Arc spray coatings using inert gases

TWI Bulletin, November/December 1992

Since 1966 when Amin joined TWI he has earned a reputation as an authority on synergic control in MIG welding. The results of his work are incorporated in MIG welding power sources the world over.

He has also studied the arc and metal transfer behaviour of various materials used in the MIG process and has originated a patent for a shielding gas mixture for optimum weld quality. More recently his attention has been directed to the arc spray process and TWI's arc spraying equipment.

The environment in which arc sprayed coatings are applied radically affects the porosity and quality of the coating. Mohammed Amin examines effects of using different gases in the process.

Arc spraying is used for various industrial applications to deposit a metallic coating on a substrate for corrosion and abrasion protection. The effectiveness of the coating is determined in part by the level of porosity produced in the coating, and by the bond strength between the coating and substrate.

Air atomisation is commonly used in this process. The main advantage is that it can be made available anywhere most easily and economically. In addition, the oxide content of the coatings is increased through oxidation of the molten wire material, which increases the coating hardness, so that the abrasion and wear resistance of the coatings is improved.

However, the oxide content could be detrimental to coating quality because in general it reduces the bond strength. Coatings with low bond strength can break or flake off prematurely in service. Therefore, the arc spray process is normally restricted to those applications where unpredictable coating failures do not cause serious damage, and where failed coatings can be replaced easily.

Furthermore, the coatings produced with air atomisation often contain a high level of porosity. Porosity is detrimental because it allows corrosive fluids to permeate through the coating and thus reduces its ability to protect the substrate.

Another disadvantage of air atomisation is that a burnoff of the alloying elements contained in the parent wires occurs. These elements are essential ingredients to produce the required coating characteristics. As a result, a coating with the specified characteristics cannot be produced reliably.

With air atomisation, the coating quality achieved is generally considered adequate for most of the common applications. However, specialised applications require coatings of higher quality. To produce such coatings, inert gas atomisation has been used by various investigators to study its effects on the coating characteristics.

In inert gas atomisation the oxide content, porosity level and burnoff of the alloying elements can be reduced by a factor of 2-4 compared with those obtained using air atomisation. Therefore, the consistency, reliability and bond strength of the coatings can be improved considerably. However, the costs of inert gas atomisation are high, and can be justified only for specialised applications.

Argon and nitrogen atomisation

Inert gases, mainly argon and nitrogen, have been used in atmospheric as well as in chamber operation to improve coating quality. ^[1-4]

Atomising gas/particle interaction

An atomising gas exerts its influence on coating quality through gas/spray particle interactions, such as oxidation and burnoff of alloy elements contained in the parent wires. Control of these interactions is the key to control of coating quality. These interactions, however, are complex and have not been formulated, because the particle characteristics, such as size, temperature and velocity, vary continuously in the spray process. ^[5-6] A great deal of effort would be required to formulate the atomising gas/particle interactions, so that the effects of the gas can be related explicitly to the coating quality.

Microstructure and oxide content

Typical microstructures produced with four operation (atmospheric or chamber)/atomisation gas combinations are shown in Fig.1 ^[1] . For atmospheric operation/air atomisation, which produced the maximum oxide content, the microstructure is distinctively layered with oxide located mainly at the interfaces between solidified droplet splats ( Fig.1a). The oxide boundaries are not so obvious for the other three combinations.

Fig.1. Typical micrographs showing effects of atomising gas on microstructure of coatings deposited on low carbon substrate, using 18-8 stainless steel wires, 160A arc current, 28V arc voltage, 0.6MPa atomising gas pressure and 250mm standoff distance;

Fig.1a) Atmospheric operation/air atomisation

Fig.1b) Atmospheric operation/nitrogen atomisation

Fig.1c) Chamber operation/nitrogen atomisation

Fig.1d) Chamber operation/argon atomisation

Table 1 Chemical composition of parent 18-8 stainless steel wires and coatings made with different operation/atomising gas combinations, using 160A arc current, 28V arc voltage, 0.6MPa atomising gas pressure and 250mm standoff distance

The oxide content of various alloy steel coatings deposited using air atomisation is substantial, varying between 15 and 30 vol%. ^[1-3] The oxide content can be greatly reduced with inert gas atomisation. ^[1,2]

Typically, for 18-8 stainless steel wires, least oxidation was produced in chamber operation/argon atomisation. Then the oxide content increased for the other combinations in the order chamber operation/nitrogen atomisation, atmospheric operation/nitrogen atomisation and atmospheric operation/air atomisation. ^[1] The oxygen contents of the coatings, together with that of the parent wires, are given in Table 1.

