What is hot-dip galvanizing?

Iron and steel rapidly rust and corrode. An excellent way to deter
rusting for a very long time is to hot-dip galvanize the parts, that
is, to dip them in molten zinc (rather like dipping strawberries in
chocolate).

Why is galvanizing such a good rust-preventer?

Galvanizing is a great rust preventer for several reasons:

• Galvanized coatings are metal (zinc), so they are
  much stronger and harder than paint or powder coating;
  they will adhere where paint may scrape off or chip off.

• Zinc is “anodic” to iron and steel. This means it
  will corrode preferentially, electrochemically protecting
  the steel. Even if the coating is scratched through and
  the steel is exposed, it will be protected the same way
  zinc anodes protect steel-hulled ships. No organic coating, and few metal coatings, will protect
  iron and steel from rusting where the coating is
  scratched.

• Galvanized coatings are thick. Alternatives like zinc plating (electro-galvanizing) are usually only something like one-fifth to one-tenth as thick as hot dip galvanized coatings. The life of the coating, and the part, is essentially proportional to the thickness of the zinc coating.

What are the down sides of galvanizing?

• Because galvanizing is thick, it can’t be done on
  standard nuts and bolts; rather the threads have to be
  specially cut to allow room for the galvanized coating, or the inside of nuts left uncoated.
  Zinc plating and most other electroplating alternatives are thin enough that nuts and bolts don’t
  have that problem.

• Most people don’t find galvanizing to be a very
  decorative finish.

• Unless special measures are taken, paint does not
  stick well to galvanizing.

• Some parts can’t be easily galvanized. For example,
  sealed tubes would be very dangerous to dip into molten
  zinc because water trapped inside could flash to steam
  and blow molten zinc out of the dip tank.

• It is dangerous to weld galvanized parts without
  proper hoods and excellent ventilation that assure the
  fumes will not be inhaled (look up metal fume fever).

Can galvanizing be reliably painted or powder coated?

Yes, galvanizing can be painted, but it may not be easy
— especially if painting was not anticipated when the parts
were galvanized.

If you are a homeowner or hobbyist trying to paint a
galvanized surface, the best advice (compiled from dozens of
public forum threads on this site) seems to be: clean the
surface with a scrub brush

⇦ on

eBay or

Amazon [affil link] and a detergent like trisodium phosphate

⇦ on

eBay

or

Amazon [affil link]
(TSP), rinse, neutralize the detergent and acidify/etch
the surface with diluted vinegar

⇦ in bulk on

eBay

or

Amazon [affil link] , rinse, apply epoxy or a
self-etching primer made specifically for galvanized
surfaces; avoid alkyd paints

If you are a manufacturer, the above will not be good
enough! The best plan, used by auto manufacturers, is to
employ galvanneal steel rather than galvanized steel if you
intend to paint it. Galvanneal has been annealed after
galvanizing in a way that causes the steel and zinc to
interdiffuse so that you are painting a steel alloy rather
than pure zinc.

Manufacturers may also wish to skip the chromate final
dip if the parts will be painted because the brightness this
dip imparts will not be required for painted parts. In any
case, the galvanized or galvannealed steel must be
phosphatized before beginning the painting process.

For detailed information on painting galvanized steel,
manufacturers and specifiers should see “Duplex Systems:
Painting Over Hot Dip Galvanized Steel”, which can be
downloaded as a ‘.pdf’ file free from the AGA at
www.galvanizeit.org/aga/aga-publications/download/non-member/931
www.galvanizeit.org/education-and-resources/publications/painting-hot-dip-galvanized-steel-1992

This is only a very brief intro to galvanizing. Here is a
partial list of related threads/ letters on the
Finishing.com Hotline public forum: