Materials that are used in engineering come with their own set of peculiarities and characteristics. Indeed, finding the right material that can both deliver the required performance and be inexpensive enough to satisfy the economic side makes for a very fine balancing act.
To tackle such a challenge, we usually have to compromise. One way found to be most efficient is to focus on the integral mechanical properties of the material and make up for any lack of surface properties by using various forms of coating.
This allows us to use a comparatively cheaper material even in fairly aggressive environments and conditions. Naturally, various paints and polymer coatings first come to mind, but these coatings often don’t provide satisfactory wear resistance or thermal durability.
On the other hand, thermal spraying is comparatively more expensive to apply, but it has been shown that the initial cost is offset by the longer service life of such a coating. In addition, new environmental standards require stricter control of the volatile organic compounds (VOCs), found in some older paints, which drove the cost of the modern paints up, thus reducing this difference in the initial cost.
These metallic coatings can serve as multi-purpose protection against corrosion and mechanical wear, such as abrasion and erosion, depending on the substrate and the chosen coating. In addition, they can change the biological, thermal and electrical properties of the part in question.
Bonding Mechanism and Material Preparation
There are numerous different thermal spraying methods, but most of them are the same in principle. A wire, or powder, of coating material is melted and shot at high speeds at the surface of the substrate we wish to protect. The molten droplets collide with the substrate surface at high speeds, and subsequently splatter on impact.
These droplets, or lamelles, fill the surface valleys of the
substrate and rapidly cool, forming a strong mechanical bond with the base
material. The strength of this bond is believed to come from a combination of
effects: Diffusion bonding and the adhesive force caused by the mechanical
hooking effect – as the coating cools down, it contracts and latches onto the
irregularities in the substrate surface microstructure.
Obviously, this process is not the same as surfacing or
hardfacing, in which standard welding methods are applied to the surface of the
base material. In such a case, the weld material fuses with the surface layer
due to the melting and mixing with base material of the substrate. Additionally,
high impact velocities are not required to create the bond.
This all plays out on a micro-scale, and the size of these droplets can be anywhere between a few micrometers up to several hundred micrometers. As the coating is applied, the droplets connect between themselves to form a uniform layer, and multiple layers can be applied to a substrate, thus increasing the thickness of the coating.
Since the bond that is made is dependent on the surface geometry of the substrate, the surface roughness has to be of a certain level, and the recommended minimum surface profile is 2 mm (Rogers, F. 1997), or 3 mm (Cunningham, T. 1996).
Besides this, the surface has to be very clean (surface pits, or valleys, need to be clear of any contamination), so chemical cleaning, followed by blast cleaning is recommended (which also has a role in roughing up the surface), and the coating should be applied before an oxide layer has time to form, ideally. (Be sure to read Questioning Current Methods in Defining Proper Surface Profile (Part 1) for more information about abrasive blast cleaning and surface preparation.)
Unfortunately, even though the substrate remains in solid state throughout the process, some change in the microstructure is to be expected, due to both the impact treatment and the transfer of heat from the coating particles. Furthermore, presence of various inclusions and oxide can be present in the coating microstructure.
In addition, cleaning itself can be problematic. Grit-blasting, necessary to achieve the appropriate surface roughness, physically deforms the material and introduces residual stresses to the system, which means that the surface preparation has to be controlled to avoid any potential problems.
Overview of Thermal Spraying Methods
Thermal spraying methods differ mostly in particle temperatures and speed, and the type of heat source. Flame (gas torch), electric arc, plasma, and detonative metallization make for the most distinct groups and most of them have temperatures of around 3,000°C (5,432°F), with the notable exception of arc metallization.
On the other hand, speed differences are far greater and they go from as low as 40 m/s for old flame metallization all the way up to 1,200 m/s for the most modern high velocity oxygen fuel (HVOF) methods.
All the different methods can be divided into five groups:
Flame spraying – The heat is generated by the combustion of the gas fuel-oxygen mixture. The wire or powder is fed into the gun, where it’s melted and expelled using compressed air or inert gas. Due to both low speeds and heat, the bond is of poor quality. The spray rate is fairly low, and productivity suffers because of this. However, this is one of the cheaper methods. The flame temperature is usually around 3,100°C (°5,612F) and particle speeds are 40-80 m/s in case of consumable powder and around 150 m/s in case of consumable wire.
Arc spraying – Two consumable electrodes are bridged by a DC arc. The material melted by the arc is then propelled by compressed gas onto a substrate at speeds similar to wire flame spraying, but at significantly higher temperatures around 7,000°C (12,632°F). This spraying method exhibits higher bond strength, less porosity, and higher productivity than flame spraying. However, it is more expensive, and it produces arc light, ozone and fumes that may pose a problem in certain environments and situations.
Plasma spraying – While this method also uses DC electric arc, the principle is different. The arc is used to heat up a gas (usually argon, hydrogen, nitrogen or a mix) to extremely high temperatures where it is in a plasma state. The plasma gas is fed into a torch, along with the coating powder that melts in the high-speed gas stream. Speeds are typically in the 200-300 m/s range and they are caused by rapid expansion of the gas when heated to the temperatures of 10,000-20,000°C (highest of all spaying methods). This method is fairly expensive but provides superior coating quality and is great for ceramic coatings, which traditionally have higher melting points. Even higher quality of coatings can be achieved by using vacuum plasma spraying) or low pressure plasma spraying (LPPS), but these methods are even more expensive.
HVOF – High velocity oxygen fuel spraying is the most modern method in use, so it isn’t very surprising that it is also one of the most expensive methods. It burns a mixture of gas of liquid fuel (usually kerosene) and oxygen in a combustion chamber and the resulting exhaust gases are expelled at extremely high speeds (800-1200 m/s). This process has lowest temperatures of around 2,700°C (4,892°F), which means that the bonding with the substrate is achieved mostly by kinetic energy of the melted particles. Similar to plasma spraying, bond strength is exceptional and coatings have low porosity.