For a Ni-45Cr alloy steel coating produced in atmospheric operation, the oxide content was 17.7% with air atomisation, 9.4% with nitrogen atomisation and 5.9% with argon atomisation. For type 420 stainless steel coatings the oxide content was 19.6% with air atomisation, 9.3% with nitrogen atomisation and 7.6% with argon atomisation. ^[2]

For a high carbon steel coating produced in atmospheric operation the oxide content was found to be 27% with air atomisation and it was reduced to 17% with nitrogen atomisation. ^[3]

That is, for alloy steel coatings, the oxide content can be reduced to about one half with nitrogen atomisation, to one third with argon atomisation and to one quarter with chamber operation/argon atomisation.

Effects of process parameters

Effects of an atomising gas do not remain constant for a variation in a process parameter. For example, effects of arc current on the oxide content of a coating, both for air and nitrogen atomisation are shown in Table 2 ^[4] for spraying 18-8 stainless steel. For air atomisation, the oxide content was reduced from 42.9 to 17.9 vol% when the arc current was varied from 50 to 350A. For nitrogen atomisation, the oxide content varied between 6.6 and 11.8 vol%. Similar effects of variation in the process parameters, would occur on the other coating characteristics, such as porosity, surface roughness, bond strength and hardness.

Furthermore, the degree of the effects may vary from one material to another. Therefore, complete information on the effects of an atomising gas on various characteristics of a coating/substrate material combination must be obtained experimentally and related with the ranges of the process parameters. Then several such relationships can provide objective and rational approaches to spray conditions to deposit coatings with optimum quality.

Table 2 Dependence of oxide content in type 420 stainless steel coating on arc current for both air and nitrogen atomisation, using 0.345MPa atomising gas pressure and 125mm standoff distance

Alloy element burnoff

Burnoff of the alloy elements by oxidation must be minimised, so that these constituents transfer to the coatings to improve their wear or corrosion resistance. Generally, the greater the heat input from the arc, the greater is the loss. ^[3] Furthermore, burnoff is greater with air atomisation, compared with that obtained with inert gas atomisation. Typically, in reclamation of worn crankshafts at a minimum rate of heat input, the carbon loss was 42% with air atomisation, but only 23% with nitrogen atomisation. Consequently, the service life of the shafts could be greatly prolonged.

Porosity

Coating porosity is greatly reduced with inert gas atomisation. With nitrogen, in deposition of a high carbon steel coating, the porosity could be halved compared with that formed in air atomisation, from 7.0 to 3.6%. ^[3] With chamber operation/argon atomisation, the coatings produced have been found to be essentially free from porosity. ^[1] Therefore, to deposit coatings for corrosion resistance, particularly for critical applications such as chemical plant and pressure vessels, inert gas atomisation should be used.

Nitrogen content

Inert gas atomisation, whether nitrogen or argon, has been found to decrease not only the oxide content, alloy element burnoff and porosity but also the nitrogen content in the coatings. ^[1,2] Typically, as shown in Table 1, the nitrogen content of the 18-8 stainless steel coatings was minimum (0.0295wt%), even less than that of the parent wires (0.0374wt%), for chamber operation/argon atomisation, which excluded oxygen from the coating operation. By contrast, the nitrogen content was maximum (0.169wt%) for atmospheric operation/air atomisation, even greater than that for atmospheric operation/nitrogen atomisation which excluded oxygen.

This is because atomic nitrogen is produced in the presence of oxygen, and diffuses in the metal more easily than the molecular form. That is, the presence of oxygen promotes absorption of nitrogen in the molten metal, and therefore must be reduced or eliminated.

