Thermally-Sprayed Active Metal Coatings
for Corrosion Protection
in Marine Environments

Eric S. Lieberman*, Graduate Research Assistant
Clive R. Clayton, Associate Professor
Herbert Herman, Professor

Department of Materials Science & Engineering
State University of New York
Stony Brook, New York 11794

January, 1984

Final Report To:

NAVAL SEA SYSTEMS COMMAND:
David Taylor Naval Ship R&D Center

and

U.S. Naval Supply Center, Puget Sound

Contract Nos. N0040682C3258 and N615338M2596

*Present address: Pratt & Whitney Aircraft, Government Products Division,
West Palm Beach, Florida

Reproduction in whole or part is permitted for any purpose of the United States Government.

1.10 Statement of the Problem

It is well established that aluminum and zinc coatings can very effectively cathodically protect steel. A new zinc-based aluminum alloy (ZA) is now being investigated in hopes of obtaining the benefits of both of these coatings.

For the metallic coated systems employed in this study, galvanic potentials are established between the mild steel substrate and the coating metals. The degree of cathodic protection is indicated by these potentials.

In a previous study by Dorfman on the cabitation-erosion resistance of thermal-sprayed coatings, it was concluded that the addition of aluminum to zinc improved the cabitation resistance of a coating by lowering porosity levels relative to aluminum and lowering work hardening rates relative to zinc. ZA, while having essentially the same electrochemical activity as does pure zinc, is much stronger as a coating, providing effective resistance against cavitation-erosion in salt water.

It has been the major goal of this study to evaluate the coatings of aluminum, zinc and ZA on steel and to ultimately understand their mechanisms of protection and failure. These coatings were subjected to comprehensive corrosion tests and evaluated in terms of adhesion, electrochemical activity, porosity, surface roughness, and interfacial contamination. Metallographic evaluation was used to characterize the coatings and their failure mechanisms.

The aforementioned coatings were flame and electric-arc-sprayed. These different application techniques were compared for all coatings for all tests carried out in evaluating the coating systems.


1.20 Protective Coatings in the Marine Environment

Aluminum and zinc are commercially used as thermal sprayed coatings for corrosion protection in marine environments. Lately, attention has been directed to a zinc-based aluminum alloy (Zn-15 wt % Al) to further enhance the corrosion protection afforded by zinc and aluminum. Due to the differing methods of corrosion protection, zinc and aluminum are alloyed to simultaneously obtain the benefits of both. However, as will be discussed later, there will emerge a synergistic effect involving the constituents.
The active protection afforded by zinc depends on the quantity of free zinc that is available; in other words, the coating protection afforded by zinc is proportional to its thickness. The advantage of using zinc is its high electrochemical activity, which conveys to it sacrificial power, thus giving zinc a high throwing potential. But due to zinc's high electrochemical activity, it has a higher rate of reaction, therefore reducing its longevity as a protective coating. Because of the low melting point of zinc, 419 degrees C (786 degrees F), and due to its sensitivity to oxidation at about 225 degrees C (436 degrees F), zinc should not be used to protect surfaces having to withstand temperatures greater than 200 degrees C (392 degrees F).

Aluminum, although more noble than zinc, is effective in anodically protecting steel because of a tenacious oxide film (Al2O3) that forms on its surface. The high corrosion resistance of an aluminum coating is attributed to a barrier effect of Al2O3 film, though always present, is able to grow on the surface under the influence of an externally applied potential difference. The Al+++ ions pass through the pores of the thin Al2O3 film on the aluminum surface, and, from the direction of the solution, O-- ions can react with the Al+++ ions, forming Al2O3 at the internal surface of the film.
Typically, metal coatings protect steel by physical shielding, the barrier effect, and electrochemically. Aluminum, with good chemical resistance, is, therefore, able to provide long lasting protection to steel. The disadvantages of the aluminum coating is that it has greater porosity than does zinc, allowing the base metal to be attacked through the coating. During service, if the coating becomes mechanically damaged, corrosion may spread through the interface between the aluminum and steel substrate, giving rise to undermining. Aluminum coatings are also more susceptible to pitting corrosion.
For reasons given above, research was undertaken to examine the synergistic effects of zinc and aluminum. In a paper by Semples and Leclerq, it was reported that zinc-based alloys, consisting of 5-22 wt % aluminum, exhibit superior corrosion protection as compared with the base metals alone. Since an increase in the aluminum weight content will lead to a significantly greater aluminum volume content, an alloy of Zn-15 wt % Al was determined by these workers to be optimum. Essentially, they developed a coating which behaves similarly to zinc, but which is aided in its corrosion protection by the more inert aluminum- rich phase.
2.00 Thermal Spraying
There are many techniques used today to produce surface coatings, but probably the most versatile is thermal spraying. The thermal spraying process deposits molten materials projected at high velocities onto a suitably prepared substrate. The coating is formed by the rapid solidification and build-up of the molten particles. A schematic of what is involved in the thermal spray process is depicted in Fig. 1 [all figures available upon request].

