Ask A Scientist Chemistry Archive

name         Adelene C.
status       student
age          13

Question -   What are the disadvantages of Plating (galvanizing)?
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Disadvantages compared to what alternative, Adelene?
Galvanizing refers to plating specifically with Zinc.
(Other metals can be electroplated, too.)
And specifically onto steel, to protect it from rust.
So I will take that to be your question.

Alternatives for rust-protecting steel are:
  - hot-dipping in molten zinc
           (* Zinc is thicker and has fewer pinholes, lasts more years and protects better)
  - plating with other metals than Zinc,   (nickel and chrome are more expensive.
            Tin might not work as well.  Cadmium and lead are toxic.)
  - using one of many types of paint    (paint includes plastic and sometimes has pores.
                    water and other chemicals go through it. does not protect as surely.)
  - bluing    ( deliberately oxidize the metal in alkaline phosphate solution.
                     The blue-black oxide is not as protective as zinc, needs care and oil.
                        * Thinner and harder, so better for sliding parts in machines.)
  - modified rust coatings    (These vary.
                   * Some last more years than zinc, and are better at resisting water and salt)
  - thick tar or plastic coatings  ( * These often last more years than zinc,
                and are better at resisting water and salt and light contact)
  - living in oil    (Machinists and mechanics do it, or used to...)
  - living in a dry or oxygen-free environment   (We may have that option in space, in decades to 
  come....)
  - living in an alkaline (opposite of acid) environment  (uh, no thanks.  Makes my skin hurt.)
  - multiple layer combinations of things above  (almost always better than one thing alone)
  - no protection ...
  - using less rustable metals ... (Stainless Steel, maybe aluminum, brass, bronze, copper, nickel,
  others...)

The *'s above are disadvantages of galvanizing.    The others are mostly advantages.

I guess the main disadvantages are, it lasts a limited time and is not very resistant to wear.
Also easily killed off, used up, by acids or water.  Zinc is even easier to dissolve in acid than iron is.
Old pieces of galvanized steel always start rusting at many tiny spots, often called "pinholes".
You will notice at playgrounds, the bars are a darker metal color where people grab with their hands.
The galvanizing is worn clear through at those places.
The lighter color is zinc, the darker color is steel, nicely seasoned by skin contact.
It should start rusting at those places, but people are handling it so often that they rub off the
rust, too.

Jim Swenson
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Adelene C.,

I see no disadvantages to plating (galvanizing) if its purpose is for sacrificial 
protection.  Sacrificial protection is the preferential corrosion of a metal coating for 
the sake of protecting the substrate metal.  For example, when zinc is in contact with a 
more noble (less reactive) metal such as steel, a galvanic cell is created in which 
electric current will flow in the presence of an electrolyte such as atmospheric water.  
Under these conditions, the zinc coating rather than the steel is affected.  I hope that 
this helps.

Sincerely,

Bob Trach
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Plating iron with zinc (galvanizing) sacrifices the oxidation of Fe to the
oxidation of Zn. This is limited by the amount of Zn that can be
economically plated onto the iron, so protection is limited. Otherwise I
do not see any particular disadvantages.

Vince Calder
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