Galvanized steel is covered with a layer of zinc to be protected from corrosion. Zinc was used in construction at the time of the destruction of Pompeii, but was first used to galvanize steel (actually iron) in 1742 and the process was patented in 1837. Galvanized steel is used for the production of shiny metal sheets, gutters and pipes for rainwater, as well as for outdoor nails. There are different procedures for galvanizing steel: hot galvanizing, electrolytic galvanizing, sherardization and spray galvanizing.
Steps
Method 1 of 4: Hot Dip Galvanizing
Step 1. Clean the surface of any contaminants
Before proceeding with any operation, the metal surface must be meticulously cleaned. The method to use for cleaning depends on what needs to be wiped off the surface.
- Dirt, grease, oil, and paint marks require the use of a weak acid, hot alkaline, or biological cleaner.
- Asphalt, epoxy, vinyl and welding slag must be cleaned with sandblasting or other abrasives.
Step 2. Wash away the rust
Washing is carried out with hydrochloric acid or hot sulfuric acid, which remove both rust and lamination scales.
In some cases, abrasion cleaning may be sufficient to remove the rust, or it may be necessary to use both the acid solution and the abrasive. In some cases, more powerful abrasives contained in cartridges are shot with compressed air at the metal
Step 3. Prepare the "flush"
In this case, a solution of zinc chloride and ammonium chloride is prepared which removes any remaining rust and any foil, protecting the metal from rust until it is actually galvanized.
Step 4. Dip the steel into the molten zinc
The molten zinc bath must be composed of at least 98% zinc and maintained at a temperature ranging between 435 and 455 degrees centigrade.
While the steel is immersed in the zinc bath, the iron contained in it reacts with the zinc, forming a series of alloy layers and an outer layer of pure zinc
Step 5. Take back the galvanized steel from the zinc bath slowly
Most of the excess zinc will drain away; the part that will not go away can be shaken and removed with a centrifuge.
Step 6. Cool the galvanized steel
Cooling the metal stops the galvanization reaction, which continues as long as the metal remains at the same temperature as when it was immersed in the zinc bath. Cooling can be achieved through one of the following ways:
- By immersing the steel in a passivation solution such as potassium hydroxide;
- Dipping the steel in water;
- Leaving the steel to cool in the open air.
Step 7. Inspect the galvanized steel
Once the galvanized steel has cooled, check it to make sure the zinc coating is in good condition, is well bonded to the steel, and is thick enough. There are several tests that can be performed to ensure that the galvanization has been successful.
The standards for performing a hot dip galvanizing and for inspecting it have been established by some specialized associations such as the International Standards Organization (ISO)
Method 2 of 4: Electrolytic Galvanizing
Step 1. Prepare the steel as for hot dip galvanizing
Steel must be cleaned and rust free before electrolytic galvanizing.
Step 2. Prepare a zinc electrolyte solution
Zinc sulfate or zinc cyanide are normally used for this solution.
Step 3. Immerse the steel in the electrolyte solution
It will react with the steel, causing the zinc to precipitate onto the steel itself, covering it. The longer the steel remains in the electrolytic solution, the thicker the cover layer will be.
While this method offers more control over how thick the zinc layer should be than hot dip galvanizing, it usually does not allow the layers to become thick in the same way
Method 3 of 4: Sherardization
Step 1. Prepare the steel as for the other galvanizing methods
Clean off dirt with acid or sandblast if necessary, and wash away the rust.
Step 2. Put the steel in a vacuum container
Step 3. Wrap the steel with zinc powder
Step 4. Heat the steel
This operation transforms the zinc powder into a liquid which, when cooled, leaves a thin layer of metal alloy.
Sherardization is the best method for galvanizing pieces of forged metal, because the galvanic layer will follow the configuration of the underlying steel. Its best use is with fairly small metal objects
Method 4 of 4: Spray Galvanizing
Step 1. Prepare the steel as for the other methods
Clean the dirt and remove the rust so that it is ready to receive the vaporization.
Step 2. Spray the metal with a well-melted layer of zinc
Step 3. Heat the coated steel to ensure a perfect bond
Galvanic coatings produced by this method are less brittle and less prone to peeling and flaking, but provide less rust protection to the underlying steel
Advice
- Galvanized steel can be further protected from corrosion if it is painted with a pigmented paint with zinc dust. However, zinc-based paint cannot be used in place of electroplating.
- Once painted, galvanized metal can have a shiny appearance.
- Galvanized steel is resistant to corrosion from contact with concrete, mortar, aluminum, lead, tin and, of course, zinc.
- Galvanization is a form of so-called cathodic protection, in which the protected metal acts as a cathode in an electrochemical reaction and the metal it protects acts as an anode or, more specifically, as a sacrificial anode, which corrodes in place of the protected metal. A metal coated with a sacrificial anode is sometimes called an anodized metal.
Warnings
- Galvanized metal has little corrosion resistance if it is in contact with any metal except aluminum, lead, tin or zinc. It is particularly exposed to corrosion with iron, steel, copper as well as with adhesives containing chlorides and sulphates.
- The zinc coating of galvanized steel is vulnerable to acid and alkaline corrosion. It is particularly vulnerable to sulfuric and sulphurous acids, which can produce a mixture of hydrogen sulfide and sulfur dioxide with acid rain, worse if rain falls from wooden shingles or moss. Rainwater can also react with the zinc coating, forming zinc carbonate. Over time, the zinc carbonate will become shiny and will eventually expose the zinc layer if not the underlying metal base to corrosion.
- Galvanized steel is more difficult to paint than non-galvanized steel.
- The zinc layer in non-galvanized steel is also vulnerable to metal weakening, because zinc tends to expand when heated and contract when cooled.