Hardness

A higher hardness is taken to imply a higher wear resistance. Therefore, a detrimental effect of inert gas atomisation is a substantial reduction in coating hardness because of a reduction in oxide content. ^[1] Typically, with argon atomisation for 18-8 stainless steel coatings, the hardness could be halved compared with that produced with air atomisation, (from HV1- 426 for air to HV1= 247 for argon). However, the coatings produced with inert gas atomisation contain less porosity and have a more consistent structure. At present, the combined effect of hardness, porosity and structural consistency on wear resistance of a coating is not well known and needs to be determined.

Bond strength

In general, the smaller the oxide content, the greater is the coating bond strength, because oxides are weak under tensile stress. ^[1] Therefore, the bond strength could be improved with inert gas atomisation. For example, in deposition of high carbon steel coatings with nitrogen atomisation, which contained 17% oxide, the bond strength was 3500 kg/cm ². Whereas the bond strength of the coating deposited with air atomisation, containing 27% oxide, was greatly reduced, ^[3] to 2371 kg/cm ². For inert gas atomisation, bond strength is increased because the coating failure mechanism comprises the combined process of brittle cracking and ductile inhibition, instead of only brittle fracture at the oxide boundaries between the solidified particles when using air atomisation ( Fig.2). ^[1]

Fig.2. Effect of atomising gas on the fracture of coatings deposited on low carbon steel substrate, using 18-8 stainless steel wires, 160A arc current, 28V arc voltage, 0.6MPa atomising gas pressure and 250mm standoff distance:

Fig.2a) Atmospheric operation/air atomisation

Fig.2b) Chamber operation/nitrogen atomisation

Coating efficiency

With inert gas atomisation, metal deposition efficiency has been found to increase by about 4-9% for aluminium coatings on low carbon steel substrate, compared with that obtained with air atomisation. ^[2] Typically, deposition efficiency was 57.9% with air, 66.8% with nitrogen and 61.9% with argon atomisation. This increase occurs because of a reduction in the oxidation and vaporisation losses, and increased adherence of the particles to the substrate. However, no data have been found for other materials such as alloy steels and stainless steels. Therefore, further work is required to determine whether inert gas atomisation offers sufficient advantage through an increase in metal deposition efficiency over air atomisation.

Fume generation

In arc spraying, fumes are produced mainly because of oxidation. With inert gas atomisation, oxidation is reduced, and therefore fume level is reduced. ^[3] Fume reduction is beneficial because it improves visibility, which is particularly useful when coating the internal surfaces of enclosed or semi-enclosed vessels. The operator can then control the critical process parameters such as standoff distance within narrow limits. This allows consistent and better quality coatings to be deposited efficiently.

Cost

For air atomisation, air is almost always supplied from a compressor, but for inert gas atomisation, nitrogen or argon must be bought as an additional item. This is supplied either in cylinders or in bulk at different tariff, see Table 3. ^[7] Use of inert gas atomisation can be justified for specialised applications when advantages are gained from improvements in coating quality and metal deposition efficiency, ^[4] and also from a higher deposition rate of the process.

Table 3 Estimated costs for nitrogen and argon atomisation for deposition of aluminium or zinc and alloy steel coatings by arc spraying ^[7]

Conclusions

Effects of using inert gas instead of air atomisation in arc spraying have been examined. With nitrogen or argon gas atomisation:

The oxide content of the coatings can be greatly reduced, by a factor of 2-4, from the level of 17-27vol% for air atomisation.

Loss of alloy elements contained in the parent wires cannot be eliminated but it can be reduced substantially, typically to about 20%, as compared with 40-50% for air atomisation.

Porosity in the coatings can be greatly reduced, typically to 3-4%, compared with 7-8% with air atomisation.

The bond strength of the coatings can be about 50% higher than that for coatings made with air atomisation.

The consistency and reliability of the coatings can be improved, compared with air atomisation.

Because of the additional cost of the gas, use of inert gas atomisation can be justified only for specialised applications. Where large bulk supplies are available, the additional spraying cost per kilogram of aluminium is about £4 for nitrogen atomisation and £24 for argon atomisation. For alloy steels, the additional cost is about £7 for nitrogen atomisation and £50 for argon atomisation. The cost of cylinder gas is about 3-10 times greater depending on the atomising gas/material combination.

References