What is Corrosion?
Many different materials are used in the architectural lighting industry, and knowing the different types of corrosion and their cause can significantly improve designs and increase the life of the project.
by Francis Julien, P. Eng, Mechanical Engineer
Many different materials are used in the architectural lighting industry, and knowing the different types of corrosion and their cause can significantly improve designs and increase the life of the project. Corrosion is a chemical reaction that tends to deteriorate material properties from exposition to environmental factors. Most metals used in everyday items do not exist naturally in their finished form. They are obtained from processed ores and combined into different alloys based on the required properties. Corrosion converts the processed metal alloys into oxides, often changing the alloys' original properties or appearance. A commonly known example of corrosion is rust. Rust is an iron oxide that forms over time from iron alloys like steel that are exposed to oxygen and water. Rusting steels turn brown-red due to the color of iron oxide. Iron oxide is weaker and reduces the structural property of a rusted part. While rust is the result of corroded iron alloys, many other metals can suffer from many types of corrosion. The first step is to understand the different environments in which architectural lighting can be installed and how they can accelerate corrosion.
Types of Environments
As previously mentioned, corrosion is caused by exposure to oxygen and water. Hence, interior environments may seem corrosion-free, but this would be a misguided assumption. While interior environments are not exposed to direct water exposure, moisture in the air can affect unprotected metals. This can be observed in areas with higher relative humidity, like bathrooms, kitchens, indoor pools, etc.
Cities / Industrial Environment Exterior
Exterior environments are exposed to water from rain or snow throughout the year. They are also exposed to condensation from variations in temperature and humidity throughout the day. Continuous exposure to water and snow accelerates the corrosion rate. De-icing salts used in the winter are corrosive for many metals. Other chemicals also impact metals' resistance in industrial environments or cities; for example, highly industrial areas can also increase the corrosion rate by exposing metals to corrosive chemicals like sulphur generated by factories.
Coastal and Marine Environments
Areas located near oceans are very corrosive environments. Sea spray and fog spread salt particles in the air. While some installations like bridges are highly exposed to waves and sea spray, buildings a few miles off the shore will also be affected by salt particles carried through the wind. Improperly protected metals exposed to coastal or marine environments can rapidly show intense corrosion levels.
It is crucial while designing products to consider the type of environment the product will be exposed to. This can impact the type of material selected, coatings, and testing requirements.
Types of Corrosion
Pitting Corrosion
Pitting Corrosion is an extremely localized corrosion that happens in spots on the surface from a damaged protective layer. This is a commonly known phenomenon in stainless steel but can also occur in other metals like aluminum. This can stay passive or spread to more extensive sections from the damaged layer. Pitting corrosion creates holes in the parts where stress is increased and can lead to failure of the part.
Pitting Corrosion Example:
Rust
Rust is the corrosion of iron alloys that turns into a red iron oxide. This oxide is brittle and can break off the iron alloy, exposing more iron to rust. Rust can easily spread through all the iron alloy and, over time, significantly weaken the part. If rust stays on the surface layer in less corrosive environments, its effect on structural properties is minimal. Surface layer rust can be removed by sanding, but rust will eventually reappear on the cleaned layer.
Rust Example:
Aluminum Corrosion
Aluminum Corrosion is noticeable by a grey powdery surface. This reaction is greatly accelerated when aluminum is in contact with salt water or enclosed spaces with high humidity levels. The deterioration of the natural protective oxide layer causes the grey powdery surface. This powder is brittle and exposes more raw aluminum underneath. It can also affect paint coatings by spreading underneath and causing the paint to peel off.
Aluminum Corrosion Example:
Galvanic Corrosion
Galvanic Corrosion is a type of corrosion that happens between dissimilar metals that are in contact with each other. For this reaction to occur, there needs to be direct contact between dissimilar materials and the presence of electrolytes (Saline water or other electrically conductive liquids). Different metals are classified on a galvanic scale from least noble (Anode) to most noble (Cathode). Noble metals are metals that are more chemically stable and less reactive to corrosion. When two metals far from each other on the galvanic scale are in contact, galvanic corrosion has a high potential. The anode will deteriorate while the cathode is protected. The anode will corrode significantly faster in contact with a cathode than alone in the same environment. The best way to prevent galvanic corrosion is to avoid mixing different metals, especially metals with high differences in galvanic scale position. When this is not possible, electrically shielding the two metals to avoid direct contact will prevent galvanic corrosion (Paint, Rubber washers, etc.) When shielding is not possible and different materials must be used, it is essential to consider the size scale of both materials. A good practice is to use significantly larger anodic materials in contact with small cathodic materials. Since the cathodic material is responsible for creating the galvanic current deteriorating the anode, the reaction will be slowed if only a small cathode attacks a large anode. Hence, using stainless steel screws (Cathode) to fix large aluminum housings (Anode) can be acceptable in some environments even if they show a risk of galvanic corrosion for the galvanic scale.
Galvanic Corrosion Example:
The Galvanic Potential Scale of Metals:
Steels
While many steel categories exist, we can classify them as stainless and non-stainless steel for corrosion purposes. Stainless steels contain at least 10.5% of chromium. Chromium is the element responsible for the corrosion resistance in stainless steel. It forms an oxide layer once in contact with oxygen that bonds strongly with the steel and protects it underneath. They offer excellent performances in exterior environments and can be used without additional protection, but they can still rust under certain circumstances. They can suffer from pitting corrosion or galvanic corrosion. Based on the type of environment, some additional protections can be used to increase their resistance. While they are used for their excellent corrosion resistance, they are typically weaker than structural alloy steels and significantly more expensive. Non-stainless steels are used in many designs, from bridges, buildings, pole lighting, etc. They are used primarily due to their excellent mechanical strength and low cost. Even in interior installations, the moisture in the air is enough to rust non-stainless steels. Hence, these types of steel must be protected from rust by coatings or paint.
