Corrosion prevention: methods to protect building structures from environmental damage
Corrosive damage is pertinent to all construction jobs and should therefore be accounted for during the design phase of any project. Often observed as "rust", corrosion is the progressive deterioration of metals through an oxidation process. In practice, corrosive damage results in an unpleasant building appearance, compromises structural integrity, and damages internal building structures. Fortunately, proper project planning can absolve architects, designers, and contractors of these potential issues
Strategic material selection to prevent corrosion in building structuresCorrosion is precipitated by environmental factors like humidity, air salinity, and chemical content. However, the use of certain materials can limit or completely negate damage introduced from these conditions.
Strategic utilization of rust-resistant steelsIron and steel are ubiquitous components in nearly all construction jobs. However, some alloy steels have limited corrosive resistance due to their high carbon content. Selecting a steel alloy with low carbon content, such as 300 series stainless steel, can diminish the chances of corrosive damage. Note, however, that as the corrosive resistance of a material is improved, its structural strength characteristics decrease and its cost tends to increase. For this reason, construction projects with less demanding needs for corrosion prevention often opt for the more affordable (but less effective) 400-series stainless steel.
Application of protective exteriorsTo decrease cost but improve rust protection in harsh environments, a number of different coatings can be applied to (typically steel) building components to prevent corrosion. One method, called galvanizing, applies a thin layer of zinc or magnesium to steel components. This coating will sacrifice itself to protect the structural integrity of the base component.
While this sacrificial property of metals can be exploited to extend the lifetime of certain parts, it can also work adversely if not considered when dissimilar metals are in contact. Known as galvanic or bimetallic corrosion, dissimilar metals, when in contact with each other and an electrolyte like water, will cause one material to rust faster than the other. Most commonly, this type of damage is seen when steel and copper are in contact on the exterior of a building, but many other combinations can also cause corrosion. The chart below can be utilized to determine which metals can safely be used in conjunction.
Construction methods to prevent corrosionFor over 80 years, architects have leveraged rainscreen concepts to extend the lifespan of exterior components on buildings against corrosive elements. This preventative method relies on the use of an exterior frame to act as the building's protective shell. The outermost shell will make direct contact with rain, wind, and corrosive exterior conditions to shield the inner walls from corrosive damage. More sophisticated rainscreens balance air pressure between the interior building and exterior frame to prevent condensation from forming on the inner structure. Certain building codes will require the use of a rainscreen, and the designer should consider this during the planning process.
To learn more about how rainscreens can protect buildings from corrosive structural damage, we welcome you to explore Nvelope systems and products.
Protective rust-proof top coatsMany components can also be specified with a rust-preventative top coat. These come in a myriad of types with varying properties. Epoxy coatings provide good strength but are susceptible to color fading. Alkyd enamels have comparable strength but are less susceptible to color damage from UV rays. Polyurethane coatings typically have the best strength characteristics and UV resistance.
As coating effectiveness improves, cost also tends to increase. A designer should consider how much corrosion protection is required for the structure, and ensure that the top coat color matches other building components.
Warranty inquiriesSome component manufacturers specify warranties on their parts regarding corrosive resistance. It's typically a good idea to ask suppliers about potential warranties, as this is a good indicator of both product performance and supplier confidence.
Frequently asked questions about preventing corrosion in construction projects
How can I determine how much corrosion protection I need?
First, it’s important to think of where in, or on the building will the fastener be used. If used on the interior of the building, where moisture is going to be controlled by the HVAC system, corrosion will likely not be much of a concern. For this reason, interior fasteners that will not see any weathering are typically installed with zinc plating or a phosphate coating.
If the fasteners are being used on the exterior of the building, whether it be on the roof or an exterior wall, then much more thought should be put into the corrosion of the fasteners. For exterior fasteners, the application of the connection and the location of the building are the driving factors in corrosion potential. While uncommon, even the contents on the interior of the building can cause corrosion. At a minimum, exterior fasteners should be coated with a zinc-rich coating like VistaCoat. This is generally acceptable for metal panel attachment in parts of the country that are not near the coastline. When the building is closer to the coastline, humidity and salt become a concern, and more corrosion protection is needed. For these applications, capped fasteners are common. Stainless steel and Zinc-aluminum alloys are common materials used to increase the corrosion protection of the heads of fasteners when this extra corrosion protection is needed. When the building gets to within a couple miles of the coast, then considerations should be made to use fasteners that are made from stainless steel, particularly 304 stainless steel which is the most corrosion-resistant and commercially available alloy of stainless steel. ‘
Stainless steel should also be considered in Rainscreen applications where moisture is considered to be always present, as well as in aluminum applications to help prevent galvanic corrosion.
What are early signs of corrosion caused by weather?
The discoloration is the first sign that corrosion is happening. For a fastener that is zinc plated, or that has a zinc-rich coating, white rust will likely be the first sign of corrosion. White rust is what happens when the zinc is sacrificing itself and corroding. This can happen quickly, and is why zinc plating is often referred to as a sacrificial coating. White rust in and of itself is not necessarily cause for alarm. However, white rust often precedes red rust, which is a sign that the fastener itself is corroding. Red rust is caused by the iron in the steel fastener oxidizing into iron oxide. The problem with red rust is that it’s effectively removing the iron from the steel, and over time this can cause fastener or connection failure. If you’re seeing red rust, it’s likely because the fastener that was installed was not selected correctly, and fastener replacement with more corrosion protection should be considered.