The coating material is introduced into the spray gun, where it is heated to a molten state, atomized in the form of fine particles, 5-200 microns in size, and accelerated towards the substrate by a high velocity gas stream. As the particles strike the surface they flatten, forming platelets which conform to the surface irregularities as well as bonding to one another.

Thermal spray coatings tend to exhibit a lamellar structure. There is, in general, little risk of thermal distortion to the substrate since the particles are small, therefore, imparting little heat to the substrate. Substrate temperatures, thus, are maintained low.
Typical applications for thermal spray coatings are to improve surface wear characteristics, to build-up or restore dimensions to the surface of worn machine elements, to provide electrical conductivity and resistance, electromagnetic interference shielding, and resistance to extreme heat, oxidation and chemical environments, etc.

There are essentially three methods used to apply thermal spray coatings: i) Flame (combustion); ii) Electric Arc; iii) Plasma. The plasma spray process will not be discussed here due to its being economically and practically inappropriate for applying active metal coatings to large surface areas.

Any material that is available in wire form, powder, or rod can be melted at 2480 degrees C (4500 degrees F) or less may be flamed-sprayed. With the wire/rod flame spray gun the material is drawn into the rear of the gun by motor driven gears, where it is then pushed through a nozzle at which point melting occurs by a coaxial flame of an oxygen-gas mixture. The fuel gas is used only to melt the material. Compressed air surrounds the flame, atomizing the material as it melts at the tip of the wire and propels it onto the work piece. A cross-section of a typical wire/rod flame spray gun is shown in Fig. 2a (all figures available upon request).

In powder flame spray gun, the powder is stored in a canister mounted on top of the gun and oxygen aspirates the powder and carries it into the oxygen-fuel flame, where it is melted and carried by the compressed-air-enhanced flame onto the work piece. The average particle velocity is 100-200 ft/s (30-60 m/s). A cross section of a typical powder flame spray is shown in Fig. 3 [all figures available upon request]. Powder can also be fed by a line from a large powder feeder or can be work-back-mounted for portable operation. However, the canister method is by far the most common method. In more recent developments, combustion guns operating hypersonically through the use of H2-O2 gas mixtures are achieving coatings competitive with plasma. But such guns are not employed to deposit active metal coatings.

The electric-arc process employs two consumable electrically conductive wire electrodes which are insulated from one another. These wires are fed at controllable speeds and meet at a point. A potential difference is applied across the wires, which causes a controlled arc to occur at the intersection, at which point the wire becomes molten. A jet of compressed air at the arc zone atomizes the molten wires, projecting the atomized particles towards the substrate in the form of a high velocity spray. A cross-section of an electric arc spray gun is shown in Fig 2b [all figures available upon request]. The wire to be sprayed is fed through wire guides into the hollow electrode tips, which provide good electrical contact between the moving wires and guide these wires into the arc zone. An atomizing nozzle is used to direct the atomizing gas through the arc zone. An atomizing nozzle is used to direct the atomizing gas through the arc zone. Different atomizing heads and wire intersection angles can be used to vary the spray particle sizes. If the arc gap is lengthened by increasing the voltage, the size of the spray particles will increase. The smaller the particles, the smoother and more dense the coating will be; therefore, the voltage is kept at the lowest level consistent with good arc stability. Arc temperatures up to 5815 degrees C (10,500 degrees F) (14) result, which melt the wire efficiently and deposit particles with high heat content and fluidity.


