Another aspect of hot-dip galvanizing is its extreme durability, a key benefit for transportation elements across the spectrum. There are three keys to hot-dip galvanizing's durability: Abrasion Resistance, Complete Coverage, and Uniform Protection.
A unique characteristic of the hot-dip galvanized (HDG) coating is the development of tightly-bonded (3,600 psi), abrasion resistant intermetallic layers. This photomicrograph (below) is a cross-section of a galvanized steel coating showing the three intermetallic layers (Gamma, Delta, and Zeta) and top layer of pure zinc (Eta). During the galvanizing process, these layers develop naturally during a metallurgical reaction between the iron in the steel and zinc in the kettle. The photomicrograph also notes the hardness of each of the layers as a Diamond Pyramid Number (DPN). As you can see, the three intermetallic layers are harder than the base steel, while the eta layer has ductility which makes damaging the HDG coating very difficult.
HDGs abrasion resistance is beneficial in several transportation markets. For instance, kicked up rocks and road debris on bridges and highways could easily scratch structure or guardrail coatings with less bond strength. Also, airport baggage handling systems need corrosion protection that will withstand the rough abuse of thrown luggage. Other corrosion protection systems, such as paint, only have bond strength of 300-600 psi. Due to rough construction environments, immersion in soil, and exposure to the elements, hot-dip galvanized steel elements in mechanically stabilized earth (MSE) structures capitalize on the dependable durability of the zinc coating. The abrasion resistant intermetallic layers of hot-dip galvanizing can withstand these rigors without damage or maintenance.
Hot-dip galvanizing is a total immersion process meaning the steel is fully submerged into cleaning solutions and the molten zinc coating all interior and exterior surfaces. This complete coverage ensures even the insides of hollow and tubular structures and the threads of fasteners are coated. As corrosion tends to occur at an increased rate on the inside of hollow structures where humidity and condensation occur, interior coverage is very beneficial. Hollow structures that are painted have no corrosion protection on the inside. Fully coating fasteners is equally important as they are utilized at connection points which are critical to structural integrity.
In addition to complete coverage, galvanized coatings also provide uniform coating thickness ensuring uniform corrosion protection throughout the piece, including vulnerable corners and edges. This is important when dealing with items such as baggage handling conveyors and pushers as well as handrail and bus/train station elements. Corners and edges of these items often have more potential for scratching or rubbing abrasion that would penetrate and weaken the already thinner areas of brush and spray applied coatings.
Complete, uniform coverage can benefit transportation infrastructure elements such as signage, electrical poles, handrails, etc. - made with hollow tubular materials which are protected inside and out by the consistent zinc coating. Protecting the entire fastener, including threads, along rail lines is critical to prevent maintenance repairs and keep tracks safe. Similarly, galvanized rebar is not allowed any voids in its coating, unlike epoxy-coated rebar, so there are no weak points when embedded in the concrete of bridges, runways, and other infrastructure.