Corrosion
Understanding Corrosion
Corrosion is a natural electrochemical reaction involving
the movement of electrolytic cells. To fully appreciate the
benefits of hot dip galvanizing, an understanding of the
cause and effect of corrosion is required.
Electrolytic cells are made up of an anodic and a cathodic
element and are found on the surface of all steels. For
corrosion to occur, the following four specific components
are required to be in contact with each other:
•an anode:
an electronegative active metal on which corrosion
occurs (the electrode where galvanic reactions
generate electrons)
•a cathode:
an electropositive noble metal protected from corrosion
(the electrode that receives electrons)
•conductive material:
the metallic connection for the anode and cathode (under
lying metal which transfers the electrical current)
•an electrolyte:
a conducting solution which carries the current (aqueous
solutions, water, moisture, dampness or other liquids)
If unprotected, the electrolytic cells of iron particles and
other impurities in the surface of steel react with moisture
and allow the formation of rust. Steel is a combination of
impurities, oxygen and metal elements both anodic (active)
and cathodic (less active). The metal elements on the
surface of a piece of steel form interlocking areas of anodes
and cathodes connected by the underlying steel which is
the conductive material.
When the steel is exposed to moisture, the electrochemical
reaction occurs. As negative electrons flow from anode
to cathode, they are charged and converted to positive
ions which in turn react negatively with hydrogen in the
moisture. The anodic area depletes and forms a pit and new
anodes and cathodes are exposed from underneath. The
cycle continues and corrosion occurs.
The greatest benefit of hot dip galvanizing is realised with
an understanding of how a zinc coating provides anodic
(sacrificial) protection to the entire steel item. This is
discussed in How Zinc Protects.
Electrolytic Cell
How Zinc Protects
When an item is hot dip galvanized, it forms a barrier
between the steel surface and moisture; without moisture
contacting the steel, corrosion cannot occur. This barrier
provides a metallurgical protective system. Bonded to the
base metal with impenetrable adhesion, it has high abrasion
qualities and shields the steel from the effects of it’s
environment.
Unique to a hot dip galvanized coating is the combination
of barrier protection and cathodic protection properties.
•Cathodic Protection
As discussed in Understanding Corrosion, the variance in
the electrical potential between zinc and steel in the alloy
layers of a galvanized coating will create an electrolytic
cell. When zinc is used to protect an item, it provides
anodic (sacrificial) properties for the base steel. In the
event a zinc coating is damaged and the base steel is
exposed to moisture, an electrochemical reaction will
occur causing the zinc anode to oxidize in preference
to the cathodic bare steel. For this reason, galvanized
coatings are referred to as sacrificial coatings or coatings
with sacrificial properties. Metals which provide sacrificial
properties to others are detailed in The Galvanic Series of
Metals on the following page.
Cathodic protection is evident in day to day fabrication and
building where:
•continuous in-line galvanized purlins, roofing sheets and
fasteners have bare metal exposed at cut ends and where
holes are punched
The cathodic reaction can also be experienced when a
hot dip galvanized item is in contact with an uncoated
piece of steel, steel filings or drill shavings. The zinc on the
galvanized item will commence to oxidize or sacrifice itself
where it is in contact with the uncoated steel.
•in-line galvanized wire or mesh is cut to length
Left in contact for an extended period, the benefits of
the zinc’s cathodic properties are unnecessarily depleted;
wasted on an object not associated with the galvanized
piece of steel.
•zinc plated nuts have uncoated internal threads.
In the above instances, the existing coating protects the
uncoated areas and will continue to do so whilst sufficient
zinc is present.
The cathodic properties of a hot dip galvanized coating will protect small areas of bare metal.
Electrolyte
(Water or Humidity)
Anodic and Cathodic Properties
H2O
H2O
A
A
C
C
A
A
A
C
C
A
C
C
Electrolyte
(Water or Humidity)
C
Anodic and Cathodic Cells
Anodic and cathodic cells are present on the surface of
the steel. When the steel is exposed to water (electrolyte),
electrons flow through the water from the anode cells to
the cathode cells. The electrons are then transferred via the
underlying steel (conductive metal) back to the anode. The
anodes subsequently dissolve and rust forms.
Zinc Coating
H2O
A
H2O
A
C
A
A
C
C
C
C
C
A
C
C
C
A
C
C
C
Steel
A
A
C
C
C
C
C
C
C
Hot Dip Galvanizing is a BARRIER coating
Hot dip galvanized coatings provide a barrier, preventing
moisture from attacking the steel surface.
ADDED BONUS — Most steel is cathodic when
compared to zinc.
As zinc is one of the most ACTIVE (electronegative) metals,
all cells on the surface of the steel under the galvanized
coating become cathodic.
