WHY GALVANIZATION MATTERS
LEARN ABOUT GALVANIZATION
Where the fabricated structural steel is viewed to ensure it has, if necessary, the proper venting and draining holes, bracing, and overall design characteristics necessary to yield a quality galvanized coating
Steel is immersed in a caustic solution to remove organic material such as grease and dirt, followed by dipping in an acid bath (hydrochloric or sulfuric) to remove mill scale and rust, and finally lowered into a bath of flux that promotes zinc & steel reaction and retards further oxidation of the steel (steel will not react with zinc unless it is perfectly clean)
The clean steel is lowered into a kettle containing 850° F molten zinc where the steel and zinc metallurgically react to form three zinc-iron intermetallic layers and one pure zinc layer
- Final Inspection
The newly galvanized steel is sight-inspected (if it looks good, it is), followed up by measurement of coating thickness with a magnetic thickness gauge
Constant exposure to temperatures below 390°F (200° C) is a perfectly acceptable environment for hot-dip galvanized steel. Good performance can also be obtained when hot-dip galvanized steel is exposed to temperatures above 390°F (200°C) on an intermittent basis.
The corrosion rate of zinc and how long it will provide protection is a function of the coating thickness and the amount of corrosive elements in the atmosphere. For example, in rural settings where there is less automotive/truck exhaust and plant emissions, galvanized steel can easily last for 100–150 years without maintenance. Industrial and marine locations contain significantly more aggressive corrosion elements such as chlorides and sulfides and galvanized steel may last for 50–100 years in those cases. The relationship between coating thickness and atmospheric conditions is contained in a popular graph developed by the AGA. Please see the publication hot-dip galvanizing for corrosion protection: a specifier’s guide.
The galvanized coating appearance may either be bright and shiny resulting from the presence of an outer layer of pure zinc, or duller, matte gray as the result of the coating’s intermetallic layers being exposed. Performance is not affected. Coating appearance depends on the amount of zinc in the coating.
Coating thickness depends on the thickness, roughness, chemistry, and design of the steel being galvanized. Any or all of these factors could produce galvanized coatings of non-uniform thickness.
The primary reason for vent holes is to allow otherwise trapped air and gases to escape; the primary reason for drain holes is to allow cleaning solutions and molten zinc metal to flow entirely into, over, and throughout the part, and then back into the tank or kettle.
How does the cost of hot-dip galvanizing compare to other corrosion protection systems, such as paints?
When compared with paint systems, hot-dip galvanizing after fabrication has comparable initial application costs and, almost always, lower life-cycle costs. In fact, the lower lifecycle costs of a hot-dip galvanized project make galvanizing the smart choice for today and tomorrow.
Hot-dip galvanized steel resists corrosion in numerous environments extremely well. It is not uncommon for galvanized steel to last more than 70 years under certain conditions.
The three intermetallic layers that form during the galvanizing process are all harder than the substrate steel and have excellent abrasion resistance.
Referred to as duplex coatings, zinc and paint in combination (synergistic effect) produce a corrosion protection approximately two times the sum of the corrosion protection that each alone would provide. Additionally, duplex coatings make for easy repainting, excellent safety marking systems, and good color-coding. Painting over galvanized steel that has been in service for many years also extends the life of the zinc coating.
First of all, the variety of things galvanized is broad. Structural steel (angles, channels, wide-flange beams, i-beams, h-beams), grating, expanded metal, corrugated sheets, wire, cables, plate, castings, tubing, pipe, bolts & nuts. The industries that utilized hot-dip galvanized steel range from:
- Bridge & highway (reinforcing steel for decks and column concrete, girders, stringers, light and signposts, guardrail, fencing),
- Water & wastewater treatment plants (walkway grating/expanded metal, handrails)
- Architectural (facades, exposed structural steel, lentils),
- Parking garages (reinforcing steel for concrete decks, exposed structural steel columns and barriers),
- Pulp and paper plants (structural steel, walkways, handrail),
- OEMs (motor housings, electrical cabinets, frames, heat exchanger coils),
- Electrical utilities (transmission towers, distribution poles, substations, wind turbine poles),
- Communication (cell towers),
- Rail transportation (poles, switchgear, miscellaneous hardware),
- Chemical/petrochemical (pipeline hardware, manufacturing buildings, storage tanks, walkways),
- Recreation (boat trailers, stadiums, arenas, racetracks), and many more.
There is no such thing as cold galvanizing. The term is often used in reference to painting with zinc-rich paint. Galvanizing by definition means a metallurgical reaction between zinc and iron to create a bond between the zinc and the steel of approximately 3600 psi. There is no such reaction when zinc-rich paints are applied and the bond strength is only several hundred psi.
Zinc metal used in the galvanizing process provides an impervious barrier between the steel substrate and corrosive elements in the atmosphere. It does not allow moisture and corrosive chlorides and sulfides to attack the steel. Zinc is more importantly anodic to steel — meaning it will corrode before the steel, until the zinc is entirely consumed.