxidation of copper during arc spraying and its influence on coating process and properties. The aim of this series of experiments was to improve coating properties of copper sprayed with the electrical wire arc spraying process by substituting compressed air with nitrogen. These experiments show that coating properties, as well as electric wire arc spraying process, are strongly influenced by the gas employed as the atomising element.

It has been generally accepted that amperage and voltage are the only variables used to determine spray rate with the twin wire arc spray process. This belief has led to the common practice of defining spray rates in terms of kgs (pounds) per amperes. The "pounds to amperes" theory has been disproved with die advent of equipment capable of successfully arc spraying 4.8 mm (3/16 inch) diameter wires of zinc, aluminum, or 85/15 (zinc/aluminum). Changes in equipment design, that allowed the use of the larger diameter wires, resulted in the issue of a U.S. patent for arc spraying wires over 3.2 mm (1/8 inch) diameter. To achieve production capabilities for spraying 4.8 mm (3/16 inch) diameter wire, numerous problems needed to be solved, which led to additional patent claims.