Steel windows and doors have made a monumental revival from the historic factory-style steel windows of the late 19th century. Today’s modern steel window and door might surprise you as thermal efficiency, durability and upkeep are better than ever. Additionally, thin lines, large glass, and maximum daylighting make these products enduringly popular with architects and designers from every design medium. But as many have seen historic steel windows succumb to corrosion and rust, the question arises:
How well do modern steel windows and doors hold up over time?
The Modern Steel Window
The evolution of the historic steel window unifies the classic look of wrought iron steel with the best window technologies of today. Integrating old world craftsmanship with new world technology creates a product that is inherently strong and timeless, while also energy efficient, and resistant to condensation and corrosion. Advanced window technologies such as innovative thermal design, high-performance glazing, and progressive finish options are a few features that have helped the steel window evolve into what it is today.
Progress Finish Options: Zinc Coating Offers Dual Protection
Applying a protective layer of zinc to a steel window or door product is the most common method used for corrosion resistance. Better known as galvanizing, zinc’s sacrificial properties buffer the steel from atmospheric oxidation that leads to unsightly rust. Zinc offers protection to steel in the following two ways:
- Barrier Protection: zinc acts as a barrier to the steel by providing an impervious seal from environmental corrosion.
- Cathodic Protection: Zinc provides cathodic protection at exposed areas. If the coating is scratched the zinc acts as a sacrificial anode by forming a galvanic cell that protects the base steel from corrosion.
Application of the zinc coating can be achieved through various methods including zinc-rich paint, hot-dip galvanizing, and zinc spray metalizing.
Hot-Dip Galvanizing for Thermally Broken Profiles?
Hot-dip galvanizing is the process of submerging steel into a pool of molten zinc to create a uniform thickness of protection. The hot-dip process is effective at providing a typical coating thickness between 2-6 mils (.002-.006 in). It is cautioned that thermally broken steel profiles should not be treated with this process because the high heat is subject to causing distortion and warping of the insulating features. For solid steel profiles, the risk of warping and distortion as a result of uneven heating is still a concern. Be sure to look for manufacturers who follow the guidelines for safeguarding hot-dip galvanizing as outlined in ASTM A384.
Added Protection Life with Zinc Spray Metalizing
Unknown to most, zinc spray metalizing or thermally sprayed zinc is a highly effective method for applying zinc to steel windows and doors. This method of application involves airstream spraying molten zinc particles onto an abrasive prepared steel surface. The zinc particles are flattened on impact and chemically adhere to form an even coating of corrosion protection. Unlike hot-dip, the spray metalizing process avoids significant heat input that may cause distortion and warping of the steel substrait, and cold rolled thermally broken steel profiles. Zinc spray metalizing also offers additional protection life to the product as all window and door components can be re-coated after all final welding, drilling, and machining is complete. With other galvanizing methods, this step is commonly skipped and could ultimately cause corrosion issues down the road.
Zinc spray metalizing is historically associated with providing steel with a thicker layer of protection. The life of the metal coating is proportional to the coating weight per unit area. The heavier coating thickness allows steel products to resist corrosion at levels far beyond its raw capabilities. For extended service life or highly corrosive climates, the zinc coating can be applied to steel windows and doors in excess of 20 mils (.020 in). To gain perspective, it is conservatively estimated that 10 mil (.010in) of zinc coating has a protection life of 25 years.
In addition, the natural porosity created by the sacrificial spray process provides an excellent layered foundation for decorative patina coatings and seals to adhere to. The final coating emphasizes the unique characteristics of steel while also extending the protection life of the window and door product by another 15 years.
Decades of Protection Effectiveness
Although zinc spray metalizing is new to the fenestration industry, this method has been successfully used for major structures for decades.
In France, the St. Denis Canal sluice and lock gates were zinc sprayed in the 1930’s and continue to be in excellent condition today with little to no maintenance over the years.
The Menai Straits Bridge in the UK is another example. The bridge’s suspension chains and components were zinc sprayed before World War II and received no maintenance while the war ensued. After the war, the zinc sprayed components were found to be in excellent condition while the painted areas were rusting.
More recently, the New Jersey Department of Transportation performed a structural coatings performance study on the Thomas Mathis Bridge. After marine exposure for 8 years, the zinc spray metalized coatings showed no deterioration over the test period while the painted zinc-rich coatings showed varying performance.
Furthermore, numerous studies performed by the U.S. Navy and The American Welding Society showed that properly applied metalized coatings of considerable thickness (at least 6 mils) provide 20 years of maintenance free corrosion resistance to steel bridges.
Progressively outperforming other galvanizing methods, the protection effectiveness of zinc spray metalized coatings is clear. While originally specified for infrastructure, use as a finish process for thermally broken steel windows and doors is cutting-edge for the fenestration industry.
The Modern Steel Window: Built to Last
Although historic steel windows have ties to widespread corrosion, the modern steel window and door, with the advantageous spray metalizing and handcrafted patina finish, will ensure a product that is extremely durable, virtually maintenance free, and expected to stand for generations.
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