Galvanization is a process used on steel to prevent rust and corrosion. Bare steel does not resist the elements well, especially moisture but also including chemical corrosion. Galvanization is an electrochemical reaction between steel and zinc. Zinc reacts with steel to form a corrosion-resistant layer atop the steel itself.
A common method of galvanization is called Hot Dip Galvanizing (HDG). Steel pieces are placed in a bath of molten zinc. The metals react together and the steel is removed from the bath, getting rid of excess zinc on the way. The newly galvanized steel is then dried and ready for use.
There is a note of caution about working with galvanized steel and that is if any of the galvanized metal is cut or deeply scratched or scraped, the galvanized layer will be lost and corrosion can occur at these points. Any disruption in the metal surface will need to be resealed.
Hot Dip Galvanizing: The Details
Of course, galvanization isn’t as simple as it sounds. Here are the detailed steps required for the process to work correctly.
Most importantly, the surface of the steel must be perfectly clean and properly prepared or the electrochemical reaction will not take place. This becomes evident when rust spots appear on the metal after a time. Surface preparation consists of three steps:
Degreasing and cleaning: the cleaning materials used are highly caustic.
- A hot alkaline (basic) solution is used to take grease, oil, dirt, and any other soluble materials off the surface of the steel.
- Pickling – It sounds like a strange name but it is similar to how pickles are made from cucumbers. A dilute solution of sulfuric acid or hydrochloric acid is applied to remove any surface rust or scale.
- Fluxing – This is a step where steel is placed into hot zinc ammonium chloride solution to rid the surface of any oxides and keep the clean metal from oxidizing before it can be galvanized.
Galvanization: Dry galvanizing consists of dipping the steel into a hot flux bath, allowed to dry, then immediately galvanized. Wet galvanizing takes the steel and dips it into molten zinc on top of which floats the flux materials. The metal is then put through flux on the way into the galvanizing bath.
The steel is completed bathed in molten zinc at an average of about 830F/445C. The bath must be at least 98% pure zinc according to standards. The zinc and steel interact forming a series of electrochemically bonded zinc-iron alloy layers topped by a layer of impact resistant zinc. Excess zinc is allowed to run off as the steel is removed; then the metal is cooled in a passivation solution, water, or in the air.
Inspection: Inspection is quick. The critical points are coating thickness, surface condition and appearance. The physical testing is generally simple.
There are other galvanization processes that result in the same type of zinc coating. Steel sheets can be galvanized “in-line” or “continuously” as the metal is rolled out.
Once steel has been galvanized its lifecycle is significantly prolonged adding a level of safety wherever it is used. The zinc coating, if galvanization has been performed correctly, is a durable coating that adds years of corrosion-free performance. When steel corrosion costs $423 billion in the U.S. alone, it would be foolish to use uncoated steel to save money.
A big plus: galvanization doesn’t interfere with steel recycling because the zinc is recyclable as well.