2.10 Surface Preparation

Surface preparation is a crucial preliminary step in the thermal spray process. Poor surface preparation will lead to inferior coating properties and ultimately to failure. The cleanliness and degree of roughness of the surface are important. It is especially critical to keep the prepared surface uncontaminated from oils (i.e., equipment and body oils).

The first step in properly preparing a substrate is to clean the work-piece by degreasing it to remove lubricants and oils. The substrate is grit blasted to roughen the surface and to remove any deposits of grease, oil, and mill scale. For steel specimens grit blasting with alumina, silicone carbide, iron grit or "Black Beauty" have been found to be the simplest, most economical and effective techniques. Grit blasting effectively removes any adsorbed oxide film present on the surface. The highly deformed region resulting from the mechanical roughening of the surface enhances the ability of the surface to "react metallurgically" with the molten particles. Care must be taken to insure that dust does not settle on the substrate surface before spraying. The surface should also not contain any embedded particles from the grit. These can serve as weakening links in the adhesive bond between the coating and the substrate.

In present study, alumina grit was used to create the irregular surface and the protuberances which are responsible for the mechanical interlocking between the coating and substrate. Safai has shown that the blasted surface should be composed of a large number of deep undercuts, which will enable the coating to be anchored to the substrate. The grit size and blasting angle will affect the geometry of the protrusions, and the surface roughness will affect the strength of the bond. Use of a large grit size will result in a surface containing a small number of shallow craters. Since it is believed that the main bonding mechanism is mechanical interlocking of the solidified spray particles with the substrate, the choice of grit is important. Also, the air supply used to grit blast must be free of oil and moisture.

The next step of surface preparation involves preheating the workpiece immediately before coating. This is done to drive off any moisture from the workpiece surface, and to expand the workpiece so that the substrate cools with the coating. This minimizes residual stresses that may be created in the coating during cooling.

The effect of surface preparation on bond strengths of flame-sprayed coatings was studied by Apps, who found that the type of grit affected bond strength and the region of failure (i.e., cohesive or adhesive). A critical factor in the bond strength was found to be the angle of blasting. A 90 degree angle (i.e., normal to the surface) proved to be the best. The blasting pressure, the distance between the blasting nozzle and substrate surface, the time and speed of blasting, were found to have no major effect on the bond strength. To achieve good bond strength, surface cleanliness, blasting angle, and grit type were considered the key factors.

Barda also found that the cleanliness of the surface has a very definite influence on adhesion. He also found that arc-sprayed aluminum does not require as efficient blasting as is needed for flame-sprayed aluminum to obtain reasonable bond strengths. We are thus led to the view that surface preparation for arc-sprayed coatings is less critical than that for their flame- sprayed counterparts. This is most probably due to the electric arc's higher spraying temperature and higher impingement velocity.


2.20 Coating Structure

Thermal-sprayed coatings are frequently found to be different in composition from that of the feedstock. The effect of the atomizing air and the surrounding atmosphere can cause chemical changes, because the particles are finely divided and can undergo oxidation. The sprayed coatings are inherently porous and oxides that surround the particles can, in fact, contribute to coating hardness. Also enhancing hardness is the fact that the particles undergo rapid solidification, this effect generally leading to hardness increases due to the small grain size that is formed. Small grain size will also contribute to an increase in tensile yield strength.

The microstructures of thermal-sprayed coatings are comprised of well-bonded layered platelets, stacked one upon another and anchored to the grit-blasted substrate. Present in a sprayed coating are numerous cavities, connected and disconnected pores, micropores, particles that were partially or totally solid before impinging onto the surface, and oxide inclusions due to oxidation occurring during spraying. Figure 4 [all figures available upon request] shows some of the individual processes that occur during the formation of a typical sprayed coating. The particles which are bonded together by strong cohesive forces form non-uniform coating layers which adhere to the substrate through both mechanical and metallurgical bonding.


2.30 Bonding Mechanisms

It is generally agreed that the bond between the sprayed coating and the substrate may be mechanical, metallurgical, chemical or a combination of these. The coating materials and substrate, and process temperature during and after spraying, as well as other processing parameters, affect the bonding mechanism.