Detonation gun spraying – Unlike most, this method doesn’t use a continuous stream of particles, but rather a series of short impulses that are caused by detonations of acetylene and oxygen. The exploding fuel produces a shock wave that propels the powdered coating onto a substrate at significant speeds of around 800 m/s. This method achieves good bond strength and low porosity, but the quality falls short when compared to plasma or HVOF spraying.
Overview of the Most Common Coatings and Substrates and Their Application
Typical materials used as coatings are zinc and aluminum and their alloys, most commonly applied to various steels (very common alloys are Zn-Al 90-10 and 85-15). Since zinc is more active on the galvanic series, it also provides for better anti-corrosion properties.
Tungsten and chrome carbide coatings have also seen widespread use due to their anti-wear properties, which makes them extremely useful in protecting parts of coal transfer chutes and various earth-moving equipment. Additional elements can be added, such as nickel, to further improve properties.
One of the most common uses is to protect various structures with long service life and demanding environment requirements, such as bridges and above ground piping. While galvanization can be used to provide corrosion protection for such structures, it is limited to the size of the galvanization plant basins, and as such can only treat secondary members. Since there is no such limitation when thermal spraying is used, primary members can be treated with no significant problems.
Thermal spraying has shown great potential in protecting reclamation equipment like radial gates, partially exposed trash racks and other elements that are exposed to a fluctuating corrosive environment. A study made by the Bureau of Reclamation, Denver, Colorado, has shown that thermal spraying is more effective and cost-efficient than traditional coatings, but further examination was recommended.
During the 1980s the Connecticut Department of Transportation (ConnDOT) participated in a study sponsored by the Federal Highway Administration (FHWA) in which five bridges were chosen to be metalized to estimate and compare the effectiveness of this method to a traditional paint coating.
All five bridges were located on the Connecticut Turnpike (I-95 and I-395) in eastern Connecticut and contained rolled steel beam girders. Since these bridges were constructed in 1950s there were a number of conditions that needed to be satisfied for the field-application of thermal spraying to be economically viable (per www.metalize.net):
- Present bridge size is adequate for future travel demands
- The deck is new, newly rehabilitated or in excellent condition
- The bridge structure is in good condition
- The girders to be re-coated are in good condition
- Existing paint is in poor condition
- Steel is exposed to a harsh salt spray or industrial environment
The coating used was Zn-Al 85-15, due to its excellent anti-corrosion properties. The work has mostly been done in the field in dry weather, to mitigate effects of moisture on both the coating and substrate properties and coating-substrate interface quality. The coating surface was prepared in a standard manner.
After metallization, an additional coating layer of high-build aliphatic urethane was applied, to protect from ultraviolet (UV) light and abrasion, and increase the aesthetic quality. The estimated average unit cost for the whole system was $10.21/square foot, but this number was hiked up due to the use of the topcoat. ConnDot decided to go with this variant as it would increase the service life for additional 15-20 on top of the already solid 25-40 that was estimated for the base coating. The cost of spraying itself was $.031 per square foot.
Since this application was done almost 20 years ago, that number would be significantly lower now, due to the advancements in the technology. Final examination of the bridges was done in July of 1995, seven years after the first bridge was coated, and none of the bridges have shown any signs of corrosion or surface degradation where metallization was properly applied.
While initially more expensive, this method is cheaper than conventional paint when the cost is calculated on a per foot per year basis.
Thermal spraying is most commonly used with metallic materials, but it should be noted that this process can be used with both non-metallic coatings and substrates, mostly ceramics and silicon based substrates. (For a rundown on selecting the appropriate coating, see Anti-Corrosion Coatings for Different Service Exposures.)
Advantages and Performance of Metallization (Thermal Spraying)
Even though metallization applies molten coatings, the substrate does not heat up significantly, and subsequently, there is no deformation of the base material or weld, but there can be changes in the material's microstructure, in heat-affected areas.
By the way of comparison, galvanization is a hot process in which the substrate is submerged into molten zinc, which means that it can change the material's microstructure due to excessive heat, and we are limited by the size of vats. In theory, metallization can be done on any size structure, and it can be done in the field, given proper environmental conditions and shielding of the surface while the coating is applied.
The performance of metallization in corrosion prevention has been well recorded and proven to be one of the most effective and longest-lasting forms we have been able to apply. Because of its high durability and great corrosion protection, this has become one of the most utilized methods of protection of various structures that have a long lifespan and are in contact with a water environment, such as bridges and dams.
In short, advantages of metallization, compared to other types of coating are (per U.S. Department of the Interior Bureau of Reclamation Technical Service Center, Denver, Colorado - Technical Memorandum No. MERL-2012-14):
- No cure time. The structure can be placed in service immediately following the conclusion of the application.
- No production of volatile organic compounds (VOCs).
- Good impact resistance (compared with epoxy).
- Good UV light resistance (compare with epoxy).
- No temperature restrictions for application.
- No humidity restrictions for application.
- Potential for long service life with less downtime for coating maintenance.
Disadvantages of Metallization (Thermal Spraying)
- Not compatible with impressed current cathodic protection (ICCP) systems found on many structures such as buried pipe.
- Higher initial cost (30–40 percent).
- Metallizing heats the substrate, which may be unacceptable in certain situations. The surface temperature will be dependent on the process and parameters used.
- Fast-flowing water can, as some studies have shown, decrease coating life (Bureau of Reclamation [Reclamation], 1966).
- Service life in immersion can vary significantly depending on water chemistry and the coating material.