Environmental corrosion combined with the damage from impact will affect an enclosure’s ability to properly protect controls, causing a multitude of problems. Those problems could include catastrophic and dangerous system collapses, production downtime, personal injury, increased maintenance costs and losses in customers. This is why it has become critical for specifiers to become more knowledgeable about enclosure materials to prevent such complications.
Even though it should be simple to make a decision on the best enclosure material for the application, it can become overwhelming. The reason - all of the enclosure materials available today will meet the needs for most applications, up to a point. Nonetheless, one material will be superior when looking for long-term reliability and cost reduction.
Most control housings are typically made up of metals, thermoplastics, or composite materials. Below is a brief overview of the three main types of enclosure materials describing how they perform relative to preventing corrosion and impact resistance. The information included here will make it easier to make the right enclosure material selection.
Material Overview – Metals
Metals such as gold, platinum and palladium typically do not corrode but are rarely used for control housing due to their high cost. Most other metals will corrode at different rates depending on a variety of factors including the type of metal and the environment.
Many specifiers for enclosures select painted carbon steel for outdoor applications based on its low initial cost. Even though painted carbon steel enclosures can be a viable option and survive in many environments specifiers need to be aware these enclosures can still fail if they are exposed to a more corrosive environment. When the adhesion of paint to the carbon steel becomes compromised, the steel can rust.
This potential for failure of painted carbon steel enclosures causes many specifiers to select corrosion resistant stainless steel instead. Stainless steel is unique because the material contains about 10-1/2% chromium. Chromium, when in contact with oxygen, forms a natural barrier or film, which protects the stainless steel from corrosion allowing it to survive in harsh environments. However, specifiers need to be aware that there are cases where the stainless steel film can be compromised.
Even though stainless steels have high strength, they exhibit low toughness allowing them to dent easily. If the stainless steel enclosure becomes dented the integrity of the box is jeopardized because it will not have a flush seal. If an enclosure is not flush creates a situation where the seal loses its capability to be air and water tight. In addition, there exists the possibility for compromised security as the controls become easy to access.
Materials Overview - Plastics
Thermoplastics—such as polycarbonate, polyester, ABS and PVC—offer a degree of corrosion protection beyond painted carbon and stainless steel. The challenge for use of thermoplastics in some applications is thermoplastics are more susceptible to UV exposure and weathering degradation over time. Certain stabilizers are now added to extend the life of the thermoplastic enclosure, but ultimately the nature of the thermoplastics will yield to extended weathering.
Composite Fiberglass is not susceptible to rust or other forms of oxidation and also offer excellent chemical resistance to halogens like chlorine and fluorine. Similar to thermoplastics, composite fiberglass materials can be susceptible to exposure from the damaging effects of UV rays. Damage from UV exposure on composite materials is often call “fiber blooming”.
Thermoplastic and Composite Impact Resistance
Thermoplastics and thermosets (composites) exhibit average strength and a high level of toughness. This means they can withstand sudden impact and maintain their shape. Non-metallic materials are often the better option when the application is located in an environment where impact damage is possible. A video of a metal crescent wrench hitting a fiberglass enclosure can be viewed at this link: http://youtu.be/WNleGNtJGD0
Each of these three enclosure materials - metal, thermoplastics and composites materials will protect the controls in a variety of applications. It is important to know the environment and the application where the enclosure will be used because there are documented cases where the material selected for the application did not provide sufficient protection for the controls. Using this information along with the material knowledge presented here, engineers can specify the best material for the project, guaranteeing long-term reliability and reducing overall cost for the controls.