The chemical and metallurgical bonding action will tend to provide the maximum strength to the bond, although mechanical interlocking is seen as the major bonding mechanism. In fact, Laszlo has proposed that the crystal structure of the metal protuberances and their shapes are the factors which control the strength of coating-substrate bonding.

An important bonding characteristic is the cohesion of the bonding of the flattened particles to one another. This cohesiveness is usually greater than the adhesive strength between coating and substrate, as evidenced in tensile adhesion test results. The existence of an oxide layer on the periphery of aluminum particles has been proposed to act as a bonding enhancing layer among the particles.

In general, the adhesion strength is dependent upon the degree of particle deformation, which depends on the degree of melting and impingement velocity, the preparation of a substrate, the ability of the substrate material to melt or to undergo a chemical reaction or diffusional process with the deposit material, and the similarity of coefficients of thermal expansion of the substrate and deposit material.


2.40 Flame vs. Electric Arc-Spraying

The flame and electric-arc spray processes have generally been viewed to giving comparable coatings. This, in fact, is not the case. The deposited coatings will actually be somewhat different, but the choice between the two techniques has generally been based on matters of practicality, such as economics and convenience.

The electric arc process has a greater impact velocity, approximately 800-1000 ft/s (240-305 m/s) and a spraying temperature of 4000 degrees C (7230 degrees F) than that for the flame spray process, which is approximately 600-800 ft/s (180-24 m/s) and 2760 degrees C (5000 degrees F). Due to the high arc temperatures, the wire is melted and the particles are deposited with high heat content and fluidity. Substrate heating is lower for the electric-arc spray process due to the absence of a flame. The arc process does not require fuel gas or oxygen. Due to the simplicity in the operation of the electric arc process, arc-sprayed deposits are generally more consistent in microstructure and, thus, properties.

Round has found that the flame-spray particles are all of similar size, with the particles being flattened, exhibiting some particle overlap. The electric-arc-spray particles within the coating are not clearly distinct, which is most probably due to the high spray temperatures. The particles appear to have been solidified together on the substrate. The flame-sprayed coating exhibits a certain degree of overlapping of individual particles, whereas in the electric-arc-sprayed coating, more of a molten mass of particles results.

Due to the high temperatures involved in the arc process, zinc and aluminum are heated to the point where some volatilization occurs. The high temperatures reached by the particles can, thus, lead to metallurgical bonding. This can significantly raise the bond and cohesive strengths of the arc-sprayed coatings. The reason the arc-sprayed coatings tend to exhibit higher bond strengths is considered to be due to highly localized partial fusion of the substrate and interactions between molten particles and the substrate have been suggested as the mechanism for the metallurgical bond, Dallaire has suggested that more general melting occurs on the impingement of the molten particles.

The high arc temperatures melt the wires faster, enabling a spray rate which is 3 to 5 times that of the flame-sprayed process. Arc-spraying is far less critical of surface preparation than is flame-spraying. The arc-spray coatings will be more dense and have less oxide content. Deposition rates are greater and more easily controlled over a wider range.

Pseudo alloys may be deposited by the arc-spraying of two dissimilar wires. Also, an economically applied wear surface can be deposited by using an inexpensive filler welding- type material as one wire.

The flame spray process delivers good deposition rates and efficiencies and is characterized by low capital investment and relative ease and cost of application and maintenance. But, again, the coatings will exhibit lower bond strengths, greater porosity, and a greater degree of heat transmitted to the workpiece.

When comparing the two processes it is important to consider energy and cost savings. The electric-arc-spray process is the least expensive to operate. Energy cost savings result because of the elimination of fuel and oxygen requirements, the relative simplicity of operation and the easy on-off capabilities resulting in a further savings in time and fuel. Use of electrical energy to melt the material is considerably more efficient (60%) than is flame heating (15-20%). Kretzchmar states that the electric-arc- spraying process has the highest degree of utilization of thermal efficiency, which, of course, is one of the reasons that arc- spraying offers higher deposition rates.