Zinc Coating
A
C
C
C
C
Galvanic Protection
H2O
A
C
C
C
C
Electrolyte
(Water or Humidity)
A
A
C
C
C
C
Hot Dip Galvanizing also offers
CATHODIC PROTECTION
Should the galvanized coating be damaged and an area of
bare metal is exposed, the anodes will detract the moisture
from attacking the cathodes. Cathodic protection occurs
as the anodes “sacrifice” themselves. As long as anodic
material exists (zinc) rust will be prevented from forming.
This table details metals in decreasing order of electrical
activity as found when submerged in seawater. It is used
as a guide to determine which metal will have a greater
tendency to lose electrons or experience galvanic corrosion
when electrically connected to another.
At the top of the list are metals such as magnesium,
zinc and aluminium. Metals located here are the most
susceptible to corrosion and can be used as a sacrificial
element to protect other metals lower on the table. They
are referred to as being anodic or less noble. In turn they
will sacrifice their properties to protect lower listed metals.
Whilst magnesium, aluminium and cadmium are also listed
above steel, zinc is cost efficient and is widely used for
this purpose.
Galvanic Series of Metals
Anodic
•Galvanic Series of Metals
As discussed in Cathodic Protection, the galvanized zinc
coating provides sacrificial properties to the surface of
the steel. Metals which provide protection to cathodic
elements are detailed in The Galvanic Series of Metals.
Mild steel
Cast Iron
Stainless steel, type 410 (active)
Lead-tin solder, 50/50
Stainless steel, type 304 (active)
Stainless steel, type 316 (active)
Lead
Tin
Brasses
Aluminium bronzes
Copper
Copper-nickel alloys
Monel
Titanium
Cathodic
Further examples of the anodic and cathodic effect are
found where other metallic objects are in contact with a
zinc coated item. This is discussed in Galvanic Corrosion.
Cadmium
Gunmetals
Metals listed at the lower end of the table are cathodic
metals. These are least susceptible to corrosive attack
and are referred to as being more noble and have less
electron activity.
The ability to provide cathodic benefits is one of the key
advantages of a hot dip galvanized coating. In contrast,
paint and other non-galvanized coating systems depend
on their ability to provide a seal over the steel surface.
Paint systems also require anti-corrosive inhibitors to
be added. Should the coating fail or become damaged,
barrier coatings offer little if any protection to the exposed
steel and corrosion will quickly commence and spread.
Magnesium
Zinc
Aluminium
Stainless steel, type 304 (passive)
Stainless steel, type 316 (passive)
Silver
Gold
Platinum
Any element listed above mild steel in the Galvanic Series
of Metals is anodic and will sacrifice itself to protect the
cathodic steel.
•Galvanic Corrosion
Galvanic corrosion occurs when an anodic element is in
contact with a cathodic metal whilst subject to moisture.
Zinc, (anodic) a more active metal than most, will rapidly
lose electrons and sacrifice its properties to corrode in
preference to the lesser (cathodic or more noble) metal.
Contact of zinc coated items with aluminium, cadmium
and stainless steel is generally fine in moderate
environments. However, in contact with metals such
as copper, the loss of properties of the zinc will be
extremely high.
An example of galvanic or electrolytic corrosion is where
dissolved particles of copper or brass in run off water from
pipes is in contact with a galvanized item. The cathodic
properties of the zinc will be activated; it will corrode in
preference to the copper; in turn wasting its energy and
depleting its ability to protect the actual steel item it is
coating. Other cathodic metals that have a detrimental
effect when in contact with zinc in a moist environment or
in liquid, include cast irons, chromium, bronze, nickel and
hard solders.
As discussed in Differentiating Zinc Coatings, other
coatings offer various levels of corrosion protection and
service life. When two or more different zinc coatings are
in contact, they will act independently. When moisture is
present on both, the thinnest zinc coating type will be the
first to oxidize and corrode. This is often evident where
zinc plated fasteners are attached to hot dip galvanized
structural steel and exposed to rain and moisture; the
electroplated items will corrode rapidly in comparison.
Hot dip galvanized items in contact with each other rarely
form issues. However, accelerated anodic reaction may
occur between two or more hot dip galvanized items in
areas which experience continual extreme high levels
of humidity.
Sacrificial properties of zinc is demonstrated where bare metal is
exposed at cut edges.
•Other Materials and Hot Dip Galvanized Coatings
Galvanized coatings perform well when exposed to
sewerage, soaps, detergents, diesel, fuel, glycerine, mineral
lubricants, refrigerants and dried timber products. When
moist, concrete and mortar may initially etch the surface of
a hot dip galvanized coating, however this should not pose
an issue to the corrosion protection properties of the zinc.
The following materials are detrimental to zinc and hot dip
galvanized items should not be placed in contact with:
•strong acid solutions
•freshly treated timber
•insecticides in solution
•unseasoned timber
•organic lubricants