Metallizing is a general term used to describe the process of applying a coating metal on the surface of an object. These coatings serve numerous functions, and can be classified as either finishing coatings (anti-corrosion and decorative coatings) or engineered coatings (wear resistant and thermal barrier coatings).
Metallizing is a relatively more complex process than anti-corrosive painting. During this method, the coating metal is liquefied, atomized and propelled onto the substrate. The coating metal comes in the form of a solid wire, which is heated and melted in a chamber. Millions of liquid particles are then ejected from the chamber, where they overlap and solidify on the surface to form a protective coating. Metallized surfaces, therefore, have a significantly longer life expectancy than those painted with traditional protective coatings.
What is Metallizing?
As stated previously, metallizing involves the projection of molten metal particles onto another surface. The molten metal particles are created by applying a heat source to a solid metal wire (usually aluminum or zinc) followed by an atomization/projection method. When the ejected molten particles come into contact with the substrate, they cool and form a mechanical bond on the surface. As the process continues, the particles bond with each other as the thickness of the coating increases.
Metallizing, like all coating processes, starts with substrate preparation. Cleaning methods, such as abrasive blasting, ensure that imperfections, defects and surface contaminants that may adversely affect the coating are removed. (Preparation techniques are examined in Substrate Surface Preparation for Corrosion Prevention.) Next, the appropriate wire material is melted while clean compressed air sprays the droplets onto the substrate to form the coating.
Metallizing protects metallic surfaces from corrosion by offering one or both of the following protection mechanisms:
Barrier Protection – In most steel structures, corrosion requires two essential ingredients: air and moisture. When the metal particles solidify and bond with the structure's surface, they form an impervious barrier, which prevents air and moisture from coming into contact with the substrate. For example, zinc, a common coating material, reacts with the atmosphere to form a thin layer of zinc carbonate, which is extremely durable and corrosion-resistant.
Galvanic Protection – Given that the right combination of coating and substrate materials are paired together, metallizing can also provide galvanic protection. During this protection method, the underlying steel substrate is protected by allowing the coating metal to corrode preferentially. For example, if the coated object is cut or scratched such that the substrate becomes exposed, then the metal coating will sacrifice itself by corroding first.
Metallizing is commonly used to protect objects that are too heavy or too large for hot-dip galvanizing. (Hot-dip galvanizing is discussed in the article Hot-dip vs Cold Galvanizing: What’s the Difference?)
Types of Metallizing Processes
While the general concept of metallizing was described previously, the process can be performed using a variety of techniques. Some of the most common variations of metallizing include:
Arc Spray Metallizing – Arc spraying involves melting the solid coating material using an electrical arc. The typical arc spray process consists of two electrically charged wires aligned in a V-shape. The wires then converge at a point to create an electrical arc. An air nozzle applies a jet of compressed air that atomizes the molten metal and projects it toward the substrate.
Figure 1. Illustration of the arc spray process. (source)
Flame Spray Metallizing – During the flame spray process, an oxy-fuel flame is used to melt the solid metal wire. The wire is propelled through the nozzle using a driven roller system that then feeds it to the flame. Similar to the arc spray process, an annular air nozzle applies a stream of compressed air to atomize and propel the molten material onto the workpiece.
Figure 2. Illustration of the flame spray process. (source)
Vacuum Metallizing – Vacuum metallizing involves melting the coating metal in a specially designed vacuum chamber. The resulting condensation from the boiling metal is then deposited onto the surface of the substrate. The coating metal can be vaporized using several techniques, including plasma or resistance heating.
Figure 3. Illustration of the vacuum metallizing process. (source)
(More information about these methods can be found in Using Metallization (Thermal Spraying) as Corrosion Prevention.)
Benefits of Metallizing
While metallizing and conventional metal painting may appear to serve similar functions, the former process has several desirable characteristics that make it ideal for some industrial applications. In addition to enhanced corrosion protection, some of the other benefits of metallizing include:
- Increased durability – Metallized coatings adhere more firmly to the substrate than conventional paints and coatings. The melting, atomization and subsequent cooling allows the melted coating to form a tight mechanical bond with the substrate. As a result, metallized surfaces are less likely to fail by peeling, blistering, cracking or flaking.
- Electrical conductivity – For applications where the transfer of electricity from one component to another is essential, metallized coatings can be used to impart conductivity. Metallizing can be used to transform non-conductive materials, such as plastic, into conductive elements. This process is especially useful in the manufacturing and electronics industry.
- Better resistance to weathering – Harsh outdoor environments can lessen the lifespan of conventional coatings and can increase the corrosion rate of metallic surfaces. The increased durability of metallized coatings can help preserve structures that are constantly exposed to external environments.
- Reduced maintenance costs – Metallized surfaces also do not require frequent upkeep or repainting. This significantly reduces the costs associated with maintenance and repair. Painted surfaces, on the other hand, need to be periodically repainted to preserve their finish.
- No surface distortion – The heat energy from the molten particles is relatively small compared to the size of the coated component; therefore, very little heat is imparted to the substrate. Because the temperature increase of the coated part is minimal, the risk of distortion is eliminated. In this regard, metallizing has an advantage over hot-dip galvanizing.
Disadvantages of Metallizing
Although metallizing is a desirable option for many steel structures, several crucial factors must be taken into consideration to determine whether this method is ideal for a given coating project.
Firstly, metallizing processes are not forgiving and do not respond well to marginally prepared surfaces. The bond between the metallized surface and the substrate is mostly mechanical; therefore, coatings can fail catastrophically if the surface is not adequately prepared to receive the coating. Proper cleaning and quality control processes must be in place during metallizing to ensure that the coating achieves its expected life cycle cost.
The initial cost of metallizing can also be significantly higher than other methods such as conventional painting and hot-dip galvanizing. This added expense is due mainly to the increased surface preparation requirements and skilled labor involved in its application. However, because metallizing requires less maintenance over its service life, it is considered to be more cost-effective in the long-term.
Also, while metallizing is ideal for many metallic surfaces, it is not suitable for reaching hidden corners, recesses, cavities and holes. Therefore, this coating technique may not be appropriate for certain metal shapes such as tubular steel sections.
Metals of almost any size or shape can be metallized. This process can either be completed off-site before construction begins, or it can be performed on-site after the structure has been erected. Engineers and contractors must weigh the pros and cons of metallizing to determine whether or not this technique is appropriate for their project.