Using high speed cinematography, Hoehle was able to photograph the melting of the tip of the wires in an electric-arc-spraying processes. For the flame process, it was found that metal accumulates on the melting wire tip first, which enlarges the melt region continuously until the streaming gases propel the particles toward the workpiece. A simultaneous separation of both finely-atomized particles and large particles was evident. Since a broad spectrum of particle size is observed, it is extremely difficult to describe the melting process. The electric-arc spray process shows evidence of different melting-off phenomena for the two oppositely charged electrodes. Rather large particles (300 - 100 microns) are propelled from the molten arc pool. The molten materials are gas atomized into fine particles which will tend to accumulate at the wire tips in irregular intervals. The large particles tend to split into many small ones upon projection toward the workpiece. Thus, large particle size distribution results.

It should be pointed out that the power source employed for the electric-arc-system is generally a silicon rectifier type constant-voltage transformer, which converts 3-phase 230- volt, 60-Hertz AC current to DC current. The open-circuit voltage is continuously adjustable over a wide range. Basically, the power source is comprised of a 3-phase main transformer, booster and auto-transformer connected to apply an adjustable low voltage to a silicon rectifier. The rectifier converts the applied AC to the direct-current needed for metalizing.

To improve the coatings deposited by the electric-arc spray method, a closed nozzle system has been developed. This design allows for the relative velocity between the particles and the compressed air to be increased by an additional radially acting air blast, as opposed to the single air blast of the open nozzle system. Closed nozzle systems have been shown to produce smoother coatings, due to improved atomization and a more focused spray, this reducing coating porosity.

Past criticisms of the electric-arc spray process have been that they produce too rough a coating. In addition, it was felt that the guns were too cumbersome for manual handling, due to heavy power cables. More recent developments, such as the closed nozzle system and modern light-weight guns, are entering the field. Today's arc gun is light in weight and capable of producing fine textured coatings comparable by changing feed wire diameter, to the extent that non-skid decks having high surface roughness, can be produced. It should, however, be noted that the electric-arc process is limited to relatively ductile electrically conductive wires.

The choice between the two competing metalization methods, electric-arc and flame- spraying, must be based on a number of complex factors, including: availability of electric power, requirement of personal expertise (i.e., the arc method is more "forgiving" from the point of view of poor workmanship), smoothness requirements, work region relative to venting of metal fumes (i.e., the arc will yield a finer particle mist than will the flame). Cost, convenience, labor, and, most importantly, technical requirements must all be weighed before a final decision is reached.


2.50 Coatings for Corrosion Protection

There are generally two types of coatings used for corrosion protection. The most commonly used method involves a barrier-type corrosion resistant coating (i.e., paint) between the steel substrate and the environment. The other type of coating protection involves an active metal coating, which protects electrochemically. These are typically metal coatings or metal-rich paints, which cathodically protect the steel substrate by acting as a sacrificial anode.

As a barrier type coating, all paint films are to a lesser or greater degree inherently permeable to moisture and oxygen; therefore, any inhibition of reaction is due to their high electrical and electrolytic resistance. If the barrier coating becomes mechanically damages or if the coating is penetrated, corrosion will occur at the point of penetration. Corrosion can then spread, uplifting the coating and eventually leading to failure.

Physical properties of paint coatings have been supplemented with provision for electrochemical activity by adding metal or metal oxide pigments. Metal-bearing paints should consist of over 90% metal to achieve adequate coverage and electrical conductivity to provide cathodic protection.

Hot-dip galvanizing is a metal coating process that is applied in liquidus form to the substrate continuously or by batch processing. It is generally used with low melting point metals. Interdiffusion occurs and forms an intermediate alloy layer which is generally composed of brittle intermetallic compounds. This intermetallic phase is necessary for good adhesion, but if present in excess may lead to brittleness and related defects, which will become apparent during subsequent fabrication processes. Control of the intermetallic compound is frequently quite difficult. Control of thickness is poor. Substrates which may be affected by heating within the molten bath are suitable. The galvanizing process is wasteful of metal, since there is no way of accurately controlling the amount of coating metal used or its distribution over the surface. In situ work generally cannot be done by the galvanized process. Parts to be coated are limited by bath size. And high strength heat treated steel (e.g. pressure vessels) cannot be galvanized. Material distortion may also occur.

Thermal-sprayed metals are much more effective in corrosion protection that are paint systems. They can be applied on site without the risk of material distortion. Unlike galvanizing, thermal-sprayed coatings are not restricted to coating bath size. Thermal- sprayed coatings, when suitably sealed, will outlast a painted coating by a factor of 4 to 8, or more, times.

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