Aluminum
Aluminum alloys used in lighting fixtures can mainly be classified into two groups: wrought alloys and cast alloys. The difference between these groups of alloys is the manufacturing techniques. Wrought alloys are processed through mechanical forming like bending, extruding, and stamping. Cast alloys are melted and cast into shape. Aluminum offers good corrosion resistance in interior or exterior installations. Corrosion resistance in aluminum comes from the natural oxide layer created when the alloy is in contact with the air. This oxide layer protects the aluminum underneath from corrosion and will prevent deterioration until the coating is damaged.
Wrought alloys are commonly used for their excellent mechanical properties and surface finish. The 6000 series is the most used and offers outstanding corrosion resistance. Due to the excellent surface finish, the oxide layer is strong and can protect the aluminum for years, but it can still suffer from galvanic corrosion.
Cast alloys also form an oxide layer in contact with oxygen, but this layer is typically less effective due to the manufacturing process. During casting or molding, micro porosities occur within the parts where oxygen is trapped. This can increase the spread of corrosion inside the part when the initial layer is damaged from scratches or an aggressive saline environment. Cast alloys are also prone to contamination during melting of the alloy. Low copper aluminum alloys offer greater corrosion resistance, but the copper concentration can vary from contamination.
How to Protect From Corrosion
Now that we have discussed the type of environments prone to corrosion and the different types of corrosion, the remaining question is how do we avoid or reduce this reaction?
Selecting Proper Material
The first step is to select the proper material based on the type of installation. As previously mentioned, stainless steel offers excellent corrosion resistance to weather but is expensive. Some stainless steels are better suited for marine environments than others. When stainless steel is not possible due to cost or strength limitations, carbon alloy steel can be used but will require additional protection. For aluminum, wrought alloy typically offers better corrosion resistance than casting alloys, but the design needs to consider shape and manufacturing limitations. When the best option for manufacturing is casting, selecting low copper aluminum alloys offers better corrosion resistance. Finally, avoiding dissimilar materials or isolating different materials can greatly reduce the risks of galvanic corrosion.
Coatings
Coatings are a great option when the raw material cannot resist a specific environment. A great example of coatings is galvanization for steel or anodization for aluminum. Galvanization is the process of coating alloy steel with a zinc layer. Hot dip galvanization can achieve this process, where the steel part is dipped in molten zinc to create a thick barrier. Hot Dip Galvanization yields exceptional corrosion resistance for years but has a poor surface finish and appearance. The zinc layer acts as a protective layer and a sacrificial layer in case of exposure of the raw steel. Galvanization can also be achieved by electro-galvanized or zinc plating. This process involves plating the steel with a thin zinc coat through electrodeposition. These parts are less corrosion-resistant than hot dip galvanization but show a greater surface finish for painting. Anodization protects aluminum parts by forming an external anodic oxide layer. This increases the corrosion resistance of aluminum parts by providing a thick, hard oxide layer. This layer can also be esthetic from metallic colors. This coating offers excellent wear resistance compared to paint.
Paint
Paint is commonly used for aesthetic purposes and better corrosion resistance. Paint acts as a barrier that shields the metal from water. This can be used as an extra protection on materials like aluminum or stainless steel that are naturally protected against corrosion. Paint can also be used to protect materials like alloy steel that would rust without any protection. Powder coating is a popular method in architectural lighting due to the high quality of the paint and the strong barrier it creates from water. While paint shields the metal, it can get damaged over time or become permeable to water. To improve the paint protection, some primers can either add protection to the raw material or improve the paint adhesion. Zinc primers are a good example that helps paint bond to the material while protecting the material underneath from corrosion.
How can we test a design to ensure proper corrosion resistance?
Corrosion is a complex reaction that can be influenced by many factors (temperature, humidity, chemicals, etc.) Hence, the best way to properly assess the corrosion resistance is to expose the design to real environments for extended periods. This is impractical since corrosion could take years to appear, slowing development.
Accelerated testing can be used to validate a design. Salt Spray testing is a standard method to evaluate corrosion resistance of parts or assemblies. Parts are exposed to saltwater fog for extended periods (from a couple of hours to thousands of hours). These tests are very popular due to their relatively low costs, and many standards like ASTM B117 control them. These tests are primarily effective at evaluating the coating effectiveness and comparing different processes or production batches. Unfortunately, there is no direct correlation between the number of hours in salt spray testing and the number of years in various environments. Some materials, like galvanized steel, which offers excellent corrosion resistance, will perform poorly in salt spray testing. Hence, salt spray testing results must be carefully interpreted to ensure different parts and coatings are compared with similar parameters. Also, comparing salt spray tests with parts exposed for years in a specific environment can help understand the resistance of a part.
Conclusion
Corrosion is a natural chemical phenomenon that will affect most metals over time. Some materials or specific conditions are more at risk of corroding. While it cannot always be avoided, it can be significantly reduced or controlled to ensure proper quality and safety for extended periods. The corrosion can be controlled by carefully selecting suitable materials and coatings and properly testing designs through accelerated testing procedures. When extreme conditions surpass the coatings or material natural corrosion resistance, proper design can reduce the corrosion impact to an aesthetic defect only.